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.

Is the Southeast really going to get an ice storm in a week?

I want to take a moment to address the chatter about the possibility of an ice storm across parts of the Southeast next week. I’ll discuss how you should interpret model guidance, and why you shouldn’t take individual model runs verbatim. However, I’ll show you why the upcoming pattern favors wintry weather. But remember, providing specifics 7 to 10 days away from a potential event is beyond the capability of this science. Focus on the pattern instead!

I’ll give you a recent example of what not to do. Several of our followers have asked us about a post that has gone viral this evening. You always want to avoid focusing on individual model output a week in advance. If Atlanta received 1.5 inches of ice, that’d be catastrophic, but there are a lot of intricate details that could change this forecast many, many times between now and then. That’s why no one should ever make a post like this. It’s pseudo-science. I hope you view this more as a learning opportunity for our followers instead of a dig at someone’s credibility.

We’ve discussed on Firsthand Weather over these last few days (especially in our supporter group) why the pattern in a week or so could actually be promising for wintry precipitation across parts of the South. Of course, that’s assuming everything evolves as we’re expecting. We also noted that we had concerns about icing issues, which hasn’t changed.

You may have heard us use the term ‘split-flow pattern.’ Sometimes, a ridge will build across the Pacific Northwest, which can also extend into British Columbia and Alaska. This type of pattern often allows colder, higher-latitude air to spill into the U.S. east of the Rockies. Of course, the amplitude of the ridge and its orientation can affect how far south the colder airmass makes it. That’s only one component of the pattern. The other component is an active sub-tropical jet stream that sets up to the south of the western ridge, which provides a pathway for storm systems to trek across the southern states (sometimes from the West Coast to the East Coast). If you check out projected winds at the jet stream level in the latest GFS, notice how the Pacific jet stream splits before reaching the West Coast. Again, the northern component of that split help drives colder air into lower latitudes, while the southern component of the split helps transport moisture across southern regions.

I want our followers to look at all of this from a probabilistic standpoint. A split-flow pattern simply increases the odds of wintry weather occurring across parts of the South. Remember in the first Dumb and Dumber movie when Mary Swanson told Lloyd Christmas that there was a one in million chance that she’d go out with him, he was ecstatic. There was a chance!! At this point, there’s a chance, and for some of you, the odds are definitely better than one in a million.

In the coming days, we’ll need to begin identifying any trends in the model guidance, which is the second step. The first step is establishing how the pattern might evolve, which tells us what to potentially look for in future model runs. We’ll soon begin pinpointing regions that will be at risk for wintry weather, and if it becomes apparent that an event will unfold, only then will we begin forecasting amounts. Regions that run along, east and even south of the Appalachians should especially pay attention to any trends. If surface high pressure does trek eastward into New England and a surface low does move across the Southeast, it’s not unreasonable to suggest that an icing event could occur as far south as parts of the Carolinas and even northeast Georgia. There still remains uncertainty about the availability of sufficiently cold air elsewhere across the Southeast (e.g. the northern Gulf coast states). However, this upcoming pattern does provide better odds for wintry precipitation across parts of the South, which hasn’t been the case for most Decembers over the last decade. But don’t forget, focus on the pattern and trends, and don’t expect specific details this soon. We promise we’ll keep you in the loop on Firsthand Weather.

Make sure you join our supporter group on Facebook if you haven’t already. Click here to join. I post details there (e.g. my recent post made on Tuesday evening) that I’m currently not comfortable sharing publicly. You often see a finished project on Firsthand Weather. In the supporter group, you get to see more of the process. Please consider joining!