Editing Storm Prediction Center (section)

==Convective outlooks==
{{multiple image
 | direction         = vertical
 | image1            = 2019-05-20 1630 UTC Day 1 convective outlook.gif
 | image2            = 2019-05-20 1630 UTC Day 1 tornado outlook.gif
 | footer            = Day 1 Convective Outlook and [[probability|probabilistic]] maps issued by the Storm Prediction Center on [[Tornado outbreak sequence of May 2019|May 20, 2019]] depicting a [[List of Storm Prediction Center high risk days|high risk day]]. The top map indicates the risk of general [[severe weather]] (including large [[hail]], damaging winds, and tornadoes), while the bottom map specifically shows the risk percentage of a tornado forming within 25&nbsp;miles (40&nbsp;km) of any point within the enclosed area. The hatched area on the bottom map indicates a 10% or greater risk of an [[Enhanced Fujita scale|EF2]] or stronger tornado forming within 25&nbsp;miles (40&nbsp;km) of a point.
}}

The Storm Prediction Center issues convective outlooks (AC), consisting of categorical and probabilistic forecasts describing the general threat of severe convective storms over the contiguous United States for the next six to 192 hours (Day 1 through Day 8). These outlooks are labeled and issued by day, and are issued up to five times per day.<ref name="convective outlooks"/>

The categorical levels of risks are TSTM (for <u>T</u>hunder <u>St</u>or<u>m</u>: light green shaded area – rendered as a brown line prior to April 2011 – indicating a risk for general thunderstorms), "MRGL" (for <u>M</u>a<u>rg</u>ina<u>l</u>: darker green shaded area, indicating a very low but present risk of severe weather); "SLGT" (for <u>Sl</u>i<u>g</u>h<u>t</u>: yellow shaded area – previously rendered as a green line – indicating a slight risk of severe weather); "ENH" (for <u>Enh</u>anced: orange shaded area, which replaced the upper end of the SLGT category on October&nbsp;22, 2014); "MDT" (for <u>M</u>o<u>d</u>era<u>t</u>e: red shaded area – previously rendered as a red line – indicating a moderate risk of severe weather); and "HIGH" (pink shaded area – previously a rendered as a [[fuchsia (color)|fuchsia]] line – indicating a high risk of severe weather). Significant severe areas (referred to as "[[hatching|hatched areas]]" because of their representation on outlook maps) refer to a threat of increased storm intensity that is of "significant severe" levels (F2/[[Enhanced Fujita scale|EF2]] or stronger tornado, {{convert|2|in|cm}} or larger hail, or {{convert|75|mph|km/h}} winds or greater).<ref name="NewCategories"/>

In April 2011, the SPC introduced a new graphical format for its categorical and probability outlooks, which included the shading of risk areas (with the colors corresponding to each category, as mentioned above, being changed as well) and [[population]], [[County (United States)|county]]/[[Parish (administrative division)|parish]]/[[Borough (United States)|borough]] and [[Interstate Highway System|interstate]] overlays. The new shaded maps also incorporated a revised color palette for the shaded probability categories in each outlook.

In 2013, the SPC incorporated a small table under the Convective Outlook's risk category map that indicates the total coverage area by [[square mile]]s, the total estimated population affected and major cities included within a severe weather risk area.<ref name="convectiveoutlook2013">{{cite web|title=Jun 12, 2013 2000 UTC Day 1 Convective Outlook|url=http://www.spc.noaa.gov/products/outlook/archive/2013/day1otlk_20130612_2000.|website=Storm Prediction Center|publisher=National Oceanic and Atmospheric Administration|date=June 12, 2013|access-date=June 14, 2013}}</ref>

Public severe weather outlooks (PWO) are issued when a significant or widespread outbreak is expected, especially for tornadoes. From November to March, it can also be issued for any threat of significant tornadoes in the nighttime hours, noting the lower awareness and greater danger of tornadoes at that time of year.<ref name="Public severe weather outlook"/>

