Thunderstorms are common here in the Valley, and we're rapidly approaching our severe weather season. Often times you'll hear WHSV meteorologists mention that the Storm Prediction Center (SPC) has placed us under "Slight Risk" of severe storms, but what does that mean to us?
Convective Outlook graphics are produced daily from the Storm Prediction Center, portraying which areas can expect severe weather. Those graphics are available at: http://www.spc.noaa.gov/products/outlook/
I encourage you to look at them while reading this blog!
There's an unlabeled, generic thunderstorm line (brown) on the map. Within this area (the right of the line), there's a 10% or greater chance of thunderstorms occurring during the valid time period. Make sure that you look at which way the arrow is pointing! These are your garden variety thunderstorms.
One step up from the brown line is to see the label "SEE TEXT" on the convective outlook. This prompts us to read the outlook discussion as to why there may or may not be severe weather. There's not enough certainly to issue a risk area, but severe storms are still possible. Reasons for not issuing a risk area include model disagreement, insufficient amounts of moisture, along with many other factors that are discussed in the text.
When there is enough certainty for severe weather, a Slight (SLGT) risk area is issued. This implies that well-organized severe storms are expected, but in small numbers and a relatively small coverage area. Often times a SLGT risk area is issued for the small chance of a more significant severe weather event. Not every severe thunderstorm will be covered with a SLGT risk, especially in the summer. Summer time is conducive for short-lived, pop up (pulse) severe storms that are common during the peak daytime heating hours of the afternoon.
The next step up is a Moderate (MOD) risk outlook. A greater concentration of severe storms are likely when this is issued. There will likely be a greater magnitude of severe weather and greater confidence in the outlook than for SLGT risk. MOD risks are reserved for days with substantial severe thunderstorm coverage, or a greater threat for a significant severe weather event. Expect multiple tornadic supercells with large hail and/or intense squall lines with damaging winds.
In cases like the May 10, 2010 severe weather outbreak, a HIGH risk area is issued. This implies that we're dealing with a major outbreak, with large coverage of severe storms and the likelihood of violent severe weather (large tornadoes and damaging wind events). These risk areas are only issued a handful of times in a given year, and are reserved for the most extreme events.
Within each outlook graphic (Day 1, 2, 3, and 4-8), there are different products to look at.
In a Day 1 convective outlook, there is the categorical outlook, and then the outlook for tornadoes, hail and wind. When you mouse over each severe threat, you'll see a percentage.
For tornadoes, the percentage is the probability of a tornado within 25 miles of a point. If the area is hatched, there's a 10% or greater probability of an EF2 - EF5 tornado within 25 miles of a point.
For hail, the percentage is the probability of hail 1" or larger within 25 miles of a point. If the area is hatched, there's a 10% or greater probability of hail 2" or larger within 25 miles of a point.
For wind, the percentage is the probability of damaging thunderstorm winds or wind gusts of 50 knots or higher within 25 miles of a point. If the area is hatched, there is a 10% or greater probability of wind gusts 65 knots or greater within 25 miles of a point.
In a Day 2 (or Day 3) outlook, there is a categorical outlook and a probabilistic outlook. You'll see percentages in the probabilisitc outlook. The percentage is the probability of severe weather within 25 miles of a point. If there is a hatched area, there is a 10% or great probability of significant severe weather within 25 miles of a point.
For the Day 4 - 8 outlook, a severe weather area is depicted only when there is a 30% or higher probability for severe thunderstorms within 25 miles of any point.
I hope this clears up any confusion with the risk areas issued by the Storm Prediction Center. We are under SLGT risk for severe weather on May 14, with the majority of the thunderstorms expected in the afternoon as a cold front passes through the Valley and higher elevations.
If you have any questions, be sure to let me know! Enjoy the weekend, and the sunshine!