Severe Weather Possible Thursday night, Friday

The Storm Prediction Center in Norman, Okla. has placed most of Florida and Georiga under a threat of severe weather Thursday night into Friday. A threat of damaging winds and tornadoes exists as a line of thunderstorms is expected to move eastward Thursday into Friday.

A closed low and trough in the upper levels were situated over the Southwestern U.S. and another shorter-amplitude trough over Wyoming and Montana Wednesday morning based on upper-level maps. At the surface, two lows and attached cold fronts were over the Midwest – one low centered over Nebraska and the other over New Mexico.

Upper level (300 hPa) map from Wednesday morning (8 a.m. EDT)
Surface map across the U.S. Wednesday morning (8 a.m. EDT)

High pressure was centered off the Southeast U.S. coast Wednesday, bringing more of a southerly flow east to southerly flow across Florida. This will bring in more moisture in the lower levels of the atmosphere.

Guidance models have this surface high moving east as the surface low in New Mexico deepens as it moves eastward over the next 48 hours. The upper-level trough is expected to deepen, providing energy to intensify the surface low and heighten the odds for rain and storms ahead of the cold front. The southwesterly flow ahead of the trough will also allow for the transport of deep moisture from south to north.

The Weather Prediction Center’s surface map forecast for Thursday evening.

That deep moisture transport is expected to start in western Florida and Georgia Thursday afternoon and evening with the rain entering the western panhandle around or after midnight with a line of showers and thunderstorms marching eastward through the night into Friday morning.

This line of storms has the potential for a few to become severe with damaging winds and a tornado or two. There are some factors going into the forecast that make this risk possible.

  • Deepening trough in the upper level of the atmosphere.
  • Diffluence spotted on guidance over South Georiga and much of Florida early Friday morning. This helps with better “ventilation” for storms.
  • Strong low-level winds (850 hPa) going through most of Georgia and the northern two-thirds of Florida during the window of rain and storms. This low-level jet can help add more low-level shear, increasing the threat of severe storms. This will also aid in moisture transport.
  • Dry air ahead of the line of storms in the mid levels. Rain and storms can rapidly cool off this mid layer, condensing the air and increasing the threat of strong and damaging thunderstorm winds. Lowering of the heights (cooling aloft) would also help with this threat, and make the atmosphere more unstable.
  • Very moist air in the lower levels. Surface dewpoints are expected to be in the mid 60s to the lower 70s, leaving more latent heat release for storm development and sustainability. The high dewpoints will also mean that temperatures Thursday night and Friday morning will stay warm. The southerly flow in the lower levels will keep the warm air advection going.
  • With the aforementioned dynamics, this will leave the stage set for higher shear parameters. An early high-res model and the American GFS is hinting at sufficient Storm Relative Helicity just ahead of the line of storms. Shear, the difference in wind speed between the surface and aloft (6 km in this case), is also sufficient.

The variable in question is how strong these storms could be as they move eastward during the overnight hours. The lack of daytime heating does leave in question the amount of instability and energy. That will have to be watched as the apparent event gets closer. Also, confidence in the timing of this line will be higher closer to the event.

It’s important for those in the low-risk zone to be weather aware – especially for those in the Panhandle, Big Bend, northern Georgia, and southwestern Georgia when the timing of arrival would be when most are asleep. Be sure to have a way to receive warnings.

Update (2 p.m. EDT April 17): The SPC outlook for Day 2, Thursday, April 18, was changed to reflect the updated outlook.

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