The Storm Prediction Center at the National Weather Center located in Norman Oklahoma is where the potential for severe weather is monitored in the lower 48 states.
Norman lies within Tornado Alley, a geographic region where tornadic activity is particularly frequent and intense. The Oklahoma City metropolitan area, including Norman, is the most tornado-prone area in the world. The Storm Prediction Center (SPC), a branch of the National Oceanic and Atmospheric Administration (NOAA) is located at the NWC. SPC forecasts severe storm and tornado outbreaks nationwide. Additionally, research is conducted at the co-located National Severe Storms Laboratory (NSSL), which includes field research and operates various experimental weather radars.
Convective Outlooks
Convective Outlooks consist of a narrative and a graphic depicting severe thunderstorm threats across the continental United States. The outlook narratives are written in technical language, intended for sophisticated weather users, and provide the meteorological reasoning for the risk areas.
This product also provides explicit information regarding the timing, the greatest severe weather threat and the expected severity of the event. The graphics that accompany the narratives provide vital information to help plan your day.
The convective outlook graphics display up to six different color categories to reflect the six likelihood of occurrences and/or increased severity of a severe weather event(s). The four convective outlooks issued (Day 1, Day 2, Day 3 and Days 4-8) are…
| Day 1 | This is the risk of severe weather today through early tomorrow morning. Day 1 forecasts are issued five times daily; 06z (around midnight), 13z (around sunrise), 1630z (mid-morning), 20z (mid-afternoon), and 01z (early evening). This is the forecast you will see on SPC’s frontpage. (What is Z-time?) |
|---|---|
| Day 2 | Day 2 continues from the ending of Day 1 (tomorrow morning) for the next 24 hours. These are issued twice daily; 07z (around midnight) and 1730z (around noon). |
| Day 3 | This is the forecast for the subsequent 24 hours. Day 3 forecasts are issued daily by 0830z on standard time and 0730z on daylight time (after midnight). |
| Days 4-8 | A severe weather area depicted in the days four through eight period. It is issued at 10z daily (early morning) and indicates a 15% or 30% or higher probability for severe thunderstorms (e.g. a 15% or 30% chance that a severe thunderstorm will occur within 25 miles of any point). |
Following are the meanings of the colors used in convective outlooks.
General Thunderstorms
The light green shading depicts a 10% or higher probability of non-severe or near severe thunderstorms during the valid period. However, just remember that a thunderstorm producing ¾” hail and wind gusts to 55 mph wind is officially a NON-severe storm but can still produce damage. So, just because you may be in an area of “general thunderstorms”, you need to keep alert for the possibility of rapidly changing weather conditions.
Severe Category 1 – Marginal Risk
The dark green shading area indicates a marginal (MRGL) risk of severe thunderstorms during the forecast period. This means a…
OR
probability for severe hail (≥1″ / ≥2.4cm) OR severe wind. (≥58 mph / ≥93 km/h).
Severe Category 2 – Slight
The yellow shaded area indicates a slight (SLGT) risk of severe thunderstorms during the forecast period. This means a…
OR
15% probability for severe hail or severe wind probability WITH OR WITHOUT 10% or greater probability of hail 2″ (4.8 cm) or greater in diameter
OR
wind gusts 75 mph (120 km/h) or greater .
Severe Category 3 – Enhanced
The orange shaded area indicates an enhanced (ENH) risk of severe thunderstorms during the forecast period. This means a…
OR
15% probability for any tornado
OR
30% severe hail or severe wind probability WITH OR WITHOUT 10% or greater probability of hail 2″ (4.8 cm) or greater in diameter, or wind gusts 75 mph or greater
OR
45% probability of severe hail or wind.
Severe Category 4 – Moderate
The red shaded area indicates a moderate (MDT) risk of severe thunderstorms are expected. This means a…
OR
30% probability for any tornado
OR
45% severe wind probability AND 10% or greater probability of a wind gusts 75 mph (120 km/h) or greater
OR
45% severe hail probability AND 10% or greater probability of hail 2″ (4.8 cm) or greater in diameter
OR
60% severe wind probability
OR
60% severe hail probability WITH OR WITHOUT 10% or greater probability of hail 2″ (4.8 cm) or greater in diameter.
Severe Category 5 – High
The fuschia shaded area indicates a high (HIGH) risk of severe thunderstorms are expected. This means a…
OR
45% or greater probability for any tornado WITH OR WITHOUT 10% or greater probability of an EF2+ tornado
OR
60% severe wind probability AND a 10% or greater probability of a wind gust 75 mph (120 km/h) or greater.
These are the official definitions. The reason for the “AND’s”, “OR’s” and “WITH OR WITHOUT’s” is the atmosphere is complicated with many different conditions that can occur. For example, there will be times when the number of severe weather events will be high but the overall intensities will not necessarily be extreme. Conversely, there may only be one or two severe events expected but the intensity of the event(s) will be extremely high.
Therefore, below is a simplified version of the official definitions. 
Public Severe Weather Outlooks
The Public Severe Weather Outlooks (PWO) are issued when a potentially significant or widespread tornado outbreak is expected. This plain-language forecast is typically issued 12-24 hours prior to the event and is used to alert National Weather Service field offices and other weather customers concerned with public safety of a rare, dangerous situation.
The Public Severe Weather Outlook is reserved for for all high risks and for moderate risks with a strong risk for tornadoes and/or widespread damaging winds. The SPC issues about 30 PWOs each year.
