Severe Weather Forecasts from the National Weather Service Storm Prediction Center

Steve Keighton, Science and Operations Officer

 

 

The most critical component of the National Weather Service ( NWS) mission is the protection of life and property from adverse weather. Severe local thunderstorms that can bring tornadoes, downburst winds, and large hail, result in a large percentage of the most dangerous of all weather phenomena. Our agency attempts to prepare the public and emergency officials for all severe weather with a “ready, set, go” incremental process, which equates to the “outlook, watch, and warning” stages using NWS terminology. There are different parts of the agency infrastructure that contribute to this process.

 

As many of you may already be aware, the “go” or warning stage is the final step in the preparation process, and essentially means “take immediate action because of an impending threat to life or property”. The local Weather Forecast Offices (WFOs), of which the Blacksburg VA office is one of 122 across the United States, has the primary responsibility for this final critical step for the 40 counties we serve. The forecasters in the local WFOs rely on rapidly updated three-dimensional Doppler radar data, satellite imagery and surface observations, as well as visual reports from volunteer weather spotters to help us make the decision to warn or not.

 

The “ready” or outlook stage, which comes first in the preparedness process, means conditions are expected to be favorable for severe weather later in the day or over the next couple of days, while the “set” or watch stage means conditions are now ripe for severe weather and storms could become severe very soon. While the local forecast offices help to coordinate the issuance of outlooks and watches for severe storms, it is the primary mission of the Storm Prediction Center in Norman Oklahoma to issue outlook areas and watch boxes for severe thunderstorm and tornado threats for the United States. The rest of this article will open up the doors of the Storm Prediction Center for you, and explain how this important NWS national center contributes to your knowledge and our knowledge of when severe weather threats are increasing.

 

The National Weather Service actually operates nine national centers that are a part of the National Center for Environmental Prediction, each with a different specialty, and they include the center that runs the sophisticated numerical weather prediction models for the entire country near Washington DC, as well as the highly visible National Hurricane Center (or Tropical Prediction Center) in Miami FL. The Storm Prediction Center (SPC) in Norman specializes in the prediction of severe local thunderstorms. The center used to be known as the National Severe Storms Forecast Center and was located in Kansas City Missouri for many years until the mid 1990s when it moved to Oklahoma and became simply the SPC. Initially, SPC was co-located with the National Severe Storm Laboratory (NSSL), a center of well-renowned cutting edge severe storms research and birthplace of the NEXRAD Doppler radar technology we use today across the NWS. In the Fall of 2006, SPC moved to a new location on the south side of Norman, in a building known as the “National Weather Center” which also houses NSSL, the Norman NWS Forecast Office, the University of Oklahoma School of Meteorology, and the Warning Decision Training Branch of the NWS National Training Division. In this brand new facility (shown in Figure 1), severe storm researchers, operational forecasters, training developers, as well as professors and students can all interact to constantly improve the science of severe storm prediction. It’s really a one-of-a-kind facility for severe storms meteorology in the world.

 

This is an image of the new National Weather Center in Norman, Oklahoma

Figure 1. The National Weather Center in Norman Oklahoma, which houses the Storm Prediction Center and several other operational and academic meteorology centers of excellence. Photo taken by Robert Fritchie.

 

 

The Storm Prediction Center is open 24 hours a day, 365 days per year like any operational forecast office in the NWS, and consists of over 30 employees, including 20 forecasters with expertise in severe storm and storm scale weather prediction, computer and electronics specialists, and several administrative personnel. In addition to severe thunderstorm specific forecasts, the SPC also issues national guidance products related to short term winter weather and fire weather hazards, but their primary responsibility is severe weather outlooks, watches, and “mesoscale” discussions which provide local forecast offices guidance on how local regions of severe weather threats are evolving from hour to hour.

 

The outlook products are issued both internally as guidance to local forecast offices and externally to other uses and available on their web site (www.spc.noaa.gov). The outlooks cover the potential for general thunderstorm and severe thunderstorm development regions for Day 1, Day 2, Day 3, and a new general Day 4-8 thunderstorm outlook, for the entire lower 48 States. The Day 1 and 2 outlooks can include an area of general thunderstorm activity, any severe storm “slight risk” areas, “moderate risk”, or “high risk” (high risk typically means the potential for a major outbreak of severe weather or tornadoes). Outlook products on the SPC web page also now include probabilities of specific severe weather threats. Text products with detailed explanations accompany the graphical outlook products and are also available on the SPC web page. Figure 2 shows an example of a graphical outlook product.

 

Figure 2. Example of Convective Outlook Day 1 graphical product from the Storm Prediction Center.

