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

Wednesday Midday Forecast Thoughts

I'm having a hard time finding a cloud in the sky here in the GTR, and the visible satellite imagery shows this nicely. Most of the cloud cover is up to our northeast associated with a large, longwave trough of low pressure.


Here's that trough of low pressure. Because we are in a negative NAO, troughs have a hard time moving through because we have a blocking high pressure over the northern Atlantic. Because of this, we'll see several more reinforcing shots of colder air in the form of dry cold fronts.


You can clearly see the temperature gradient associated with our next frontal passage poised to occur tomorrow afternoon. Notice the blues over Missouri; those are pretty cold temperatures and that air is on the way here!

Notice the winds in the lower levels are out of the west in advance of that front. That's because winds don't have time to back around to the south to allow any moisture return. Because of this, I think we'll only see some mid to high level clouds associated with this frontal passage--certainly no rain to worry with.


Strong high pressure builds in behind this next frontal passage. Subsidence will be abundant with this feature, so no clouds are expected Friday or Saturday with very, very dry conditions with dew point temperatures well down in the 30s. Cold air advection will be wide open, and we'll struggle to get to the low 60s despite maximum sunshine.


As discussed in yesterday's post, the NAO looks to be trending towards neutral by the end of the month. We may be seeing the first effects of this as that pesky trough finally begins to weaken and lift out to the northeast. It looks like we get some return flow off the Gulf and w'ell see moisture slowly return and also temperatures will respond as well. Some clouds will be likely by the first part of next week along with the return of low to mid 70s!

IW

Thursday Afternoon Forecast Thoughts

We are socked in with low clouds this afternoon across the GTR, but we notice the clearing line is not all that far away! Places like Memphis, Little Rock, and Shreveport are getting lots of sunshine. Because of all the clouds, we're quite cool across the area as well! 

Temperatures are holding in the 60s with northwesterly winds funneling in drier air. That breeze is making it feel kind of chilly out there, and if you step outside it feels more like a late November day versus a mid October day. 

If we look at moisture in the lower levels (850mb), we can see drier air relative to the moist conditions we saw during the middle of the week. However, this will not last all that long, but we should squeeze out a predominantly sunny day tomorrow before more clouds build in tomorrow night and into Saturday. 

Here's why. We have another broad, longwave trough evolving over the eastern half the country. Associated PVA in the base of said trough should lead to an increase in cloud cover, but I'm not expecting a whole lot of rain with this system simply due to the forecasted lack of moisture. 

By Saturday morning, which is likely when the associated energy will be moving through with the trough, moisture is lacking. The richest moisture content is located well off to our southeast, so that's why I'm only expecting an increase in cloud cover and maybe a few spot, isolated showers. 

The overall pattern looks rather progressive toward next week. Check out this whopper of a trough by Wed/Thu of next week. Raw temperature data from the GFS suggests some 30s are likely to accompany that trough, so we'll just have to see! 

Because the NAO is forecast to dip a little more negatively over the next couple of days, the idea of a dose of some cold air is not all that unheard of. By the end of the month, the NAO is forecast to trend back toward neutral, so that would indicate this forecast 'cold snap' should be relatively short lived. 

Have a great Friday and weekend! 

IW

Wednesday Afternoon Forecast Thoughts

Here's a snapshot of the morning skew-T sounding launched by NWS Jackson. Notice the copious amounts of moisture located from the surface to around 400mb. We've got lots of cloud cover across the region and intermittent rain showers thanks to all of this moisture. 

Here in the GTR, we are right ahead of a fairly potent cold front. Notice all of the warm air across the southern halves of Mississippi and Alabama, and the noticeably cooler air back behind this front. 

A strong shortwave will be passing to our north tomorrow, but associated uplift from being in the right-rear quadrant of a 100kt jet streak (not shown) will keep rain chances in our forecast at least through about lunchtime tomorrow...but clouds will likely be slow to clear out. 

Moisture at 700mb lingers through lunchtime, but it begins to decrease by tomorrow evening, so we should see the sky begin to clear out and we'll bottom out right around 50 degrees for Friday morning's low temperature! 

Another strong shortwave will pass to our north on Saturday, but most of the rain shower activity is forecast by the models to be to the south of our area. But evidence continues to suggest a big-time cool down may be coming by the end of next week, so stay tuned!!!

