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Storm Prediction Center

The Storm Prediction Center (SPC) is a national forecast division of the National Weather Service under the National Oceanic and Atmospheric Administration, tasked with issuing timely predictions for severe thunderstorms, tornadoes, and extreme fire weather threats across the United States.^[1]^[2]
Headquartered at the National Weather Center in Norman, Oklahoma, the SPC employs a staff of meteorologists who produce convective outlooks extending up to seven days ahead, mesoscale discussions for short-term developments, and coordinated watches for imminent severe weather events.^[3]^[4]
Originating from early centralized severe weather units in the 1950s, the SPC has evolved through advancements in radar technology, numerical modeling, and data integration to enhance forecast accuracy and lead times, thereby supporting public safety and emergency response efforts.^[5]^[2]
Its products, including categorical risk assessments for hail, wind, and tornado potential, serve as critical guidance for local National Weather Service offices and enable proactive measures against convective hazards that annually cause significant loss of life and property.^[1]^[4]
While the SPC's operations have been recognized for improving severe weather warning efficacy, recent proposals under federal efficiency initiatives have targeted its facilities for potential relocation or consolidation, raising concerns about impacts on forecasting continuity amid ongoing threats like tornado outbreaks.^[6]^[7]

History

Origins and Predecessors

The development of centralized severe weather forecasting in the United States accelerated in the late 1940s following major tornado outbreaks that highlighted deficiencies in existing systems. On March 20, 1948, an F3 tornado struck Tinker Air Force Base near Oklahoma City, Oklahoma, destroying over 100 aircraft and causing $10 million in damage (equivalent to approximately $130 million in 2023 dollars). This event, combined with a subsequent F3 tornado on March 25, 1948, that again targeted the base, prompted Air Force meteorologists Major Ernest J. Fawbush and Captain Robert C. Miller to issue the first successful operational tornado forecast earlier that day, predicting severe thunderstorms capable of producing tornadoes across central Oklahoma. Their forecast, based on pattern recognition from upper-air data and surface observations, was verified when the tornado materialized, marking a breakthrough in applying scientific methods to short-term severe storm prediction.^[8]^[9] In response, the U.S. Air Force established the Severe Weather Warning Center (SWWC) at Tinker Air Force Base in 1948 to provide tornado and severe thunderstorm advisories specifically for military sites nationwide, employing techniques like hodographs and instability indices derived from radiosonde data. The civilian U.S. Weather Bureau, facing pressure to develop comparable capabilities for public protection, created the Severe Local Storms (SELS) unit in 1952 as its dedicated severe weather forecasting arm. Initially headquartered in Washington, D.C., SELS issued experimental tornado watches and severe thunderstorm warnings, drawing on Air Force methods while emphasizing broader public advisories; it relocated to Kansas City, Missouri, in the mid-1950s to leverage central U.S. weather patterns.^[10]^[5] SELS operated until 1966, when it was reorganized and renamed the National Severe Storms Forecast Center (NSSFC) under the newly formed Environmental Science Services Administration (predecessor to NOAA). The NSSFC, still based in Kansas City, expanded SELS's mandate to include probabilistic convective outlooks and mesoscale analyses, integrating radar and satellite data as they became available, and coordinated with local Weather Bureau offices for watch issuance. This center functioned as the direct institutional predecessor to the Storm Prediction Center, refining forecasting protocols amid growing computational resources and observational networks through the 1970s and 1980s.^[11]^[5]

Establishment and Early Operations

The Storm Prediction Center (SPC) was established in January 1997 through the relocation of its predecessor, the National Severe Storms Forecast Center (NSSFC), from Kansas City, Missouri, to Norman, Oklahoma, where it was colocated with the National Severe Storms Laboratory (NSSL).^[12] This move, completed by early 1997, marked the formal transition of forecast operations to the new facility at the University of Oklahoma's National Weather Center campus, enabling enhanced collaboration between forecasters and researchers.^[5] The renaming to SPC had occurred in October 1995, under the leadership of Joseph T. Schaefer as director, reflecting a shift toward predictive emphasis over mere forecasting.^[13] Early operations focused on maintaining continuity in severe convective weather forecasting while integrating advanced computational tools. By early 1997, the center transitioned from legacy mainframe systems to UNIX-based workstations under the NAWIPS (NAWIPS AWIPS Interactive Processing System) framework, improving real-time data analysis and product dissemination.^[13] Staff, numbering around 20-30 meteorologists initially, issued daily convective outlooks, mesoscale discussions, and severe thunderstorm/tornado watches for the contiguous United States, drawing on radar, satellite, and numerical model data from the National Centers for Environmental Prediction (NCEP).^[5] This period saw operational testing of probabilistic tornado forecasts and enhanced fire weather outlooks, amid events like the May 27, 1997, Texas tornado outbreak, which tested the center's nascent capabilities in high-risk environments.^[14] The colocation with NSSL facilitated immediate access to experimental research, such as dual-polarization radar prototypes and storm-scale modeling, which informed refinements to watch issuance criteria and outlook probabilities during the first years.^[12] Operations ran 24/7 with three shifts, emphasizing causal analysis of synoptic patterns, instability parameters (e.g., CAPE values exceeding 2000 J/kg), and shear vectors to predict supercell and squall line development, prioritizing empirical verification over model-dependent assumptions.^[5] By late 1997, these efforts laid groundwork for expanded product suites, though challenges persisted in data latency and forecaster training on new interfaces.^[13]

