WARNING: This post contains meteorological phrases and terminology, which may not be suitable for non-meteorology majors.

Warm moist air will surge northward out ahead of a strong cold front over the deep south on Friday (New Years’ Eve). Dewpoints in the mid 60s will be widespread over extreme eastern Texas, Louisiana and southwestern Mississippi, while dewpoints in the upper 50s to low 60s will be widespread over Mississippi, central/eastern Arkansas, eastern Missouri, and western Illinois. The atmosphere will be most unstable over extreme eastern Texas and central/southern Louisiana where temperatures will rise into the mid to upper 70s.

Surface dewpoints at 18Z Friday, December 31, 2010, per the Dec. 30th 00Z GFS.

So we know there is plenty of surface moisture and warm temperatures, along with sufficient forcing (the strong cold front), but what about the rest of the parameters necessary for severe weather? The other parameters I will look at are: Convective Available Potential Energy (CAPE), Convective Inhibition (CIN), upper-level support, mid-level moisture, Lapse Rates, and wind shear (both speed and directional). (I personally have looked at much more than this, but for the sake of this post, this is all I will talk about)

Let’s look at the mid-level moisture. I am looking at this just to make sure that if a storm forms it will have a sufficient depth of moisture to work with. There are a few ways to do this, but I find the easiest way is to look at a forecast sounding. Below is the forecast sounding for 18Z this Friday, December 31st. This forecast sounding is for an area about 50 miles north-northwest of New Orleans, LA.

The atmosphere is saturated up to nearly 850mb, so we now know we have a sufficient depth of moisture to work with. This sounding can tell us so much more than just the depth of moisture, so I will continue to refer to it throughout the rest of my blog. Earlier I mentioned that I would look at both speed and directional wind shear, so once again let’s refer to the sounding above.

The wind at the surface is 10 knots out of the south-southeast. At 850mb the wind is 40 knots out of the south-southwest and it is 35 knots out of the south-southwest at 700mb. We have decent speed shear and alright directional shear (you don’t necessarily need directional shear to get severe thunderstorms, but directional shear is necessary for the development of tornadoes). If you look closely you can also notice that between about 800mb and 600mb the winds slightly back, indicating weak cold air advection at that level. This brings me to the next point, lapse rates.

Based on this sounding (the NAM is very similar) the mid-level lapse rate is approx. 6 deg C/km and the low-level lapse rate is approx. 5 deg C/km. These aren’t the greatest, but they are more than sufficient for severe weather. Next, lets take a look at the upper-levels for this same time period.

At 500mb there is really nothing to speak of in the way of support over the deep south. Here the flow is out of the southwest between 60 and 75 knots (can be seen on the sounding above as well). The best upper-level support is well to the northwest associated with the closed 500mb low (as pictured below). There is a large jet streak present at 250 mb stretching from the southern plains to the upper midwest. This jet streak, however, is cyclonically curved, so the best upper-level divergence associated with this is over the upper-midwest, not the deep south (I do not have this pictured, but you can take my word on it).

While we have concluded that there really isn’t much in the way of upper-level support over this region, there also isn’t much in the way of upper-level hindrance. What does this mean? The storms will likely be low-topped and there also won’t be anything preventing the storms from forming (aloft, anyway).

Finally, let’s take a look at CAPE and CIN. The NAM and GFS are a bit different when it comes to the amount of CAPE. The GFS has 750-1000 J/Kg of CAPE over most of extreme eastern Texas, southern Louisiana, and southwestern Mississippi, whereas the NAM has anywhere from 200-750 J/Kg of CAPE. The true amount of CAPE will depend on the cloud cover during the day on Friday, as well as if there is any precipitation out ahead of this front. The same holds true for the amount of CIN. If the clouds break up and the sun is able to sufficiently heat up this region, then there will be more than enough CAPE for severe weather and minimal CIN. If clouds or precipitation is over the area during the day (before the main line of storms get there) this will severely hinder the instability (low CAPE) and will increase the CIN, limiting the potential for severe storms.

My current thinking on this is that the precipitation will not be a factor, but the clouds will be. I think low-level clouds will move in Friday morning and will greatly hinder the potential to have wide spread CAPE values over 500 J/Kg. However, I still think there will be some breaks in the clouds over parts of eastern Louisiana, western Mississippi and perhaps southeastern Arkansas. This should allow for this area to have sufficient CAPE values and minimal CIN. Nonetheless, I believe the entire region I mentioned in the first paragraph will have the potential for a slight risk of thunderstorms. The main threats would include damaging winds, hail to the size of golf balls, and perhaps even a tornado or two. I will have more on my thinking of what will actually happen in tomorrows post.

P.S. There will be a significant winter storm associated with this 500mb closed low over the western half of the country (ongoing right now) and in the northern plains and portions of the upper midwest.