Wednesday Afternoon Forecast Thoughts - Severe Weather Update

We start with water vapor imagery early this afternoon. The blues, greens, and whites indicate copious amounts of mid-level moisture, originating from the southeastern Pacific/Gulf of California. An area of enhanced moisture associated with a fort max is seen over eastern parts of Texas, and is likely contributing to the veil of high-level cirrus that is blanketing us here in the GTR and all across central and northern Mississippi. 

Severe Weather Discussion 

The Storm Prediction Center maintains a slight risk of severe weather for much of the state of Mississippi for Thursday and Thursday night, and an enhanced area of severe weather probabilities are defined for north Mississippi up through western Tennessee and Kentucky. 

Synoptically, this system remains impressive with a significant dynamic wind field. A strong negatively tilted shortwave trough will be ejecting out of the southern Plains northeastward, and PVA associated with this feature will be abundant. 

In the upper levels, an area of enhanced diffluence will coincide with downstream PVA and result in widespread and significant dynamic uplift. In fact, some of the wind fields are very potent, including a 50-70 kt low-level jet and a 120+ kt upper-level jet streak. 

Moisture will be far from meager with this event thanks to the aforementioned strong low-level jet transporting Gulf moisture northward in advance of the approaching frontal boundary. Additionally, southwesterly flow in the mid-levels (as shown in the water vapor imagery) is also supplying copious amounts of moisture in the mid-levels of the atmosphere as well. PWAT values will exceed 2" for most locations as well, so heavy rainfall is also a hazard.

However, thermodynamically speaking, this system is just lacking. With all of the moisture in the air, lapse rates are barely moist adiabatic, if that. Given that we have relatively warmer air in the 700-500mb layer, mid-level lapse rates remain poor (< 6 C). This greatly inhibits instability and the ability for storms to be sustained into the mid-levels of the atmosphere. Therefore, these showers and embedded storms will most likely be low-topped with very low cloud bases with LCL heights likely lower than 1km AGL in some cases. 

That being said, with such incredible dynamic uplift, it is likely that at least some of the thermodynamic insufficiencies will be overcome. As SPC's enhanced risk shows, this is more likely to the north of the GTR as mid-level temperatures/heights are a little bit lower relative to what they are here in central MS. 

Here's the modeled lapse rates from the 12z NAM. As discussed, this graphically depicts the relatively higher lapse rates (6 - 6.5 C/km) well to our north, and values < 6 down this way. 

Given all of this information, sustained WAA will steadily increase our dew point temperatures and despite widespread cloud cover expected tomorrow, surface air temperatures should manage to achieve the mid 70s. However, as the mid-level ridge 'battles' with the approaching trough (700mb temps/heights warmer/higher), appreciable CAPE is not able to be realized in the warm sector, at least here in MS. Still, models can underdo instability even this short in advance, but the prospects of achieving CAPE over 250 J/kg is very unlikely at this point. 

However, even with very marginal CAPE/instability, the dynamic forcing cannot be ignored. With that said, I'm still expecting a strongly forced line of convective showers with a few lightning strikes embedded to prelude the cold front tomorrow late afternoon through tomorrow night. Within this line, provided the boundary layer remains well mixed (which it should given very strong low-level winds prevailing and even increasing nocturnally), some of those stronger winds located just a couple thousand feet off the ground could be transferred downward. Behind this forced line, moderate rains will likely continue right along the front and some of it being post-frontal as well.

Additionally, winds remain relatively backed around to at least southerly as a weakening meso-low moves northeast out of Arkansas. This combined with the aforementioned very strong low-level jet winds is creating helicity in the lowest 2km AGL of over 300-400 m^2/s^2, favorable for tornadogensis. However, because the instability is so low, the excessive shear in the atmosphere may act to 'shear out' the showers and embedded storms that try to grow vertically. Nonetheless, very quick, spin-up tornadoes are possible down this way, but not likely, within the leading, strongly forced line of showers and embedded storms. 

The highest isolated tornado and potentially gusty wind threat should be located north of us here in the GTR, basically where the SPC has defined a 30% chance of severe weather (in the graphic above). The thermodynamics down this way are severely lacking, and the best wind field seems to be out of phase with the extra uplift associated with the approaching front. Details on the mesoscale tomorrow will be important to this event's evolution, so stay tuned!

IW

Tuesday Midday Forecast Thoughts - Severe Storms Thursday

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

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

Thursday Afternoon Forecast Thoughts

Here's the radar as of 4 PM this afternoon. We're actually noticing some showers and embedded thunderstorms to the east of I-55, north of I-20, and south of Highway 82 across east-central MS. I have to ask myself why this activity is here, and one potential explanation is this: 

Notice the relative higher pressures to our east, and how the lower pressures are kind of angled toward it. This is an inverted surface trough, and oftentimes can result in showers and storms over a limited spatial area, and in our case, it would be south-central into east-central Mississippi. 

Here's the latest water vapor imagery of Tropical Storm Karen. Notice the vast amounts of dry air on the north and west side of the system, while copious amounts of tropical moisture exists to the northeast and southeast. Tropical systems, for one, like lots of moisture because dry air disrupts their circulation...much like what Karen is experiencing now. How will Karen impact our weather in the GTR? 

Here's the latest national weather vapor imagery. Notice the large swirl of clouds over the Intermountain West region. This is associated with a large trough of low pressure that will also be influencing our weather pattern this weekend as well as influencing Karen's path. 

Let's jump to Saturday. I'm showing the GFS model here, but please remember there are several different models... 

For your Saturday, we have Karen approaching the central Gulf Coast while the aforementioned strong trough continues to lift northeast into the Great Lakes region. If Karen weren't in the picture, this in of itself would likely reduce any severe weather threat that might would otherwise be a possibility with a system of this stature. 

Here's the 500mb projection at 18z Sunday which is 1pm. Karen is in the process of getting picked up/absorbed by the trough, but the overall trough axis remains very deep. However, the precipitation forecast is interesting: 

Here's the same time, 1pm Sunday. We have a cold front approaching from the northwest, and Karen to our southeast near Dothan, AL (according to this model). If this were correct, dry air from the northeast being wrapped into Karen juxtaposed with subsidence often associated with the outer parts of a tropical system, would probably reduce our rainfall potential. 

This is slightly counter-intuitive. We still have uplift from the approaching trough/front, but when it is more or less "offset" by the dry air/subsidence, our rain potential MIGHT be lessened due to this. 

However, the European model has Karen significantly west relative to the GFS, but still has it passing to our south and southeast. However, given its closer proximity, our rain chances would be increased. The NAM has a similar track as well. 

Here's the NHC track as of the 4pm advisory. There's no doubt that the trough approaching us will pick Karen up and steer it northeast, but just how fast this happens is still uncertain. I think the best course of action is, for now, give all models equal credibility and watch how Karen interacts with the shear/dry air over the next 24 hours....

BOTTOM LINE: Rain still is a decent bet for us here in the GTR Saturday evening into Sunday afternoon, either in association with Karen or the front, or both. But given the potential fly in the ointment I discussed, our potential accumulation could be significantly reduced, or maybe enhanced depending on the ultimate track of Karen. 

The joys of forecasting...

IW