===Categories===
[[File:June 27, 2020 SPC Day 1 Outlook.gif|thumb|left|An example of an Enhanced Risk day overlaid with the radar with Severe Thunderstorm Watches in effect.]]
A '''marginal risk''' day indicates storms of only limited organization, longevity, coverage and/or intensity, typically isolated severe or near-severe storms with limited wind damage, large hail and possibly a low tornado risk. Wind gusts of at least {{convert|60|mph|km/h}} and hailstones of around {{convert|1|in|cm}} in diameter are common storm threats within a marginal risk; depending on the sufficient wind shear, a tornado – usually of weak (EF0 to EF1) intensity and short duration – may be possible. This category replaced the "SEE TEXT" category on October&nbsp;22, 2014.<ref name="NewCategories"/>

A '''slight risk''' day typically will indicate that the threat exists for scattered severe weather, including scattered wind damage (produced by straight-line sustained winds and/or gusts of 60 to 70&nbsp;mph), scattered severe hail (varying in size from {{convert|0.25|in|cm}} to {{convert|1.75|in|cm}}) and/or isolated tornadoes (often of shorter duration and varying weak to moderate intensity, depending on the available wind shear and other sufficient atmospheric parameters). During the peak severe weather season, most days will have a slight risk somewhere in the United States. Isolated significant severe events are possible in some circumstances, but are generally not widespread.<ref name="convective outlooks"/>

An '''enhanced risk''' day indicates that there is a greater threat for severe weather than that which would be indicated by a slight risk, but conditions are not adequate for the development of widespread significant severe weather to necessitate a moderate category, with more numerous areas of wind damage (often with wind gusts of {{convert|70|mph|km/h}} to {{convert|80|mph|km/h}}), along with severe hail (occasionally over {{convert|2|in|cm}}) and several tornadoes (in some setups, isolated strong tornadoes are possible). Severe storms are expected to be more concentrated and of varying intensities. These days are quite frequent in the peak severe weather season and occur occasionally at other times of year. This risk category replaced the upper end of "slight" on October&nbsp;22, 2014, although a few situations that previously warranted a moderate risk were reclassified as enhanced (i.e. 45% wind or 15% tornado with no significant area).<ref name="NewCategories"/>

A '''moderate risk''' day indicates that more widespread and/or more dangerous severe weather is possible, with significant severe weather often more likely. Numerous tornadoes (some of which may be strong and potentially long-track), more widespread or severe wind damage (often with gusts over {{convert|80|mph|km/h}}) and/or very large/destructive hail (up to or exceeding {{convert|3|in|cm}} in diameter) could occur. Major events, such as large [[tornado outbreak]]s or widespread straight-line wind events, are sometimes also possible on moderate risk days, but with greater uncertainty. Moderate risk days are not terribly uncommon, and typically occur several times a month during the peak of the severe weather season, and occasionally at other times of the year. Slight and enhanced risk areas typically surround areas under a moderate risk, where the threat is lower.<ref name="convective outlooks"/>

A '''high risk''' day indicates a considerable likelihood of significant to extreme severe weather, generally a major tornado outbreak or (much less often) an extreme [[derecho]] event. On these days, the potential exists for extremely severe and life-threatening weather. This includes a large number of tornadoes - many of which will likely be strong to violent and on the ground for a half-hour or longer, or widespread and very destructive straight-line winds, likely in excess of {{convert|100|mph|km/h}}. Hail cannot verify or produce a high risk on its own, although such a day usually involves a threat for widespread very large and damaging hail as well. Many of the most prolific severe weather days were high risk days. Such days are rare; a high risk is typically issued (at the most) only a few times each year (see [[List of Storm Prediction Center high risk days]]). High risk areas are usually surrounded by a larger moderate risk area, where uncertainty is greater or the threat is somewhat lower.<ref name="convective outlooks"/>