Key Developments and Expansions

In October 1995, the National Severe Storms Forecast Center (NSSFC) was renamed the Storm Prediction Center (SPC) to better reflect its focus on predictive forecasting for severe convective weather events across the contiguous United States.^[15] This rebranding occurred amid broader National Weather Service (NWS) modernization efforts, which included upgrades to observational networks and computing infrastructure to enhance forecast accuracy and lead times.^[12] In 1997, the SPC relocated from Kansas City, Missouri, to Norman, Oklahoma, co-locating with the National Severe Storms Laboratory (NSSL) to facilitate closer integration between operational forecasting and severe weather research.^[12] This move, part of the NWS's consolidation of specialized centers, improved access to advanced radar data from the newly deployed WSR-88D Doppler network and supported collaborative development of forecasting tools.^[16] By 2006, the SPC shifted a short distance to the National Weather Center campus in Norman, further embedding it within a hub of meteorological expertise including university partners.^[13] The SPC expanded its convective outlook products during this period, introducing Day 3 forecasts around the early 2000s to provide earlier guidance on severe thunderstorm risks up to 72 hours ahead.^[17] In October 2014, outlook risk categories were refined from four (marginal, slight, moderate, high) to five by adding an "enhanced" level, allowing finer differentiation of severe potential based on empirical verification data.^[18] These changes stemmed from ongoing evaluations showing improved skill in probabilistic guidance, enabling better public and emergency manager preparedness.^[19] The SPC also broadened its mandate to include fire weather outlooks, with dedicated forecasting beginning in the early 2000s to address critical fire spread risks from dry thunderstorms and wind events.^[20] By the 2010s, this expanded to issuance of Extremely Critical ratings for high-confidence extreme fire weather days, incorporating ensemble model data for national-scale predictions.^[21] These developments aligned with NWS priorities for multi-hazard forecasting, leveraging SPC's convective expertise to mitigate wildfire threats in western states.^[22]

Organizational Structure

Mission and Mandate

The Storm Prediction Center (SPC), a component of the National Weather Service (NWS) within the National Oceanic and Atmospheric Administration (NOAA), has the core mission of providing timely and accurate forecasts and watches for severe thunderstorms and tornadoes across the contiguous United States. This includes issuing probabilistic convective outlooks that delineate risks of severe weather hazards such as tornadoes, large hail (typically ≥1 inch in diameter), and damaging wind gusts (≥58 mph), extending up to eight days in advance.^[23] The center's forecasting efforts prioritize the identification of organized convective activity capable of producing these threats, drawing on numerical weather models, satellite data, radar observations, and surface analyses to generate guidance that supports emergency managers, media, and the public.^[24] SPC's mandate aligns with the broader NWS objective of delivering weather forecasts, warnings, and decision-support services to protect life and property while enhancing economic resilience.^[25] Established as the national hub for severe convective weather prediction, the center issues mesoscale discussions to highlight evolving threats and coordinates with local NWS forecast offices for watch issuance, ensuring a seamless transition from national-scale outlooks to localized warnings.^[26] This operational framework emphasizes probabilistic risk communication over deterministic predictions, reflecting the inherent uncertainties in mesoscale convective systems, and mandates continuous verification against observed events to refine forecast techniques.^[27] In fulfilling its mandate, SPC maintains a focus on the contiguous United States, excluding Alaska, Hawaii, and U.S. territories, due to the distinct meteorological regimes in those regions handled by other NWS centers.^[1] The center's products, updated multiple times daily during peak severe weather seasons (typically spring and summer), serve as authoritative guidance for mitigating impacts from events that, on average, produce over 1,200 tornadoes and thousands of severe thunderstorm reports annually in the U.S.^[28] This mission-driven approach underscores a commitment to evidence-based forecasting, prioritizing empirical data from historical climatology—such as the 1982–2011 severe weather database—to inform risk assessments without overreliance on unverified models.^[29]

Location, Staffing, and Collaboration

The Storm Prediction Center (SPC) is headquartered in Norman, Oklahoma, within the National Weather Center, a collaborative facility shared with the local National Weather Service (NWS) Weather Forecast Office and the National Severe Storms Laboratory (NSSL).^[30] This location, selected for its proximity to severe weather-prone regions and research infrastructure, supports integrated operations and real-time data access.^[31] SPC maintains a core staff of 22 full-time forecasters supplemented by 10 fill-in and support personnel, enabling continuous monitoring and forecasting across multiple shifts.^[32] Shifts are typically staffed by 4-6 forecasters, collectively offering over a century of severe weather expertise to handle high-volume convective seasons.^[32] SPC collaborates extensively with NWS Weather Forecast Offices nationwide, conducting in-person coordination with the Norman office and virtual watch conferences with affected local offices to issue timely severe weather watches.^[31] It also partners with NSSL and NWS headquarters for research-to-operations transitions, including experimental forecasting tools and hazardous weather testbeds.^[33] These efforts extend to broader NOAA entities for data integration and verification, enhancing national severe weather response.^[30]

