From there, it seems that a surface low forms over central or western Texas,
and tracks northeastward, into the Mid MS Valley and Lower Ohio Valley.... with
the classic Alabama severe weather track of Arkansas to Kentucky/Illinois. The
surface low will be deepening along the way, from 1004-1006mb to under 996-mb as
it heads toward inland New England areas. This puts the Deep South in the warm
side of the storm.....
Now, beyond a few days, there's really no way whatsoever to know the mesoscale details of a weather system...and it's the mesoscale details that make or break a severe thunderstorm/tornado day.... but there are many larger-scale signals that we look for to know if the overall setup is favorable or unfavorable. Let's break down those signals with this system and see how we stack up....
I usually start off by looking at the jet stream, to get an overall feel for the large scale pattern in the upper-levels, as it has a major major role in how the other mandatory levels behave. During the winter months, I use the 200-mb level to find the core of the jet stream, as the colder air is denser, which means the atmosphere is "thicker", placing the core of the jet stream higher in the atmosphere. Below is the 12Z GFS 200-mb prog for 180 hrs.
We see a major upper-level system over the center of the nation with a 130-140 kt jet streak from eastern Texas to southeastern Kentucky. Note that the northern branch of the jet and the southern branch of the jet are combined here. This is called phasing, and this adds power to a storm system as you basically have two branches of the jet stream phasing into one stronger jet. Also note the diffluence over the Deep South, with wind barbs over Arkansas pointing northeast to north-northeast and wind barbs over the northern Gulf of Mexico pointing east-northeast. This evacuation of air in the upper levels in one specific area promotes rising motion in the atmosphere, and many major severe weather outbreaks are found in that diffluent area. Note also that Alabama is in the right-front quadrant of this upper jet. Now, I know that we are taught in standard meteorology books that rising motion occurs in the left-front and right-rear quadrants of the upper jet, and sinking motion is found in the right-front and left-rear quadrants. In the Deep South though, we are much closer to the Gulf of Mexico, and much further away from the source region of the capping inversion, than say...Oklahoma. We usually end up with a weaker cap in a severe weather situation here, than in Oklahoma. That means, that in order to get discrete, strong updrafts here, when such a dynamic system is present, there has to be something to counteract all the rising motion on such a weak cap. The subsidence in the right-front quadrant of the upper jet can often do this, by "weeding out" the weaker updrafts trying to grow around supercell thunderstorms. As a matter of fact, all four F5 tornadoes in the state of Alabama occurred in the right-front quadrant of the upper-level jet...as well as many other major tornado outbreaks... We all know the days... April 3, 1974; May 27, 1973; April 4, 1977; March 27, 1994; April 8, 1998; November 10, 2002; March 1, 2007.
We move a little lower in the atmosphere, to 500-mb, to get a good picture of the trough/ridge pattern...get details on shortwaves...areas of positive vorticity advection and associated rising motion...etc...
Note again the major system over the center of the nation. The upper-level low is centered over southern Missouri here, with a 110-kt jet from eastern Texas to western Tennessee. To put those speeds into perspective, 100-kts is called the "barb of death" in severe weather forecasting. Also note that the Deep South is again in the right-front quadrant of the jet, as mentioned above. As with the upper-levels, we are in the center of strong diffluence, and this will help promote large scale rising motion in the atmosphere, necessary in aiding thunderstorm development. Another thing to note is the vorticity pattern here. Note that we don't have one large vort max, but rather two. This tells me that the forcing may not be overly strong, so strong that we have all the convection just instantly forced into a line. The vorticity maximum itself isn't the whole story though, we look at the advection of that vorticity downstream, to get an idea of rising motion. Note that the secondary vort max upstream from the Deep South is parallel to the upper-level winds. The advection pattern associated with this is known as channel vorticity advection, and is much more subtle. This also makes me think things may not be instantly forced into a line. Also note the height pattern. There's an old rule of thumb with synoptic level severe weather forecasting, that the greatest tornado threat will lie ahead of the shortwave, between the 564dm and 576dm H5 heights. On the model prog above, that would be about the northern two-thirds of Alabama.
We move down a little further, to the 700-mb level.
We note the very strong system again centered over Missouri, with a 70-kt jet at this level punching in from the southwest. Something very important to note is the very strong dry punch coming across Mississippi and Alabama. Mid-level dry air will do two things in an environment like this... A) Create subsidence that will help not only allow breaks in the clouds, but spacing of storms...so that discrete convection is favored. B) The dry air will allow more evaporational cooling as moist air is brought upward in developing updrafts, and this evaporational cooling will make the air surrounding the rising air colder...meaning that the rising air will continue to rise...and rise faster. Also note the relative humidity gradient over Mississippi and Alabama. I typically look for the greatest severe weather threat to be in the gradient from 40% to 80%, and that is much of Mississippi and Alabama on this image.
We move down into the lower-levels of the atmosphere now, by looking at 850-mb.
Several things to note here. We have the cyclone at this level centered right over St. Louis. This puts Alabama deeply within the warm sector of the system. Note that 12 deg C 850-mb temperatures extend northward to the TN/KY state line. 12 deg C at 850-mb is often a very good indicator for unstable low-level air. Also note, ahead of the strong cold front over Mississippi, the very strong south-southwesterly low-level jet of 50-60 kts from southern Louisiana to central Kentucky, with some of the highest wind speeds over Alabama. This will mean that not only warm, moist, unstable air races rapidly northward, but there will be very strong wind fields through the atmosphere, given the winds at the higher levels as well. This is very favorable for sustained updrafts and updraft rotation. Note also that the wind barbs originate from the southern Gulf of Mexico and northwest Caribbean. This is a signal that very rich low-level moisture will be present in the warm sector of this system. 65 degree dewpoints look VERY doable to me, along and south of wherever the warm front happens to setup.
We look at the surface now...
We have a deepening 1000-mb surface low very near Paducah, Kentucky with a trailing cold front into Mississippi and Louisiana. This is classic surface low placement for a severe weather outbreak day in Alabama. It appears that the main warm front is over the Ohio Valley, with a secondary warm front over north central Alabama. This secondary warm front will be the northern edge of the most unstable air....and this boundary will also be an area of enhanced low-level shear. Note 1000-500mb thickness of 564dm well into Alabama at this time. 1000-500mb thickness of 564dm is often a signal for significantly unstable air in the cool season. No, you won't see 3000 SBCAPE, but you probably never will in Alabama. This is the signal for air sufficiently unstable for significant severe weather, IF all other parameters are favorable. Also note that 1008-mb surface pressures extend well into South Alabama. I typically look for surface pressures at or below 1008-mb for a significant tornado threat. One of the "old school" rules of thumb. Also note the isobars... To get the surface wind direction, draw a wind barb 30 degrees inward from an isobar for low pressure, as winds converge inward toward a low pressure area. That makes the surface winds over Alabama out of the southeast. Now, compare that to the wind directions and speeds in the other progs through the different layers, and you have a strongly veering wind profile with height. This kind of veering wind with height is very favorable for rotating thunderstorms.
That's all what we can see right now..... but as stated above, it's the mesoscale details that make or break an event, and we've all seen that before too. What if the unstable air doesn't make it far enough northward? What if convection forms on the coast to block inflow from further northward? Where does that secondary warm front setup? What will the track of the surface low be? What's the timing of the system? Those are all VERY important questions, that can't be answered right now. All we can see is the larger scale indicators, and they seem to be becoming consistent on an idea that favors a POTENTIAL severe weather event before Christmas. There's still plenty of time for this all to change, but it needs to be monitored closely in the coming days.