|The NOAA logo, the agency under which all other meteorological agencies operate.|
The only thing is that this government agency is in the middle of Oklahoma, and it's responsible for just about every official severe weather forecast issued in the last 5 decades. This agency is called the Storm Prediction Center.
A damaged plane, along with damaged vehicles, after the Tinker AFB tornado.
Back on March 20, 1948, two Air Force meteorologists -- Captain Robert C. Miller and Major Ernest J. Fawbush -- made history by issuing the first accurate tornado forecast. A few hours after they issued their forecast of tornadoes, a tornado swept through Tinker AFB in Oklahoma City, OK doing considerable damage. There were some injuries, but thanks to the few hours of warning before the storms came, there were no fatalities. Using the techniques of Fawbush and Miller, the US military created a unit devoted solely to forecasting tornadoes.
After public pressure, the military agreed to start issuing civilian tornado forecasts, and the military set up a branch of its weather bureau dedicated to tracking tornadic weather for both military and civilian use. This unit evolved throughout the 1950s and 1960s and moved from Washington DC, to Kansas City, MO, to its eventual home in Norman, OK, to become the National Severe Storms Laboratory (NSSL) in 1964. As technological advancements grew in meteorology (the advent of satellite imagery and Doppler radar being the most important), the NSSL grew in importance to both the meteorological community as well as American public.
The NSSL was renamed the Storm Prediction Center (SPC) in 1995, and the rest is history.
(Link to a more detailed, official history of the SPC)
The SPC is responsible for numerous tasks, the most important of which include severe weather forecasting, fire weather forecasting, and helping local NWS offices pinpoint flash flood threats. To most people, though, the severe weather products they issue are the most important, and that's what I'll cover in this diary.
The first thing to mention is what constitutes a severe thunderstorm. A severe thunderstorm is one that produces any wind damage (branches down, power lines down, building damage, etc.), hail 1.00" (quarter size) in diameter, or produces a tornado.
To forecast for severe thunderstorms, the SPC issues 5 main severe weather products:
- Convective Outlooks: Severe weather forecasts issued 1 to 8 days in advance. These convective outlooks include "risks" for severe weather -- generalized risks (slight risk, moderate risk, or high risk) as well as actual percentages (15% chance of wind damage, 30% chance of hail, 2% chance of tornadoes).
- Watches: These are severe thunderstorm watches and tornado watches. They are short term (usually 6-8 hour) forecasts that indicate that conditions in a given area are favorable for the development of severe weather.
- Mesoscale Discussions: These are unscheduled statements issued by the SPC to discuss current weather and areas that the SPC is watching for possible severe weather development. In a severe weather context, they're issued to talk about potential severe weather (before they issue a watch) or ongoing severe weather. Mesoscale Discussions are also issued for severe weather weather and heavy rain events, but they're more rare.
- Storm Reports: The storm reports are an aggregate of all severe weather (wind damage, large hail, or tornadoes) reported to the National Weather Service and relayed to the Storm Prediction Center.
- Mesoscale Analysis: The mesoscale analysis page is a way to get a detailed weather analysis from 10 fixed sectors across the country. You can get everything from simple surface observations (temperature, wind, pressure) to detailed severe weather parameters overlain on current Doppler radar images.
Convective outlooks are both graphical and text-based. The graphical forecasts are the most used and great for a summary, but the text-based forecasts are what's really useful.
Here is an example of a Day 1 convective outlook, from the April 27, 2011 tornado outbreak:
The area highlighted in green is an area at risk for general, non-severe thunderstorms.
The area highlighted in yellow is an area with a slight risk for severe weather. A slight risk means that there is expected to be some hail, some wind, and maybe a tornado or two, but not a huge severe weather outbreak.
The area highlighted in red is an area with a moderate risk for severe weather. A moderate risk means that a concentrated severe weather outbreak is likely, with numerous large hail, damaging wind, and/or tornado reports expected.
The area highlighted in purple/pink is an area with a high risk for severe weather. A high risk is reserved for only the most volatile days, and is only issued once or twice every year. A high risk means that a major tornado outbreak is expected, or an extreme derecho (straight-line wind storm) is expected. Along with the high probability of tornadoes, a large amount of extreme hail or wind reports are expected.
