A very potent severe weather setup appears poised to unfold across parts of the Lower and Middle Mississippi River valleys and points eastward, namely Friday afternoon through early Saturday morning, with all modes of severe weather possible, including destructive winds and damaging tornadoes. At the helm is a rather potent, progressive, and stalwart upper-level disturbance that will gyrate eastward, with a 110kt mid-level jet streak poised to take aim on large parts of the greatest threat area in the exit region of this seasonably strong jet core as it rounds the basal side of the trough during the early afternoon hours on Friday. This will be juxtaposed with a strengthening low-level jet maxima of 50-60kts coalescing with a strengthening surface cyclone that will deepen rapidly as it peregrinates ENE from somewhere near St. Louis, MO to northeast Indiana by ooz/03 (6 PM CST), yielding SFC-500 mb bulk shear values on the order of 60-100 kts across parts of MO/IL/KY/TN/IA/OH by 21z/4 PM CST Friday! This should rapidly draw in copious gulf moisture, with surface dewpoints well into the 50s progged to reach NC Indiana by late Friday evening. A surface warm front should extend ESE of the surface cyclone, with a strong cold front moving across portions of IL and points southward.
In the warm sector, SBCAPE values of 1000-2000 J/KG will be found across a large swath of Dixie Alley, namely from the Ohio River Valley S/SE. To the north of this, instability values remain questionable due to the models showing ongoing precipitation for parts of the day north of I-64/I-70, perhaps lingering into the very early afternoon hours. Nonetheless, seasonably favorable lifted indices of -3 to -7 C are progged across a large portion of the risk area. Notwithstanding, there is potentially a brief window for discrete supercells INVO I-70 in eastern IL and Indiana, which will lie proximal to the advancing warm front, prior to an EML at 700 mb “capping” the atmosphere until the advancing cold front rushes eastward and stabilizes the atmosphere as it gyrates eastward. If the 00z/02 GFS is to verify, this CINH could remain a problem for convection in the northern target region, extending southward along the front towards the Ohio River Valley.
We expect initial discrete supercell initiation to occur by 18z across portions of SW IL an.d into SE MO and N AR, with the situation rapidly evolving into a QLCS that may extend as far south as Mississippi, before rapidly advancing eastward into portions of Indiana, Ohio, Kentucky, Tennessee, Alabama, Mississippi and other parts of western Appalachia by Saturday morning. Folks in the primordial risk area will want to keep abreast of the latest watches and warnings, as this is a potentially dangerous situation, namely due to the strongly sheared environment in what will largely be a nocturnal event across a region all too familiar with such hazards; progged low-level CAPE profiles support a dangerous, sustained SVR event across Dixieland tomorrow night!
On a terminal note, we do not see as many parallels between this event and the recent “super outbreak” of April 27, 2011. First, widespread instability are progged to be somewhat lower across the coterminous geographical area, though it is worth noting that these instability values, while seasonably respectable, are lower than they were on 04/27/2011 with a caveat being that comparing early March instability to late April instability values should be viewed with obvious incongruity. Furthermore, the timing and placement of surface features differs; however, of utmost differentiation are the speed and directional shear parameters between the two setups. The streamwise vorticity with Friday’s setup is a little bit less favorable for the maintenance of long-track discrete supercells over a very wide geographic region for an extended period of time. The favorable 0-1 km directional shear on April 27, 2011 was enhanced by strongly backed, southeasterly surface flow across the region, along with a plethora of outflow boundaries from morning convection, whereas the more favorable hodographs on Friday tend to show the best 0-1 km concavity owed to somewhat weaker surface flow that will remain parallel to the surface cold frontal boundary, so this probably favor splitting storm modes with the highest tornadic threat also coming from supercells embedded in squall lines and the eventual evolution of a QLCS, though it should be noted, as evidenced by what happened in Harrisburg two days ago, that, given favorable shear profiles, QLCS tornadoes can be strong and deadly. None of this is to suggest that this event is to be taken lightly; however, we don’t think it will be an April 27th doppelganger per se.
Again, folks in the risk area should be prepared for dangerous weather tomorrow afternoon and evening; Godspeed to those in the greatest risk areas on FRI and SAT morning.
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