Core Forecasting Products

Convective Outlooks

The Storm Prediction Center (SPC) issues convective outlooks to forecast the likelihood and intensity of severe thunderstorms, including risks of tornadoes, large hail, and damaging winds, across the contiguous United States up to eight days in advance.^[34] These outlooks delineate areas of general thunderstorm activity and escalating severe weather risks using a color-coded categorical scale: general thunderstorms (light green), marginal (dark green), slight risk (yellow), enhanced risk (orange), moderate risk (red), and high risk (magenta).^[35] Day 1 outlooks, covering the current day from 1200 UTC to 1200 UTC the next day, are updated five times daily at approximately 0600, 1300, 1630, 2000, and 0100 UTC, providing the highest resolution guidance.^[36] For Day 1 and Day 2 outlooks, forecasters specify areal probabilities of severe weather hazards within 25 miles of any point, including overall severe thunderstorm probability (any hail ≥1 inch, wind gust ≥58 mph, or tornado), alongside subtype probabilities: tornado (EF2+ intensity), hail (≥2 inches), and damaging wind (≥74 mph gusts).^[37] These probabilities inform risk category assignment; for instance, a marginal risk indicates isolated severe storms with limited organization, longevity, or coverage, typically corresponding to low probabilities such as <5% for severe thunderstorms. Slight risk denotes scattered severe storms with some organization, enhanced risk widespread severe potential with isolated very large hail or significant tornadoes, moderate risk organized clusters or lines producing widespread severe weather, and high risk rare widespread, long-lived outbreaks with multiple intense tornadoes or extreme hail and winds.^[38] Day 3 outlooks, issued once daily around 0730 UTC, retain categorical risks but omit subtype probabilities due to increasing forecast uncertainty.^[36] Days 4–8 outlooks, updated daily, focus on severe thunderstorm probabilities of 15% or 30%+ within 25 miles, without categorical severe risk areas, emphasizing climatological analogs and model ensembles for longer-range convective threats.^[34] The categorical system, refined over time with input from verification studies, prioritizes spatial depiction of severe potential to guide emergency managers and media, though high risks are issued sparingly—typically in peak spring seasons like March through May—to reflect exceptional threat levels.^[39] Verification archives since 2003 demonstrate probabilistic skill, particularly for Day 1–2 outlooks, outperforming persistence forecasts.^[36]

Mesoscale Discussions

Mesoscale Discussions (MCDs) are concise, short-term guidance products issued by the Storm Prediction Center (SPC) to address areas of current or anticipated hazardous convective weather on the mesoscale, typically spanning tens to hundreds of kilometers.^[40] They focus on evolving features such as convective clusters, outflow boundaries, or drylines that could produce severe thunderstorms, including large hail, damaging winds, or tornadoes.^[26] MCDs serve to notify National Weather Service field offices, emergency managers, and the public of potential severe weather threats when broader convective outlooks indicate elevated risk but specific watch issuance remains uncertain.^[41] MCDs are issued on an as-needed basis throughout the year, often multiple times per day during active severe weather periods, with numbering reset annually starting from 0001 in January.^[40] Criteria for issuance include observational evidence from radar, satellite, and surface data combined with short-term model guidance indicating imminent severe potential over a limited area, typically when a watch is not yet justified but monitoring is warranted.^[26] They are generally released 1 to 3 hours prior to potential watch initiation, providing lead time for preparation while allowing forecasters to refine assessments as conditions evolve.^[26] Since April 9, 2013, responsibility for heavy rainfall MCDs has transferred to the Weather Prediction Center, concentrating SPC efforts on convective and severe wind threats.^[40] Each MCD follows a standardized text format, beginning with the issuance time in UTC, affected states or regions, areas of primary concern, and a validity period usually lasting 1 to 2 hours.^[42] This is followed by a summary paragraph outlining the severe weather hazards and confidence in development, succeeded by a technical discussion paragraph detailing supporting evidence from mesoscale analyses, such as instability parameters, wind shear profiles, and ongoing storm modes.^[42] Accompanying graphics, when included, delineate the discussion area and may overlay radar or model data for visual context.^[40] Updates or superseding MCDs can be issued if threats intensify or shift, ensuring timely communication.^[40] In the SPC forecasting process, MCDs bridge the gap between probabilistic convective outlooks and deterministic watches, enabling rapid response to mesoscale convective system initiation or upscale growth.^[41] Archives of MCDs date back to January 1, 2004, allowing verification of forecast rationale against observed outcomes.^[40] While primarily text-based, they incorporate probabilistic language on watch likelihood, such as "watch possible" or "watch unlikely," to guide local warning decisions without preempting them.^[26]

Severe Weather Watches

The Storm Prediction Center (SPC) issues severe weather watches to alert the public and emergency managers to areas where organized severe thunderstorms or tornadoes are expected to develop, typically providing 4 to 8 hours of lead time for preparation.^[43] These watches cover approximately 25,000 square miles on average and are delineated in collaboration with local National Weather Service (NWS) offices to specify affected counties or parishes.^[44] SPC issues roughly 1,000 such watches annually, focusing on threats that persist for hours rather than isolated or short-lived events, which are handled via local warnings instead.^[44] Severe weather watches encompass two primary types: tornado watches and severe thunderstorm watches. A tornado watch is issued when conditions favor multiple tornadoes, including the potential for intense (EF2 or stronger) tornadoes, alongside severe hail (≥1 inch diameter) or damaging winds (≥58 mph).^[43] ^[45] In cases of high confidence for long-track violent tornadoes, SPC includes "Particularly Dangerous Situation" (PDS) wording, targeting a verification rate of at least 75% for multiple intense tornadoes.^[44] A severe thunderstorm watch applies to organized convection expected to produce at least six severe events, such as widespread significant hail (>2 inches) or strong winds (>75 mph), but without a primary tornado threat.^[43] PDS designation for severe thunderstorm watches is reserved for destructive scenarios like large bow echoes with winds ≥80 mph or baseball-sized hail.^[43] ^[46] The issuance process begins with SPC forecasters monitoring convective outlooks and issuing mesoscale discussions to highlight emerging threats, often 1-2 hours before a watch.^[43] Watches are coordinated with local NWS forecast offices to ensure accurate geographic boundaries, with lead time goals of 45 minutes for severe thunderstorm watches and up to 2 hours for initial tornado events.^[43] Unlike warnings, which indicate imminent hazards and are issued by local offices, watches emphasize the need for heightened awareness rather than immediate action.^[44] Extensions, cancellations, or replacements are managed through consultation between SPC and local offices, reflecting evolving storm conditions.^[43] Probabilities stated in watches represent the likelihood of severe events across the entire area, such as a 70-95% chance for high-confidence tornado watches, differing from point-based outlook probabilities.^[44] Verification studies indicate low false alarm rates, with approximately 10% of tornado watches producing no severe reports, underscoring their role in balancing coverage and specificity.^[47]

