There is a threat for severe storms (even rotation) late in the day Wednesday, especially across Southside. The threat for very heavy rainfall in locally strong storms is also something to watch starting Wednesday evening lasting into Thursday and Friday.
For those of you who haven't kept up with the forecast from only a couple of days ago, things are changing fast. It was just earlier this week when it looked like pleasant, fall-like air would keep us nice and quiet going into the weekend. Now all of that has changed.
The culprit is an upper level low, moving in our direction from the Great Lakes region. This low was earlier forecast to move east with time and usher in cooler weather for the end of the work week. While low temps will eventually get chilly late this week, the main issues for us our now severe storms and very heavy rain bands.
Most computer models over the past 36-48 hours have been trending further WEST with the upper low. This pool of swirling cold air aloft will be cut-off from the main jet stream, but will also have its own wind field associated with it.
Upper lows in the Spring and Fall are often behind periods of very active weather. That's because the circulation around the slow-moving lows allow for rain to develop day after day, forming in bands that sometimes train over the same areas. By Thursday night, the upper low for this week is forecasted to be centered in eastern Tennessee, with the circulation pulling in heavy thunderstorms into Virginia from the south Wednesday, Thursday, and even into Friday.
The other issue with upper lows, and the one this week is the cold air upstairs. When the sun heats the ground and the air near it during the day, the cold air sitting above us will make the air bouyant, meaning it can rise fast into strong and even severe storms. With colder air to work with upstairs, hail is possible with any strong storm. And since cold air is heavier, strong winds are possible as well.
In fact, the winds will be blowing at different directions and speeds, something called wind shear, Wednesday and Thursday. This combination means we have to watch for severe storms with rotation for all areas, especially Southside VA late in the day Wednesday.
Rainfall amounts will range greatly through Friday, depending on where the heavier rain and thunderstorm bands develop. In general, .50" to 2" of rain are possible area-wide through the day Friday. However, amounts underneath heavier bands could exceed 3-4", with some models showing more than that.
As this pattern sets up Wednesday, be prepared for rapidly changing weather. Have a plan of action in case warnings are issued. The threats are two-fold... severe storms and flooding in spots through Friday.