Storm Prediction Center maintains moderate risk - ABC 33/40 - Birmingham News, Weather, Sports

Storm Prediction Center maintains moderate risk

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From the ABC 33/40 Weather Blog:

We continue to highlight an enhanced severe weather threat for a large part of Mississippi, northwestern Alabama and southern Tennessee. The SPC kept basically the same area under a Moderate Risk severe weather outlook for today. There had been concern that it might have to be upgraded to a High Risk, their highest threat level. High Risks are very rare and indicative of a high potential for severe weather.

The enhanced area is surrounded by a large standard risk forecast (Slight Risk) that extends all the way from southeastern Texas to western North Carolina way up to southeastern Iowa.

INSTABILITY: A warm, moist airmass covers Alabama and the Deep South this afternoon. Clouds have been thick, but there has been just enough sunshine warm temperatures into the upper 70s and lower 80s by early afternoon. They will rise a couple of more degrees this afternoon. Dewpoints are rising into the middle and upper 60s. So, if you have been paying attention to Alabama weather over time, you know that is a juicy airmass and fuel for the storms will not be a problem. We always have to look next at the octane of the fuel, and that isn’t a problem either, with instability values running over 2,500 j/kg. If those CAPEs are realized, that will represent high instability and strong updrafts will have no trouble being maintained. One thing in our favor is that mid level lapse rates are poor, which could help keep the storms from running out of control. Keep your fingers crossed.

WIND SHEAR: We look at a number called bulk shear, which is just the difference in wind velocity from the ground up to about 18,000 feet. Those numbers will be increasing from around 40 knots to around 50-55 knots as we go through the afternoon and evening, which is sufficient for those updrafts to lean over with height, which keeps the storms from choking themselves out. Low level helicity will start out low this afternoon, but it will be increasing over West Alabama through the late afternoon and into the evening hours. So with these two parameters increasing as we get into the late afternoon and evening hours, the threat will be increasing as well.

EHI: Energy Helicity Index values (which combine instability with low level shear) will also be increasing and pushing toward 3 across the I-20 corridor by this evening. Anything great than 1 is a problem and values over 2 are trouble.

LCL: The height of the Lifted Condensation Level is a real important parameter to Central Alabama tornadoes. It is the height at which rising air becomes saturated. When that number is less than 1500 meters, it means the chance of tornadoes is higher. Than number will be in the less than 1,000m through the event.

DIFFLUENCE: One thing we look for is winds spreading apart at the top of the top of our atmosphere, where the storm anvils will be spreading out. This helps to make it easier for the updrafts to be intense. Our winds in the upper levels will definitely be spreading apart, or diffluent, so this is a concern.

SPECIAL BALLOON: The National Weather Service in Birmingham is launching a special balloon at shortly before 1 p.m. that will sample the atmosphere and give us a good profile of the atmosphere.

STORMS FIRING OVER NORTHEAST LOUISIANA: The storms intensifying rapidly now over Northeast Louisiana are very concerning. Ping pong sized hail has already been indicated with these. Skies have cleared ahead of them along I-20 into the Jackson area, are this means instabilities and the potential for them to be severe is increasing.

SO IN SUMMATION: There is the potential for significant severe weather outbreak across much of Alabama this afternoon and tonight. All modes of severe weather are possible, including strong tornadoes, damaging winds and large hail. The highest tornado threat is from Birmingham north and west late this afternoon and this evening. The threat will shift to more of a straight line wind/flooding issue after midnight.

FLOODING: Rain amounts of 3 to 5 inches are likely across North/Central Alabama between today and Wednesday morning. Keep in mind some places saw one inch of rain yesterday, so flash flooding is a very real possibility, and if you live in a flood prone area pay attention to flash flood warnings. A flash flood watch will go into effect for Central Alabama at 4 p.m. today.

TIMING: The storms should reach western Alabama’s Lamar and Marion Counties as early as 3 p.m. They will slowly spread eastward through the late afternoon and evening hours, affecting mainly areas west of I-59 through 7 p.m. The back edge of the activity may not reach I-59 though until after midnight. Areas southeast of Birmingham could see the storms until sunrise. So this will be a prolonged event lasting through the overnight hours.



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