2020 going out with a bang, so to speak. Would you expect any less? Severe weather remains a possibility for later today across east Texas, Louisiana, Mississippi, Alabama and the Florida panhandle.
It still looks like the first storms will fire across Texas and Louisiana late in the morning, those same storms will push across Louisiana and into Mississippi and Alabama during the afternoon, evening and into the overnight hours.
To me, it look slike we are going to be looking for storms to develop behind a warm front and ahead of a pseudo-dry line. That should limit the severe potential the farther east you go and the later you get after peak heating.
However, there will be a pretty stout low-level jet that develops at night along the pseudo-dry line as it presses east, and that may help to boost the waning threat after peak heating and into the overnight hours.
Full disclosure: This is a rather tricky setup and there may still be unknowns leading up to the beginning of the event later this morning.
From the SPC
The Storm Prediction Center has made some minor changes to the Severe Weather Outlook for later today. It trimmed the northern extent of the Merginal and the western extent of the Slight. It also removed the Enhanced area from southern Louisiana.
Here is the latest discussion:
…Southeast Texas through the lower Mississippi Valley…
Cold front will stall from the TX coastal area through northern LA and central MS. This boundary might advance slightly northward as a warm front or remain stationary through a portion of southeast TX and the lower MS Valley today into the evening. Cyclogenesis will commence along the front near the central TX coast later this morning. This cyclone will subsequently develop northward through east TX within corridor of deep forcing for ascent associated with a northward-ejecting shortwave trough.
A moist warm sector will reside south of the front with mid to upper 60s F dewpoints from southeast TX into southern LA and MS. However, widespread clouds and weak lapse rates will support only a modest thermodynamic environment with MLCAPE from 500-1000 J/kg. Deeper forcing for ascent will begin to overtake the western portion of the warm sector across southeast TX later this morning.
This will result in more robust thunderstorm development in vicinity and east of the developing surface low near the central TX coast and southward along the cold front into the western Gulf. Activity will subsequently spread into southeast TX and southwest and central LA during the day before reaching southwest LA by early evening. Vertical wind profiles will strengthen as a coupled jet structure overspreads the warm sector in association with the ejecting shortwave trough.
Mixed storm modes are likely. Large low-level hodographs with 300-500 m2/s2 0-1 km storm relative helicity and 50+ kt effective bulk shear will support organized storms, including bowing line segments and supercells with low-level mesocyclones capable of producing tornadoes and damaging wind.
…Central Gulf Coast states…
A subtle warm front will move onto the central Gulf coast later today. The boundary layer will moisten with dewpoints increasing to the mid to upper 60s F. Showers and thunderstorms are expected to gradually increase and develop northward through the evolving warm advection regime later today into the overnight.
Primary severe threat is expected during the evening and tonight as low-mid level winds increase downstream from the shortwave trough that will remain well west of this region. Hodograph size will increase substantially overnight, along with effective bulk shear supporting potential for a few supercells.
The deeper forcing for ascent will remain west of this region, and most storms might remain slightly elevated given the most favorable wind profiles will evolve overnight. Nevertheless, the environment will support a supercell/tornado threat, assuming updrafts can root in the boundary layer.
SPC.NOAA.GOV
Quick look at model data
Right now, it looks like the HRRR model is doing a reasonable job at producing realistic outcomes. Here is a look at the 00z data showing the progression of thunderstorm coverage during the day.
This type of coverage would go a long way toward limiting – but not completely reducing – the threat for severe weather across eastern Louisiana, Mississippi and Alabama. There would be so much rain and so many storms, it would be difficult for a single storm to get rooted and become dominant.
That isn’t to say it can’t happen. It is just difficult.
This same pattern has developed in other model guidance tonight, too. the HRW suite of models as well as the NAM-3km shows a wide cluster of regular thunderstorms with very few – if any – becoming dominant and severe.
Karrie Meter
The Karrie Meter highlights the conditional and limited threat outlined by the HRRR model guidance above and the messaging from the SPC
Notice that the highest numbers only exist in little splotches. For the most part the numbers are in the 2s and 3s, but a few tiny specks of 4s do exist. That is the model picking out where the stronger storms will be and trying to resolve what the interior of the storm may look like mathematically and then expressing it as a Karrie Meter number.
In English? The splotches are showing a reasonable approximation of where the stronger storms may develop and move, but it is only an estimation. And given the higher numbers are showing up as tiny specks is further evidence that the threat for severe weather is conditional and limited, but certainly not zero.
The Bottom Line
Timeline
Not much has changed here. I’m going to leave the timeline as-is. and the same as before, with two hours of wiggle room on each end.
Keep in mind, due to the nature of this event, the timeline does not indicate how long it will be raining. Instead it indicates the timeline for the highest risk for the threat for severe weather.
10a – 2p: Mainly a Texas and Louisiana issue
2p – 6p: Eastern Louisiana and southern Mississippi
6p – 10p: Mainly southern Mississippi
10p – 2a: Southern Mississippi and western Alabama
2a – 6a: Mainly in Alabama and the Florida panhandle
Expectations
–- Heavy rain and gusty wind will accompany nearly every rain shower and thunderstorm. Wind will be sustained at greater than 10mph in general through the afternoon. Wind gusts will exceed 30mph in most rain showers and would be up to 75mph in the strongest thunderstorms.
— There will be some localized flooding, with water coming up in people’s yards and the creeks may rise a bit for a short period of time during some of the heavy rain within the rainbands.
–- Power outages will be likely. Prepare to be without power for up to 6 to 12 hours. The combination of a constant breeze and higher gusts as well as some of the rain means trees are likely to come down.
–- Based on the latest available data, this will not be a widespread catastrophic event for our area. But a some places will still see damage from gusty wind, downed trees and possible tornadoes.
Tornado Threat
The tornado threat will likely be confined – mostly – to Louisiana and southwestern Mississippi. The SPC has placed that are under a 10-percent tornado risk.
While the rest of southern Mississippi and Alabama remain in a 5-percent tornado risk area, as noted by the data above and the SPC discussion, the potential for tornadoes to develop is going to be conditional.
Another way to think about it is we have all the ingredients to make a tornado, the question is if all of those things can line up in the right place at the right time to make it happen. And that is something we can’t know.
