As a professor, this is the type of weather event that I would probably use on a test. Because at first glance, and on paper, it looks like a guarantee for some severe weather. But if you look behind the curtain things get a bit more uncertain. And it would be the level of uncertainty juxtaposed against the potential risk that would really test my students forecasting muscles.
And thus, in this case, mine, as I’m trying to do the forecast for you guys now.
This is a tricky one to nail down.
From the SPC
The Storm Prediction Center is maintaining a Slight Risk for severe weather for the Gulf Coast on Thursday and into Friday morning.
The Slight has been pulled back to nearly Houston and clipped on the eastern edge to include just east of Pensacola now.
The SPC Discussion:
An upper-level low will move from northern Mexico into the southern Plains on Thursday. Divergent flow aloft will overspread eastern Texas and Louisiana as the negatively-tilted system deepens. A large cluster of thunderstorms appears likely to be ongoing in southeast Texas at the start of the period.
This convection is forecast to move eastward as a low deepens and moves northeastward into the Arklatex. Although instability will be weak along the attendant surface trough, strong deep-layer shear will create conditions favorable for severe storms. Lift associated with a mid-level jet, rounding the base of the upper-level system, will provide additional support for organized storms.
As the mid-level jet moves eastward across the Lower Mississippi Valley on Thursday, thunderstorm development is expected to occur across parts of Louisiana and Mississippi. Instability is forecast to gradually increase across the moist sector, which should be located from Louisiana to southwestern Alabama.
Discrete thunderstorms that develop ahead of the cold front across the moist sector could have a wind damage and tornado potential.
During the late afternoon, the most favorable area would be in far southeast Louisiana, where the models maximize instability at the northern end of the moist axis. The corridor of maximized low-level moisture is forecast to shift eastward across southern Mississippi and southern Alabama during the evening and overnight period.
Models suggest the possibility of a convective line located along the moist axis. This line would be associated with a continued wind damage and tornado threat during the overnight period.
spc.noaa.gov
Currently, the main question is ‘Will the necessary instability make it farther north, or will is be stuck closer to the coast?’
And for that question, we have no answer. Yet.
The Data
I’m going to walk you guys through something that is very important. This is a behind the scenes, sciencey, and physicsy deal, but I think it is worth noting. We are going to look at the atmosphere from the top down and look for hints as to why this forecast may be a bit tricky, and where- I think – the data may be failing a bit.
300mb
Looking up at 300mb in the atmosphere (about 25,000ft up), on Thursday night around 11pm, there are two big things to note. The first is the divergence happening at this level across parts of Louisiana, Mississippi and Alabama. This means air from lower in the atmosphere can freely ascend (rise) and produce clouds and shower and storms.
The other thing to note is a pronounced Jet Streak (not Jet Stream, that is different) from just off the coast of Lake Charles, LA to about Vicksburg, MS.
I haven’t written a “WX Info” post about Jet Streaks, but I really need to. The physics that go into these things is fascinating stuff and is a big deal when trying to assess the potential for the low-level wind fetch.
You can see the Jet Streak on the map above. And some important things happen in a Jet Streak.
In the atmosphere, the exit region of a Jet Streak (or the end of it) pulls air into it from below from the right. And air is lifted on the left side. That means, according to the textbook, on the map above, as the air leave a Jet Streak, air underneath that (like down around 850mb) is going to be moving from SE to NW. And then lifting on the left side. But we will get back to that in a second.
500mb
I just want to take a look at the vorticity map quickly to highlight that often we look at this map to figure out where the ‘umph’ in the atmosphere is going to be. The 500mb map is looking up at about 18,000ft. This isn’t an end-all-be-all or a ‘silver bullet’ by any means, but it does do a good job at ID’ing where to look for the development of “weather” in general.
On the map above we are still looking at 11pm on Thursday night. Notice that a lot of the ‘umph’ is still back across parts of Texas and Louisiana. That doesn’t mean “weather” can’t be happening out ahead of that point, but it does mean that any storms that do develop will not have the added boost of nearby vorticity.
Not a big deal, but it is something to keep in the back of your mind.
850mb
Down at 850mb, recall I saw the wind should be moving from SE to NW in response to the Jet Streak up at 300mb.
Notice that it is not.
This is not a failing in the modeling, this is real life. Real life does not always work like the textbook.
However! I would expect the wind to have a bit more of a tug to the NW at this level near Vicksburg and point to the SE. Because, while life doesn’t always work out like it is shown in textbooks, life cannot totally defy physics. Also there should be an increase in the wind speeds in this area, too.