The Storm Prediction Center began asking for public comment on proposed categorical additions to the Day 1-3 Convective Outlooks on April 21, 2014, for a two-month period.<ref>{{cite web|title=Experimental SPC Day 1, 2, 3 Convective Outlook Change Public Comment Page|url=http://www.spc.noaa.gov/exper/dy1-3example/|website=Storm Prediction Center|publisher=National Oceanic and Atmospheric Administration|access-date=April 21, 2014|archive-date=April 24, 2014|archive-url=https://web.archive.org/web/20140424123623/http://www.spc.noaa.gov/exper/dy1-3example/|url-status=live}}</ref> The Storm Prediction Center broadened this system beginning on October&nbsp;22, 2014 by adding two new risk categories to the three used originally. The new categories that were added are a "marginal risk" (replacing the "SEE TEXT" contours, see below) and an "enhanced risk". The latter is used to delineate areas where severe weather will occur that would fall under the previous probability criteria of an upper-end slight risk, but do not warrant the issuance of a moderate risk. In order from least to greatest threat, these categories are ranked as: marginal, slight, enhanced, moderate, and high.<ref>{{cite news|title=Forecasters Adding New Layers of Storm Outlooks|url=https://abcnews.go.com/m/story?id=21567394&sid=81|agency=[[Associated Press]]|website=[[ABC News (United States)|ABC News]]|publisher=[[The Walt Disney Company]]|date=January 17, 2014|archive-date=March 4, 2016|access-date=June 29, 2020|archive-url=https://web.archive.org/web/20160304194954/http://abcnews.go.com/m/story?id=21567394&sid=81|url-status=live}}</ref><ref>{{cite news|title=Forecasters adding layers of storm outlooks|url=http://www.arkansasonline.com/news/2014/jan/17/forecasters-adding-new-layers-storm-warnings/|agency=Associated Press|newspaper=[[Arkansas Democrat-Gazette]]|date=January 17, 2014}}</ref><ref>{{cite news|title=A better outlook: SPC revises its severe categories|url=http://www.wusa9.com/story/weather/2014/03/27/spc-severe-thunderstorms-tornadoes-hail/6974379/|website=[[WUSA (TV)|WUSA]]|publisher=[[Gannett Company]]|date=March 28, 2014|access-date=March 30, 2014|archive-date=April 26, 2014|archive-url=https://web.archive.org/web/20140426215509/http://www.wusa9.com/story/weather/2014/03/27/spc-severe-thunderstorms-tornadoes-hail/6974379/|url-status=dead}}</ref>
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===Issuance and usage===
{| class="wikitable" style="text-align:center; font-size:90%; margin-left:1em; float:right; clear:right;"
|+ Day 1 and Day 2 probability to categorical outlook conversion<ref name="NewCategories">{{Cite web|title=SPC Convective Outlooks|url=http://www.spc.noaa.gov/misc/SPC_probotlk_info.html|work=Storm Prediction Center|publisher=National Oceanic and Atmospheric Administration|last1=Grams|first1=Jeremy|last2=Bunting|first2=Bill|last3=Weiss|first3=Steve|date=October 22, 2014|access-date=2014-10-22|location=Norman, Oklahoma}}</ref><ref>{{cite web|title=Product Description Document (PDD): SPC Day 2 Convective Outlook|url=http://products.weather.gov/PDD/SPC_Day_1to3_Cat_Conv_Outlook.pdf|website=Storm Prediction Center|publisher=National Oceanic and Atmospheric Administration|date=November 14, 2019|access-date=February 4, 2020|archive-date=February 18, 2017|archive-url=https://web.archive.org/web/20170218111731/https://products.weather.gov/PDD/SPC_Day_1to3_Cat_Conv_Outlook.pdf|url-status=dead}}</ref>
! Outlook probability!!TORN!!WIND!!HAIL
|-
| 2% ||style="background:lime;"|MRGL||<small>not used</small>||<small>not used</small>
|-
| 5% || style="background:yellow;"|SLGT||style="background:lime;"|MRGL||style="background:lime;"|MRGL
|-
| 10% || style="background:orange;"|ENH||<small>not used</small>||<small>not used</small>
|-
| 10% <small>Sig. Severe</small> || style="background:orange;"|ENH||<small>not used</small>||<small>not used</small>
|-
| 15% || style="background:orange;"|ENH|| style="background:yellow;"|SLGT|| style="background:yellow;"|SLGT
|-
| 15% <small>Sig. Severe</small> || style="background:red;"|MDT|| style="background:yellow;"|SLGT|| style="background:yellow;"|SLGT
|-
| 30% || style="background:red;"|MDT|| style="background:orange;"|ENH|| style="background:orange;"|ENH
|-
| 30% <small>Sig. Severe</small> || style="background:magenta;"|HIGH|| style="background:orange;"|ENH|| style="background:orange;"|ENH
|-
| 45% || style="background:magenta;"|HIGH|| style="background:orange;"|ENH|| style="background:orange;"|ENH
|-
| 45% <small>Sig. Severe</small> || style="background:magenta;"|HIGH|| style="background:red;"|MDT|| style="background:red;"|MDT
|-
| 60% || style="background:magenta;"|HIGH|| style="background:red;"|MDT|| style="background:red;"|MDT
|-
| 60% <small>Sig. Severe</small> || style="background:magenta;"|HIGH|| style="background:magenta;"|HIGH|| style="background:red;"|MDT
|}
{| class="wikitable" style="text-align:center; font-size:90%; margin-left:1em; float:right; clear:right;"
|+  Day 3 probability to categorical outlook conversion<ref name="NewCategories"/>
! Outlook probability!!Combined TORN, WIND, and HAIL
|-
| 5% || style="background:lime;"|MRGL
|-
| 15% || style="background:yellow;"|SLGT
|-
| 15% <small>Sig. Severe</small> || style="background:yellow;"|SLGT
|-
| 30% || style="background:orange;"|ENH
|-
| 30% <small>Sig. Severe</small> || style="background:orange;"|ENH
|-
| 45% || style="background:orange;"|ENH
|-
| 45% <small>Sig. Severe</small> || style="background:red;"|MDT
|}
{| class="wikitable" style="text-align:center; font-size:90%; margin-left:1em; float:right; clear:right;"
|+ Day 4–8 probability to categorical outlook conversion<ref name="NewCategories"/>
! Outlook probability!!Combined TORN, WIND, and HAIL
|-
| < 15% || ''No Area''
|-
| 15% || style="background:#ffee66;"|Severe ''(15%)''
|-
| 30% || style="background:#ff8888;"|Severe ''(30%)''
|}
Convective outlooks are issued by the Storm Prediction Center in Zulu time (also known as [[Universal Coordinated Time]] or UTC).<ref name="OldCategories"/>

