Nasty Ice Storm to Unfold Across Mid-South, Southeast, and Mid-Atlantic

A strong shortwave approaching the Southern Plains has already resulted in a precipitation shield developing across much of Oklahoma and upper Texas. As the wave treks across the two states overnight, precipitation will fill in across the eastern half of Texas tonight before doing the same across Arkansas, Missouri, and Louisiana overnight into Wednesday morning. The Arctic air mass remains well entrenched over much of the Southern Plains and Mid-South. Surface high pressure over the Ohio Valley will begin departing eastward of Wednesday, but with recent snow/sleet cover, plenty of cold air will remain in place to help pull off another powerful winter storm across recently impacted areas.

In this article, I will focus primarily on the ice storm threat with the coming system. A mixture of freezing rain and sleet will likely lead to scattered to widespread power outages across parts of eastern Texas, lower Arkansas, Louisiana, and Mississippi on Wednesday/Wednesday night and across parts of North Carolina and the Virginias on Thursday.

Southern Plains & Mid-South Ice Storm Impacts and Timeline

Temperatures will continue to plummet overnight tonight across the Plains and Mid-South. Widespread 20s already overspread the region of interest, and many of those areas will drop into the teens overnight. Precipitation will reach Arkansas, far northern Louisiana, and northwestern Mississippi early enough Wednesday to fall mainly as snow at first. An associated surface low will skirt along the southeast Texas and then across the Louisiana coast on Wednesday. This low will transport warmer air overtop colder air at the surface. This warm nose will lead to a nasty freezing rain moving across eastern Texas, the northern two-thirds of Louisiana, lower Arkansas, and western/central/northern parts of Mississippi.

Projected temperatures by overnight early Wednesday morning
Projected temperatures by overnight early Wednesday morning

Due to strong, low-level warm air advection, the latest NAM guidance results in freezing rain/sleet transitioning to a cold rain more quickly across much of Louisiana and Mississippi later on Wednesday. Warm advection will partially be offset by melting on Wednesday, since the ice-to-liquid phase change is a cooling process. Plus, dry air in the low levels will result in evaporational cooling keeping colder air in place for longer. For these reasons, I favor the colder HRRR model guidance, which has an ice storm impacting areas farther to the south. The latest HREF ice accretion projections for Wednesday and Wednesday night provides some idea as to where the heaviest freezing rain accumulations will occur. A large swath of 0.5-1+ inch freezing rain accumulations could fall, resulting in widespread power outages. Due to the self-limiting nature of ice storms, freezing rain accumulations these high can be challenging to reach. However, surfaces already well below freezing will allow accumulations to occur immediately. To provide some perspective, amounts as little as 0.05-0.1 inches can cause major issues on roads, especially on bridges and overpasses.

HRRR Ice Accumulation Projections over the next 48 Hours
HRRR Ice Accumulation Projections over the next 48 Hours
HREF Ensemble Projections from Tuesday evening into Wednesday evening
HREF Ensemble Projections from Tuesday evening into Wednesday evening

Carolina and Mid-Atlantic Ice Storm Impacts and Timeline

The surface high across the Ohio Valley on Wednesday will shift into New England late Wednesday into Thursday. Surface ridging will build down the east side of the Appalachians, setting up a cold air damming setup. As mid-level ridging along the East Coast amplifies on Thursday, the surface low along the Gulf coast will take on a northeastward trajectory. As the feature moves across central Georgia and South Carolina during the day, rain will overspread much of the Southeast. However, enough cold air at the surface will allow a major ice storm to unfold across western/central/northern parts of North Carolina, much of Virginia, southeastern West Virginia, and into the Delmarva peninsula.

Strong warm air advection in the low levels will eventually result in a transition from sleet and/or freezing rain to a cold rain across upper South Carolina and lower parts of North Carolina. The timing on this transition will determine ice totals in that region. The latest HRRR guidance doesn’t depict any significant totals since it has sleet transitioning directly to rain, but HREF ensembles indicate a more prolonged period of freezing rain down to the North Carolina/South Carolina border. Ice totals in northern Virginia into the Delmarva peninsula should receive lower ice totals relative to areas to the south due to more snow in the beginning. For areas in between, the ice storm could bring totals reaching or even exceeding 0.25-0.5 inches.