Mesoscale Discussions
When conditions appear favorable for severe storm development, SPC issues a Mesoscale Discussion (MCD), normally 1 to 3 hours before issuing a weather watch.
SPC also puts out MCDs for mesoscale aspects of hazardous winter weather events including heavy snow, blizzards and freezing rain. MCDs are also issued on occasion for heavy rainfall or convective trends.
The MCD basically describes what is currently happening, what is expected in the next few hours, the meteorological reasoning for the forecast, and when/where SPC plans to issue the watch (if dealing with severe thunderstorm potential). Severe thunderstorm MCDs provide you with extra lead time on the severe weather development.
Severe Weather Watches
Tornadoes can occur in either type of watch, but tornado watches are issued when conditions are especially favorable for tornadoes. Severe thunderstorm watches are blue with tornado watches in red.
Watches are large areas, 20,000 to 40,000 square miles, and are issued by county. They are numbered sequentially (the count is reset at the beginning of each year). A typical watch has a duration of about four to six hours but it may be canceled, replaced, or re-issued as required. A watch is NOT a warning, and should not be interpreted as a guarantee that there will be severe weather!
When a watch is issued, stay alert for changing weather conditions and possible warnings. Any warnings will be issued by your local NWS Weather Forecast Office.
When a severe weather watch is issued close to your location but does not include your county, you should still remain alert.
The atmosphere rarely follows straight lines, and thunderstorms do not always remain within the man-made boundaries we create around them. When SPC feels confident about the possibility of severe weather in a specific area, they try to issue a watch at least one hour prior the onset of severe weather.
In some instances the phrase “THIS IS A PARTICULARLY DANGEROUS SITUATION” will headline a watch (called a PDS watch). The “particularly dangerous situation” wording is used in rare situations when long-lived, strong and violent tornadoes are possible.
PDS watches are issued when, in the opinion of the forecaster, the likelihood of significant events is boosted by very volatile atmospheric conditions.Usually this decision is based on a number of atmospheric clues and parameters, so the decision to issue a PDS watch is subjective.
There are no hard threshold or criteria. PDS watches are most often issued in association with “high risk” convective outlooks.
My Daughter, as a requirement of her profession, spent four days training/learning for weather awareness/conditions at the SPC, in Oklahoma. She considered it a “great week” since she is part of a long family tradition of running toward storms, as opposed to, running away. She is now in direct contact with the SPC during large, outdoor public gatherings. Dad ended-up with a couple cool t-shirts 😉
Get ready to rock n roll!
https://www.instantweathermaps.com/GFS-php/conussfc.php?run=2019040500&time=PER&var=ASNOWI&hour=240
Enjoy the mini weekend warm up, because by the middle or end of next week the bottom drops out! Get ready for multiple days of below normal temps! Incredible!
Tigers Win..!!!
Rock on!
The weekend warm up will be lived, then get ready for a long stretch of below normal temps again. The cool pattern keeps rocking!
38* degrees in thee D today no thanks! INDY!
Better skiing weather than baseball! The cold just keeps rocking!
Just another cold, late winter day in MI. Unfortunately, as has been the case since the year began, April holds no hope of improving conditions. 2019…cold, routinely below average, and of course…storm free.
At least you face reality! Michigan usually has cold Springs and colder Winters!!
April 4th and the current wind chill in is is 29 degrees! What a cold April day and so far this year the average temp is well below nprmal! Incredible!
You also said yesterday was cold and yet it was above average! Hmmm….
Back to back above average days with a beautiful and warm weekend upcoming! I love it!
I am making some changes to the front page so ignore the man behind the curtain 🙂
Wizard of OZ….love it …INDY!
As Indy stated it has been a long time since there has been a warm day for opening day at Detroit. Here is the opening day temperatures going back to 2000. 2018/39. 2017/42, 2016/28,2015/52, 2014/52, 2013/45, 2012/43, 2011/43, 2010/38, 2009/51, 2008/51,2007/62, 2006/54,2005/68, 2004/46, 2003/35, 2002/37, 2001 and 2000/36. So the range has been from 35 to 68.
Slim
Tiger staidum should have a contractual roof especially for this time of year and if the ever make the post season again in September..Great stats Slim as always!! INDY!
Well just another below normal temperature day with wind chills in the 20’s again! When will it ever end? What an INCREDIBLE cool pattern we are locked into! Wow!
Our cool spring continues today..lots of Severe weather is going to start rolling but way to our south we will be on the nothern edge of some pretty good storm systems and with that thought stay on the cool shower side of things looks like next couple weeks after a beautiful warm weekend sets in enjoy it! wow another blustery Tigers home opener today what’s new I can’t even remember a warm opening day seems like it’s been forever I will be watching for my 70* degree mancave this afternoon with a Sprite or 2 and where it will feel… Read more »
The Tiger pitching has been good so far this year!
The pitching has been good so far but he hitting not so much.
Slim
Since 2000 the only mild opening days in Detroit have been 62 in 2007 and 68 in 2005.
Slim
Forever ago lol! Go Tigers! INDY!
I view the SPC website often when severe weather is a possibility. This info helps me to better understand what I’m looking at. Nice work, MV.
Agreed…thanks MV!!!!
A lot of good information MV. Here in West Michigan we will see several times each year with marginal, and slight risks. With a very few Enhanced and even less moderate risks and high risks are uncommon here in Michigan. It is now cloudy and a cool 36 here at my house.
Slim