Upslope Flow Decreases Tuesday/Wednesday, Confidence Increasing on Another Heavy Rain Threat Late Week

Lead Forecaster Daniel Crawley

Good Monday afternoon and Happy Labor Day to everyone. Unfortunately the weather hasn’t been very cooperative as we have seen a lot of clouds and rainfall across the region. Let’s break down what’s happening…

A broad trough with deep southwest flow at 500 mb exists across the Southeast US, that along with southeast upslope at the low levels has resulted in a lot of rainfall across the Foothills and Western Piedmont. We have also been dealing with some flash flooding of small streams along the Blue Ridge.

While the Eastern US has been active, the Rockies and Western US has been under the control of high pressure aloft. Shortwave energy on the northeast side of the ridge is located across the Dakotas, its that piece of energy that will create another complex weather situation for the late week.

Our shortwave energy is going to break off from the main jet stream as we get to tomorrow and Wednesday. We will have a weak boundary slide southeast toward the Carolinas, the main impact from this is changing the low level flow to a more westerly component for about 36-48 hours, that will lower precipitation chances to a more diurnal fashion.

However, the shortwave we’re focusing on is going to capture the current feature across the Lower Mississippi River Valley and it will ultimately re-energize things once we get to late Thursday and Friday. A new, closed upper low is going to form over the state of Louisiana once we get to the weekend, this will be the first ingredient in the recipe for more heavy rainfall late this week.

With the upper low located where it is supposed to be, and heights slowly rising in the Southwest Atlantic the 500 mb flow is going to have Western Caribbean and deep Gulf of Mexico origin to it this week. Deep moisture is the second ingredient to heavy rain potential.

There is growing concern of an extensive “atmospheric river” setup for someone in the Southeast US. After a brief lull in precipitable water values locally Wednesday, deep moisture will return quickly as the cutoff low settles in.

As we focus in more on the Western Carolinas, the wind flow which briefly is West/Northwest on Tue-Wed will shift back to easterly on Thursday and then by Friday, a stout southeast fetch develops once again over the Blue Ridge. This fetch may infact be a tad stronger than what we are currently seeing. This is the third big ingredient needed for heavy rain later this week.

Needless to say, the ingredients are lining up for more heavy rains later this week across the Upper Southeast US and the Southern Appalachians. Some of the current high water will run off but the ground is going to remain saturated by the time our next big weather maker has evolved.

Current guidance is unfortunately focusing rains on the Western Carolinas. This specific map is a possible scenario for late week where 1-3 inches of rain could fall across the Western Piedmont and 3-5 inches in the escarpment region. Again, this is focused on Friday and Saturday, not the current rain event. We will definitely keep an eye on this…