A shortwave impulse lifting out of Northern Mexico on Friday should provide the energy for the formation of a low pressure system to develop along the Gulf Coast. This system could bring two major impacts in the form of heavy rain and the potential for severe thunderstorms across parts of the southeastern U.S on Saturday and Saturday night.

The HPC is currently forecasting a large area of the southeastern U.S. to be affected by some heavy rainfall from this system. Many areas along the Texas Gulf Coast, southern Louisiana, southern Mississippi, Alabama, Georgia, and South Carolina have the potential to see in excess of an inch and a half or rain between now and Sunday morning. It would not be out of the question if some localized areas were to pick up three inches or more of rainfall, with most of this falling on Saturday.

Taking a look at the NAM model’s 850 mb map for Saturday morning, strong southerly winds will be blowing in warm and moist air from the Gulf of Mexico providing for plenty of moisture and the uplift needed to create some heavy rain as well as thunderstorms. With models varying between each other as well as on consecutive model runs on the exact track of this system, there is still uncertainty in the areas for greatest risk of heavy rain as well as greatest risk for severe thunderstorms. Judging from the current NAM model run depicting the 850 mb level, warm air advection would place the greatest uplift across basically the same areas that are forecasted by the HPC to receive the heaviest amounts of rainfall.
That being considered, taking a look at the NAM Model’s 500mb map for the early afternoon on Saturday, shows a large area of positive vorticity located over Texas associated with the upper level trough. However, out in front of this across Mississippi, Alabama, and Georgia we do see varying pockets of vorticity which are being advected downstream of their respective locations. Baring this in mind, I would expect the potential for embedded heavier rain and thunderstorm bands within the large swath of precipitation that will be associated with the surface low as well as the potential for short line segments of thunderstorms outside of this large zone of precipitation. However, keep in mind that the image below is just model output and therefore specific areas of vorticity could easily change miles in one direction or another so try not to get carried away with predicting exact locations of these rain or thunderstorm bands.
Below is the current Day 3 Categorical Outlook from the SPC depicting a large area under slight risk of severe thunderstorms Saturday and Saturday night. The area at risk is pretty much the area the models have defined as being under warm air advection, receiving strong southerly winds from the Gulf of Mexico, and as having those pockets of vorticity go through. I would expect that the SPC will not sway in the area of this forecast too much though slight changes may be needed if the warm and moist air makes it farther north than expected or not as far north. The main threats right now would be damaging winds but also depending on the exact setup, we could end up seeing at least a few tornadoes spawn from this event. The hail threat should not be as large as these two factors just due to the amount of warmer air aloft however this does not mean that in some storms with stronger updrafts, severe hail could not occur.
Another factor that could really narrow down the severe threat area for us is the location of the jet stream. The current NAM model image below shows a large 150 knot plus jet streak centered over Mississippi and Alabama. The main area of concern here will be the right rear quadrant of this jet streak as it will provide for an area of uplift. If you take a look at the model loop, you would see that this area basically carves out a spot across the southern portions of Mississippi, Alabama, and Georgia as well as the Panhandle of Florida.
Taking a look at the SPC’s Probabilistic Day 3 Outlook, this is exactly where the SPC has highlighted an area for a heightened slight risk chance and if things keep progressing the way the models think they will, this area or an area very close to this may well be highlighted as a moderate risk by the SPC as we get closer to the event in my opinion.
The strength of this event will depend on the development of this surface low and how much warm and moist air it can pull in from the Gulf. If the system can pull in a lot of this air and line up nicely will the jet streak aloft, we could be looking at an outbreak of storms across parts of the southeast this weekend. Another factor to consider as this system sets up will be the extent of the storms northward which will be mainly dependent on the track of the low and how far north the warm front can extend. It is definitely something everyone should keep an eye on, especially those that live in these areas. I will do my best to post again on this subject before the event occurs to update everyone on the situation.