The well-advertised cold front is now moving into northern Alabama after some very warm temperatures across our area earlier today. Some folks have reached the low 80s today in a few spots, and with such colder air moving in tonight, that has a lot of folks wondering why severe weather (and especially tornadoes) isn't a part of the forecast. To understand why that is the case, you first need to understand what ingredients are necessary for severe weather, and then we will look at why they just are not in place today. In order to get thunderstorms, you need three basic ingredients: moisture, instability, and a lifting mechanism. In most cases, in order to get that storm to become severe (and especially rotating), you need a fourth ingredient, which is wind shear... or the change of wind direction and/or speed with height. Keep those four ingredients in mind as we diagnose this situation...
This surface analysis from late afternoon shows the surface cold front has moved into north central Mississippi and far northern Alabama. This is a very strong cold front, with a 30 degree temperature change across it in under 100 miles of distance. However, arctic air masses like that are usually very shallow near the surface front. This causes the surface front to "outrun" what would be better severe weather ingredients associated with the storm system.
As we look at the jet stream level, you will quickly notice that the strongest upper-level winds are behind the cold front. Also notice that the pink outlined contours, which show upper-level divergence (upper-level support for rising air), are also located behind the front in the colder air. In order to get severe thunderstorms and tornadoes, you absolutely need an upper-level system that is configured in a way so that the upward motion and strong winds aloft overspread the warm, moist air at the surface ahead of the cold front.
We continue to track a strong storm system with a spring side that Alabama is enjoying today with highs in the 70s and a winter side where there is snow, ice and temperatures in the 20s. That cold snap and wintry precipitation moves in on Thursday!
Winter storm warning: (PINK) 3AM Thursday - 3PM Thursday
Which means accumulating wintry precipitation will create slick driving conditions.
Freezing rain advisory: (Purple) 3AM Thursday - 3PM Thursday
Which means a period of freezing rain is possible that might create some slick spots on elevated surfaces like bridges and overpasses and across the higher terrain.
Travel troubles likely tomorrow in the red shaded area where the most ice is expected and where temperatures will be colder for a longer duration.
When to expect freezing rain?
Lamar, Marion, Winston, Fayette, Walker, and Cullman counties any time after 3pm from northwest to southeast. The threat increases southeastward slowly during the mid-morning hours.
The changeover by noon may extend as far south as the I-59 corridor, especially across elevated surfaces. The amount of icy spots will decrease the farther south you go.
Precipitation wraps up quickly on Thursday afternoon and the threat for frozen precipitation drops off dramatically. At that point will be watching for the freezing of residual moisture, especially up north.
Here's a look at how much freezing rain and sleet we are expecting. Due to a very warm nose aloft we are now leaning more towards a freezing rain rather than a sleet or snow event at this point in time.
Marion, Winston, western Cullman, northern Lamar, northern Fayette, & northern Walker counties could see over a quarter of an inch of freezing rain (coating of ice) and up to 1 inch of sleet (ice pellets that bounce).
The amounts possible drop off significantly the farther southeast you live and so the impacts won't be as extreme. That's not to say that you won't encounter slick spots if you are traveling tomorrow. Remember that bridges and overpasses freeze first and especially elevated surfaces like trees, power lines, vehicles etc.
With the higher threat for freezing rain, the concern for potential power outages increases and mainly across the counties under a winter storm warning. Gusty winds during the morning and afternoon hours will increase that threat. So if you live in Lamar, Marion, Fayette, Walker, Winston or Cullman county, be ready for potential outages and if you don't have to travel tomorrow then don't.
We will continue to keep you posted online, on the blog and on Fox 6! Stay with us for more updates around the clock on this approaching winter storm system.
Jill Gilardi Fox 6 Meteorologist
We continue to track a winter storm system that's slowly approaching. We'll remain on the warm side of the system through Wednesday night with highs in the 70s most places. Off and on showers and even a few storms will be possible during that time frame.
Cooler air filters in from the northwest overnight and lagging precipitation behind the cold front will changeover to freezing rain and sleet. The greatest accumulation and travel impacts are expected across the counties under a winter storm warning including, Marion, Lamar, Fayette, Winston, Walker, Cullman and Blount counties.
Counties to the south and along the I-59/I-20 corridor are under a freezing rain advisory from 3am Thursday through 3pm Thursday.
