... Low pressure will approach the area this evening before passing through late tonight into Saturday. The low will move northeastward into New England Saturday night through Sunday and high pressure will return for the early and middle portions of next week.
NEAR TERM /THROUGH TONIGHT/... Mid afternoon surface analysis shows an area of low pressure near Peoria, Illinois, beneath an upper low tracking northeastward toward southern Lake Michigan. A frontal occlusion extends southward through the Ohio and Tennessee River Valleys to a new low developing near Pensacola, Florida. A warm/coastal front extends northeastward up the southeast coast, with cool air damming (CAD) noted in the lee of the Appalachians due to high pressure situated over the upper Saint Lawrence River Valley. Despite the wedge, the distance of surface high pressure has rendered CAD weak enough to allow temperatures over virtually all of the Mid-Atlantic to rise above freezing.
Southeasterly low-level flow between the low over Illinois and the high north of New England is banking abundant Atlantic moisture into the Mid-Atlantic, with southwest flow aloft advecting in moisture from the Gulf of Mexico. Periods of rain (mainly light through nightfall) are expected across parts of the central Virginia Piedmont and central Shenandoah Valley, enhanced in the adjacent higher terrain due to upslope flow. Further to the northeast over northern Virginia and central Maryland including the DC/Baltimore metro areas, any rain should hold off until later this evening.
Upper troughing will take on a negative tilt as it moves into the Great Lakes, and its associated surface front (occlusion) will approach from the lower Ohio River Valley tonight. The second area of low pressure will deepen as it moves over the piedmont of North Carolina this evening, and then track northeastward generally along or just east of I-95 overnight. Moisture advection will continue to increase and funnel northward along and ahead of this frontal system. Given the deep forcing and abundant moisture, a band of moderately heavy rain is expected to pivot west to east across the entire region overnight. The heaviest rain will probably last about 2 or 3 hours in any one location (generally 9PM to midnight west of I-81, midnight to 3AM between I-81 and US-15, 3AM to 6AM for the I- 95 corridor, lingering until 9AM near the Chesapeake Bay).
Rainfall rates in excess of half an inch per hour are possible, with areal average amounts of 1 to 2 inches expected. Taller, eastern facing slopes along the Blue Ridge will likely eclipse 3 inches. Flash Flood Guidance (FFG) values are lowest across portions of eastern West Virginia between the Allegheny Front and the foothills just west of I-81, and in the DC/Baltimore metros. These are the areas most likely to see any isolated or nuisance flooding issues, though widespread flooding or flash flooding are not currently expected due to the relatively progressive nature of the heaviest rain, lack of deeper convective instability and below normal rainfall over the last few weeks. Therefore, no Flood Watches have been issued.
Temperatures should hold generally steady overnight with warm air advection on southeasterly flow offsetting diurnal cooling.
Side note: some of the high resolution model guidance indicates there may be just enough dynamic cooling to result in a brief burst of snow on the back edge of the frontal precipitation band over the summits of the Allegheny Front (generally above 3500 feet). Impact wise, this seems minimal (not to mention the low probability of occurrence), but have added a small area of a chance of snow regardless after about 11PM.
SHORT TERM /SATURDAY THROUGH SUNDAY NIGHT/... Low pressure will continue to move across the region early Saturday morning as the parent H5 shortwave moves into New England. Periods of heavy rain will develop mainly after midnight Saturday before tapering off from west to east by Saturday AM. While the bulk of the rain will have moved to our east by daybreak Saturday, additional rain showers are likely to continue for the metro areas until late Saturday morning. Drying conditions will ensue Saturday afternoon as high pressure gradually works its way back into the region.
With strong westerly flow aloft, upslope snow is expected for extreme western portions of the Allegheny Front late Saturday into Sunday. Although with a lack of atmospheric moisture along with subsidence working its way back into the region, any upslope induced snow will be light in nature and be more confined to the higher elevations. Elsewhere, expect dry conditions to continue into early Monday.
LONG TERM /MONDAY THROUGH FRIDAY/... The early part of next week will be characterized by a broad area of upper-level low pressure over northern New England and eastern Canada. Several pieces of upper-level energy will be rotating around this upper-low, coming fairly close to our area.
A fairly robust piece of upper-level energy swings through late Monday, bringing the chance for some upslope snow sowers along the Allegheny Front. Otherwise, much of the area stays dry. Some rain will be possible in central/southern VA, but for now, looks to stay to our south. Meanwhile, high temperatures reach the mid 40s, so right around average. Much of the same Tuesday, but slightly cooler and drier. Upslope snow may continue, but thinking the chances are lower due to drier air in place, and less upper-level support.
Meanwhile, the southern branch of the jet stream is becoming more active during this time as well. An upper trough will be ejecting out of the southwest into the southern Plains Tuesday. This will induce low pressure along the Gulf Coast on Tuesday night into Wednesday, which will track to our south during the day on Wednesday. Additionally, a piece of shortwave energy will be diving out of the midwest on Wednesday as well, directly over the Mid- Atlantic. So, all this being said, with a marginally cold airmass in place Wednesday night, could see some rain or snow across the area. This could linger into Thursday, but would be too warm for snow at that point for most. High pressure will then briefly build into the region through Friday. Highs on Wednesday through Friday will stay in the low 40s.