This weekend, we could see the return of severe weather to parts of the deep south. Its difficult to rely on too many model specifics 120+ hours out, but we are at the timeframe to start becoming aware. Like any of these cool season high shear setups, this needs to be watched closely. Even the smallest amount of instability can cause major problems in these environments and as we have already seen multiple times this past Fall. I want to briefly look at what the GFS is showing synoptically. Not much has changed from the 12z to the 18z runs today, so I will be using a mix of the two.
As a disclaimer, I am just presenting what the GFS is showing at this moment and I am not making any sort of prediction as to what will really happen this far out. As of now, the main threat looks to be on Saturday in the East Texas, Louisiana, southern Arkansas, Mississippi, and possibly western Tennessee areas. Overnight and into Sunday, the threat may extend into Alabama, central Georgia, and Florida in the form of a fast moving squall line. Out ahead of the low pressure system, we should see very juicy air with strong warm air and moisture advection aided by a very impressive 850 mb low level jet. Dewpoints could be in the 70′s near the Gulf Coast and approaching 65 as far north as southern Arkansas, western Tennessee, northern Mississippi, and northern Alabama.
Let’s look briefly at the synoptic setup. By 00z Sunday, the 500 mb trough is shown to be taking on a neutral to slightly negative tilt. At the 850 mb level, we see a very impressive low level jet approaching 60 knots, and at the surface a developing midlatitude cyclone is shown in the Texarkana region. Looking at surface Theta-E, we can also see a well defined warm and cold front extending from the low pressure center.
Synoptically, if the last few GFS runs verify, all the players are on the field for some kind of severe event. Just for fun, let’s look at some of the finer details. Could we see discrete supercells or mainly a QLCS type event? With any high shear setup that is characterized by strong forcing, there is always a risk for a big squall line. However, we do see some potential for discrete supercells looking at the surface-500mb crossover shear. Note that many angles between the surface and 500mb wind vectors are approaching 30-45 degrees in the areas of interest. This could suggest a more discrete mode if cells can fire ahead of the cold front or along the warm front.
Another fear is that this setup could be a rainy mess. PWATS could be approaching 1.9 inches in many of the Gulf Coast states which would suggest very heavy rainfall and HP supercells. Anyone chasing needs to be aware of flash flooding potential and possible fast moving rain wrapped tornadoes should the discrete mode dominate. Instability is the biggest question mark, as with most cool season events, but the usually conservative GFS is still showing values of 1000 J/kg up into parts of northern Mississippi/southern Arkansas and up to 1500 J/kg across most of Louisiana. Parts of eastern Texas could even see values of up to 2000 J/kg or greater. These CAPE values are more than enough to spark a regional severe weather outbreak. As with most cool season events, shear and helicity are not a problem. The combination of the available instability and shear is showing some eye candy with the supercell composite parameter (SCP). Let’s look at how the 18z GFS progresses throughout the day Saturday into Sunday morning:
Those are some pretty impressive values for the middle of December. As the time gets closer we should get a better grasp on the details. As the models stand now, there is definite potential for a regional severe weather outbreak with all modes of severe weather possible. How large that potential is remains to be seen but it should be fun to follow. I may or may not be able to give a blog update as the event gets closer as I will be traveling for the holidays, but I will try.
As far as the northern side of the system goes, the GFS shows some minor to moderate snowfalls starting in eastern Kansas, into Missouri, Northern Illinois, and Indiana again. The GFS suggests this system should not be as potent as the last system with regards to winter weather but that could change quickly. Finally, I leave you with a snow accumulation image from the 12z ECMWF 162 hours out. If you believe the Euro, there will be crippling snows in parts of MO/IL/IN/MI/OH and eventually into the northeast. Again, there are a lot of details to be worked out, but the solution is definitely one to keep an eye on:
Remember to stay tuned to LiveStormsNow.com this weekend as this event unfolds. We will have many chasers out documenting every angle of this potentially significant storm.
Parts of the country are gearing up for the next round of wintry weather this weekend. Portions of Missouri and Illinois could be expecting snow accumulations to begin as early as this afternoon (12/13). Winter weather advisories, winter storm watches, and warnings span a large swath of the US, extending from extreme eastern Kansas up into Maine.
Heavy rain with some freezing drizzle has already begun in parts of Missouri and Illinois which should slowly transition to a wintry mix later this afternoon/evening and eventually to all snow. NOAA’s Weather Prediction Center is suggesting a blend of the GFS and ECMWF, which would hint at a rain/wintry mix transition for the St. Louis and central Illinois regions beginning around 6pm-9pm CST. Precipitation could come in two rounds, with the initial round being associated with the warm air advection out ahead of the developing cyclone. A secondary and possibly more significant wave of precipitation may accompany a deformation band as the cyclone continues its track. The GFS is not showing strong cyclogenesis but there is some development of the surface cyclone underneath the divergent region of an expansive jet streak with speeds upwards of 140 knots at the 250 mb level.
As the system advances, expect moderate to heavy snowfall overnight in central IL, where 3-5 inches could fall. The system will then begin to make its way into Indiana and Michigan. While things will take a bit longer to get started in central Indiana, this is where we will most likely see our greatest Midwest accumulations (5-8 inches with higher amounts possible locally). As we approach 00z Sunday, snow will begin to taper in Indiana but will pick up into Ohio and Pennsylvania. Finally, as the weekend closes, many northeastern states should get a healthy dose of snow, including New York, Massachusetts, Connecticut, Vermont, New Hampshire, and Maine. After a brief warm up for much of the nation next week, more arctic air from Canada should filter its way down through much of the nation again. The cold pattern looks to remain in tact for the foreseeable future.
For event totals, St. Louis could see 3-5 inches along with most locations in central Illinois. Expect to see a wide swath of 5-8 inches extending through most of central Indiana into western Ohio with locally higher amounts possible. When all is said and done, Pennsylvania also looks to get hit with some of the heavier snowfalls in the 5-8 inch range. Parts of the upper Northeast can expect totals up to 12 inches, especially in higher terrain. Below are the percentages the Weather Prediction Center is giving for at least 4, 8, and 12 inches of snow respectively through 12z on Monday:
Be sure to check in with Live Storms Network throughout the weekend, as we will have a number of streamers in the field giving updates and catching footage as the event unfolds!
As the graphic above shows, we are dealing with a wintery system moving across portions of the Central Plains this afternoon that may cause some issues for the rush hour in places like Norfolk, Yankton, and Sioux City as rainfall changes or has already changed to snowfall. Some totals out of central Nebraska are already at 3”+ and climbing. The snow will begin to taper off across the SW this afternoon but is expected to continue across most of the major metro areas for quite some time this evening. Some 6” totals are expected across central sections of the state into SE South Dakota.
We have Chris Allington out in Nebraksa gathering some storm footage and we should have some available to show you shortly. Stay tuned…