... High pressure will move offshore to the north through tonight as low pressure near the ArkLaTex region lifts into middle Tennessee. This area of low pressure is expected to re-develop over eastern North Carolina and southeastern Virginia by Saturday morning before moving out to sea. Another wave of low pressure will follow taking a bit further north track across the region Sunday. High pressure will build from the Northeast toward the Middle Atlantic Monday into Tuesday. Unsettled weather may return during the middle of next week.
NEAR TERM /THROUGH SATURDAY/...
The area of clear skies has dropped down into central Virginia this morning. I still expect this clearing to be short-lived as cloud cover increases from the south due to an approaching low pressure system. I have kept snow totals mainly the same with the morning update with a light trend upwards along the ridge tops. No headline changes at this time other than a possible Small Craft Advisory for our lower Chesapeake Bay zones for Saturday morning.
.Previous Discussion 08Z surface observations depict a sprawling 1028-mb ridge extending from the Great Lakes into the Mid-Atlantic region. Relatively calm to weak north/northwesterly flow is being observed early this morning with a mid-level cloud deck streaming from west to east. This uptick in cloud cover has kept most locations at/or above freezing aside from locations along the Allegheny Front. While some of the guidance clears the region out this morning, that appears less certain given the coverage of clouds moving in from the west. Regardless, increasing high clouds are expected by this afternoon ahead of a relatively flat wave emerging out of the Ozarks. High temperatures are to max out in the mid/upper 40s which is on the lower end of the guidance. Stuck a bit closer to the 00Z NAM given its better depiction of a cold air damming setup.
Warm advection precipitation arrives by as early as the late afternoon across central Virginia back into the Potomac Highlands. Some of this will commence as snow, especially at higher elevations which currently have a snowpack where temperatures will be locally cooler. Enhanced low-level frontogenesis coupled with southeasterly upslope flow will augment amounts into the night. 2 to 4 inches of snow are possible across the Virginia Blue Ridge. The southeasterly flow will also advect milder air aloft bringing freezing rain into the picture, especially after midnight across elevations above 2,000 feet. Any elevation below will likely see temperatures rise above freezing leading to a cold rain during the overnight hours. A Winter Weather Advisory is currently in effect from the Blue Ridge westward to the Potomac Highlands, including I-81. By after sunrise, all precipitation should turn to rain across the region. The back edge of the shield of rainfall will exit by the late morning/early afternoon with some residual showers possible from I-81 westward into the evening. A mild day is expected with high temperatures topping out in the mid/upper 50s on Saturday.
SHORT TERM /SATURDAY NIGHT THROUGH SUNDAY NIGHT/... After a brief lull in the action, the next wave of precipitation takes aim at the Mid-Atlantic late Saturday night into Sunday. With the initial system exiting into the Atlantic, the flow aloft flattens to more west-northwesterly in nature. A series of perturbations in the flow are forecast to race eastward, each being an impetus toward organized rainfall. Per the 06Z/00Z NAM, precipitable water values spike to above an inch. It is difficult to discern whether the attendant west-east oriented frontal zone will make it up to the I-66/Highway 50 span. With light to moderate rainfall expected much of the day, rain-cooled air north of the boundary may thwart any such attempts. Total rainfall amounts are likely to be in the 0.50 to 1.25 inch range, locally higher in the terrain. Given the potential for rapid snow melt and rainfall earlier in the weekend, a non-zero flood threat exists for these locations. The NOHRSC snow water equivalent graphic shows 2 to 3 inches of liquid sitting in some of the snow pack over eastern West Virginia.
Overall temperatures on Sunday are somewhat uncertain with ensembles showing anywhere from the low 50s to mid 60s around DC. Believe the lower numbers are more likely as the frontal zone stays south of the region. Precipitation continues into Sunday night before winding down into the following morning. Overnight temperatures will sit above average given the mild air mass in place.
LONG TERM /MONDAY THROUGH THURSDAY/... Guidance is in good to excellent agreement at the beginning of the medium range period. Broad ridging is forecast over the interior western CONUS with subtropical ridging stretching across the Gulf of Mexico to the Bahamas. An upper-level low is forecast to be caught between the two ridges pressing eastward out of the Four Corners region, while a deep upper-level trough pivots out of Canada across the Great Lakes and into the Northeast. Without a pronounced blocking ridge over Greenland, and subtropical ridging to the south, the cold trough approaching from Canada should have only short residence time over the Northeast U.S. early next week.
At the surface, a cold front demarcating the leading edge of a colder airmass is expected to have largely cleared the area by Monday morning, though with a trailing upper-level jet, and ridging to the south, would not be surprised if the front slowed a bit causing precipitation to linger at least during the morning hours.
A transient shot of cooler temperatures probably won't fully encroach on the local area until Monday night into Tuesday. At this point, it does not appear that temperatures will stray too far below normal. Still, there may be enough cold air in place especially across the higher terrain as low pressure approaches Tuesday night into Wednesday to result in a wintry mix. This low pressure will be the result of the aforementioned cutoff low over the Four Corners region. Guidance notoriously struggles with cutoff lows even in the short range, but given the synoptic pattern of ridging to the south and surface high pressure dropping into the northwestern Atlantic Ocean ahead of it, believe a surface low track should be far enough north to bring precipitation to the area (non-GFS solution favored).
As is to be expected at increasing time ranges, guidance diverges toward the end of the forecast period. The EC guidance strongly favors another deeper trough diving into the Northeast U.S. by the end of next week, while the GEFS is much more subdued. The GEFS (Canadian) guidance is somewhere in between. Overall, a consensus favors a cooling trend with perhaps another wave or two bringing modest precipitation chances late next week.