Figure 2. Example of Convective Outlook Day 1 graphical product from the Storm Prediction Center.

 

 

Many people are familiar with severe thunderstorm and tornado watch boxes, which represent the “set” stage of the severe weather preparation process. During times of active or expected widespread severe weather, the SPC is often very busy issuing these watch boxes for various parts of the country, in coordination with the local forecast offices who then issue the final watch county notification text product. You may notice on some web pages or television graphics a watch box appearing as a large parallelogram, and on other graphical images you may see the outline of the official watch which follows individual county boundaries instead. If a watch is issued for part of the Blacksburg County Warning Area (CWA), our web page would show the counties within the watch shaded in rather than a straight edge to the boundary of the watch. The local forecast offices are responsible for the clearance of counties within a watch as storms move through the original watch box and the threat is determined to no longer exist for a portion of it, although there is usually coordination with the SPC when this occurs.

 

An example of a tornado watch box is shown in Figure 3 with both the original parallelogram from SPC as well as the official watch area outlined by the county borders. Watches are usually valid for about a 4 to 8 hour time window from when they are issued (the one in the example is a little longer than normal). When the decision to issue a watch box is made, there are general severe weather thresholds that the SPC expects to have a reasonable chance of being met within that area and during the lifetime of that watch. Specifically, when a tornado watch is issued they are expecting a good chance of two or more tornados, or one or more strong tornados, somewhere within that area. When a severe thunderstorm watch is issued they are expecting a good chance of 10 or more reports of either large hail (penny size or greater) or wind events (damage or measured thunderstorm winds of over 60 mph), or at least one report of extreme wind (80 mph) or very large hail (golf ball size or larger).

 

 

 

 

Tornado watch box and corresponding county outline

Figure 3. Tornado watch box and corresponding county outline issued by the Storm Prediction Center, after coordination with local NWS Forecast Offices.

 

 

Follow up statements to watches and mesoscale meteorology discussions are issued on a frequent basis so that local forecast offices and other users have an idea if the SPC forecasters are thinking of issuing a new watch, thinking that the threat is ending or continuing or perhaps even changing within a current watch. As portions of watches are cleared by the local forecast offices, SPC issues summary statements once per hour with the status of what areas are still covered by watches. Also, SPC carefully monitors all warnings issued by the local offices and keeps track of all severe reports sent our from local offices to help maintain situational awareness of ongoing threats, and also so they can verify the outlooks and watches they issue (just as local offices verify our warnings). This is another reason why your timely and detailed reports of severe weather (i.e., specific hail size or measured or estimated wind speed) that you provide to us are so important.

 

Forecasters at the Storm Prediction Center use a wide variety of datasets and tools to make their forecasters, including climatology and pattern recognition developed from many combined years of experience. They not only use a lot of objectively analyzed maps showing a wide variety of severe weather related parameters, but also hand analyze maps on a frequent basis to maintain a close eye on all the individual observations. Many of the tools and parameters they are using now to make their forecasts have come from years of research that SPC forecasters have conducted, or that others in the research community (many of them in the same building now) have conducted. Forecasters work on much of their research activities during the winter season when severe storm activity is at a minimum, and they are often attending local seminars or national and regional conferences to continue enhancing their professional skills.

 

One of the SPC forecasters recently made a trip to the Blacksburg office not only to share with our staff information about their forecasting process, tools, and data sets, but also to work with a couple of us on a joint research project involving the study of organized lines of severe thunderstorm activity that approach the Appalachians from the west, and the various environments and patterns which tend to either result in their decay or their survival as they move across the mountains and into the Piedmont. This particular forecast problem is not only important to us locally to help us anticipate when we will need to keep warnings going, but to SPC on a larger scale since they are tasked with issuing the outlooks and watch areas for the country, and so the domain of our study covers from northern Georgia into Pennsylvania. While he was visiting, he even got to experience our first severe weather event of the season with nickel size hail falling outside the office right at the end of his presentation (the event on March 28). Some of us don’t think the timing of an SPC forecaster’s visit and the severe weather in our back yard was a coincidence!

 

As you can tell, NWS local forecast offices and national centers such as the Storm Prediction Center have to collaborate effectively since we share the same overall mission of protecting lives and property, and our specific responsibilities overlap somewhat as well (with the SPC, the overlap involves the issuance and communication to the public, media, and emergency officials of severe weather hazards that may potentially develop in the next couple of days or next couple of hours). Working together on applied research and becoming more familiar with each others specific jobs and tools of the trade is vital to effective collaboration, and ultimately helps us improve how we accomplish our mission to protect lives and property.