The Storm Prediction Center has put areas northwest of the Natchez Trace Parkway in a Slight Risk of severe weather for late tomorrow. The Slight Risk area shifts to cover areas along and east of the I-55 corridor from Tennessee down to the I-20 corridor for Monday morning. I'm not entirely confident this will be that big of a severe event for us. Let me explain...
Storms will have a hard time firing off during the daytime hours on Sunday. This will be due to a layer of warmer air about 4,000-5,000 feet above the ground. We call this layer of warmer air a "cap" or "lid". It essentially keeps the atmosphere stable and storms from forming. The cap can be broken by strong lift. This can be in the form a cold front or even daytime heating. Though temperatures will warm to near 80 on Sunday, it looks like the cap will be fairly strong, so it will likely need to wait on the cold front, that will be developing to the west on Sunday, to break it. Once that happens, we will have storms. The National Weather Service will use data collected from weather balloons to gauge the strength of the cap. I won't rule out an isolated storm during the daytime hours Sunday, but I would not bet on it. In fact, most of Sunday evening and quite possibly most of Sunday night will be storm-free for many of you. Model data shows that cold front will not arrive until Monday morning, so it is possible storms will impact your Monday morning commute.
Let's dive into the severe chances with this system...
As I noted above...the SPC has put north Mississippi in a Slight Risk of severe storms. It looks like the greatest severe threat will actually be up around the Memphis area, once storms actually do form. The map below shows EHI. This stands for the Energy Helicity Index. That's one of those fancy "meteorological" terms, but it essentially shows us where the best instability (how easy it is for the air to rise) will combine with the best wind shear (changing of wind direction and speed from the ground into the clouds). Areas where these two combine forces will have the best chance to see tornadoes and severe storms.
The model data in the map shows areas from the Mississippi Delta, around Greenville, all the way up to southern Illinois has the higher EHI values. Those EHI values are highest along the Arkansas/Tennessee/Mississippi border. It is these areas that have the highest tornado potential and there may even be a few strong tornadoes in those areas. Golf ball size hail and damaging straight-line winds are also likely. We may see the SPC upgrade this area to a Moderate Risk tomorrow.
The reason the best tornado risk will remain to our north is because of the track of the low pressure system. If it were to track further south (possible, but not likely), then our tornado risk would increase significantly in central Mississippi. Right now it appears the same areas that saw tornado warnings on Thursday night will have a chance again late Sunday night and Monday morning. Areas south of I-20 will see some heavy storms, but not much in the way of severe weather and I really don't see much of a tornado risk in those areas.
The time frame for storms on Monday looks to be about 3AM-7AM for the Delta, 5AM-9AM for west-central and south Mississippi, 7AM-11AM for the I-55 corridor, and 10AM-2PM for east Mississippi.
Check back tomorrow by early afternoon for an update on this storm system and be sure to follow me on Twitter for my thoughts as new data comes in today and tomorrow.
Eric - elaw@wlbt.com
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