Wednesday Midday Forecast Thoughts

Late this morning, visible satellite imagery shows a clear sky with just a few high level, wispy cirrus clouds across the state of Mississippi. A southeasterly wind flow, however, will gradually increase our moisture content over the next couple of days; subsequently, we'll see a gradual increase in cloud cover by the end of the week in advance of our next storm system. 

An active southern branch of the jet stream has established itself from California eastward toward the nation's midsection. Some ridging is seen trying to nudge in from the southeast, so temperatures will actually warm up slightly through the end of the week. We'll see readings near 70 degrees by your Friday! 

By Saturday, a strong upper-level trough will be evolving across the eastern half of the country, sending a strong cold front our way. Moisture will have increased throughout the week, so some showers are possible especially Saturday morning. However, the big story is the cold blast of air that will accompany this cold frontal passage. 

MOS is not handling this bout of cold air well at all. I think raw guidance has a much better handle on the situation. Strong arctic high pressure (~1038mb) will build in behind this front, and lows Sunday morning will dip well down into the 20s! 

It looks like we have some interesting weather issues setting up next week. Of course, this is a week out, but models agree on a low pressure moving northeast along the Gulf Coast during the middle of the week next week. The GFS (shown above) shows some precipitation on the backside of the low.

A quick peek at forecast soundings show a saturated dendritic growth zone, but some warming and drying in the 1-2km layer. However, we are saturated at the surface as well as below freezing. So, based on this sounding alone, I would expect some freezing drizzle or light freezing rain as the system ejects to the northeast. It should be noted that the Euro has a similar setup, except it's a good 6-12 hours faster than the GFS…so I'll be watching trends throughout the week and providing more updates as the weather situation gets clearer! 

Have a great hump day Wednesday! 

IW

Friday Midday Forecast

We've certainly had a rainy start to our Friday across the GTR as rain has filled in nicely across the area this morning. Why all this rain? 

Here's the 500mb vorticity analysis. There is a short wave in the mid-levels of the atmosphere, and the associated fort max is located on the AR/LA border. Significant downstream uplift from this feature is likely driving force  for the rain this morning. 

Also, if you notice in the surface observations, a weak surface low seems to be spinning up over parts of southwestern Alabama, just to the southeast of Meridian, MS. Additionally, a psuedo-warm front-type structure extends to the northeast, and moisture is, in essence, overrunning this feature and is also combining with the aforementioned uplift to produce widespread rain this morning. 

Here's the potent shortwave for the last part of the weekend. Notice a slight negative tilt to the trough, with over a 100kt wind max in the base of this trough that is progged to spread across the Ohio River Valley. 

This shortwave will bring some severe weather from the Tennessee Valley all the way to the Great Lakes. 

The primary dynamics associated with this system will be well to our north, but the SPC has included us here in the GTR in their standard slight risk of severe weather. An analysis of the severe weather parameters reveal, again, the highest threat will be to our north. 

But, instability will be higher (relatively) than the past few events we have. Forecasted CAPE values are between 500-1000 J/KG but with weaker dynamics. 

The 0-6km shear values are much higher to our north, but notice the shear vectors are due west across our area. This suggests the deep layer wind field will be flowing across the approaching frontal boundary, so isolated supercell storms seem to be the most probable convective mode, especially for areas in north MS into the Ohio River Valley. 

For us here in the GTR, I can't rule out an isolated severe storm, but the highest threat will be in areas to our north. 

Have a great Friday, and stay dry!

IW

Monday Afternoon Forecast Thoughts

We're enjoying a very pleasant, warm day across the GTR and really the entire Southeast. The sky is mostly clear and it feels great. But notice off to our northwest the blue dots and much colder temperatures; that's associated with a frontal system that will be bringing in the coldest air so far this Fall season to our area by the middle of the week. 

The NAM/GFS are in remarkable agreement through the middle of the week with the timing of this frontal passage. I think we will see clouds increase overnight tonight into tomorrow morning, but with such dry air in place, I don't see much chance of rain…but I suppose a sprinkle is possible! 

Also notice the very strong (1044mb) area of high pressure building in behind this front. That strong pressure gradient that results will cause our winds to be rather gusty tomorrow, maybe exceeding 20-25mph at times. Temperatures will also be struggling to get out of the 50s with strong cold air advection. 

