The winter storm everyone is talking about is still on track and the expected impact on the region remains unchanged. This is not likely to be one of those blockbuster storms but the combination of snow, sleet, freezing rain and then rain, in that sequence, will disrupt highway travel for the next 24 hours. The bulk of the Tuesday precipitation will be in the form of snow in the 1-3 inch range. The most disruptive part of the storm will likely be the overnight transition to sleet and freezing rain.
The morning satellite image shows the main storm is still developing in the Plains states but a disturbance running out ahead of the storm is generating precipitation in the east. In Maryland temperatures are cold enough that snow is the result.
By evening, storm development is shifting into Tennessee and Kentucky with a warm front reaching into the Carolinas. The Mid Atlantic region is still trapped in cold air at the surface at this time. The purple dashed line reaching from southern Maryland into northern West Virginia depicts the rain/snow dividing line. The situation isn’t that clear cut though. Warmer air (above freezing) several thousand feet up in the atmosphere will begin pushing northward while subfreezing temperatures remain locked in at the surface. This means that an area around and south of the rain/snow line will see a wintry mix of sleet and then freezing rain as precipitation falls into a shallow layer of cold air at the surface. This south to north transition will be in progress during the overnight hours.
The chart above depicts the areas west of the Bay as having a 40% chance of receiving at least 4 inches of snow. This indicates that computer models are pointing toward a snow accumulation roughly in the 1-3 inch range.
The map above shows the Winter Storm Warning areas for the most significant snow/sleet/ice accumulations in dark blue. The light blue shaded counties are under a Winter Weather Advisory for lesser accumulations.
By Wednesday morning the storm center will be shifting toward the upper Ohio River valley. Cold air at the surface remain locked in over Maryland with the warm front hung up to the south. The rain/snow line has moved into Pennsylvania but with continued cold temperatures around freezing at the surface, freezing rain is likely to persist during the morning, eventually changing over to rain from south to north. How quickly this will occur is the uncertain factor in the forecast.
The chart above indicates that much of central and northern Maryland has a 40% probability of receiving at least a .25 inch accumulation of ice in freezing rain and this is why the Winter Storm Warning is in effect. Areas north, northeast and northwest of Baltimore will be the slowest to warm above freezing and thus stand the best chance to receive the heaviest ice accumulations in this storm.
Additional information added at 12:00 noon, 1/27/09
I have mentioned that the difference in temperatures a few thousand feet up in the atmosphere and the surface will be critical as to the type of precipitation we receive. At 7am this morning the surface temperature at BWI-Marshall was 28 degrees and at 5,000 feet it was roughly 20 degrees (all Fahrenheit). All readings below freezing so snow was in the air. Forecast models indicate that by 1:00am Wednesday temperatures at 5,000 feet will have risen to around 32 F over Baltimore but remain several degrees below freezing at the surface. This is a recipe for sleet from Baltimore southward.
By 7:00am Wednesday, computer models forecast that the temperature at 5,000 feet over Baltimore will rise above the freezing point but that temperatures at the surface will be at or just below 32 F. This is a recipe for sleet or freezing rain, depending on the vertical depth of the freezing temperatures. If the freezing air mass is fairly deep, rain drops will freeze into sleet before hitting the ground. If the freezing air mass is fairly shallow, say under 1,000 feet, rain drops will hit the ground as a liquid but freeze on contact with cold surfaces.
The map above shows one computer’s forecast for surface temperatures at 7:00am Wednesday. The darker shaded blue areas show the cold air locked in place over Maryland and northern Virginia. This is called “cold air damming” and frequently occurs in winter storms moving through this area. The lightest blue shading on the map indicates temperatures at roughly 32 F. The green, yellow and orange shadings show temperatures above freezing. Notice that the mountains in West Virginia are warmer. The surface elevation is high enough that it is in the warmer air. The forecast for the stubborn pocket of cold air to still be in the area around the Bay Wednesday morning is the reason icy conditions are expected.
Check in frequently with WBAL TV-11, wbaltv.com and Insta-Weather Plus on 11-2, Comcast Digital 208 and Verizon FiOS 460 for updates.