The categories at right refer to the risk levels for the specific severe weather event occurring within {{convert|25|mi|km}} of any point in the delineated region, as described in the previous section. The Day 1 Convective Outlook, issued five times per day at 0600Z (valid from 1200Z of the current day until 1200Z the following day), 1300Z and 1630Z (the "morning updates", valid until 1200Z the following day), 2000Z (the "afternoon update", valid until 1200Z the following day), and the 0100Z (the "evening update", valid until 1200Z the following day), provides a textual forecast, map of categories and probabilities, and chart of probabilities. Prior to January 28, 2020, the Day 1 was currently the only outlook to issue specific probabilities for tornadoes, hail or wind. It is the most descriptive and highest accuracy outlook, and typically has the highest probability levels.<ref name="convective outlooks"/>

Day 2 outlooks, issued twice daily at 0600Z in Daylight Saving Time or 0700Z in Standard Time and 1730Z, refer to predicted risks of convective weather for the following day (1200Z to 1200Z of the next calendar day; for example, a Day 2 outlook issued on April 12, 2100, would be valid from 1200Z on April 13, 2100, through 1200Z on April 14, 2100) and include only a categorical outline, textual description, and a map of categories and probabilities. Day 2 moderate risks are fairly uncommon, and a Day 2 high risk has only been issued three times (for [[Tornado outbreak of April 6–8, 2006|April 7, 2006]], [[Tornado outbreak of April 13–16, 2012|April 14, 2012]], and [[Tornado outbreak of March 13-16, 2025|March 15, 2025]]).<ref name="convective outlooks"/> Probabilities for tornadoes, hail and wind applying to the Day 1 Convective Outlook were incorporated into the Day 2 Convective Outlook on January 28, 2020, citing research to SPC operations and improvements in numerical forecast guidance that have increased forecaster confidence in risk estimation for those hazards in that timeframe. The individual hazard probabilistic forecasts replaced the existing "total severe" probability graph for general severe convective storms that had been used for the Day 2 outlook beforehand.<ref>{{cite web|title=Service Change Notice 19-94|url=https://www.weather.gov/media/notification/scn19-94spc_risk_graphics.pdf|website=National Weather Service|date=November 15, 2019|access-date=February 4, 2020|archive-date=February 5, 2020|archive-url=https://web.archive.org/web/20200205000703/https://www.weather.gov/media/notification/scn19-94spc_risk_graphics.pdf|url-status=live}}</ref>