IW

Wednesday Afternoon Forecast Thoughts

Here's a look at afternoon water vapor imagery. Lots of dry air at the mid levels is located over us here in the GTR and normally, we would expect sunshine with this...but we have an interesting setup today...

No doubt, the morning sounding from Jackson, MS confirms the presence of dry air in the mid levels that the water vapor imagery is showing us. However, if we look closest to the surface, we see nearly saturated conditions and fairly moist conditions all the way up to almost 850mb. After (above) that, a hydrolapse occurs and there is a significant decrease in moisture. This means all the moisture is located in the lowest 5k feet of the atmosphere. 

Low and behold, the visible satellite imagery shows lots of clouds particularly focused in the eastern half of Mississippi. There are a lot of factors associated with this channeling of moisture. First...

The remnants of Karen have actually developed a coastal low off the Carolinas. Air channeling on the west side of this feature is being funneled southward then turning to the west, resulting in an easterly to southeasterly flow for us here in the GTR. That's increasing our moisture just enough to result in low-level clouds. Furthermore: 

Here's the 294K isentropic surface. It appears moisture located on this surface juxtaposed with the wind fields associated with this surface is resulting in isentropic upglide across our state, thereby reinforcing the potential for clouds in our area. 

For tomorrow, that coastal low is persisting and it depends on our surface wind direction as to whether or not the clouds will stick with us tomorrow. It looks like that our overall wind should remain the same, so we may can expect additional low clouds that are slow to mix out tomorrow as well. 

IW

Wednesday Afternoon Forecast Thoughts

Here's a look at afternoon water vapor imagery. Lots of dry air at the mid levels is located over us here in the GTR and normally, we would expect sunshine with this...but we have an interesting setup today...

No doubt, the morning sounding from Jackson, MS confirms the presence of dry air in the mid levels that the water vapor imagery is showing us. However, if we look closest to the surface, we see nearly saturated conditions and fairly moist conditions all the way up to almost 850mb. After (above) that, a hydrolapse occurs and there is a significant decrease in moisture. This means all the moisture is located in the lowest 5k feet of the atmosphere. 

Low and behold, the visible satellite imagery shows lots of clouds particularly focused in the eastern half of Mississippi. There are a lot of factors associated with this channeling of moisture. First...

The remnants of Karen have actually developed a coastal low off the Carolinas. Air channeling on the west side of this feature is being funneled southward then turning to the west, resulting in an easterly to southeasterly flow for us here in the GTR. That's increasing our moisture just enough to result in low-level clouds. Furthermore: 

Here's the 294K isentropic surface. It appears moisture located on this surface juxtaposed with the wind fields associated with this surface is resulting in isentropic upglide across our state, thereby reinforcing the potential for clouds in our area. 

For tomorrow, that coastal low is persisting and it depends on our surface wind direction as to whether or not the clouds will stick with us tomorrow. It looks like that our overall wind should remain the same, so we may can expect additional low clouds that are slow to mix out tomorrow as well. 

IW

Monday Morning Forecast Thoughts

This morning we're seeing picture-perfect, fall-like weather conditions across the GTR as a cold front passed through yesterday giving us much, much drier air. The oranges in this image depict the dry air in the mid levels of the atmosphere that has built in behind the departing shortwave trough. Subsidence on the back side of this feature will be giving us lots of sunshine for much of this week. 

High pressure to our north, combined with the remnant low of Karen over Florida, will look to enhance the transport of cooler air to spill south from the Ohio River Valley. We'll likely see 40s across much of the GTR tonight with a clear sky, so enjoy it!

The primary shortwave axis has lifted well off to our northeast, but a piece of energy is still lagging behind and has become kind of 'cut off' from the upper flow; this is likely the remnant low of Karen being absorbed into the mid-level flow. Notice the large counter-clockwise motion in the Florida Panhandle. This is common for big troughs to sometimes leave spokes of energy behind. But, since we're on the back side of this feature, we'll continue seeing subsidence associated with negative DPVA and continue to see lots of sunshine through this week. 

A strong shortwave is forecast to move northeast across the nation's midsection, but should continue toward the northern Great Lakes. However, subtle, weak waves of energy (as seen in the first image) are likely to be leftover in the overall weakness in the upper heights. The second image shows associated moisture (at 700mb, indicative of clouds) increasing as a result of the weak uplift from the impulses of energy riding ENE in the mid-level flow. 

Overall, a fantastic week is expected with lots of sunshine and temperatures in the upper 70s during the day and in the 50s overnight. 

IW