Tuesday, September 30, 2014

90 again? When is REAL fall coming??

We've had a few tastes of fall in the past month, but between those, warm days have left many of you wondering "when is fall here to stay?"

We'll have another shot at it with a large-scale cold front that will move quickly through the region Thursday night. Prior to then, the Mid-South will be toasty the next couple of days. In addition, with southerly wind returning, humidity will also increase. Today's high of 90 could be matched tomorrow with enough sunshine, which I think is a good possibility. With increasing clouds and southerly wind Wednesday night, lows will remain above 70, but those clouds and a chance of thunderstorms Thursday afternoon will hold temperatures in the upper 80s.

On Thursday night, a cold front will sweep through the region, bringing showers and thunderstorms area-wide. The severe weather threat is low with this system, but not non-existent, particularly in Arkansas. This time of year, instability is typically the wild card in potential severe weather scenarios. The dynamics (for instance, upper level wind energy) associated with these systems tend to get stronger as the fronts get stronger in the transitional seasons. However, without instability (the gasoline that causes the fire to explode), strong storms have a harder time becoming severe.

Forecast instability, as measured by CAPE, at 1am Friday morning as the probability of storms increases. Values are more than sufficient for thunderstorms, but the severe weather threat is fairly low. Graphic courtesy WeatherBell.
The timing of Thursday's storms will be during the overnight hours when instability (brought about by heating of the atmosphere) will be lower than during the afternoon hours when the sun is high (but still at moderate levels). Therefore, while a marginal damaging wind threat is possible, the best scenario for severe storms will be to our west where the storms will be during peak heating. Rainfall amounts could reach an inch or more Thursday night, but widespread flooding is not expected either.

"Forecast radar" from the mid-day run of the NAM model valid at times indicated. This is an early indication of timing and is NOT a forecast of exactly what will occur. Note the general decrease in line strength between the two images. Graphic courtesy WeatherBell.

The storm system will likely be gone by the time you head to work or take the kids to school on Friday morning. Behind it, a much cooler airmass overtakes the Mid-South. Skies will quickly clear and, even with lows in the mid to upper 60s Friday morning, mid 70s may be as high as we go in the afternoon. The weekend looks simply stunning! Some outlying areas may see lows in the 40s with highs in the 70s across the board. A great weekend to get out and enjoy fall in the Mid-South!

Erik Proseus, MWN Meteorologist

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