Saturday, December 9, 2023

Severe Storms Possible Saturday Night, Then Clearing and Turning Sharply Colder

(Forecast)

Saturday (High 69, Low 54): Mostly cloudy and breezy. Thunderstorms are likely in the evening and night hours, and some storms may become severe. 

Sunday (High 50, Low 42): Rain ending early in the morning. Staying breezy and turning sharply cold.

Monday (High 50, Low 27): Sunny. Very cold.

(Extended Outlook)

Tuesday (High 54, Low 25): Sunny.

Wednesday (High 56, Low 29): Mostly sunny.

Thursday (High 58, Low 33): Mostly sunny.

Friday (High 60, Low 35): Partly cloudy with a 20% chance of showers.

(Tea Leaves)

Saturday (High 60, Low 38): Partly cloudy with a 20% chance of showers.

Sunday (High 61, Low 40): Mostly sunny with a 20% chance of showers.

Monday (High 57, Low 36): Mostly sunny. 

(Pronóstico)

Sábado (Máxima 69, Mínima 54): Mayormente nublado y con brisa. Es probable que haya tormentas eléctricas en las horas de la tarde y la noche, y algunas tormentas pueden volverse severas.

Domingo (Máxima 50, Mínima 42): La lluvia terminará temprano en la mañana. Mantenerse ventoso y volverse muy frío.

Lunes (Máxima 50, Mínima 27): Soleado. Muy frío.

(Perspectiva Extendida)

Martes (Máxima 54, Mínima 25): Soleado.

Miércoles (Máxima 56, Mínima 29): Mayormente soleado.

Jueves (Máxima 58, Mínima 33): Mayormente soleado.

Viernes (Máxima 60, Mínima 35): Parcialmente nublado con un 20 % de probabilidad de lluvias.

(Hojas de Té)

Sábado (Máxima 60, Mínima 38): Parcialmente nublado con un 20 % de probabilidad de lluvias.

Domingo (Máxima 61, Mínima 40): Mayormente soleado con un 20 % de probabilidad de lluvias.

Lunes (Máxima 57, Mínima 36): Mayormente soleado.

(Discussion)






It was a mostly overcast day in the Tennessee Valley with a southerly breeze. The High in Cullman was 64 after a morning Low of 34, quite a temperature gradient today. Saw a High of 64 and Low of 28 in Jasper. Haleyville had a High of 60 and Low of 38. 

The High was 61 after a morning Low of 28 in Fort Payne. Then on the other side of the state, Muscle Shoals had a High of 64 and Low of 40. Decatur saw a High of 63 and Low of 35. Huntsville had a High of 63 and Low of 39. Nashville had a High of 64 and Low of 42. Memphis got up to 65 after a Low of 47. And Tupelo made it up to 62 degrees today after a Low of 40. 

It's been a while since I posted a forecast, and the weather may get active Saturday night into early Sunday morning, at least a lot of rain and wind, but some potential for thunderstorms becoming severe as well. Which you can have in early December around here, though it's more common in November for North Alabama into Tennessee. 

And I am still recovering from COVID-19, fairly nasty bout of it, when I thought the virus had pretty much run its course and wasn't catching all that often with people anymore. So this is the first forecast in a while. Got sick right after Thanksgiving. And then over the past week or so, about half the people I know seem to have come down with at least a mild case of it. At least now you hear more stories of people recovering from it, usually without the hellacious complications that marked the first year of it. Many thanks to all the doctors and other medical folks who helped make this easier on us in various ways. 

And now we have a warm front bringing some light showers overnight and into tomorrow (Saturday) morning. Before the cold front pushes through Saturday night and Sunday, bringing first a threat for some storms, and then some sharply colder air. 



The rain will likely taper off for a while as we go through tomorrow morning, and we'll have a few hours of clear weather, just a lot of clouds and windy conditions. The main line of showers and thunderstorms should hold off until tomorrow evening into the overnight hours. But we can't rule out a few isolated cells or clusters forming ahead of the main line during the peak of the afternoon heating. Those would probably initiate back in Mississippi and then track up toward Northwest Alabama. It'll be a really breezy day, looking for a High near 69. Low tonight should be in lower 50's.



Then Sunday is a tricky day to write a forecast for, because our High will probably be about 50 degrees and come toward the beginning of the day, and then by evening, falling into the lower 40's. One of those days the temperature is going backwards. Should stay breezy, but I think the rain ends early enough to prevent any significant wintry mix that we might otherwise have some concern about. 



Then on Monday, high pressure settles in, and despite plenty of sunshine, we will struggle to reach 50 degrees, after starting the day in the upper 20's. We could still have some breezy periods, and the wind chill factor could get brutal. This is some seriously cold weather behind this front. 



Then on Tuesday, looks like sunny skies again, daytime temperatures moderating into the mid-50's, Low toward the mid-20's, very cold and dry air in place.



On Wednesday it looks like the high pressure will be centered up around the Great Lakes. Around here we'll stay mostly sunny, High rebounding to upper 50's, Low to about 30.