Specialized Products

Fire Weather Outlooks

The Storm Prediction Center's Fire Weather Outlooks delineate areas across the contiguous United States susceptible to significant wildfire ignition or rapid spread risks arising from the interaction of pre-existing dry fuel conditions and forecasted adverse weather elements, such as low humidity, strong winds, high temperatures, and dry lightning.^[48] These products serve fire management agencies by providing probabilistic, categorical assessments to inform resource allocation and suppression strategies.^[21] Outlooks are issued for operational periods spanning Day 1 (current day, valid from approximately 1200 UTC to 1200 UTC the next day), Day 2, and extended forecasts through Day 8, with update cycles tailored to each: Day 1 at around 1517 UTC, Day 2 at 1746 UTC, and Days 3-8 at 2042 UTC.^[21] Graphical depictions highlight risk zones using color-coded categories—Elevated, Critical, and Extremely Critical—to indicate escalating potential for extreme fire behavior.^[21] For dry thunderstorm threats, subcategories include Isolated Dry Thunder (ISODRYT) for elevated risks and Scattered Dry Thunder (SCTDRYT) for critical levels.^[48] Issuance criteria for non-thunderstorm-driven (wind and humidity-focused) events emphasize sustained wind speeds, relative humidity (RH), temperature, fuel dryness, and event duration. Critical areas require winds of at least 20 mph (15 mph in Florida), RH at or below regionally variable thresholds (e.g., 15-35%), temperatures exceeding 50-60°F seasonally, dry fuels per Geographic Area Coordination Center definitions, and persistence for three or more hours.^[49] Extremely Critical designations apply to rarer, high-confidence scenarios with winds of 30 mph or higher (25 mph in Florida), RH one-third below thresholds, very dry fuels, elevated temperatures (60-70°F or more), and similar durations, often amid exceptional drought or significant deviations from climatological norms.^[49] For dry thunderstorm scenarios, Critical (Scattered) criteria include lightning coverage of 40% or greater with minimal rainfall (≤0.10 inches), RH at or below regional thresholds, dry fuels, temperatures above 50-60°F, and at least three hours of conditions conducive to fire starts from dry lightning.^[49] Isolated Dry Thunder alerts feature lower lightning coverage (10-39%) under similar dry fuel and temperature constraints but without mandatory low RH persistence.^[49] Fuel dryness thresholds draw from the National Fire Danger Rating System, utilizing Energy Release Component (ERC) percentiles and 100-hour fuel moisture levels aligned with GACC standards for "dry" or "very dry" classifications.^[49] Supplementary indices like the Fosberg Fire Weather Index exceeding 40-50 often support delineations.^[49] Web archives of Fire Weather Outlooks date back to June 4, 2002, enabling historical review and verification analyses.^[21] A dedicated 2021 verification study of Day 1 outlooks quantified their performance in anticipating realized fire weather threats, underscoring the products' role in probabilistic forecasting amid variable fuel and meteorological inputs.^[50]

Experimental and Emerging Tools

The Storm Prediction Center maintains an experimental forecast tools webpage featuring specialized analyses and model outputs to support severe weather prediction, including hourly mesoanalysis graphics blending surface observations with model data across regional sectors, observed sounding analyses with overlaid forecast parameters, and soundings climatology for over 60 parameters derived from historical radiosonde data.^[51] These tools, updated as of October 2022, enable forecasters to evaluate atmospheric instability and shear in real time, though they remain non-operational and subject to ongoing refinement based on model performance.^[51] Advanced ensemble model guidance, such as the High-Resolution Ensemble Forecast version 2 (HREFv2), provides probabilistic outputs for severe, winter, fire, and precipitation hazards using convection-allowing members, while the Short-Range Ensemble Forecast (SREF) offers 22-member ensemble plumes for point forecasts at over 1,000 stations, verified against observations. High-Resolution Rapid Refresh (HRRR) hourly graphics further aid in short-term convective evolution assessment. Through collaboration in the NOAA Hazardous Weather Testbed's Experimental Forecast Program, the SPC evaluates emerging convection-allowing systems like the Warn-on-Forecast System (WoFS), which assimilates radar, satellite, and surface data to generate rapidly updating (every 15-30 minutes) probabilistic forecasts of tornadoes, hail, and severe winds out to 3-4 hours, tested annually in Spring Forecasting Experiments since 2000 to inform operational transitions.^[52]^[53] Machine learning applications, integrated into WoFS prototypes, enhance hazard probabilities; for instance, the WoFSCast model processes WoFS outputs to predict thunderstorm initiation and intensity, routinely reviewed by SPC forecasters during 2024-2025 experiments, demonstrating skill in 30-minute increments beyond traditional guidance.^[54]^[55] Colorado State University-developed ML models, calibrated on SPC mesoanalysis parameters, further assist in next-day severe probabilities, improving forecaster confidence via explainable outputs during Hazardous Weather Testbed evaluations.^[56]^[57] The Severe Weather Extended-Range forecasting and Verification Experiment (SWERVE), initiated in 2025, tests subseasonal models for severe weather signals up to three weeks ahead, building on SPC's week-long outlooks by identifying heightened risk patterns in weeks 2-3, with initial results validating detectable signals in operational and experimental ensembles.^[58]^[59]

