Late this Tuesday morning we've got a veil of cirrus clouds over us here in the GTR, with the more expansive areas of stratus/cumulus to our north and northwest associated with a developing/strengthening storm system that will be impacting us Thursday. Despite the somewhat 'filtered' sunshine, temperatures should make it into the upper 70s and low 80s in some spots.
Severe Weather Discussion:
We continue to get closer to Thursday's potential severe weather episode. The Storm Prediction Center has carried over their slight risk from Day 4 to Day 3, while also maintaining a 30% circle of severe weather within 25 miles of a point delineated in the map above.
At the time of this writing, the 12z GFS was still running, but most models are in decent agreement with respect to the intensity and timing of this event. Even though, a quick peek at the fresh 12z GFS shows the threat much earlier in the day, potentially in the afternoon. I think the GFS bias of too progressive is being seen here, and for now the course of action will be to bank on timing sometime during the evening hours. The GFS/NAM continue to suggest the main action will be between 6 PM through Midnight or so, while the Euro is a little slower.
Regarding intensity, this continues to be a fairly impressive system especially examining from a dynamic standpoint. Wind fields continue to be strong at all levels, including a 40-50kt LLJ out of the south-southwest. This will act to transport copious amounts of low-level moisture northward out of the western Gulf.
A strong negatively tilted shortwave trough is forecast to swing northeast out of the south-central Plains, with significant PVA downstream contributing to lots of uplift. Upper level jet structure continues to look impressive as well as a diffluent jet pattern overspreads us as well as us being at least loosely in the right rear quadrant of a 120+kt jet streak.
All of this would point to significant dynamic uplift in the warm sector of this storm system. As we get better frontal forcing in the afternoon, lift will even be higher as it interacts with a relatively warm and very moist environment with temperatures likely holding in the 70s with dew point temperatures in the mid 60s.
However, thermal instability continues to look very marginal and this fact in of itself is a significant limiting factor to what otherwise be a fairly potent severe weather threat. Moisture looks to run throughout the entire atmospheric column with no areas of notable drying seen in modeled skew-t soundings. This translates to moist adiabatic or even less than moist adiabatic lapse rates in the 850-500mb layer, so atmospheric cooling is not happening as fast. As a result the lifted index is also weak with values at or above -1 C.
Furthermore, a couple of small capping inversions were seen in the 12z NAM modeled soundings as well around 800 and 600mb, respectively. Any warming, especially at the 700mb level, would certainly hamper convective processes. However, because the thermal instabilities are so low and cloud bases being so low to the ground (<1000m AGL), some of these storms may survive beneath this inversion because they will be very low-topped. Nonetheless, thermal instability remains very marginal and will be a limiting factor in the intensity and spatial extent of this severe weather threat.
One potential caveat I'm noticing in the models is a lead shortwave earlier in the day. This is seen in both the GFS and NAM models. Depending on when this moves through, enhanced uplift could lead to scattered shower and perhaps storm development by late morning into the early afternoon across central MS extending northeast into northwest AL. This could serve to hamper any daytime heating that would have occurred despite the clouds and reduce our diurnal instability to almost zero. On the other hand, if that shortwave lifts out of here by early afternoon, subsidence immediately on the back side of that feature may act to break up some clouds and allow pockets of enhanced surface heating to develop. This will be something that will have to be watched over coming days and especially on the mesoscale level on Thursday.
Concerning storm motion/type, 0-6km shear vectors continue to be at least loosely perpendicular to the approaching frontal boundary. Also, values of 60kts certainly suggest storm organization and maintenance is likely. I still think an expansive area of showers and storms will be ongoing just ahead of and just behind the approaching front, with a strongly forced line/QLCS/LEWP of storms on the eastern edge.
Providing the boundary layer can stay well mixed, which seems plausible considering the expected nocturnal increase in low-level winds at the onset of the loss of daytime heating, some of these stronger winds may get brought down to the surface. Furthermore, shear in the lowest 2km AGL continues to be very high, on the order of 40kts which certainly favors low-level storm rotation. Quick spinup tornadoes on an isolated basis also seem possible, especially with any breaks in the line or embedded supercell in the line as well.
Also to note: there remains an area of relatively low surface pressure extending down through parts of Arkansas and Missouri (~1004mb). While the surface low is well to our north over the Great Lakes, this area of pressure falls may be enough to keep our surface winds at least backed to the south, maybe a little east of due south (say ~160-170). This helps keep the isolated tornado potential in the cards, especially north of the Highway 82 corridor in MS.
Bottom line: this remains a complex situation and exact placement of certain key features remains a little uncertain. There still remains time for parameters to change, for the better or worse, concerning this severe weather threat. Stay abreast of weather changes throughout the week!
IW
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