Posted by Meteorologist Tory Farney
It is that time of year again when many companies as well as states publicize their Severe Weather Awareness Week. After such tragic losses from last year’s tornado season, the first step toward not allowing something like that to happen again is education. Many steps have been taken this past year to try to improve the warning of citizens but it is also equally as important for citizens to be prepared to take shelter when they are told to do so. This should involve more than being surprised by the warning at the last minute and scrambling to find where you can shelter. It is much smarter to know when your area has a chance of severe weather days in advance of the actual event. This allows you the ability to continue to check back in on the situation as it nears as well as being able to make and change your plans accordingly for the day and/or night when the event may occur. In this post I will be giving many of you a first look at a threat which has just begun to really catch the eyes of meteorologists last night into this morning. I will take a look at what the NAM model and the GFS model are predicting and talk about any differences between the two as well as give you what area I think will experience the greatest threat from this two day event. However, as I stated before, it is important to keep up to date on situations like this and two of the best sources for this are the Storm Prediction Center and the National Weather Service.
To this event now, the Storm Prediction Center has just issued its new Day 3 Convective Outlook and its new Day 4 through 8 Convective Outlook this morning. They have highlighted a large area of the south to be under the threat for severe weather on Thursday (Day 3) and Friday (Day 4). Cities stretching from Brownsville to Charlotte and Bowling Green may see a good chance at some severe weather Thursday into Thursday night with cities such as Jackson and Atlanta being placed under the greatest risk currently. Friday, cities from Virginia beach down the east coast into the Jacksonville and Tallahassee areas would have the greatest chance for severe weather at this point.
With those general areas in mind, I am going to look some now at the models and see what they are forecasting as far as processes that may help lead to a severe weather event. First, let’s just take a look at the surface temperatures. Below is the NAM model’s output for Thursday evening showing some very warm air over the south with temperatures well into the 70s across most of Texas, Louisiana, Mississippi, Alabama, Georgia, Florida, and South Carolina. Additionally, the model depicts a nice south to southwest breeze across this area which will allow warm and moist air to flow in off of the Gulf of Mexico.
As you may remember from my Spring Severe Weather Outlook post, I cited above normal Gulf of Mexico water temperatures as one of the things that may help out severe weather this season and looking at the current sea surface temperature anomalies below, one can see that the temperatures in the Gulf are currently above normal in most spots.
Returning to the modeled surface now, by the midnight area, the NAM model really shows this moisture working its way into the Gulf states and temperatures remaining quite warm with many values in the mid and upper 60s. You can also now see the very dry air behind and large temperature gradient along the surface cold front which will be pushing through allowing for lines of thunderstorms to form along and ahead of it. Continuing into Friday which I am not showing here, the NAM continues to show this cold front pushing towards the coast removing the warm and moist air with it. The GFS model, which I did not show here, paints a very similar picture but pushes the cold front through maybe a hour or two earlier than the NAM is.
Looking to the upper air now, the 850 mb NAM model depicting conditions for the early night on Thursday, shows us a few important features. First off, the cold front is still very pronounced at this level both in temperature and moisture differences between ahead and behind the front. Additionally, very warm 850 mb temperatures in the mid 60s are occurring out in front of the cold front across portions of Texas, Louisiana, Mississippi, and Alabama. This is due in part to warm air advection occurring in this area blowing very warm temperatures at this level across southern Texas and Mexico where readings are in the upper 60s and even reaching the 70 mark. Maybe the most important feature though is a very strong low level jet with winds over 50 knots centered over Mississippi and Louisiana. The GFS shows pretty much the same set up but does show the colder air seeping into Texas much faster which may limit the amount of warm air advection that could occur across the Louisiana and Mississippi areas.
Looking towards Friday morning now, the NAM model shows some warm air advection still occurring but most of it is over Florida and not nearly as strong as it was on Thursday night. Also while the low level jet remains fairly strong, it is mostly advecting in cold air now. The GFS shows this same thing happening.
Looking to the upper levels of the atmosphere, the NAM model depicts a large jet streak which will form throughout the day Thursday and into Thursday night. Below is the model output for what the jet streak should look like around noon on Thursday. The jet streak is not very pronounced at this time with basically two smaller jet streaks occurring, one over the Midwest into the mid-Atlantic and another across the south, in a somewhat unorganized jet stream.
This all changes Friday night with the formation of a large 170 knot plus jet streak. This will not only create for very strong winds aloft but the right rear quadrant is located right over the area of concern for severe weather. This will provide for lots of extra uplift and is the one feature that really continues into the next day giving strength to the prediction for severe weather to continue on Friday along much of the southern half of the east coast.
The GFS model has this jet streak in almost the same location, however it never really creates it with such intensity. The GFS maxes the wind speeds out around 150 knots and does not have these strong wind speeds spread out across as great of an area. This can be seen in the image below for Friday morning.
This is just an early look at what may be happening as we head into Thursday and Friday this week across the south. Slight changes in model runs are still occurring and there are some differences between the GFS and the NAM as you have seen. Based on the current information, I would say the Day 3 Convective Outlook issued by the SPC is a very good representation of where the threat areas lie Thursday into Thursday night. However heading into Friday, the Day 4 Outlook may be relying to heavily on the strength of the jet streak to suggest a severe weather threat. While some areas in this area will likely be hit with severe weather, I think it is the southern portion of this Day 4 Outlook that will see the greatest threat. So please stay aware in the next couple days, especially if you live across Louisiana, Mississippi, and Alabama as most of the severe weather affecting these states will hit under the cover of night, providing for even more danger to life. Additionally, besides the lines of thunderstorms that are expected to occur there appears to be some chance for supercells to form with this system.
Meteorologist Tory Farney