Convective Addiction

Forecast: Friday March 2nd, Potential Significant Severe Weather Outbreak.

Posted on 29. Feb, 2012 by Adam in Uncategorized

Just two days after being affected by an early season deadly outbreak, parts of the Midwest and OH/TN Valleys could be facing yet another, even more significant outbreak this coming Friday.  We have been monitoring this system for several days now, as people who follow us on social media are aware.  The event is drawing closer, hence, we wanted to analyze the system more in depth now that 00z data is arriving for the March 1st model suite.  In this forecast you will see mostly plots of the 0z NAM and 0z GFS, with a noteworthy observation that the 00z NAM is aligning more with the previous GFS and EURO model trends.  Those runs have not completed yet, but we wanted to put something out there as early as possible.

So what are we examining here?  First and foremost, the models are depicting a deep surface low tracking across IL and into the lower great lakes by Friday evening.  There are still some differences in timing, placement and strength of the surface cyclone, with the 00z/01 GFS being the most aggressive with a 996 mb low centered just south of CHI at 18z FRI, whereas the 00z/01 NAM has a somewhat synchronous surface low centered near St. Louis at 18z/FRI, with the 12z/29 ECMWF showing a 1000 mb low centered over SC IL .  The eventual timing, strength and placement of the surface cyclone will be crucial to the evolution of the event; these differences should be refined in the next 24 hours or so.

ECMWF 18z/FRI per 12z/29 model run, showing position of the surface low

This low, as it deepens, will draw very warm and moist air all the way up into Central Illinois and Indiana.   Such magnanimous dewpoints this early in the year are always an eyebrow raiser, and it’s worth noting that the Gulf’s moisture supply has not been pillaged as incessantly as it was last winter, leaving rich moisture more readily able to peregrinate poleward and feed into strong cyclones that traverse regions to the north.  A cold front will be draped southwest from the surface low, stretching into parts of TX.  Consequently, this system will have a large and very moist warm sector, which has the potential to bring severe weather to a large area once again.

Aided by the ample moisture noted above, the models are predicting generous amounts of CAPE for early March, approaching 2000 j/kg in some locations.

While this may seem low compared to bigger outbreaks later in the year, this is still concerning due to the impressive dynamics the system will bring with it.  Bulk shear values will be very high, upwards of 80kts as a 100kt H5 jet streak rounds the basal side of the trough and exits stage right, with the entire lower Mississippi River Valley in its crosshairs.  The mid and upper-level jet streaks appear stronger than they did on the 12z/29 numerical model runs, per the GFS, and the surface low has responded in a similar fashion with a slower, stronger ejection on the 00z/01 run as compared to the 12z/29 run, now being shown as 4-6 millibars deeper than it was 12-hours ago.  The mid-level wind profiles are contributing to the very favorable shear profiles with this setup.

00z/01 GFS 500 mb forecast for 18z/FRI 03/01

When you have that kind of shear in place it won’t take much CAPE to get storms rotating and producing severe weather, as we saw with the storms on February 28/29th. One potential limiting factor for a more widespread tornado outbreak, however, is the positive nature of the trough. This in turn will keep directional shear profiles largely unidirectional across much of the warm sector.  Not to be outdone, the recent runs of the SREF are progging copious amounts of seasonably ample CAPE profiles across the region by early Friday afternoon (see below).

This aforementioned scenario would favor a more linear event.  Notwithstanding, given such impressive speed shear in place, this would certainly support embedded areas of rotation that could potentially produce strong tornadoes.  The best low-level turning will be confined to a narrow corridor just south of the warm front, and this is the area that would probably be most favored for initial supercell development early on.  If you are a chaser looking for an ideal tornado catch, this is probably the area you want to gamble with.  There also appears to be an inversion around 850mb.  This may thwart storm development somewhat, but if the CAPE values being predicted are realized along with the strong forcing along the cold front, it seems probable that this will be overcome.

00z/01 GFS 850 mb profile at 18z/FRI 03/02

Our expectation is that convection will initiate by early afternoon southeast of the favored triple point, with the best chance for a chase able tornadic supercell INVO of the warm front, southeast of the surface low, across S IL or far SE MO (i.e., K1H2 – KFWA).  A massive QLCS is likely to erupt by late afternoon and early evening, extending from SE IL into SC AL and points ENE.  All modes of severe weather are possible, though the threat for tornadic supercells, especially those embedded within QLCS structures, will remain a bona fide threat given the directional shear profiles found across the region.  Wind damage is also a threat, along with some large hail.  Given the impressive speed shear profiles, some substantial wind damage appears likely; consequently, assuming that the instability profiles materialize, and it looks as if they will, this is going to be a very dangerous situation for anyone residing in the middle and lower Mississippi River valleys, extending eastward across the Ohio River Valley.

Caveat:  there is still much data to pour over and the forecast will need to be refined in the next 24 hours, but people living in IL, IN, KY, TN, OH, AR, MS, AL need to be watching this system closely as it has the potential to cause significant damage to life and property.

AL/JLR/RW/DN