The GFS/NAM also are in good agreement with the temperature forecasts as well. With the core of the high pressure not too far to our northwest, and a clear sky, radiational cooling will be highly effective in lowering our temperatures into the upper 20s area wide Tuesday night. The only potential caveat to this freeze would be the winds staying up higher than forecast overnight which would probably result in temperatures reaching the low 30s but creating wind chill readings well down in the lower 20s. 

This high will be firmly entrenched across the Southeast. As a result, we'll see another night (Wednesday) of sub-freezing temperatures across the GTR. Both evenings we should see temperatures range from 25 to 30 for most spots, so take preventive measures to protect sensitive plants or pets. 

It looks like the pattern will get a little progressive toward the weekend. The cold conditions will not last long, and we'll see temperatures rebounding into the 60s and upper 60s by Saturday. The flow aloft will evolve to southwesterly, and it looks like individual pieces of energy could bring some rain chances to our forecast late Friday lasting through the first part of next week, stay tuned! 

IW

Thursday Morning Forecast Thoughts

The parent trough that brought us the showers last night has since lifted well of to our north and east this morning. However, the base of the trough still is close enough to our area, and when combined with subtle PVA remaining, some clouds are hanging around and will likely persist through the overnight hours. However, they are certainly not widespread and we've got about a 95% blue, sunny sky! 

High pressure is forecast to be right on top of us tonight. With efficient radiational cooling and calm winds, temperatures will bottom out in the mid 30s for most locations. But, I wouldn't be surprised to see some areas of north-central Mississippi get close to the freezing mark! 

By tomorrow, the GFS is forecasting more of a flat ridge in place across the area with a piece of energy associated with the subtropical jet over the Gulf of California. Friday should be a nice, sunny day with moderating temperatures with highs in the 60s; it should feel fantastic! 

By Saturday, despite a very dry low level airmass, some mid-level PVA associated with the aforementioned piece of energy in the Gulf of California will bring us some clouds by Saturday, but no rain is expected thanks to the dry surface airmass. 

The GFS is forecasting a fairly strong long wave trough over the eastern half of the country by the first part of next week. Because moisture return will be weak, albeit nonzero, we probably can squeeze out a few showers but for now, no widespread rain is expected. 

Looking ahead past this trough, the NAO is forecasted to go mostly negative once again. The Euro and GFS both suggest very strong high pressure associated with a rather cold airmass by the middle and end of next week, which would support this negative NAO forecast. Some of the raw temperature guidance suggests we'll be well down below freezing into the 20s! Brrrr! We'll watch things, stay tuned! 

IW

Monday Morning Forecast Thoughts

Visible satellite imagery shows a fetch of clouds representing a channel of low and mid-level moisture originating in the eastern Pacific. This is in advance of our next storm system that is organizing over the Four Corners region. 

A longwave trough will be evolving eastward by Tuesday while a ridge of high pressure in the mid-levels of the atmosphere will be anchored over the Florida Peninsula. This type of setup favors a funneling of moisture in from the eastern Pacific, and this is already seen in visible imagery shown above.

Meanwhile, the low-level airmass will remain dry through at least Wednesday. You can see the cold front off to our west and associated showers. Notice the tightly packed isobars across the GTR/Southeast - this suggests slowly retreating high pressure and a residual wedging effect, especially for areas just to our east. 

So, through Wednesday, all of our clouds will be high based into the mid/upper levels of the atmosphere thanks to the dry characteristics of the low level airmass. 

Most models agree that a band of rain will work in here Wednesday evening into the predawn hours of Thursday. Models aren’t quite as bullish on overall precipitation this go-around as they were last week, but they might be undergoing it just a tad (I’ll show PWATs below). By the same token, the models could actually be spot on thanks to the relatively delay in our surface moisture flux that will not commence until Wednesday afternoon. 

Still, we cannot ignore these high PWAT values (exceeding 2” here in the GTR). But also, just like last time, this might be an event where the heaviest rain will be confined to the Delta of Mississipppi into southwest MS/central Louisiana. 

Behind this system, strong high pressure builds into the Tennessee Valley Friday morning. This will likely lead to temperatures falling to around 40 or into the upper 30s, marking the return of the crisp, cool feeling we saw this past weekend!

IW

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