Monday, October 28, 2013

Monday Morning Forecast Thoughts - Severe Storms Thursday?


Current visible imagery this morning shows clouds lingering this morning, with most of the showers located to our south near Meridian. We should stay dry through the day and warm up into the 70s. 
We will still have some low-level moisture lingering, but a ridge of high pressure will be building in from the southwest. Subsidence aloft will try and suppress cloud development, but steady WAA in the low levels should result in fair weather cumulus across the area for both Tuesday and Wednesday. With a warm fetch of air coming from the Gulf, temperatures should warm effectively into the low 80s both days. 
Severe Weather Discussion:
The SPC maintains a severe weather risk area for a large part of Mississippi up into parts of the Ohio River Valley for Halloween Thursday. 

First off, dynamically speaking, this system is rather impressive as a strong negatively tilted shortwave pivots northeast of our area. Plenty of PVA ongoing in advance of this feature as well as enhanced diffluent flow at the 250mb level. So, dynamic uplift will certainly not be lacking with this event. In fact, winds at all levels are quite supportive of severe storms, including a 45-55kt south-southwesterly low-level jet atop south-southeasterly winds at the surface. This creates directional and speed shear; as a result, helicity values are quite high, on the order of exceeding 400 m^2/s^2 in the lowest 2km AGL. 

Thermodynamically, there are certainly some limiting factors that will most likely hamper what otherwise would be a fairly substantial severe weather threat. Let me disclaim and say that, of course, these parameters could change and one would need to be abreast of such changes as any increase in thermodynamic instability will certainly lead to a higher severe weather threat. 

With that said, moisture looks to be in fair abundance as PWATs climb to over 1.5". Furthermore, dew points will climb into the 60s in advance of the cold front, pooling to near 65-67 F right ahead of the front. However, moisture looks to exist throughout the entire column, so the lapse rates are looking meager at best. Moist adiabatic lapse rates are found in the 850-500mb layer, and this will greatly inhibit instability as LI's remain at or above -2 C throughout the event, at least here in the GTR. A little higher instabilities are noted off to our west near the Delta. 

Because of all the moisture and dynamic uplift, clouds should fairly widespread and inhibit surface destabilization. However, there could be pockets of cloud breaks, albeit isolated, that could lead to areas of relatively enhanced instability for which models cannot possibly account. Additionally, models oftentimes underdo instability this far in advance and 'come around' right before the event and show higher values. However, I'm skeptical that such will happen just yet due to all of the cloud cover expected. 

That being said, we all know that significant dynamic uplift can compensate for meager thermal instability. Even though surface CAPE values are below 500 J/kg, with such excessive shear, severe weather can and often still happens. 

Considering all factors, I think we here in the GTR are on the eastern side of the severe threat, as SPC's risk suggests. I think the better juxtaposition of modest instability and shear will be over the Delta into southwestern Tennessee, but some severe storms are certainly still possible this far east as we'll still be feeling the effects of daytime heating. 

I think by the time the activity gets to us here in the GTR, there will be an expansive area of rain with a strongly forced line of heavier rain and embedded thunderstorms on its leading edge. It may wind up being either a QLCS or LEWP-type event, and some spots within that forced line could go into rotation, and quick, spin-up tornadoes are possible as well as some potentially gusty winds given the strong wind field in place. 

Remember, we are still 4 days out and a lot can change in that amount of time. This is our first severe chance this Fall season, so we'll have to be aware of how things evolve throughout the week, and pay extra close attention to any mesoscale details on Thursday that cannot be resolved by numerical models this far in advance. 

Stay tuned!!

IW

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