Area Forecast Discussion
Issued by NWS Sterling, VA

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478 FXUS61 KLWX 200755 AFDLWX Area Forecast Discussion National Weather Service Baltimore MD/Washington DC 355 AM EDT Sat May 20 2017 .SYNOPSIS... A cold front will stall out to the south of the area through Sunday. The front will then lift north into the area as a warm front late Sunday before another cold front passes through from the west Monday. High pressure will briefly return for Tuesday before another cold front impacts the area Wednesday. && .NEAR TERM /THROUGH TONIGHT/... As of 330 AM...cold front currently draped from west-to-east across the northern half of the forecast area (i.e., just north of DC) as depicted by surface observations and local area radar data. Front will continue to drop south of the area through daybreak...and remain south of the area through much of the weekend...leading to a couple of days of below normal temperatures. As the front remains to our south...low pressure over the plains will eject northward into the Great Lakes area...while a surface high sinks southward across the northeast beneath upper-level ridging. These features will foster persistent onshore flow...with extensive cloudiness...as a saturated marine layer develops between 2-4 kft AGL. Presence of cloud cover will greatly inhibit quality insolation...with highs today 20+ F below yesterday. Cold front will also shunt moisture to the south...with dewpoints dropping into the low 50s by Sunday morning. While most areas will remain dry...showers and a few thunderstorms are possible closer to the cold front across the SW (e.g., mainly Highland, Augusta, and Nelson counties) this afternoon. Any thunderstorms that do develop across this area would weaken rapidly as they move into a convectively-poor environment north of the front. && .SHORT TERM /SUNDAY THROUGH MONDAY NIGHT/... Easterly onshore flow continues through much of Sunday as CAD wedge continues to dominate with surface high pressure centered off the NY/NJ coast. Once again...cloud cover will hinder insolation and keep temperatures in the 60s. Warm front will begin to lift back northward late Sunday...with southerly return flow developing to its south. Rain chances return into Monday as moisture quickly streams back into the area ahead of frontal passage. Poor timing suggest limited instability ahead of cold front. One exception could be across central VA...where some destabilization could develop with a few thunderstorms possible. We begin to dry out late Monday afternoon/evening as cold front shifts away from the area. && .LONG TERM /TUESDAY THROUGH FRIDAY/... A deep, large scale trough axis will be dropping into the Central Plains Tuesday, and sharpening as it heads toward the East Coast Wednesday and Thursday. The only differences are in the fine-scale details, which will drive whether a secondary cutoff low can form in Appalachia (GFS) or the parent low rotates south (ECMWF). Regardless, they dynamic nature of this system should be able to induce cyclogenesis. Therefore, the midweek is looking wet. Tuesday will be the driest day of the stretch, as weakening high pressure passes over the area. Guidance has been consistent that the day would start out sunny, but clouds would be advancing through the day. Latest guidance, however, is suggesting that showers will be on the doorstep of central Virginia by the end of the day. Occasional showers will be the rule through Thursday. Areal coverage (scattered vs numerous) will be the biggest challenge...something that can`t be sorted out at this time frame. For the most part, will be emphasizing a precip type of showers. However, as the potential upper low nears, there should be sufficient instability for some thunder as well. The deepening cyclone should migrate toward New England by the end of the week, for considerable improvement by Friday. && .AVIATION /08Z SATURDAY THROUGH WEDNESDAY/... Cold front will stall to our south through Sunday...with high pressure sliding to our NE. This will allow for persistent easterly onshore flow...which will keep the area entrenched with low clouds. Long-lasting MVFR ceilings appear likely with cloud bases near 2-4 kft AGL. Warm front lifts northward into the area late Sunday and a cold front will cross the area Monday. These features will allow rain to develop with periods of sub-VFR with low ceilings. Improvement is expected later Monday as cold front pushes east of the area. Tuesday should be VFR for most of the day. Showers will spread north late Tuesday or Tuesday night, and will be scattered or numerous through Wednesday. Flight restrictions quite likely, but details uncertain this far out. && .MARINE... Winds will continue to increase over the waters through morning as north/northeast winds develop with pressure surge translating across the area behind departing cold front. Small Craft Advisory remains in effect for the potential for low-end SCA gusts near 20 knots through afternoon. Pressure surge weakens by evening...with winds diminishing overnight. Winds turn southerly late Sunday as warm front lifts back north through the waters...wind gusts near SCA criteria are possible (the low probability) as this occurs. Low-end SCA gusts remain possible Monday as cold front slides across the area. Transitory high pressure Tuesday will be replaced by showers associated with an upper low Tuesday night into Wednesday. Gradient flow not that strong, and mixing should be poor. Therefore, no headlines anticipated at this time. && .TIDES/COASTAL FLOODING... Easterly onshore flow develops this weekend as high pressure nudges into the area from the northeast. Flow turns southerly by Sunday night as warm front lifts into the area. Water levels will gradually increase today through Monday, with anomalies supporting elevated/minor coastal flooding at Annapolis, Straits Point, and DC by Sunday night. && .LWX WATCHES/WARNINGS/ADVISORIES... DC...None. MD...None. VA...None. WV...None. MARINE...Small Craft Advisory until 6 PM EDT this evening for ANZ531>534-536-537-539>543. Small Craft Advisory until 11 AM EDT this morning for ANZ530- 535-538. && $$ SYNOPSIS...MSE NEAR TERM...MSE SHORT TERM...MSE LONG TERM...HTS AVIATION...MSE/HTS MARINE...MSE/HTS TIDES/COASTAL FLOODING...MSE

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