day2-20110521-1730

SPC Outlooks will change starting this Wednesday

The Storm Prediction Center has used the same risk categories for a large part of most of our meteorological careers. Although they have adjusted the probabilities, percentages, and yes even the colors — the categorical risk areas have remained the same. On this coming Wednesday, that will change. The Storm Prediction Center currently uses a simple categorical risk system which highlights the potential for severe thunderstorms across the country. The risk areas of Slight, Moderate and High are triggered by percentage chance of a type of severe weather within 25 miles of any point. And while the newer outlooks also highlight the potential for general non-severe thunderstorms across the country, Wednesday will mark a dramatic change in the way these outlooks are viewed and understood.

The main change will be the addition of two risk levels, both falling on opposite sides of the “Slight” risk of severe thunderstorms. “Marginal”, a new risk category, will highlight the potential for severe thunderstorms that includes a 5-10% chance of wind and hail and a 2-5% chance of tornadoes — but does not trigger a Slight Risk. “Enhanced” falls on the other side of “Slight”; in other words the risk levels are enhanced (30-45% risk) but not organized or widespread enough to trigger a Moderate Risk.

In addition to the added risk categories, the Storm Prediction Center will remove the “See Text” category from their outlooks. Used since the 90′s or earlier, See Text was placed on the graphical outlooks to alert users that a percentage risk area for thunderstorms was triggered, but it was too low to trigger a Slight Risk. So the user could scroll to the text discussion for more information. The new outlooks “Marginal” category does a fair job in replacing the old See Text descriptor.

The new SPC  probability to category conversion, active as of Wednesday October 22nd.

The new SPC probability to category conversion, active as of Wednesday October 22nd.

In general, these changes seem to positively favor the interpretation of the Storm Prediction Center’s outlooks. Initially, however, there may be some confusion as to what exactly the new categories mean for severe weather chances in any given area.

The Storm Prediction Center has provided a link with images and comparisions between the new outlooks, coming Wednesday, and the old outlook graphics and format. You can view it here.

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