TIMING THE POTENTIAL CHANGEOVER:
The air will be colder for a longer period of time across the counties under a winter storm watch which means a greater threat for accumulation and problems developing on roadways. Farther southeast, the biggest concern may set up on elevated surfaces and above 1,000 feet elevation where the air will be at or below freezing first and for the longest. Remember, we are coming off of a very mild stretch so it's going to take several hours below freezing for precipitation to really become an issue.
MARION, LAMAR, WINSTON, WEST FAYETTE, WEST WALKER AND WEST CULLMAN COUNTIES:
MARION, LAMAR, WINSTON, FAYETTE, WALKER, CULLMAN, & BLOUNT COUNTIES:
ALONG AND SOUTH OF I-20:
HERE'S WHAT'S HAPPENING IN THE ATMOSPHERE REGARDING TEMPERATURES AND IMPACTS ON PRECIPITATION TYPES:
ICY PATCHES ON THURSDAY:
We are going to continue working on this tricky accumulation of freezing rain and sleet forecast. The heaviest sleet looks to set up for counties under the watch. Perhaps 1" at quick glance, especially across Marion and northwest Winston counties. First on elevated and grassy surfaces. Freezing rain amounts could range from as low as a tenth of an inch to a quarter of an inch. Due to gusty winds and the threat for freezing rain, power outages will be possible. I don't think a widespread threat, but enough of a threat that you should prepare for it.
Ok that's the scoop so far.... We are going to continue to look over data and fine tune the forecast.
Jill Gilardi Fox 6 Meteorologist
We are tracking a warm front slowly lifting northward across Alabama and is visible both by looking at the temperature sensors and satellite imagery.
Not everyone is going to get to enjoy highs in the 70s this afternoon, but areas especially south of I-20 should. Unfortunately it looks like quite a dreary day continuing to the north of the front. Expect low clouds, fog and showers to continue to the north of the warm front.
Wednesday, the warm front continues to lift northward and allows all of Central Alabama to warm into the upper 60s and lower 70s.
We'll be tracking the movement southward of a strong cold front on Wednesday night through Thursday. The model assessment of the cold air still wavers some between models but we believe that enough cold air will filter in at the surface across northwest Alabama for precipitation behind the front to changeover to sleet and freezing rain. This changeover may take until daybreak across our northwestern counties and closer to 9AM across the higher elevations of Alabama near the I-59/20 corridor.
That's threat number 1 of a 2 punch happening on Thursday! If you live or plan on traveling at elevations above 1,000 ft, anticipate icy spots developing during the mid morning hours on Thursday. During the evening hours, below freezing air pushes even farther southward and any residual moisture around will likely freeze resulting in slick spots anywhere across the Fox 6 viewing area. That's punch number 2!
So at this point, it looks like two zones set up for the greatest chance for travel impacts on Thursday.
THURSDAY MORNING IMPACT ZONES:
Northwest: Longer duration below freezing and a longer duration of sleet and freezing rain accumulation. Marion, Winston, Cullman, Lamar, Fayette and northwest Walker counties.
Higher Elevations: Temperatures look to dip below freezing from 1,000 ft and higher during the mid-morning hours and any lingering precipitation falling during that time frame would likely freeze into a light glaze on elevated surfaces and bridges and overpasses could see some slick spots developing too. Higher elevations in Jefferson, northern Shelby, St. Clair, Blount, Etowah and Cherokee counties.
THURSDAY EVENING AND NIGHT IMPACTS:
All of the Fox 6 Viewing Area: Any residual moisture will likely freeze as temperatures plummet below freezing. The only saving grace from widespread issues will be gusty winds to help dry up area roadways.
We'll continue to update you as we track this developing storm system... Subtle changes to the forecast and fine tuning will likely occur leading up to the event.
STRONG WIND GUSTS PLUS AND MINUS:
PLUS= Wind gusts over 20 mph will help to dry roadways faster
NEGATIVE= Could knock down tree limbs and cause sporadic power outages
Just prepare for the worst case scenario so you aren't caught off guard if you happen to lose power for a few hours on Thursday.
Jill Gilardi Fox 6 Meteorologist
New higher resolution model data is suggesting our highest risk for sleet and freezing rain will remain in Northwest Alabama Wednesday night into Thursday morning. Since last week, we've been trying to get a good handle on a wintry weather threat being hinted at by the GFS model. At Fox6, we have been very reluctant to "buy into" a significant freezing rain threat because of the GFS model performance this Winter season.