Looks like the moisture stays down at the Gulf Coast for Thursday too and high pressure close enough to dominate our weather up this way. The High could get up to near 60 again with a Low of 30 or so. Mostly sunny skies. 



Friday the GFS wants to rush a lot of Gulf moisture up this way. 


And is often the case, the ECMWF is far less enthusiastic in any such a scenario. It has us dry, and usually this time of year, I'll defer to it on that. 

For now will just mention an increase in clouds and continue the gradual climb of daytime temps toward the 60 degree mark, Low moderating to about 35 at night. 



Beyond that, we get into serious model madness. Above is the GFS for Saturday, a week from today. 



And the European model is seeing a whole different scenario. 

Toying with the idea of 20% minimal rain chances here for the far extended period. But I think the ECMWF is probably right here. 

So will probably see a High near 60 and a Low climbing into the upper 30's here, rain chance staying minimal.



Models do agree with the basic look that any rain we might get late in the period will be on its way out on Sunday. Will probably see a High near 60, Low near 40 here. And please understand that when you get out beyond seven days in advance, it is an educated guess at best, more looking at trends. But it can be fun to try every now and then. 



If the model trends are worth anything by Monday the 18th, then looks like drier air again, so a High more toward the mid-50's and Low more in upper 30's or maybe easing back down to mid-30's.




Now for tomorrow's severe weather potential, meaning Saturday's. The NAM shows the main squall line coming in to Northwest Alabama and neighboring parts of Tennessee at about 9 PM Saturday night. However, some potential does exist for isolated storms to try to form in the afternoon/evening ahead of that main line. So those would need to be monitored whether they stayed out to themselves as supercell thunderstorms or whether they formed into clusters. That's a dicey aspect of it. It's more likely we'll just see the squall line at night. 




Having said that, parameters may support severe thunderstorms as early as 3 PM for at least Northwest Alabama and adjacent parts of Tennessee and Mississippi. So if we do see any supercells in the afternoon, probably would form in this vicinity and track through these areas, and will of course bear watching. 



It's more likely to see thunderstorms going severe along the actual squall line after dark though. That's the best timing where all the ingredients will probably be in the right amounts. The surface-based CAPE could meet or exceed 1,000 joules/kilogram, which is higher than we typically see in December. 



On the other hand, the wind shear values are a little on the modest side for what we normally see in a December setup like this. But they are still respectable. Even if I saw this in the Spring time, 0-1 km Helicity going over 200 m2/s2 over a large area, combined with this kind of unstable air and a strong cold front like this, I would have some concern for severe thunderstorms capable of producing damaging winds, large hail, and isolated tornadoes across the region. 




Even the SREF, not as aggressive as the NAM, brings the Significant Tornado Parameter up to 1-2 tomorrow evening and night. That is enough to support an isolated tornado threat and is worth respecting. 



The Storm Prediction Center has outlooked basically all of North Alabama up into Central Tennessee for a basic Level 2 out of 5 risk of severe storms tomorrow evening and night. This risk, which you can see also as a 15% damaging wind threat, extends back through West Central Alabama and back into most of the state of Mississippi, even back into parts of Arkansas and Louisiana where supercell thunderstorms may initiate before forming into clusters and then a line. 


The risk for large hail is more marginal, a 5% risk until you go into Western Tennessee back into Arkansas, Northern Mississippi and Louisiana, where they have a 15% risk. Usually these cool-season events do not produce as much large hail as we see in the main Spring severe weather season. If we do see instances of a lot of large hail somewhere, it'll probably be an isolated supercell that managed to break off of, or form ahead of, the main squall line. 


And we've all got a 5% risk for isolated tornadoes across the Tennessee Valley region too. This is not a really widespread threat, but it is a basic risk that isolated tornadoes could occur. And where they do, people need to be prepared, especially since it is an overnight event. 

The simple version is:

* Use a reliable method to get warnings overnight, not an outdoor siren. At least enable WEA on your cell phone, but a weather radio is best. 

* If you go under a Tornado Warning (or Severe Thunderstorm Warning), you want to be in a sturdy house or other strong building, not a mobile home. Get as low down in the house as you can and as near the center as you can, preferably in a small area like a bathroom, hallway, or closet, and shield your body from debris, especially your head. Of course stay away from any windows. Stay in a safe place until sure the danger has passed, and if there is damage afterward, take things slow and stay calm, just do the next right things. 



For a full rundown on severe weather safety, see here. Even though Saturday night's threat is not anything overwhelming, it is a routine severe weather threat that deserves to be respected. And if you live in a mobile home, or another weak structure such as a top-floor apartment, I urge you to plan ahead and make some good decisions as a course of least regret. 


We'll probably see about an inch or so of average rainfall out of this system, with isolated amounts up to about two inches possible. We needed this for the drought. That situation is easing up with each event. Just be careful in case any storms do become severe with this system. It only takes one in the whole region, and if somebody sleeps through it without proper shelter, then we've got issues. So let people know. No need to hype it up, this is a routine severe weather threat that we've seen many Decembers before. Actually we've seen a lot worse than this in the late Fall before. No need to get overly alarmed, but to completely dismiss it and go to bed without a way to wake up and get the warning, would be a mistake. 

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