[[File:Hazard Intensites in SPC MDs and Outlooks PowerPoint at the 2025 Central Region Spring Symposium.pdf|thumb|"Hazard Intensites in SPC MDs and Outlooks", a presentation on the future of convective outlooks by the SPC]]
Day 3 outlooks refer to the day after tomorrow, issued twice daily since August 13, 2024<ref>{{Cite web |title=Storm Prediction Center Aug 13, 2024 1930 UTC Day 3 Severe Thunderstorm Outlook |url=https://www.spc.noaa.gov/products/outlook/archive/2024/day3otlk_20240813_1930.html |access-date=2025-01-08 |website=www.spc.noaa.gov}}</ref> at 0730Z in Daylight Saving Time or 0830Z in Standard Time and 1930Z and include the same products (categorical outline, text description, and probability graph) as the Day 2 outlook. As of June 2012, the SPC forecasts general thunderstorm risk areas.<ref>{{cite web|title=Service Change Notice 12-26|url=http://www.nws.noaa.gov/om/notification/scn12-26spc-day3-thunder.htm|website=National Weather Service, D.C. Headquarters|publisher=National Oceanic and Atmospheric Administration|access-date=April 13, 2013|archive-date=February 20, 2013|archive-url=https://web.archive.org/web/20130220071309/http://www.nws.noaa.gov/om/notification/scn12-26spc-day3-thunder.htm|url-status=live}}</ref> Higher probability forecasts are less and less likely as the forecast period increases due to lessening forecast ability farther in advance. Day 3 moderate risks are quite rare; these have been issued only twenty-two times since the product became operational (most recently for April 28, 2025).<ref name="convective outlooks"/><ref>{{cite web |last1=Goss |first1=Steve |date=20 Mar 2022 |title=Mar 20, 2022 0730 UTC Day 3 Severe Thunderstorm Outlook |url=https://www.spc.noaa.gov/products/outlook/archive/2022/day3otlk_20220320_0730.html |access-date=24 May 2019 |website=Storm Prediction Center |archive-date=20 March 2022 |archive-url=https://web.archive.org/web/20220320073248/https://www.spc.noaa.gov/products/outlook/archive/2022/day3otlk_20220320_0730.html |url-status=live }}</ref> Day 3 high risks are never issued and the operational standards do not allow for such. This is most likely because it would require both a very high degree of certainty (60%) for an event which was still at least 48 hours away and a reasonable level of confidence that said [[severe thunderstorm outbreak]] would include ''significant severe'' weather (EF2+ tornadoes, [[Beaufort scale|hurricane-force winds]], and/or [[Severe weather terminology (United States)#Hail diameter sizes|egg-sized hail]]).

Day 4–8 outlooks are the longest-term official SPC Forecast Product, and often change significantly from day to day. This extended forecast for severe weather was an experimental product until March 22, 2007, when the Storm Prediction Center incorporated it as an official product. Areas are delineated in this forecast that have least a 15% or 30% chance of severe weather in the Day 4–8 period (equivalent to a slight risk and an enhanced risk, respectively); as forecaster confidence is not fully resolute on how severe weather will evolve more than three days out, the Day 4–8 outlook only outlines the areas in which severe thunderstorms are forecast to occur during the period at the 15% and 30% likelihood, and does not utilize other categorical risk areas or outline where general (non-severe) thunderstorm activity will occur.<ref name="convective outlooks"/>

Local forecast offices of the National Weather Service, radio and television stations, and emergency planners often use the forecasts to gauge the potential severe weather threats to their areas.<ref name="convective outlooks"/> Even after the marginal and enhanced risk categories were added in October 2014, some television stations have continued to use the original three-category system to outline forecasted severe weather risks (though stations that do this may utilize in-house severe weather outlooks that vary to some degree from the SPC convective outlooks), while certain others that have switched to the current system have chosen not to outline marginal risk areas.

Generally, the convective outlook boundaries or lines – general thunderstorms (light green), marginal (dark green), slight (yellow), enhanced (orange), moderate (red) and high (purple) – will be continued as an arrow or line not filled with color if the risk area enters another country ([[Canada]] or [[Mexico]]) or across waters beyond the United States coastline. This indicates that the risk for severe weather is also valid in that general area of the other side of the border or oceanic boundary.
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