As we head into next week, there will be a few rounds of storms. Right now it looks like Monday, Tuesday and Wednesday will feature the possibility for storms. Monday may even feature the chance for some severe weather in Arkansas, Mississippi and Tennessee.
But, Wednesday is the main event, as it appears right now.
Wednesday will be the day that will feature the potential for more widespread – and perhaps significant – severe weather developing across parts of the southeast.
From the Storm Prediction Center
Yesterday, the SPC introduce a Slight Risk for severe weather on Wednesday. Today, the SPC has increased the risk for severe weather to an Enhanced Risk for some sections and expanded the Slight Risk. That is a “2” (slight) and a “3” (enhanced) on the 1-to-5 scale, where “5” is the highest risk for severe weather.
For comparison to yesterday’s risk area, it has been trimmed farther south and squeezed to the east. That leaves Mississippi covered by the “30-percent” / Enhanced / “3” out of “5” area.
Here is the discussion:
…DISCUSSION…
Medium-range models have come into somewhat better agreement with respect to synoptic features through the early half of the period, though smaller-scale differences persist. Still, confidence has increased that a fairly widespread severe weather event will occur Day 4 (Wednesday) across the lower Mississippi Valley/central Gulf Coast region and northward into at least parts of the Tennessee Valley, and then continuing into Day 5 (Thursday) east of the Appalachians.
In the wake of this severe-weather episode, high pressure/stable air should spread across the U.S. east of the Rockies, yielding at least a couple of days of quieter weather with respect to convective potential.
On Wednesday, convection will likely be ongoing across the eastern Texas/Arkansas/Louisiana areas, with some severe risk possible during the morning hours. By afternoon, an increase in coverage and intensity is expected as closed upper system shifts out of the Plains toward the Mississippi Valley. With a rather large/destabilizing warm sector and ample shear across a broad region, a rather large 15% area will be maintained, representative of an all-hazards severe risk.
Corridors of greater risk — including possibly more concentrated tornado potential — may evolve during the afternoon, possibly near the northward-moving warm front. While the ECMWF is farther south with this west-to-east front (central MS/AL) during the afternoon than the GFS (southern TN), will nonetheless introduce a 30% risk area — encompassing a range of possible warm-frontal locations — given likelihood for widespread severe weather that warrants greater probability at this time.
The severe risk may diminish some into the evening/overnight across the southern Appalachians area, but should increase somewhat Thursday east of the mountains, possibly as far north as southern Virginia and extending southward into north Florida. Timing differences in progression of the surface front increase between the GFS and ECMWF with time, so when the threat finally subsides — as storms move offshore — remains uncertain.
However, this should certainly occur prior to the start of Day 6 (Friday), resulting in what appears likely to be at least a couple of days of substantially decreased convective potential.
Courtesy: spc.noaa.gov
Looking at the CIPS Analogs
There have been a ton of these CIPS maps posted to social media in the recent days highlighting the potential for severe weather across a wide area of the southeast. And while there is a lot of utility with the Analogs, let’s take a deeper look at these an unpack some extra information they can reveal.
The latest from the CIPS Analogs shows a bullseye severe weather potential across parts of northeastern Louisiana and a wider area in Louisiana, Arkansas, and Mississippi. In the graphic above, the CIPS Analogs data is using the same percentage as the SPC. So, you can interpret this as where the potential risk on the same 1-to-5 scale would be.
Underneath that, though, we can break it out into individual threats.
This shows an interesting – but also predictable – trend that the tornado risk will develop farther east than the hail threat. And the hail threat develops farther east than the wind threat.
This doesn’t mean that the only risk for tornadoes is rather east and the only risk for wind is farther west. Instead, we can use this to predict the evolution of an event.
Often, when storms develop, the first threat is wind, then the updraft becomes more established and hail becomes a concern, then finally once a storm is full mature, a tornado threat may – or may not – develop. So, seeing a tornado risk develop farther east than hail, and hail farther east than wind is a predictable outcome within the data.
We can also pull a timeline of events out of this data, too. Since the storms need to take time to mature, it can be predicted that the highest risk for tornadoes – but not the only risk – may be later in the day. Adding to that, these maps are within 24 hours of a specific time.. The time chosen is 7pm on March, 17th. So, as things move east on the map, they get later in the day.
That is also supported by history. The SPC data for the area shows the most likely time for tornadoes to occur in this area is between 20z (3pm) and 1z (8pm).
So, historically, tornadoes are most likely to develop during the afternoon and evening. And, the data for this particular event, suggest tornadoes are most likely later in the day That means the environment will not be fighting ‘against’ any extra inertia holding back tornado development.
