8:00 PM Saturday, January 15, 2022
Updates only to the snow/sleet and freezing rain graphics. The timing remains unchanged (future radar below the snow and ice graphics.)
Models continue to warm the mid and upper levels, meaning more sleet and freezing rain. There is a concern along US Route 29 where the freezing rain will continue for a prolonged duration with one quarter to locally one-half inch of freezing rain. With a combination of wind gusts up to 25 mph (with higher gusts across higher elevations,) scattered to widespread power outages are expected.
Snow and sleet forecast has been nudged back due to more freezing rain. However, a general 4 to 6 inches of snow remains possible west of US Route 29 before a changeover to sleet and freezing rain. Higher amounts where it stays snow the longest. There is still the potential for up to 10 inches of snow (locally higher) in the mountains.
Precipitation Timing:
7:00 to 10:00 a.m. Sunday: Snow will overspread Southern Virginia and the Roanoke and New River Valleys. There is the potential for 1 inch per hour snowfall rates for several hours. Across Far Southwestern Virginia, mixing issues due to the mid-levels rising above freezing, with a mixture of sleet and freezing rain.
10:00 a.m. to 2:00 p.m. Sunday: Heavy snow across much of Central Virginia and the Central Shenandoah Valley. Snow will change over to sleet and freezing rain across Southern Virginia and the North Carolina Piedmont as low pressure moves across Eastern North Carolina.
2:00 to 5:00 p.m. Sunday: Very heavy snow continues across Central and Northern Virginia. Meanwhile, sleet and freezing rain continue to move northward along US Route 460. Mixed precipitation changes back over to snow across Far Southwestern Virginia. Across Metro Richmond, any mixed precipitation changes over to all rain by late afternoon or early evening. Heavy rain is possible across Hampton Roads.
5:00 to 8:00 p.m. Sunday: Low pressure moves into Southeastern Virginia. Central Virginia mixes or changes over from sleet and snow to freezing rain and plain rain. Along Interstate 95 and east, any mixed precipitation will also change over to rain. Further north and west, it remains cold enough for all snow.
8:00 to 11:00 p.m. Sunday: Low pressure moves into Central Virginia. Precipitation begins to wind down along and south of Interstate 64. To the north and west (including the Central and Northern Valley) snow and sleet will change over to a period of freezing rain.
6:00 PM January 14, 2022
Written by Meteorologist Peter Forister
Snow and ice graphics created by Denver Murray
This weekend brings another major winter weather event to the Mid-Atlantic, with the potential for snow, sleet, ice, rain, and wind. Just about everything that you can cram into a single storm system is expected with this very dynamic low pressure, and this brings incredibly difficult forecasting challenges into play. This article is a technical discussion on the storm setup and will break down the forecast details to be considered from a meteorological and geographical perspective.
Synoptic Overview:
To start, a low pressure is expected to develop in the Deep South on Saturday with a very deep trough that is currently diving in from the Pacific Northwest. The polar jet stream will “dig” south all the way to the Gulf of Mexico. This is a “typical” mid winter storm pattern, with a deep anomalous trough in the central United States, and an established high pressure over New England. The pattern is a classic setup for winter weather in this region, and has been well forecasted for over a week!
(Model graphics courtesy of TropicalTidbits.com)
The high pressure to the northeast forces an area of cold arctic air to the north to stick around Virginia in a process known as “Cold Air Damming” (CAD). Due to the clockwise flow around high pressure systems, cold air from Canada will be pushed south and hug the Appalachian Mountains. Our low pressure system will then track from the south to the north and overrun this cold air. In areas where the cold air is deep, through the entire height of the atmosphere, precipitation will fall as snow. However, CAD tends to be pretty shallow, often only keeping the arctic air in the lowest 3,000 ft of the atmosphere. Above this shallow layer, the ”mid levels” of the atmosphere (from about 900mb-700mb, or 3,000ft to 10,000ft) offer the greatest forecasting challenge.
Occlusion Forecast:
The evolution of the low pressure system on Sunday as it starts to overrun the CAD is what makes this forecast more complex. The low pressure will fully mature over the Deep South because of the strong jet stream, and undergo a process known as occlusion. This occlusion means that the storm will appear to move backwards for a few hours before a new low pressure center forms further east. Over the course of the day Sunday, this process will happen twice; once over Georgia, and again over Virginia.
(Model graphics courtesy of PolarWX.com)
The process of occlusion entrains warm and dry air into the middle of the storm system. Dry air is apparent on many models, and can be seen on the mid level relative humidity plots. From a forecast perspective, it is likely that this so-called “dry slot” will shut off precipitation somewhat earlier than models might expect, and serve to temper the overall snow totals. If the system were further southeast or less prone to occlusion, this would be less of an issue (as with what happened with the early January storm or the blockbuster 2018 winter event).
Warm Air Advection Forecast:
Despite the occlusion, the storm system will strengthen to a relatively strong 995-985mb central pressure over land. The strong high pressure sitting to the north will create a hefty pressure gradient over a short distance, and where there is a strong pressure gradient, strong winds will follow. The strongest winds in the storm system will occur in an area known as the Low-Level Jet (LLJ) which occurs at 850mb (or about 3,000ft – 6,000ft above the surface). The LLJ transports deep moisture from the Gulf of Mexico and Gulf Stream and encourages the development of widespread precipitation within the system. However, the LLJ also transports abundant warm air from as far south as the Caribbean and will warm this layer of the atmosphere by as much as 20 degrees in just a few hours. This warming is known as Warm Air Advection (WAA), and throws yet another challenge in the mix for forecasters.
