Multiple thunderstorm squall lines capable of producing scattered to widespread damaging winds and a few erratically moving tornadoes will develop across eastern Texas late Friday, with storms potentially continuing into the overnight hours before strengthening over Mississippi and Alabama on Saturday.
Meanwhile, the same moisture-rich warm air mass fueling the storms will also feed a strip of heavy rainfall and potential flooding across the Mississippi Valley into Illinois, Indiana and perhaps the Northeast.
The severe thunderstorm risk
An appreciable risk for severe weather exists Friday from eastern Texas into Louisiana, Arkansas and southeastern Oklahoma. This includes the northern Houston suburbs, Shreveport and Alexandria, La., — hard hit by an EF3 tornado on Dec. 16.
An area of low pressure will develop to the northwest along the Texas-Oklahoma border near the Red River early Friday, enhancing southerly winds ahead of it and aiding the buildup of a warm and humid air mass. Cold air at the mid-levels will lend itself to an unstable atmosphere, in which pockets of air can ascend and transform into severe thunderstorms.
Meanwhile, a ripping low-level jet stream will be racing north through Mississippi at highway speed, barely a mile above the ground. Some of that momentum will mix down to the surface, with wind-driven rain possible ahead of the main storm complexes.
It’s that same river of swiftly moving air that will bolster wind shear — or the change of wind speed/direction with height. That in turn will encourage some of the storms to rotate.
The result will be discrete rotating thunderstorms or supercells, as well as thunderstorms embedded within a squall line.
The former will precede the latter. The supercell thunderstorms bubbling up ahead of the squall line will have the potential to produce at least isolated tornadoes before the main line of storms along the front arrives.
The squall line itself will likely be a QLCS — or quasi-linear convective system. That’s basically a squall line with embedded areas of spin that twist and contort the shape of the line itself.
In environments like this, characterized by moderate instability but high shear, tornadoes can develop quickly and erratically. Straight line winds exceeding 70 mph are possible in a number of locations.
That’s what much of the Interstate 20 in eastern Texas and northern Louisiana will be facing from Friday into Saturday. Storms will likely continue overnight, posing the danger of damaging winds or twisters in populated areas. By Saturday, the threat shifts east into Mississippi and Alabama.
A few storms may rumble across the Southeast on Sunday, but by then severe storms will be more the exception than the rule.
Flood concerns will also accompany the system, primarily north of the severe weather zone. In fact, the greatest risk for significant rainfall and potential flooding exists from East Texas and Arkansas into the central Mississippi Valley, eventually encompassing parts of Illinois, Indiana and the Ohio Valley before perhaps affecting the Northeast into Sunday.
Friday night and Saturday look to be the days for heaviest rainfall. A widespread two to four inches is possible with localized six-inch amounts.
Meanwhile, additional heavy rainfall is possible over Alabama and northern Georgia as the strong southern storms congeal into a clumped mass of heavy rain Saturday and Sunday.
After that, an additional round of flooding is possible as more rainy weather targets Alabama, Mississippi and the Georgia Appalachians early next week from Monday through perhaps Wednesday. That could elevate some total rainfall amounts to greater than five inches.
The sneaky wintry surprise?
On the storm’s backside, the encroaching cold front could flip some moisture to snow in northern areas. Some models have been indicating excessive amounts of freezing rain, but that’s unlikely. With high precipitation rates, it may be challenging to build the layers of glaze necessary for ice accretion. Moreover, the freezing of supercooled water droplets would likely release enough heat to offset accreting precipitation.
In Iowa, Wisconsin and northern Michigan, a plowable snowfall may occur Saturday. It’s too early to supply exact amounts, however. We’ll continue to refine our forecasts.
Eventually, accumulating snow may also fall in northern New England on Sunday.
The bottom line
This system is likely to feature a number of threats for many, whether in the form of flooding, damaging wind or even tornadoes — some at night. Anyone potentially facing these hazards ought to make sure they have a way to be notified if a warning is issued for their location, and a plan of what to do to keep out of harm’s way.
If you live in the Deep South, for example, and know that your home does not have place inside that offers safe shelter during a tornado warning, craft a plan of what to do and where to go and communicate that plan with those with whom you live.