Verification and Performance

Accuracy Metrics and Verification Studies

The Storm Prediction Center (SPC) evaluates the accuracy of its probabilistic convective outlooks using standard metrics such as the Brier skill score (BSS), which measures skill relative to climatology, and reliability diagrams, which assess calibration between forecast probabilities and observed frequencies.^[36] These methods account for the spatial and probabilistic nature of forecasts, often verified against local storm reports (LSRs) within specified radii (e.g., 40 km for tornadoes, 25 miles for severe hail and wind).^[36] Verification distinguishes between traditional grid-based approaches (80-km resolution without interpolation) and interpolated methods that refine probabilities between forecast contours, the latter typically yielding 10-40% higher skill.^[36] For Day 1 outlooks analyzed from 2009 to 2016 (2922 cases), BSS values indicated modest positive skill: 0.049 for any tornado probability (traditional method), rising to 0.059 with interpolation; 0.076-0.096 for severe hail; and 0.093-0.130 for damaging wind.^[36] Significant tornado probabilities showed BSS of 0.028, while significant wind exhibited near-zero or negative skill (-0.001 traditional).^[36] Tornado probabilities displayed underforecast bias, with observed frequencies exceeding forecasts at higher probability bins, whereas wind probabilities were well-calibrated overall.^[36] No consistent year-to-year trend in skill emerged, though geographic patterns favored higher BSS in the central and northern Great Plains, and seasonal peaks occurred in spring and late autumn.^[36] Day 2-3 outlooks (2012-2016, ~1568 cases each) demonstrated lower skill, with BSS of 0.055 for Day 2 any-severe (traditional) and 0.028 for Day 3, improving modestly to 0.066 and 0.040 via interpolation.^[36] Underforecast bias persisted, particularly for tornadoes, and reliability weakened at longer leads, reflecting challenges in environments with low convective available potential energy (CAPE) or mismatched shear.^[36] Earlier studies of Day 1 outlooks from 1986 to 2000 confirmed ongoing improvements in accuracy and skill, using similar probabilistic measures alongside practically perfect hindcasts for event-specific assessment.^[60] More recent evaluations (2016-2024) incorporate self-organizing maps of North American Mesoscale model outputs to link forecast errors to synoptic patterns, treating LSRs as areal events to quantify risk zone coverage versus forecasted probabilities, though specific updated BSS values remain tied to ongoing analyses.^[61] Overall, Day 1 forecasts exhibit reliable positive skill exceeding climatology, while multi-day products highlight room for refinement in bias correction and resolution.^[36]

Historical Case Studies of Forecast Outcomes

The Storm Prediction Center's (SPC) convective outlooks and related products have been evaluated in several major tornado outbreaks, revealing strengths in anticipating broad-scale severe weather potential despite challenges from model uncertainties and evolving storm modes. Verification studies highlight that SPC Day 1 outlooks generally exhibit higher probabilistic skill for severe wind events compared to tornadoes, with issues arising from underestimation of discrete supercell development or overreliance on ensemble guidance that underplays instability.^[36] Historical cases demonstrate instances where early recognition of high-risk setups led to rare "high risk" designations, enabling advance warnings, though local-scale forecast refinements remained limited by real-time data gaps. May 3, 1999, Oklahoma-Kansas Outbreak: SPC forecasters issued a Day 1 outlook at 1200 UTC on May 3 predicting a 10% probability of tornadoes, including a hatched area for significant (F2+) tornadoes, across central Oklahoma and southern Kansas, based on anticipated supercell-favorable parameters like strong low-level shear and high CAPE exceeding 3000 J/kg.^[62] Despite numerical model discrepancies—such as inconsistent dryline forecasts and underpredicted storm initiation timing—the outlook accurately captured the outbreak's potential, which produced 74 tornadoes, including an F5 tornado near Oklahoma City causing 36 fatalities and $1 billion in damage. Challenges included operational reliance on sparse upper-air data and model biases toward linear squall-line modes rather than discrete supercells, yet the forecast's emphasis on extreme instability and shear alignment validated the high-end risk assessment post-event.^[63] This case underscored SPC's ability to integrate subjective reasoning with available guidance amid forecast inconsistencies, contributing to timely tornado watches issued less than 30 minutes before initial storm development.^[64] April 25–28, 2011, Southeastern U.S. Super Outbreak: SPC began highlighting severe potential five days in advance, escalating to multiple Day 1 high-risk outlooks, including a rare continuous high-risk coverage for April 27 covering Mississippi, Alabama, and Tennessee, with 15–30% tornado probabilities and hatched sig-tornado areas reflecting veered hodographs and CAPE values over 2500 J/kg.^[65] The forecasts accurately delineated the threat corridor, where 360 tornadoes occurred across four days, killing 316 people and causing $11 billion in losses, with the April 27 segment alone producing four EF5 tornadoes. Verification confirmed strong performance, as the outlooks aligned closely with observed supercell dominance and outbreak scale, aided by consistent model signals of a potent mid-level trough and Gulf moisture return.^[66] Minor discrepancies involved slight underestimation of eastern extent on April 26, but overall, the proactive multi-day messaging enhanced public preparedness, demonstrating SPC's efficacy in high-conviction setups where synoptic forcing overwhelmed typical uncertainties.^[65] These cases illustrate SPC's verification strengths in categorical risk communication for major outbreaks, where empirical parameters like storm-relative helicity exceeding 300 m²/s² reliably signaled violent tornado potential, though persistent challenges in Day 2–3 lead times for sig-tornado hatching persist due to ensemble spread in convective initiation.^[62] Post-event analyses from peer-reviewed sources affirm that such forecasts, when issued, correlate with observed event magnitudes better than baseline climatology, informing ongoing refinements in outlook probabilities.^[36]