HRRR Ice Accumulation Projections over the next 48 Hours
HRRR Ice Accumulation Projections over the next 48 Hours
HREF Ensemble Projections from Wednesday morning into Thursday morning
HREF Ensemble Projections from Wednesday morning into Thursday morning

A Plethora of Winter Storm Potentials to Monitor

Ice storm potential next week

Synopsis

A longwave trough with quite a broad base will remain almost stationary across the northern half of the U.S. this upcoming week into at least next week. A ridge will persist just off the West Coast, while a block sits over western Greenland. Another ridge will remain positioned over the southeastern quadrant of the U.S. This ridge will initially keep temperatures well above average across the Southeast; however, northern troughing will prohibit the ridge from amplifying unabated. A baroclinic zone will become established across the Southern Plains, Mid-South, Missouri Valley, Tennessee Valley, Southeast, Ohio Valley, and Mid-Atlantic. This setup will bring several opportunities for snow, ice, and rain across the mentioned regions over the next two weeks.

We discussed around a week ago how longwave troughing can suppress the storm track too far southward to bring any meaningful wintry precipitation. Instead, conditions are generally very dry and cold. If you recall, model guidance had a lobe of the tropospheric polar vortex digging as far south as the Tennessee Valley for early this week. Instead, we got a flatter trough with some Southeast ridging. In most guidance now, we’re stuck with a long-lasting broad-based trough that likely won’t keep the southern stream storm track suppressed.

Temperature outlook february 14-18, 2021
Probability of above/below average temperatures over February 14-18, 2021

Cross polar flow extending from Siberia over into western Canada has allowed Arctic air to pool over western Canada. That brutally cold airmass has already begun spilling into the upper Plains and Midwest. But essentially, we now have the available cold air to tap as numerous storm systems parade from the Southwest/Southern Plains in an east or northeastward direction. One major drawback to the expected pattern configuration is that ice (sleet/freezing rain) could become the more predominant frozen precipitation-type across the Southern Plains, Mid-South, Southeast, and even into the lower Ohio Valley and Mid-Atlantic. With broad troughing centered across the central U.S. and a Southeast ridge to the east, southwesterly flow will transport warm air above the surface. However, Canadian high pressure will wrap around very cold air at the surface. This will produce atmospheric profiles that support sleet/freezing rain versus snow.  

Systems We’re Currently Watching

We have a slew of systems we’re currently watching that will bring impacts in the foreseeable future. We’re going to post articles and social media updates on each system individually, but we will introduce those threats here.

February 10-12, 2021 (Wed.-Fri.): Shortwaves embedded in mostly westerly flow will bring widespread precipitation across the Southern Plains, Mid-South, Missouri Valley, Southeast, Tennessee, along and south of the Ohio River, and the Mid-Atlantic. Canadian high pressure has continued to advect cold air at the surface across northern portions of where precipitation will develop. We expect a prolonged period of sleet/freezing rain to fall across northern Texas, central/eastern Oklahoma, northern/central Arkansas, lower Missouri, western Tennessee, upper Mississippi, and Kentucky. North of the Ohio River and areas across much of the Mid-Atlantic will experience mostly snow, although lower and central parts of the Virginias may get a mixture of snow/ice. For areas south, expect rain.

February 13-14, 2021 (Weekend): Forecast model guidance indicates HIGH uncertainty for this potential event. The outcome of this potential winter storm depends on the interaction of three features: a shortwave entering lower California late week, a shortwave entering the Pacific Northwest around the same time, and a lobe of the tropospheric polar vortex spinning over the northern Plains/Midwest. The European model continues to indicate that the California wave gets suppressed so far south that it passes across the Gulf of Mexico. This feature would bring rainy conditions to Florida and areas along and relatively close to the Gulf coast over much of the weekend. On the other hand, the GFS often has the California wave phasing with the Pacific Northwest wave somewhere over the central U.S. This scenario would result in the phased systems eventually taking on a northeastward trajectory. This scenario would potentially bring a significant winter storm to the central/southern Plains, the Mid-South, the Missouri Valley, the lower Midwest, the Ohio Valley, the lower Great Lakes, and Northeast over the weekend. I will admit that this is a tough forecast, and at the moment, I need additional time to study this event.

February 15-17, 2021 (Mon.-Wed.): A strong shortwave will enter the western U.S. later in the weekend and dig southeastward into the Four Corners region. As the wave continues eastward, the tropospheric polar vortex lobe will move eastward across the Great Lakes and Northeast. These two features will create a region of confluent flow across the Ohio Valley and Mid-Atlantic/New England, which will support Canadian high pressure moving eastward into the Northeast. This high will likely result in cold air damming as far south as the Carolinas and Georgia. A surface low will likely develop along the Gulf coast in response to the shortwave approaching the region. Cold air will already be in place across the Southern Plains, Mid-South, and much of Tennessee to support wintry precipitation. With cold air damming in place, the Carolinas, northern Georgia, and northeastern Alabama need to closely monitor the potential for an ice storm next week. This system could potentially bring an impactful winter storms to parts of the Ohio Valley, Mid-Atlantic, and (maybe) the Northeast. We have about a week to get into specifics. I advise against making any changes to current plans until confidence increases over the next two to three days.