Knowing how to track severe weather on your own is a crucial part of keeping you and your family safe year-round. The best tool to use in your quest for severe weather information is the United States Government. Yeah, that nasty, evil, Federal, socialist, communist, Kenyan entity that sits in Washington DC.
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.
(Note: The orange section headers are all links. Click them to go to the corresponding SPC page. Just about all links open in a new window.)
Convective outlooks are day-long forecasts for thunderstorms and severe weather. The Day 1 (today) outlook is updated 5 times per day, with the most updates coming during day to account for daytime heating and instability. The Day 2 forecast is updated twice per day. The Days 3-8 forecasts are updated once per day.
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:
On the individual watch pages, you'll see a closer view of the watch, with the current radar superimposed over it. The text below the watch will tell you what kind of weather the SPC expects to develop, as well as a technical forecaster discussion of the environment and why they issued the watch.
A mesoscale discussion is a special statement from the SPC for a generally small area. They're used for a wide variety of topics, from alerting other forecasters that they're about to issue a watch, to giving an update on ongoing severe weather, to areas where they expect heavy snow or heavy rain to impact an area.
They're incredibly useful if you want to stay one step ahead of whatever weather is coming your way.
The mesoscale discussion page works the same as the watches. You can click the circled areas to view the page. This mesoscale discussion was issued as the devastating April 27, 2011 tornado outbreak was underway:
...and this was the associated text:
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/.
*Note: Words written /like this/ are meant to be parenthetical (like this). It's a standard not to write parenthesis in NWS products, just as it's international standard TO TYPE IN ALL CAPS.
The storm reports page shows all of the wind damage, large hail, and tornado reports the NWS and SPC have received over the span of the day. Blue dots represent wind damage, green dots represent large hail, and red dots represent tornadoes. Black triangles represent large hail over 2.00" in diameter, and black squares represent wind speeds recorded or estimated to be over 75 MPH.
The SPC archives all storm reports back through June 1, 1999.
The storm reports are available for download in KML format (used by Google Earth), viewable online with Google Maps, as well as viewable in table form on their site.
Here's an example of one of the worst severe weather days in history, April 4, 2011, when a record 1,303 reports of severe weather were collected.
The mesoscale analysis (mesoanalysis) page is the real beef of the site if you're a diehard weather geek. The page has hundreds of variables you can superimpose over the current radar map. They just completely overhauled the mesoanalysis page last week, so I'm still getting my bearings on the new features they've added.
On the mesoanalysis homepage, you'll see a US map with 10 colored boxes drawn on it. These are called "sectors" -- the ten areas of the US most prone to severe weather. Clicking on one of these boxes (or the links above the image) will take you to that region's mesoanalysis page.
The dropdown menus let you look at everything from CAPE (Convective Available Potential Energy...a measure of instability in the atmosphere) to the freezing level (at what height the atmosphere reaches freezing) to all the layers of wind shear that are relevant to severe storms.
It's above my pay grade to go into detail of every variable on this site. The SPC provides good explanations of each at this webpage.
Here's a screenshot of the mesoanalysis page, showing the MUCAPE (Most Unstable CAPE) for Thursday afternoon during the severe weather across the southeast. Click to enlarge in a new window.
Other Pages
You can access upper atmospheric maps, soundings (the upper air data collected from weather balloons), short range model forecasts, as well as fire weather forecast maps on the Forecast Tools page.
The severe weather events archive is a must if you're looking for information about a previous severe weather outbreak. It has information on just about every severe weather event that's occurred since January 3, 2000. For example, here are a few of the more significant outbreaks of the last few years:
The WCM page is the place to get all sorts of cool statistics relating to severe weather. From maps on how frequently tornado watches are issued, to tracks of tornadoes dating back to 1950, it's a really good place if you're looking for (or just curious about) the hard data relating to severe weather.
The "About Tornadoes" page is a detailed page answering most of the common questions relating to tornadoes.
The "About Derechos" page is similar to the About Tornadoes page -- it gives pretty good information about a weather phenomenon known as a derecho.
This is a valuable page if you want to know more information about the Enhanced Fujita Scale -- the system used to determine the strength of a tornado based off of the damage it produces.
This is nowhere near being an exhaustive list of all the products and services the Storm Prediction Center offers. I encourage you to go to their site and just click around. You never know what you'll find.
Tomorrow evening, I will post Part II: Using Radar, describing how to look for severe weather and tornadoes using radar.