The GFS has not been very good this Winter with the fine details, but it continues to be a reliable source of the big picture. We have known for about five days now that parts of Alabama may see some icy weather. The RPM (which is a WSI product), the NAM, and the 4km NAM are all in agreement that Northwest Alabama will experience at least a 3-4 duration for freezing rain and sleet. In the Fox6 viewing area, travel issues will be likely early Thursday morning in Marion, Winston, Cullman, Lamar, Fayette, andWalker Counties. The graphic below shows the 4km NAM out for 3am Thursday.
The frozen precipitation should be wrapping up just after sunrise on Thursday. Looking at the model data, we should be switching back to all rain briefly before the post-frontal moisture quickly dries up.
Notice the graphic below comparing the NAM and GFS models. The Y axis indicates precipitation amounts and the X axis is time. The green columns show all rain. The yellow and pink indicate freezing rain and ice pellets, respectively. Even the GFS has backed off on the duration for freezing rain/sleet.
This Fox6 created impact graphic will show the probability of significant travel issues for Thursday morning. As you can see, we have much more confidence in travel problems in Northwest Alabama. It's also possible with an extended period of freezing rain and higher winds we could see some power outages.
So does this mean we will be without travel issues in other areas? Not exactly. There is still a significant concern areawide for bridge and higher terrain icing. If you live in higher terrain (especially north of I-20) - you could see a glaze by early Thursday morning. Plus, with heavy rain the night before and temperatures falling quickly we could see some "flash freezing" of residual moisture on Thursday. Again, I think this will be most likely on elevated roadways. Note the sharp gradient in temperatures prior to sunrise Thursday. This is one of the sharpest temperature gradients I've seen. We could see temperature drops between 30 and 40 degrees within just a few hours.
Stay with Fox6 for updates on this forecast. As always, this is a Winter weather forecast with a some level of uncertainty. We could see the situation improve significantly or the freezing precipitation threat shift farther to the south. The best forecast skill comes closer to the event.
Have a great Tuesday!
Fox6 Chief Meteorologist
Sleet and/or freezing rain is likely by Thursday morning across Alabama and parts of the Fox6 viewing area will be significantly impacted. The challenge right now is figuring out where. As new model data begins to arrive, we are getting a better idea on the where and when. The GFS model gave us our first hint at this possibility last week and now as the more reliable (at least this Winter) NAM data rolls in we'll take our first real attempt at breaking down the highest impact areas. This is the NAM out for 6am Thursday. The green is rain, the orange is sleet, and the blue is snowfall.
The Bufkit sounding for Northwest Alabama (Russellville) supports sleet over rain or freezing rain. The orange columns indicate sleet. The bottom axis is the time scale. Bufkit indicates a changeover from rain to sleet sometime before midnight and last through at least 7am.
HIGHEST IMPACT: Marion, Winston, Cullman, Lamar, Walker, Fayette, Pickens, and Tuscaloosa Counties. Some of these places will see wintry precipitation fall for a duration of 6-7 hours. The sleet will likely be a long enough duration to result in some travel problems as temperatures fall into the upper 20s Thursday morning. It's possible we could see power outages in these areas.
By 5am Thursday morning, it looks like the threat will shift south and east based on the NAM. This means Etowah, Jefferson, St. Clair, Calhoun, Blount, and Jefferson Counties will be impacted during this time. This will be a shorter duration precipitation event with rain and sleet mixed for a bit becoming all sleet at times. Travel problems are possible during this time, but not as severe as our counties to the north and west. Many issues in these counties will likely be on elevated roadways and higher terrain.
The Bufkit sounding for Birmingham shows the peak time between 7am and 10am. The rain should end by 10am leaving us with cloudy skies and temperatures around 32 degrees.
WHAT TO EXPECT: As with any Winter weather forecast, we'll have a lot tweaking to do before now and Wednesday. We have a few limiting factors with this event that will play a huge role in minimizing icing on roads. We will be coming off several days of temperatures above freezing. In fact, Tuesday we'll reach the lower to mid 70s in many areas. Despite that, because of the the long duration of possible Winter weather West of I-65 and North of I-20/59 - it's possible we can overcome that and see some roads ice over. The general theme is the farther south and east you go the better your situation. The farther north and west in Alabama and the situation worsens. Just like some of the previous events - some of us will only have a cold rain while others will have some significant issues. There is significant model variation on the placement of the freezing line.
I would expect widespread travel issues in some places and likely another day with delayed schools in others. Temperatures on Thursday will likely stay below freezing and with water still on roadways from the previous day's rain - it's likely we could see that freeze as well.
As always, stay close to weather information this week. We'll have plenty of updates on this weatherblog and on Fox6.
Fox6 Chief Meteorologist
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