Thursday, October 30, 2014

The Storm Prediction Center new convective categorical outlook system.

I've posted on my Facebook Weather Page a few times about the Storm Prediction Center (SPC)  proposing to change the Convective Categorical Outlooks.  Well earlier this month that became official.  The SPC  has gone from the old four overall risk levels to six overall risk levels. The SPC has also dropped the SEE TEXT portion of the outlook; something I'm a big fan of.

The Outlooks are  products that I often use when I post  when I'm talking about the likelihood of severe weather. So I wanted to take a moment to discuss the new changes.

Why the change? The old system was often imprecise. Also, the See Text was very confusing for laypeople to understand. The SPC hopes the new system will better delineate low end severe weather treats.   

Here's an example of the old outlook. ( this is an example and doesn't reflect the current weather)

As you can see it has the slight, moderate, high risk, as well as SEE TEXT.

Under the old system, areas highlighted  in the green color  could indicate anything from garden variety storms to strong thunderstorms ( hail smaller than 3/4 of an inch and winds less than 58 mph).   In order to know where the greater risk was, one had to go to the dreaded SEE TEXT. When reading the SEE TEXT, it helped to have a degree in Meteorology to better understand all the jargon.

Here's an example of the new outlook. ( this is an example and doesn't reflect the current weather)

You can see,  SPC has changed the categories to now include the risk for general storms and then marginal, slight, enhanced, moderate, and high severe weather risks.

Having more risk categories may seem confusing. So let me explain the changes.

General Thunderstorms will be split into two new categories...General and Marginal.

The General category: is where conditions support the  possibly of garden variety storms.  This area is colored pea green. 

The Marginal (MRGL) category:  replaces the SEE TEXT, this means there is the possibility of stronger thunderstorms, maybe a few that reach severe criteria. This area is colored green. 

The Slight (SLGT) category: this means there is the chance for organized severe weather. But the likelihood of widespread severe weather is very unlikely. This area is colored yellow.

The Enhanced (ENH) category: This one  may seem more confusing. The ENH risk is assigned when conditions are becoming more favorable for severe weather, but the likelihood of a severe outbreak is fairly low.  Greg Carbin with SPC suggests the best way to utilize this new outlook is by examining probabilities that are associated with each category and the individual probabilistic threats. This area is colored mustard

The Moderate (MDT) category: when the risk for a widespread severe outbreak is moderately high, including the risk for  several tornadic supercells,  large hail, and squall lines with damaging winds.   This area is colored red

The High category:  means the risk for a major severe weather outbreak is extremely high. This category is reserved for extreme severe weather setups, including the risk for  violent EF4 and EF5  tornadoes,  extremely large hail, and very damaging wind events like significant derechos. This area is colored  fuschia

You can better see how the new system works in the tables below (note the colors are as close as I can get them).



The SPC updates the Day 1 Outlook five times a day. It is used to provide specific guidance based on type of threat:  Tornado, Damaging Wind and Hail

For Day 2 and Day 3 outlooks, probabilities for specific type of storm impacts are not provided.

Well that  should give you a good understanding of the new Categorical Outlooks. As always I welcome your feedback and questions.


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