SEVERE WEATHER 101: Understanding Severe Weather Forecasts

In Part 1 of our week long series, we gave you a rundown of the chain of command involved in a severe weather event, starting with the Storm Prediction Center and end with storm chasers and storm spotters on the ground witnessing the weather first-hand.

day1otlk_20080529_1630_prtNow for part 2, the goal of this article is to help you better understand the forecasts put out specifically for severe weather across the country.  This part of the severe weather prediction process takes place at the Storm Prediction Center in Norman, OK.  As I explained in Part 1, the SPC is basically the first line of defense in advance of a severe weather event.  The greatest example of this is the convective outlook.

Many probably know these convective outlooks by a more basic name, the severe weather outlook.  These forecasts come in both text and graphical form.  The graphical version is commonly seen on television and right here on  Although basic looking, these graphical forecasts have a very complex method to their madness.  In the most basic use of these convective outlooks, the forecasts is broken down into three levels of severe threat:  Slight, Moderate and High Risk.

For those in severe weather-prone areas, thunderstorms carrying a slight risk of severe weather are rather common.  But once the threat of wide more dangerous outbreaks becomes apparent, moderate and high risk forecasts come into play.

prob_to_cat_day1_seetextNow some might believe that if you are under a moderate or high risk that it means big, killer tornadoes are possible.  That’s not always the case.  See these forecasts are based on a very complex criteria table.  This table (seen to the right) is comprised of three different types of severe weather.  Those three are tornadoes, large hail (1” in diameter or greater) and strong winds (in excess of 58mph).

You can see as the percentages for each type of severe weather go up, the type of risk changes.  Note that if strong winds are the only forecasted severe weather, a high risk can NOT be issued.  On the flip side if the only risk were to be a 30% risk of a tornado a high risk is warranted.

So what does the SPC mean by “Outlook Probability?”  Let’s say the risk for tornadoes in an area is 10%.  That means that the area circled by that graphic stands a 10% chance of seeing a tornado develop within 25 miles of any point.

So how does the SPC come up with these forecasts?  Meteorologists tasked with predicting severe weather for the entire country are using the most advanced and state of the art forecasting models to help them accurately predict what will happen, where and when.  With each storm, scientists are able to better fine tune these special computer algorithms and hopefully bring us closer to more accurate forecasts with each passing day.

These convective outlooks are issued up to a week in advance.  They are updated once per day until the day before the severe weather event.  Known as the Day 2 convective outlook, that forecast is updated twice per day.  The Day 1 Outlook, or the day the severe weather is expected to occur, is updated five times during the course of the day.

Now this all might sound a bit technical, but I believe it’s a great resource for both experts and amateurs alike.  Most of all it provides some of the best advance warning that dangerous weather is possible in the days ahead.  So how can you look at all of these convective outlooks for yourself?  You can find links to these forecasts by clicking going to our links page.  You can find a link at the top of this page.

The convective outlooks I’ve been talking about in this post can warn of possible severe weather days in advance.  But what happens when the development of severe weather becomes imminent?  In our next segment of Severe Weather 101, we’ll take a look at the role mesoscale discussions play in the final hours before severe weather strikes.

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