Global models have been indicating that the LLJ will reach speeds of 70-80kts (80-95mph) over Virginia as the storm lifts north on Sunday afternoon. The WAA is expected to warm the mid-levels at a rate up to 10 degrees per hour, and race to the west at speeds nearing 90 mph. This means that the mid-levels of the atmosphere could go from 20 degrees F (plenty cold for snow) to 40 degrees F in just two hours. The surface is expected to remain below freezing for most areas west of I-95 for many hours due to the CAD discussed above, which means that rain falling through the very warm layer will re-freeze as sleet and ice. A noted issue with global model performance in this region is that they often under-estimate the strength of the LLJ and fail to accurately account for the timing of the mid-level warming. All of these factors will encourage more widespread “icy” conditions than many models may indicate.
Overall Forecast Implications:
Because of the strong CAD that will be in place on Sunday morning, areas along and west of I-95 will start the day with a strong thump of snow. Surface temperatures will be quite cold (the mid-20s for most) so the snow will be fluffy and accumulate quickly. This thump of snow will last for several hours for most areas, especially along and west of Route 29.
After the initial snow, the WAA process will begin to take over the storm system. Mid-level warm air will force a changeover from snow to sleet over all areas to the east of Interstate 81. Snow totals are completely dependent on how quickly this changeover to sleet occurs. Once it starts sleeting, it will most likely not change back to snow. Instead, a gradual transition from sleet to freezing rain is expected overnight, and then the dry slot takes over, completely cutting off precipitation east of the Blue Ridge.
The only areas where this changeover to sleet will not occur will be along and to the west of Interstate 81, where the LLJ may hold off long enough for snow to continue through the night. In areas where this happens higher-end snow totals are possible, but the corridor for this enhanced snowfall continues to narrow as models get closer to the actual event.
Snowfall & Sleet Forecast
We have cut back our snowfall totals due to temperatures above freezing in the mid-levels of the atmosphere. The precipitation begins as snow, then as it travels through the middle column with temperatures above freezing, the snow melts into the rain, then refreezes before reaching the surface. With low-pressure moving across Eastern North Carolina and Virginia, warmer air from the Atlantic would work into the region, allowing for snow to mix or changeover to sleet.
Freezing Rain Forecast
There is a concern for significant icing along and east of the Blue Ridge. Some models suggest even the mid and upper levels rise above freezing with the surface temperature remaining below freezing for several hours Sunday into Sunday evening. Our forecast is in the middle of the European and NAM (North American) models. Sleet and freezing rain on top of snow would cause significant issues on roads, bridges, and power outages.
Gusty Winds: The strongest winds will occur once low-pressure moves along Interstate 95 Sunday evening into Sunday night. Wind gusts up to 45 mph along the coast will be possible. Wind gusts 20 to 30 mph will be likely inland. This is enough wind to cause power outage issues on top of heavy snow and freezing rain. There is a MODERATE TO HIGH POTENTIAL FOR POWER OUTAGES BETWEEN U.S. ROUTE 29 AND INTERSTATE 81. There is also the threat of coastal flooding along the Chesapeake Bay.
“Bust” Scenario:
Snow totals could significantly underperform their current forecasts if a few conditions verify. 1). If the precipitation shield ahead of the system is smaller than forecast. This would hold off the snow until the afternoon on Sunday, and limit the amount of time that snow would have to accumulate before the changeover to a rainy mix. 2). The WAA is much stronger than forecasted, which would lead to a near-immediate changeover to mixed precipitation. 3). The dry slot is stronger than forecast, which would cut off overall precipitation earlier than expected (this is of especially high concern along I-81).
“Boom” Scenario:
Snow totals could overperform in a few areas if a few conditions verify: 1). If precipitation arrives earlier than expected, or a deformation band forms in the initial thump of precipitation, there will be a chance for very rapid snowfall accumulation rates. The changeover to sleet would still occur, but more snow would already be on the ground beforehand. 2). In western areas, especially along I-81, the potential exists for no changeover to sleet at all and a reduced dry slot. If both of these can occur on Sunday evening, snow totals could significantly exceed forecasts due to 12+ hours of constant heavy snowfall rates.
Precipitation Timing:
7:00 to 10:00 a.m. Sunday: Snow will overspread Southern Virginia and the Roanoke and New River Valleys. There is the potential for 1 inch per hour snowfall rates for several hours. Across Far Southwestern Virginia, mixing issues due to the mid-levels rising above freezing, with a mixture of sleet and freezing rain.
10:00 a.m. to 2:00 p.m. Sunday: Heavy snow across much of Central Virginia and the Central Shenandoah Valley. Snow will change over to sleet and freezing rain across Southern Virginia and the North Carolina Piedmont as low pressure moves across Eastern North Carolina.
2:00 to 5:00 p.m. Sunday: Very heavy snow continues across Central and Northern Virginia. Meanwhile, sleet and freezing rain continue to move northward along US Route 460. Mixed precipitation changes back over to snow across Far Southwestern Virginia. Across Metro Richmond, any mixed precipitation changes over to all rain by late afternoon or early evening. Heavy rain is possible across Hampton Roads.
5:00 to 8:00 p.m. Sunday: Low pressure moves into Southeastern Virginia. Central Virginia mixes or changes over from sleet and snow to freezing rain and plain rain. Along Interstate 95 and east, any mixed precipitation will also change over to rain. Further north and west, it remains cold enough for all snow.
8:00 to 11:00 p.m. Sunday: Low pressure moves into Central Virginia. Precipitation begins to wind down along and south of Interstate 64. To the north and west (including the Central and Northern Valley) snow and sleet will change over to a period of freezing rain.