There is an interesting wrinkle to the data, though. Looking at the “Top 5 Most Similar Analogs” to the upcoming event, the CIPS data show:
2/28/2012 – Significant severe weather event for Kansas / Missouri
2/24/2011 – Significant severe weather event for southeast
4/17/1995 – High Risk day in Southern Plains
2/10/2009 – Significant severe weather event for Texas / Oklahoma
2/21/2007 – Bust
So there is a 1-in-5 chance, historically, that a similar set-up produces a significant severe weather day int eh southeast. And a 1-in-5 chance it is a bust. And a 3-in-5 chance there is significant severe weather elsewhere.
So, you may be asking, “what gives, Nick? Does that mean we are in the clear?”
Not really. In fact, the data showing multiple events in February gives me an extra bit of confidence indirectly. In general, mid-march features much more instability than mid-February along the Gulf Coast. Often in February, to get severe weather in the Southeast the environment is “High Shear / Low CAPE” and storms are very spinny, but they lack the necessary instability (CAPE) to have widespread severe weather.
That will not be the case with this upcoming event. There will be ample instability. So on days that historically featured severe weather elsewhere, but nothing in the southeast, may have been a function of other factors at play that will not be in play this time.
But there are two sides to this. Because the CIPS Analogs come in two “flavors” so to speak.
And the shorter-term analogs (the other flavor) highlight this a bit better. Instead of looking at Ensembles, which can average out potentials, the shorter-term analogs look at deterministic data.
You can see there is a much more robust spread within the severe weather potential. And zero “busts” within the data. And that supports a different-looking map from the CIPS Analogs
That is a bullseye of greater than 70-percent in central Mississippi. Meaning that looking at the top 15 most similar events to the upcoming event within the available deterministic data, that 70-percent of those events (at least 10 of the 15) have produced severe weather. And within that, a threat for tornadoes continues. As well as long-track tornadoes.
And that risk for long-track tornadoes stretches from southwestern Mississippi to northern Alabama. with a 40-percent chance for a long-track tornado in parts of southern Mississippi.
So events like this within the first flavor of CIPS (based on ensemble data) show as high as a 60-percent risk for severe weather and a 45-percent risk of a tornado across parts of Mississippi. And the second flavor of CIPS (based on deterministic data) show as high as a 70-percent chance that historically severe weather has happened in similar situations with a 40-percent chance that historically a long-track tornado has developed.
More data; Some good news
There may be some good news and there is a chance this may not end up being some kind of intense, crazy, severe weather outbreak. Why?
The Blob.
The Southeast has been saved by The Blob before. Many times, in fact. The Blob, is just a cluster of showers and storms that forms along or near the coastline and robs places farther inland of any extra warm air and moisture from the Gulf of Mexico.
The Blob is often why severe weather busts occur.
This is the good news. The data has went back-and-forth about The Blob. Sometimes it will be there, other times not. Sometimes it will be pronounced (like the data above) other times it will be there, but not as robust. And the size of The Blob matters. The deeper, wider and stronger it can become, the more it will save places inland from as much (but not all) severe weather.
So, The Blob is something we will have to watch closely within the data during the next 48 hours.
Fringe data; other possibilities
This is something I’ve usually kept to myself. But I want to share two other possibilities that I consider “fringe” outcomes. These are the low-possibility, but unable-to-ignore outcomes. I won’t get into the weeds here, as much, but I want to share these in case the data starts to support these, so people aren’t caught off-guard.
Double-Decker
There is a chance that there is some morning convection on Wednesday that can break through the weak cap of warm air aloft. This would mean a few isolated storms in the AM across parts of Louisiana, Mississippi and Alabama before the other storms (likely more robust) develop in the afternoon and evening.
Too many storms
There is a chance that the cap of warm air I mention above is not as strong and storms start to fire earlier in the day. All over the place.
And while that might sound bad, it is actually good. Because in this case, storms would be fighting each other for “dominance” in their area and wouldn’t be as able to fully mature. This would lower the overall risk for severe weather and tornadoes. But it wouldn’t likely drop it to zero.
Warm front outbreak
Along the warm front as it lifts through northern Mississippi, northern Alabama and into Tennessee, there is a chance that is where a lot of the hailers and a few or the tornadoes develop.
This would be independent of The Blob, too. That means, even if The Blob forms and cuts off the flow for other parts of the southeast, it wouldn’t matter for places along the warm front in this scenario.
The Bottom Line
It is still too early for a timeline because we are still a bit too far out. But be prepared for storms on Wednesday. Some of these may be severe. A few may be pretty potent if things like up like we are thinking.
The main concern with severe weather on Wednesday will be heavy rain, frequent lightning, wind gusts up to 60mph, hailstones up to the size of tennis balls (maybe larger), and the potential for a few tornadoes – some may be strong (EF2 or EF3) or long-track.
The threat extends from east Texas all the way to western Georgia. And from the Gulf Coast north into southern Missouri and Tennessee.
