550 FXUS61 KLWX 020903 AFDLWX Area Forecast Discussion National Weather Service Baltimore MD/Washington DC 403 AM EST Wed Dec 2 2020 .SYNOPSIS... Low pressure will lift further away across southeastern Canada as high pressure builds in from the Midwest through Thursday. The high will shift offshore late Thursday into Friday as low pressure develops over the Tennessee River Valley and heads in the general direction of Virginia. This low should shift offshore by late Saturday, then a clipper-like system may affect the region early next week. && .NEAR TERM /THROUGH TONIGHT/... The early week expansive upper low has finally begun to exit the region with the circulation center near Lake Ontario. Embedded shortwaves continue to spiral around in a counterclockwise fashion. The accompanying mountain showers have finally begun to wind down with radar echoes staying up in western/central Pennsylvania. As of 345 AM, some light radar returns are evident across north-central Maryland which may foster snow flurry development. Camp David, MD (KRSP), which sits around 1,800 feet, is currently reporting light snow accompanied by some reduction in visibility. Any remnant snow showers should wind down during the early morning hours as forcing from the upper trough lifts away from the Mid-Atlantic. Unlike the previous day, abundant sunshine is expected for much of Wednesday. Temperatures aloft will continue to rise as the dome of cold air retreats into the higher latitudes. In spite of the pronouced warming within the upper levels, boundary layer mixing is not particularly deep in the cool season. As such, expect high temperatures to only be about 3 to 6 degrees warmer than Tuesday. This will bring temperatures into the mid 40s to low 50s from along I-81 and east of the Blue Ridge. 30s will be more commonplace in the mountains, locally lower where snow cover exists. With brisk winds in the forecast another day, it should feel a bit colder than the thermometer reads. Westerly wind gusts through the afternoon hours may reach 20 to 30 mph at times. Residual wind heading into the overnight should mitigate attempts at any decoupling processes. Low temperatures will be at, or below freezing in all spots aside from right along the Potomac River and Chesapeake Bay. && .SHORT TERM /THURSDAY THROUGH FRIDAY NIGHT/... As the upper low lifts farther into Quebec, a quick moving shortwave ridge crosses the Mid-Atlantic region on Thursday. This will encourage a more pronounced increase in temperatures with more of a southerly component to the wind vectors. Low 50s are likely with perhaps even a few 55 degree readings given appreciable sunshine. Clouds will be on the increase for the second half of the day. This is in response to both a northern stream clipper crossing the Great Lakes, and a deepening trough over the Ozarks. The latter will bear watching given impacts late in the work week and continuing into the weekend. Until then, dry conditions should persist into Thursday night with milder temperatures given the increasing clouds. Despite the limited sunshine, Friday could prove to be a bit milder than Thursday with the steady south to southwesterly flow. However, this warm advection regime may also introduce some rain into the forecast ahead of the advancing upper low. The global guidance remains adamant in bringing the low from the Ozarks toward the Central Appalachians by late Friday. Timing issues are evident but it seems like rainfall will push into the region late Friday and into the overnight hours. Some light snow accumulations are possible over the mountains if enough cold air is in place. && .LONG TERM /SATURDAY THROUGH TUESDAY/... A highly amplified upper-level pattern is forecast to continue at least through the weekend, and possibly well into next week. A very strong ridge is progged to be positioned over the Rocky Mountains roughly along 120 W longitude to start the weekend. Downstream, an upper-level low moving out of the Tennessee River Valley will be shifting eastward toward the Middle Atlantic coast, while leaving a piece behind trailing across Texas into northern Mexico. This latter piece will set up a rex block over the western United States into early next week (where an upper-level low undercuts a ridge). Lastly, ridging over the North Atlantic Ocean will begin to weaken as it is bombarded by leftovers from the area of low pressure currently lifting away from our region as of early Wednesday. An area of low pressure at the surface will accompany the system pulling out of the Tennessee River Valley to start the weekend. Model guidance has come into better agreement regarding a deeper evolution of the surface low due to better phasing/interaction between the northern and southern branches of the jet stream, but the exact track and strength details still need to be ironed out. Given the cutoff nature of the low and an open feed of moisture from the Gulf of Mexico, another soaking rain seems plausible near the low track, with snow perhaps on its far northwestern periphery over the higher terrain. Despite the better agreement, there is still a wide array of possible outcomes in terms of exact precipitation amounts. But, given the trends, flooding may again be a threat on Saturday. Accumulating snow is also possible over the higher terrain. Once this system departs, brisk conditions are likely to close out the weekend in strong northwesterly flow behind the low. That same northwesterly flow will likely bring a clipper-like system across the region early next week. The column looks cold enough for all snow, but moisture will probably be lacking. Even so, a few flurries or snow showers can't be ruled out even for the lower elevations. It appears as if the upper-level pattern may start to retrograde a bit by the middle of next week, casting further uncertainty as pattern shifts often offer very changeable weather patterns. && .AVIATION /08Z WEDNESDAY THROUGH SUNDAY/... VFR conditions are expected at all terminals through at least Friday afternoon. Brisk westerly winds with gusts up to 25 knots are possible this afternoon before diminishing in strength by the evening hours. Winds turn more south to southwesterly on Thursday into Friday, while lighter in nature. Rain returns to the region by Friday evening and into the overnight which would support the potential for sub-VFR flight rules. Uncertainty exists with the timing of this next upper low. Sub-VFR likely Sat in RA. E flow becomes NW and could gust 20-25 kts. VFR likely Sun-Mon with continued gusty NW flow. && .MARINE... Cold advection from the exiting upper low continues to keep the waters well mixed. Westerly wind gusts of 20 to 25 knots are possible at times over all waters. A Small Craft Advisory is in place through early this evening. There is some potential for such conditions to persist into tonight, particularly over the Chesapeake Bay and lower Tidal Potomac. A brief period of high pressure will keep winds below SCA criteria on Thursday with the winds turning more south to southwesterly. Such winds do pick up in strength later on Friday which is worth monitoring. Low pressure will likely track near or just south of the region Saturday. Depending on its track and strength, E/SE flow ahead of it could gust 20-25 kt. There is a higher level of certainty in gusty NW winds Saturday night through Sunday, possibly lingering into Monday as the low pulls away. && .LWX WATCHES/WARNINGS/ADVISORIES... DC...None. MD...None. VA...None. WV...None. MARINE...Small Craft Advisory until 6 PM EST this evening for ANZ530>534-536>543. Small Craft Advisory from 7 AM this morning to 6 PM EST this evening for ANZ535. && $$ SYNOPSIS...DHOF NEAR TERM...BRO SHORT TERM...BRO LONG TERM...DHOF AVIATION...BRO/DHOF MARINE...BRO/DHOF