Some thunderstorms may eventually sneak into parts of the Mid-Atlantic states Monday.
The impending severe weather comes at arguably one of the worst possible times, falling on a weekend and major holiday amid the novel coronavirus pandemic that has prompted some states to shutter their community storm shelters.
There’s also the risk of flooding for some in the mid Mississippi and Tennessee Valleys, potentially extending toward the Eastern Seaboard next week. Heavy, persistent rain to the north of severe thunderstorms could fall on already saturated soils from spring rains across the Deep South.
The upper-level system that will spark the potentially violent storms was approaching the West Coast on Thursday, hovering off California. That upper-level disturbance will move across the Baja Peninsula and Mexico on Friday, while at the same time, a surface low will dive southeast across the Intermountain West and Four Corners region.
These weather features will meet up with and intensify the surface low as it emerges into the Southern Plains by Saturday. That’s when trouble begins to brew.
Ahead of the growing storm system, a screaming southerly low-level jet stream will draw north a plume of warm, humid air from the Gulf of Mexico. Temperatures in the 60s and 70s will combine with dew points in the lower 60s to juice up the atmosphere. The only thing needed to touch off severe thunderstorms is a trigger, such as a cold front.
That triggering mechanism comes in the form of a combined cold front and dry line. A dry line marks the boundary between dry, parched air — usually from the Desert Southwest — and moisture-rich air that frequently originates from the Gulf of Mexico. The approaching low will drag a dry line out of New Mexico and across Texas on Saturday, whipping up a round of severe thunderstorms as it does so.
Meanwhile, cold, dry air filtering down from the northwest behind the low will eventually surge southeastward and reinforce that clash between the air masses. This will become especially important on Sunday.
Saturday: Texas and Oklahoma are main targets
On Saturday, Texas cities like Dallas, Fort Worth, Austin and Houston will be in the main threat in the region.
Tornadoes don’t look like the main storm-related hazard on Saturday. Instead, damaging straight-line winds and large hail look to be the main threats.
Sunday: Danger along the Gulf Coast
On Sunday, the atmosphere becomes more conducive to producing a wide array of severe storm hazards. An influx of warmth and moisture over Louisiana, Mississippi, Alabama and western Georgia will help lead to a concerning atmospheric setup.
Simultaneously, a mid-level jet stream will swing through with strong west-southwesterly winds overhead. The contrast between southerly or south-southwesterly winds at the surface and more westerly winds aloft will enhance the amount of wind shear in the atmosphere. Wind shear describes a change of wind speed and/or direction with height.
In environments characterized by strong wind shear, any storms that tower high enough into the sky will be subjected to a twisting force, which favors rotating thunderstorms with a tornado risk.
It’s likely that a number of rotating supercell thunderstorms will develop across Mississippi and Alabama on Sunday. Such storms can produce damaging winds, large hail and tornadoes.
On Thursday, the Storm Prediction Center took the unusual step of highlighting the potential for “long-track supercells and a corresponding strong tornado risk” four days ahead of time, due to the consistent signals that conditions will be ripe for supercells.
That risk is contingent on storms forming in a manner that maximizes tornado chances, such as discrete supercells rather than a single squall line.
Any squall line more closely linked to the front will also produce at least scattered strong to locally damaging winds, and a few quick-hitting tornadoes are possible.
Flooding is also possible in the cooler air north of the severe weather risk area, including in portions of northern Mississippi, Alabama and Tennessee. These areas have already seen unusually heavy precipitation so far this year.
Storms may make it as far as central or eastern Georgia late on Sunday
Monday: Carolinas, Southeast at risk
Monday is a trickier day to predict, as the outcome depends on the timing of specific weather features, such as the main cold front. If the front were to swoosh through earlier in the day, it would spell an end to any severe weather risk in the Southeast. However, a slightly delayed frontal passage — as currently anticipated — would favor an opportunity for the development of more storms.
Damaging winds, hail, and a few tornadoes are possible in the eastern Carolinas, eastern Georgia and possibly into parts of northern Florida on Monday.
It’s not out of the question that portions of the southern Mid-Atlantic manage a bit of severe weather on Monday depending on how far north the narrow plume of Gulf air is able to travel. Even if no severe thunderstorms occur, places like Washington and Baltimore could end up dealing with heavy rainfall and localized flooding.
It’s vital that those in the risk areas have a severe weather plan in place this weekend. It’s a good idea to have multiple ways to be notified of adverse or dangerous weather, and to know where to seek shelter if need be.
Officials in Alabama have urged residents without private storm cellars to check ahead of time to ensure their desired community shelter is open. Some communities have chosen to close the doors to their storm shelters in response to social distancing concerns stemming from the threat of covid-19.
Some National Weather Service offices have even begun mentioning the location of hospitals and outside covid-19 testing locations in severe weather warnings when they are likely to be impacted by dangerous weather.
Statewide stay-at-home orders are up in Texas, Louisiana and Alabama. A shelter-in-place order imposes similar rules in Georgia.
Mississippi has a softer “healthy-at-home” order. Restaurants are still open for drive-through, for instance.
No statewide orders are in effect in Oklahoma or Arkansas, but a number of communities are enforcing their own regulations.
The dwindled populations in public buildings, including shopping malls or schools, for instance, could potentially be helpful in a severe weather context.
When a destructive tornado raged through Jonesboro, Ark., in late March, no lives were lost — partially owing to the emptiness of the businesses that were struck.