Impact and Criticisms

Contributions to Severe Weather Response

The Storm Prediction Center (SPC) contributes to severe weather response by issuing convective outlooks and watches that provide advance guidance to local National Weather Service (NWS) offices and emergency managers, enabling coordinated preparations across large regions. These products delineate areas at risk for severe thunderstorms, tornadoes, and related hazards, often 1 to 3 days ahead, allowing for the activation of emergency operations centers, resource prepositioning, and public alerts before threats materialize. For instance, Day 2 convective outlooks are critical for emergency managers to initiate planning, such as coordinating with local authorities and mitigating potential impacts, while Enhanced Risk designations prompt heightened readiness measures.^[67]^[27] SPC's tornado and severe thunderstorm watches, covering multiple counties or states, signal imminent threats and trigger local NWS offices to issue precise warnings, streamlining the transition from forecast to immediate action. This national oversight ensures consistent threat assessment, as SPC meteorologists analyze meso-scale features and environmental conditions 24/7 to refine watch boundaries and probabilities. During events like the tornado outbreak associated with Hurricane Milton in October 2024, SPC's specialized outlooks outlined risks, timing, and tornado potential within the tropical system, supporting local response efforts in Florida by informing shelter decisions and evacuations.^[68]^[69]^[69] By integrating radar, satellite, and model data into mesoscale discussions and probabilistic forecasts, SPC enhances response efficacy, reducing the burden on under-resourced local offices during high-impact outbreaks. This layered approach has facilitated more proactive societal responses, as evidenced by emergency managers' reliance on SPC products for operational decisions over climatological baselines.^[70]^[27]

Challenges, Limitations, and Debates

Forecasting severe convective weather events at the Storm Prediction Center (SPC) faces inherent challenges due to the rarity of such phenomena and the chaotic dynamics of the atmosphere, which limit predictive skill even under ideal observational conditions. Rare events, such as significant tornadoes or widespread severe hail, occur infrequently, complicating the development of robust verification baselines and leading to objective limits on forecast accuracy; for instance, the Critical Success Index (CSI) for practically perfect forecasts of rare events with a standard deviation of occurrence around 3% ranges from approximately 0.12 to 0.30, with actual SPC outlooks, like that for April 26, 1991, achieving a CSI of 0.18, representing about 33% of the maximum possible skill.^[71] These limits arise because forecasts must cover large areas to encompass uncertainty, inevitably producing false alarms and misses, as SPC watches are rarely issued for regions smaller than 10,000 km².^[71] Verification of SPC products is further hampered by inconsistent observational data, reliant on volunteer spotter reports that lack uniform spatial and temporal coverage, and by metrics like CSI that equally penalize false alarms and missed detections despite differing real-world costs. Developing meaningful verification procedures requires accounting for varying forecast difficulty, as climatological baselines alone fail to capture environmental nuances, prompting ongoing research into asymmetric scoring rules tailored to decision-making contexts.^[71] Skill levels degrade in marginal convective environments, such as those with low convective available potential energy (CAPE below 500 J/kg) paired with moderate shear (15-20 m/s), where probability of detection (POD) for hail and tornadoes is notably poor, or in weakly sheared regimes (≤10 m/s) with moderate instability (CAPE ~1000 J/kg), yielding low POD for damaging winds.^[72] Communication of SPC convective outlooks presents additional limitations, as categorical risk levels (e.g., Marginal, Slight, Enhanced, Moderate, High) are meteorologically calibrated but prone to misinterpretation by non-experts, who often incorrectly rank Enhanced risk above Moderate, inverting the intended severity order. While probabilistic or numeric formats (e.g., "Level 2 of 5" or specific percentages) mitigate some confusion for lay audiences, low numeracy exacerbates persistent anchoring to categorical labels, and combined formats do not fully resolve misordering.^[73] Debates persist over optimizing risk category design and presentation to balance meteorological precision with public comprehension, particularly as outlooks extend to days 3-8 with decreasing reliability, though empirical verification confirms overall skill in capturing severe weather outcomes despite interpretive gaps.^[73]