In the Day 1 outlook, the SPC breaks down the wind, hail, and tornado threats by probability percentage. A 30% probability in winds means that there's a 30% chance of wind damage within 25 miles of a point. The same goes for tornadoes or hail. Black hatching on the threat area means that there's an increased chance of either significant winds (75+ MPH), significant hail (larger of 2.00"), or significant tornadoes (EF-2 or stronger). The tornado risk probabilities from the April 27, 2011 tornado outbreak is seen below.
A 5% risk of tornadoes or a 15% risk of wind damage within 25 miles of a point doesn't sound high, but it's all relative. A 5% risk for tornadoes means that tornadoes are 5 times more likely than climatology. That's big.
The SPC is the agency responsible for issuing severe thunderstorm and tornado watches. These watches usually last for 6 to 8 hours and mean that weather conditions in the area under the watch are favorable for producing severe weather. A watch is a heads-up that severe weather is likely to develop in the short term future, and a cue for you to keep an eye on the weather.
The main SPC site has a watch overview much like this one, that allows you to click the individual watches:
THE SEVERE WEATHER THREAT FOR TORNADO WATCH 232...235...CONTINUES.PARTICULARLY DANGEROUS SITUATION /\PDS\/ TORNADO WATCHES 232/235 CONTINUE UNTIL 00Z/03Z RESPECTIVELY. THIS INCLUDES THE POTENTIAL FOR LONG-TRACK STRONG/PERHAPS VIOLENT TORNADOES INTO THIS EVENING AS A SEVERE WEATHER OUTBREAK ONLY INCREASES IN MAGNITUDE/RISK.AN EXTREMELY DANGEROUS/LIFE-THREATENING SITUATION CONTINUES TO UNFOLD THIS AFTERNOON ACROSS A LARGE PART OF MS/AL...WITH ADJACENT PORTIONS OF TN/NORTHWEST GA ALSO EXPECTED TO BECOME A CONCERN LATE THIS AFTERNOON/EVENING. CURRENT OBSERVATIONAL TRENDS...REASONABLY SUPPORTED BY EXPERIMENTAL HRRR GUIDANCE...IMPLY THAT SCATTERED SUPERCELLS WILL CONTINUE TO FORM IN BROKEN NNE-SSW ORIENTED CORRIDORS OF SUBTLE CONFLUENCE AHEAD /\EAST\/ OF MORE STORMS/SUPERCELLS THAT ARE DEVELOPING ALONG A PRE-COLD FRONTAL TROUGH/DRYLINE GENERALLY NEARING I-55 IN MS.THE WARM SECTOR AIRMASS HAS AGGRESSIVELY DESTABILIZED THIS AFTERNOON AMID NEAR 70F/LOWER 70S F SURFACE DEWPOINTS...REFERENCE SPECIAL 18Z OBSERVED RAOBS FROM JACKSON MS/BIRMINGHAM AL...WITH A WIDE/HIGHLY SHEARED MOIST SECTOR IN PLACE ALONG/SOUTH OF A MODIFYING WEST-EAST OUTFLOW BOUNDARY /\NOW AN EFFECTIVE WARM FRONT\/ ACROSS FAR NORTHERN PORTIONS OF AL/MS. EXTREME LOW LEVEL SHEAR...VIA LONG/CURVING LOW LEVEL HODOGRAPHS...WILL REMAIN HIGHLY CONDUCIVE FOR SUPERCELLS CAPABLE OF LONG-TRACK STRONG/VIOLENT TORNADOES INTO THIS EVENING AMID 0-1 KM SRH OF 300-500 M2/S2 OR GREATER /\ESPECIALLY NEAR THE AFOREMENTIONED NORTHERN MS AND AL BOUNDARY\/.
- March 2 2012 outbreak across TN, IN, KY and OH
- April 27 2011 major southeast tornado outbreak
- May 22 2011 Joplin tornado
- June 4 2008 major Washington DC straight-line wind event (derecho)
- May 4 2007 Greensburg KS EF-5 tornado
- April 28 2002 La Plata MD F4 tornado