*Christmas Week Severe Weather Outbreak Slated for Tonight.*
SW Louisiana Forecast Discussion...All is calm this morning in anticipation of tonight's severe weather event. A few showers moved across the area overnight as a weak disturbance out in front of the main system moved from SW to NE across the forecast area in the established SW flow aloft. Most of the shower activity was on the light side. Mild conditions prevailed overnight with temperatures closer to the normal high for late December as opposed to the normal low. Overnight minimums were around 60. The mild weather will continue today in advance of the vigorous late December storm. Overcast conditions exist at present, and this be the case throughout this last full work day before Christmas. Air mass or streamer showers will occur at random across the forecast area today. The widespread shower and thunderstorm activity is slated for tonight into the morning hours of Christmas Eve.
The sharp trough and associated cold front will be advancing SE through the day. As this occurs, a convergence zone will be established across the area. Showers and thunderstorms will develop across the area late this afternoon, and become more widespread and increase in intensity and areal coverage in the overnight hours. The dynamics and kinematics associated with this system continue to be very impressive, and all the parameters to produce severe weather are in place. This is a very dynamic system, and you will see just how dynamic it is over the next couple of days with severe weather around here, and a big blizzard through the Mid West. Along with the sharpening and increasingly negatively tilted trough, there is the presence of a upper level low and surface low as generated by Jet Stream energy. This has occurred as the Polar Jet and Subtropical Jet came together out West. The upper level low currently over AZ/NM will move Eastward through the day, and add to the lift and instability across the area with its associated cold air aloft. The trough digs into West Texas this evening, and then heads Eastward via a very strong Jet Stream which will be in place overhead (winds up to 100 kts. aloft). The trailing cold front and surface low will be marching Eastward at this same time. The lifting mechanism will be moving into an environment with strong dynamics from the Jet Stream, with plenty of instability and wind shear. Storms will fire along and out ahead of the front across Texas later this afternoon.
The environment in place is favorable for severe weather genesis. At present, the atmosphere remains largely capped in between last night's short wave and the advancing storm. The cap will erode this afternoon as forcing increases across the area ahead of said front. I expect showers and thunderstorms to increase in earnest after 22Z (4p.m.) An environment favorable for supercells will be in place initially out ahead of the front. A squall line (MCS) will initiate over Texas late this afternoon. The low-level jet in place will create a sheared environment, and thus lead to a tornado threat across the forecast area. However, the highest shear environment should be North of the I-10 corridor across East Texas into the ARK-LA-TEX and down towards the Northern half of SW Louisiana. There's no skill in pinpointing a specific location where a tornado will occur, but it certainly appears that the most likely area to see a tornado will be in the region highlighted above. Tornadoes are possible across the entire area, but points not mentioned above including Lake Charles & Vicinity should reside in an environment that will be less sheared. All modes of severe weather will be possible across the entire forecast area, and while the amount of shear may limit the tornado threat across the Southern half of the area, the damaging wind threat is greater. Hail will be a threat as well with the cold air aloft as judging by the advancing ULL. The environment will transition from a supercellular environment to an MCS environment overnight. What this basically means is that the mode of severe weather will shift from more of a tornado threat to a damaging wind and hail threat with the squall line. Tornadoes can't be entirely ruled out with the squall line as well, as the environment will favor what is known as a LEWP (Line Echo Wave Pattern). A LEWP is basically an area of rotation embedded in the squall line. This is caused by the presence of wind shear in the upper levels. The tornado threat is typically higher in advance of a squall line, because the supercells feed off of the CAPE values (Convective Available Potential Energy), and moisture from the Gulf. Whereas, the squall line generally takes up all the available energy as it moves in. There is a large amount of energy that will be displaced later tonight into early Christmas Eve.
Aside from the severe weather threat, a heavy rainfall threat will be realized as well. 2-4" of rainfall is anticipated especially since we should see numerous showers and thunderstorms in advance of the main component of the system overnight. This will add insult to injury with the grounds still very saturated from all the rain during the first half of this month. Flash flooding will likely be an issue to contend with whether we get off unscathed with the severe weather or not. The front currently extends from Eastern OK down into North Texas between Wichita Falls and Dallas, and further South into the Rio Grande Valley. The squall line should form late this afternoon advance into SE Texas up towards the Shreveport area between 0Z and 6Z Thursday (6p.m.-Midnight CST). The large ridge of high pressure that controlled our weather over the weekend and Monday is still centered off the SE U.S. coast, and is only slowly pushing away. This will likely result in a slow Eastward progression of the MCS overnight. I expect it to be near Lake Charles between 5 and 7a.m., and into the Acadiana area between 8 and 10a.m. The severe weather threat will come to an end as the squall line pushes on by. The severe weather threat will be maintained overnight, and it stands to be a very long night tonight in the weather office. The severe weather threat ends comes to an end from West to East after daylight Thursday, and before noon for all. The heavy rain threat will come to an end as well during this time, and we'll transition to light rain and/or drizzle Thursday afternoon in response to wrap-around moisture as a CAA takes over. The associated surface low will be moving from West Texas to SE Missouri by Thursday morning.
Temperatures will remain on the mild side until fropa Thursday morning. Beyond this point, a sharp drop in temperatures will occur, and the warm weather will be a distant memory for Christmas. Highs today in the 70s will be replaced with falling temperatures on Christmas Eve. Morning temperatures around 60 will fall into the 40s by Christmas Eve evening. High pressure builds in just in the "Ole Saint" Nick of time for Christmas. Christmas Day looks clear and cold with lows in the low to mid 30s, and highs close to 50. A much colder pattern will be established behind this boundary. It looks quite cold through New Years. I will have all the complete details with the full forecast package tonight. More updates will be forthcoming this afternoon.
Stay tuned for more!
-DM-
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