12/31/20 9a update: Morning model guidance holding, forecast becoming more certain

There wasn’t much of a change between the overnight and mid morning data. That is good news. It means the model guidance is finally getting a good handle on things. Or it gave up. Ha! I’m kidding, of course.

The SPC forecast hasn’t changed much, either, so it does look like things are getting a bit more certain.

From the SPC

The Storm Prediction Center only made very small changes to the Severe Weather Risk.

Here is the latest discussion:

A midlevel closed low south of the Big Bend will eject northeastward toward the Ozarks by early Friday, in response to an upstream shortwave trough digging south-southeastward from northern CA toward northwestern Mexico. At the surface, a 1004 mb low as of 12z will develop inland across the upper TX coastal plain today along a quasi-stationary front, and the low will continue north-northeastward toward the Ark-La-Tex tonight.

The warm sector (with boundary-layer dewpoints in the mid-upper 60s) will spread inland to the east of the cyclone and contribute MLCAPE of 500-1000 J/kg today across extreme southeast TX/southwest LA, and into tonight across southern MS and southwestern AL. Thunderstorm development this morning will be focused along the front near the developing surface cyclone along the upper TX coastal plain.

Convection will spread northeastward through tonight as the cold front surges eastward across LA/MS to the south and southeast of the cyclone. Effective bulk shear of 45-60 kt will be favorable for organized severe storms on the nose of a 70-90 kt midlevel jet, and low-level shear/hodograph curvature (effective SRH of 200-300 m2/s2) will be maximized in the northern portions of the warm sector in conjunction with a southerly 40-50 kt low-level jet.

The combination of linear forcing for ascent along the cold front and substantial cross-boundary flow and deep-layer shear vectors will support a somewhat broken band of supercells and line segments. Embedded supercells will also be possible in a loose band along a secondary moisture plume from the southeastern Gulf of Mexico toward southwestern AL by tonight.

Surface-based buoyancy and the strong vertical shear will support the potential for a few tornadic supercells, damaging gusts, and isolated marginally severe hail.

However, the magnitude of the tornado threat could be tempered some by rather modest low-level lapse rates inland, and less-than-ideal phasing with the ejecting midlevel trough and surface cyclone glancing the northwest side of the surface warm sector.

spc.noaa.gov

That last line is important. The overall tornado threat to the east (across Mississippi and Alabama) may be tempered inland by some not-as-favorable parameters.

The Bottom Line

Timeline

Not much has changed here, either. But I still think the two hours of wiggle room on each end of the timeline below is needed.

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 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.

Threats

The main concerns with storms today will include…

• Heavy rain
• Localized flash flooding
• Frequent lightning
• Wind gusts up to 75mph
• Hail up the size of quarters
• A few tornadoes

Tornado potential

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. The 5-percent risk stretches into Alabama.

The above map shows where the strongest storms of the day after looking at the morning guidance. This doesn’t necessarily mean those areas will see a tornado, but if there is going to be one, most likely it will be with the strongest storms.

So if you live inside one of those circled areas – or near one – keep an extra close eye to the sky.

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.