That increase in wind speed does show up (yellow shading)!
This may also mean, though, that the Jet Streak – which happens in 3D, not just on a flat plane at one level of the atmosphere – may be deeper in the atmosphere. And we may be seeing piece of it all the way down at this level back across parts of Louisiana (the slightest bit of yellow shading).
So we may need to go down to the surface to find that SE wind.
Surface
Did we find it here? Sort of.
Not exactly a full fetch from the SE but a bit more bend for sure.
You can also see where the boundary for storms will likely be at this point. The dewpoint front between the green shading and the blue shading. That is the difference between dewpoints in the 60s (humid) and dewpoints in the 50s (not humid).
It is also important to note that the purple shaded area (dewpoints in the 70s and very humid) is well out into the Gulf.
How far that moisture makes it north will likely dictate the threat for severe weather.
What decides that? … I’m so glad you asked.
What dictates the severe risk?
In this case the plume of moisture off the coast will likely be the limiting factor. And as shown above the fetch from SE to NW underneath the Jet Streak doesn’t fully materialize within the model guidance shown.
However, within some of the higher-resolution model guidance, that can “see” things a bit clearer, that does develop. And, on top of that, in between the 300mb 500mb and 850mb levels that I showed… at 700mb there is ample vertical motion.
So! The combination of the materialization of the Jet Streak, the atmosphere’s response to that materialization through the column regionally, and the ability of that response to pull low-level moisture in from the Gulf underneath the Jet Streak and in concert with the vertical motion that develops due tot he instability created by the influx of warmer, moist air underneath the negatively-tilted 500mb low injecting cold air aloft will determine the general severe risk for the area.
Makes total sense, right?
I know, I know, I can see some of you guys now…
But that is to say, it isn’t just a simple “slam dunk” for severe weather in this situation.
Karrie Meter and CIPS Analogs
This is why I like to rely on the Karrie Meter. It is a bit more straightforward and easier to understand. And it is showing some higher numbers at times, but things are pretty hit and miss.
Which is to be expected given the explanation above.
You can see that values between “2” and “4” sweep through the area in the evening and into the overnight hours. That is consistent with a Marginal to Slight Risk for severe weather from the SPC. So the SPC and the Karrie Meter are seeing the same things.
That is good news! It means the forecast has a bit higher confidence.
You can also see that there are ‘hot spots’ where the numbers are a bit higher than in other places. This isn’t because the model knows that one particular spot will see more severe weather than another spot. It is just the model data lining up correctly for a single point in time, at a single spot.
We are still a bit too far out in time for the models to resolve specifics like, “this town will see worse weather than that town”. Instead we are still at the point where we have to talk about general areas.
And the general area is highlighted perfectly – given the data we’ve looked through so far – by the Slight Risk from the SPC.
It looks like the CIPS Analogs is having a tough time identifying the higher risk area between Thursday afternoon and overnight.
It shows a few spots where severe weather is possible, but has a bit of a dead zone in the area that -within the raw data – would suggest a threat remains (eastern Louisiana and southwestern Mississippi).
This is likely due to the Ensemble data being split with one group producing a faster system (east) and half producing a slower system (west).
The operational NAM data from CIPS shows that “Boom or Bust” potential I was talking about on WDAM.
Notice that the Top 15 most similar situations we have been in, historically, produced either a total NothingBurger or reasonably significant severe weather day. And there are eight busts and seven booms.
Almost split down the middle.
The Bottom Line
I know that a lot of you are thinking, “Oka, Nick, we get it. Just give us the forecast…”
Right now, it looks like showers and storms will develop as early as Thursday afternoon across parts of Louisiana and Mississippi. Some of these storms may be severe, but the threat will be limited until we get a bit later in the evening.
A line of storms will develop by late afternoon across central Louisiana nad press – perhaps relatively slowly – east.
In the evening, SE Louisiana, southern Mississippi and SW Alabama will have a slightly higher risk from 6pm until 6am as discrete cells try to develop ahead of a line of storms and then with the line of storms itself.
The main concern will be heavy rain, localized flooding, frequent lightning, wind gusts up to 74mph, hail up to the size of quarters, and the potential for a few tornadoes. And within that tornado threat, depending on how things line up, there will be the potential for an EF-2 or EF-3 tornado, too.
Things will clear out by 6am on Friday, January 1st, 2021.