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[13]
A Brief History of the Storm Prediction Center
Feb 13, 2010 · Although SPC's immediate history dates to the early 1950s, the roots of severe weather prediction in the United States may be traced much ...
[14]
[PDF] texas tornado outbreak on may 27, 1997
This paper examines the pre-storm environment and radar observations of the event. 2. PRE-STORM SYNOPTIC ENVIRONMENT a. Early Morning Environment. During the 12 ...
[15]
Frequently Asked Questions (FAQ) - Storm Prediction Center
Jul 30, 2025 · SELS became the National Severe Storms Forecast Center (NSSFC) in 1966. NSSFC was renamed to the Storm Prediction Center (SPC) in 1995. For more ...
[16]
HWT History - NOAA Hazardous Weather Testbed
1984–1985: The Preliminary Regional Experiment for Storm-Central (PRE-STORM) takes place. This exchange of ideas, support of field operations, and real-time ...
[17]
Evaluation of the Storm Prediction Center's Convective Outlooks ...
Oct 1, 2014 · The Storm Prediction Center issues four categorical convective outlooks with lead times as long as 48 h, the so-called day 3 outlook issued at ...
[18]
Changes to Storm Prediction Center Outlooks Effective Today
Oct 22, 2014 · The SPC's Day 1, Day 2 and Day 3 severe weather outlook categories will change from the current four (see text, slight, moderate, high), to five.
[19]
Colorful Language: Investigating Public Interpretation of the Storm ...
The Storm Prediction Center (SPC) convective outlook is one of the oldest continuous severe weather forecasts, having existed in one form or another since 1955 ...
[20]
Abstract: Storm Prediction Center's Fire Weather Verification (Fourth ...
Nov 14, 2001 · There are five dedicated forecasters who issue these forecasts once a day at 4 AM Central Local Time. The criteria and forecasting techniques ...
[21]
SPC Fire Weather Outlooks - Storm Prediction Center - NOAA
No information is available for this page. · Learn whyMissing: history | Show results with:history
[22]
Fire Weather - National Weather Service
Storm Prediction Center's Fire Weather Outlook. Current Fire Potential - Click for NIFC All Fire Potential Outlooks · Current Large Wildfire Map · Incident ...Wildfire Weather Safety · Fire Weather Forecasts · National Agreements
[23]
Severe Weather 101: Tornado Forecasting
Meteorologists at the NOAA Storm Prediction Center (SPC) issue daily forecasts, or convective outlooks, for organized severe thunderstorms over the US.
[24]
Focus on Severe Weather
Within NOAA, the National Severe Storms Laboratory (NSSL) and the Storm Prediction Center ... The mission of the SPC is to provide timely and accurate ...
[25]
About the NWS
NWS Mission Provide weather, water and climate data, forecasts, warnings, and impact-based decision support services for the protection of life and property ...We are the National Weather... · NWS Partners · NOAA Observation SystemsMissing: mandate | Show results with:mandate
[26]
Staying Ahead of the Storms - NOAA
Jun 2, 2023 · ... Storm Prediction Center (SPC) located in Norman, OK. Use the ... mission of protecting life and property as much as possible. The ...
[27]
The Impact of the Storm Prediction Center's Convective Outlooks ...
The results show EMs prefer to wait until an Enhanced Risk for severe thunderstorms is issued to prepare for severe weather. In addition, the Day 2 Convective ...Missing: response | Show results with:response
[28]
https://www.weather.gov/about/forecastsandservice
[29]
https://www.spc.noaa.gov/new/SVRclimo/climo.php
[30]
[PDF] noaa weather partners
The Storm Prediction Center provides hazardous weather forecasts including critical tornado and severe thunderstorm watches for the contiguous United States ...
[31]
May 6, 2024: Being co-located with NWS Norman allows us to do in ...
May 6, 2024 · Being co-located with NWS Norman allows us to do in-person watch collaboration. The remaining affected local NWS offices join the watch collaboration process ...
[32]
SPC has 22 full-time forecasters and 10 fill-in/support staff. In honor ...
Aug 30, 2024 · A typical shift at SPC with 4-6 forecasters can have well over a century (100+ years) of severe weather and forecasting experience at one time.Missing: size | Show results with:size
[33]
[PDF] O. Collaboration with other national and international research ...
NSSL scientists collaborate routinely with the National Centers for Environmental Prediction. (NCEP) Storm Prediction Center through the Hazardous Weather ...
[34]
SPC Convective Outlook - Storm Prediction Center - NOAA
No information is available for this page. · Learn why
[35]
[PDF] Storm Prediction Center Severe Risk Categories
• The categories: TSTM (light green) non-severe, 1-MRGL (dark green), 2-SLGT (yellow), 3-ENH. (orange), 4-MDT (red) and 5-HIGH (magenta). • SPC Severe ...
[36]
Probabilistic Verification of Storm Prediction Center Convective ...
Convective outlooks are produced for days 1–8, when the valid period for a forecast day spans 1200–1159 UTC, but the outlook specifics vary as a function of ...
[37]
IEM :: Features Tagged: spc - Iowa Environmental Mesonet
For the "Day 1" product, individual threat risks are forecast for tornadoes, damaging wind gusts, and hail.
[38]
The SPC 5-point severe thunderstorm risk category scale explained
Mar 14, 2025 · The "High" risk category is reserved for the most dangerous of severe weather outbreaks and means widespread severe storms are expected.
[39]
[PDF] A Detailed Analysis of Storm Prediction Center Convective Outlook ...
May 24, 2025 · A Detailed Analysis of Storm Prediction Center Convective Outlook Patterns. David Stang. Page 2. Introduction. The Storm Prediction Center (SPC) ...
[40]
SPC Current Mesoscale Discussions - Storm Prediction Center
No information is available for this page. · Learn why
[41]
[PDF] 1. INTRODUCTION Co-location of the Storm Prediction Center (SPC ...
When the threat level becomes elevated over a mesoscale area in time and space, the SPC may issue a Mesoscale Discussion (MD) product to notify local National ...
[42]
SPC - Mesoscale Discussions (CloudGIS) - Overview - ArcGIS Online
May 31, 2023 · All MDs contain an area, affected line, concerning line, valid time, a paragraph for a summary, and a paragraph for a technical discussion, ...Missing: Convective | Show results with:Convective
[43]
About the SPC - NOAA/NWS Storm Prediction Center
### Summary of Severe Weather Watches from SPC
[44]
Storm Prediction Center Frequently Asked Questions (FAQ)
### Summary of FAQs Related to Severe Weather Watches
[45]
Severe Weather 101: Thunderstorm Basics
A Severe Thunderstorm WATCH is issued by the NOAA Storm Prediction Center meteorologists who are watching the weather 24/7 across the entire U.S. for ...Thunderstorm Types · FAQ · Forecasting · Detection
[46]
New "Destructive" Severe Thunderstorm Warning category to trigger ...
Jul 22, 2021 · The criteria for a destructive damage threat is at least 2.75 inch diameter (baseball-sized) hail and/or 80 mph thunderstorm winds. Warnings ...
[47]
[PDF] 1 A Multi-Tiered Verification of SPC Tornado Watches (2003–08 ...
The FM rate of 10.57% is somewhat surprising, as it indicates that approximately one out of every ten tornado watches has no severe storm reports associated.
[48]
National Weather Service Fire Weather Outlooks | Drought.gov
Historical Information · Outlooks and Forecasts · All Data. Explore More ... The National Weather Service (NWS) Storm Prediction Center's Fire Weather Outlooks ...
[49]
None
### Summary of SPC Fire Weather Outlook Criteria
[50]
'2021 NOAA/NWS SPC Day 1 Fire Weather Outlook Verification' - ADS
This study aims to quantify and document the performance characteristics of the SPC Day 1 Fire Weather Outlook during the entirety of 2021.<|separator|>
[51]
SPC Forecast Tools - Storm Prediction Center - NOAA
No information is available for this page. · Learn whyMissing: products | Show results with:products
[52]
NOAA Hazardous Weather Testbed
... new forecast models, techniques, and products to support NWS Storm Prediction Center forecast operations. Since the “Spring Program,” the HWT has evolved to ...The Spring Experiment · About HWT · Experimental Forecast Program · HWT History
[53]
Warn on Forecast:WoFS
Warn-on-Forecast is a National Oceanic and Atmospheric Administration research project tasked with increasing lead time for tornado, severe thunderstorm, and ...
[54]
WoFSCast: A Machine Learning Model for Predicting Thunderstorms ...
May 19, 2025 · While not yet operational, a WoFS prototype is already routinely used by NOAA's Storm Prediction Center, Weather Prediction Center, and ...
[55]
Predicting Thunderstorm Hazards With WoFS and Machine Learning
... Storm Prediction Center. These forecasts project out to four hours and can be examined in 30-minute increments to focus on areas of immediate concern. The ...
[56]
CSU machine learning model helps forecasters improve storm ...
Feb 26, 2023 · CSU machine learning model helps forecasters improve confidence in storm prediction · Working with Storm Prediction Center forecasters · Nine ...
[57]
Exploring the Usefulness of Machine Learning Severe Weather ...
We developed an artificial intelligence (AI) system to predict tornadoes, large hail, and damaging straight-line winds. The AI system was leveraged in real time ...
[58]
SWERVE: Predicting severe weather weeks in advance - Inside NSSL
Sep 30, 2025 · The NOAA Storm Prediction Center currently issues extended severe weather outlooks up to a week in advance. These critical forecasts shed ...
[59]
Forecasting tornado risk three weeks ahead is becoming possible
Oct 3, 2025 · Early results from SWERVE show that forecasts in weeks two and three can sometimes detect meaningful signals of heightened severe weather ...
[60]
Evaluation of the Storm Prediction Center's Day 1 Convective Outlooks
Aug 7, 2025 · The Storm Prediction Center's convective outlook slight risk areas ... development of the National Weather Service's Storm Prediction Center ...
[61]
Evaluation of SPC Convective Outlook Accuracy and the Weather ...
The NOAA Storm Prediction Center's Day 1 Convective Outlook is an important daily severe weather forecast, the accuracy of which is relied upon for safety ...
[62]
Storm Prediction Center Forecasting Issues Related to the 3 May ...
Forecasters at the Storm Prediction Center (SPC) were faced with many challenges during the 3 May 1999 tornado outbreak.
[63]
The Great Plains Tornado Outbreak of May 3-4, 1999
On May 3, 1999, multiple supercell thunderstorms produced many large and damaging tornadoes in central Oklahoma during the late afternoon and evening hours.Storm B · Storm D · Storm E · Storm I
[64]
NSSL Podcast:Remembering May 3, 1999
Apr 20, 2009 · Less than 30 minutes later, the Storm Prediction Center issued a tornado watch for western and central Oklahoma. Explosive thunderstorms ...<|separator|>
[65]
[PDF] Service Assessment - The Historic Tornadoes of April 2011
Weather Forecast Offices (WFOs) issued 96 tornado warnings during this outbreak with a. Probability of Detection (POD) of 87.1 percent, a False Alarm Ratio ...
[66]
The historic tornadoes of April 2011
This tornado outbreak was anticipated and forecast days in advance. The SPC began focusing on the affected area in its convective outlook products 5 days ...Missing: verification | Show results with:verification
[67]
[PDF] The Impact of the Storm Prediction Center's Convective Outlooks ...
Jun 11, 2021 · In addition, the Day 2 Convective Outlook serves as the threshold for higher, value-based decision ... (Storm Prediction Center 2021). Once ...
[68]
Understand Tornado Alerts - National Weather Service
What is the difference between a Tornado Watch, a Tornado Warning and a Tornado Emergency? ... Watches are issued by the Storm Prediction Center for counties ...
[69]
Storm Prediction Center enhances severe weather safety - WPBF
May 30, 2025 · The SPC's biggest contribution during the tropical season in Florida is outlining risks with an approaching tropical system, providing timing, ...
[70]
Strengthening America's Resilience to Extreme Weather
Sep 10, 2025 · SPC provides expert guidance and warnings to help keep people safe before storms hit, sometimes days in advance. Local forecasters, emergency ...
[71]
[PDF] objective limits on forecasting skill of rare events
Forecasting rare, severe weather events is challenging. Equally challenging, however, is the problem of developing meaningful verification procedures that ...
[72]
9A.2 Forecast challenges at the NWS Storm Prediction Center ...
One major challenge in forecast evaluation is the simple fact that some forecasts are more difficult than others. In the case of forecasting severe convection, ...Missing: limitations | Show results with:limitations
[73]
Exploring the Differences in SPC Convective Outlook Interpretation ...
While previous work has shown that the Storm Prediction Center (SPC) convective outlooks accurately capture meteorological outcomes, evidence suggests ...<|separator|>