Area Forecast Discussion
Issued by NWS Sterling, VA

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000 FXUS61 KLWX 180844 AFDLWX Area Forecast Discussion National Weather Service Baltimore MD/Washington DC 344 AM EST Thu Jan 18 2018 .SYNOPSIS... Low pressure off the Carolina coast will push out to sea today. High pressure will build south of the area through the weekend. A cold front will cross the area early next week. && .NEAR TERM /THROUGH TONIGHT/... Surface low pressure is intensifying off the NC coast this morning as an upper level low moves along the NC/VA border. As the upper low is departing to the east, mid level clouds have been dissipating. Over much of the area, temperatures in the teens with a 5-15 mph wind are resulting in wind chills in the single digits. High elevation stations indicate wind chills less than -10, where a Wind Chill Advisory continues until 9 AM. For today, the gradient between the departing low and high pressure over the deep south will result in continued breezy conditions with 20-25 mph gusts at times. A wind speed max resides in a subsidence inversion a few thousand feet above sea level. Am thinking the inversion results in poor mixing (reference PIE/2G4 forecast soundings), negating the need for a high elevation Wind Advisory, though can`t rule out criteria being met on the highest exposed peaks. Otherwise, subsidence behind the departing trough should result in mainly clear skies. Warm advection aloft and downsloping flow will result in highs making it above freezing for the lower elevations, and into the 40s south of I-66 in VA. Tonight, the gradient will relax a little, which should allow our typical colder valleys to decouple. A weak shortwave trough could bring some cirrus but otherwise skies will be mostly clear. Have gone near or lower than colder guidance in these locations, with dew points supporting lows in the mid teens. Lower to mid 20s can be expected in the cities and locations which maintain a light breeze. && .SHORT TERM /FRIDAY THROUGH SATURDAY NIGHT/... High pressure will slide by to our south Friday and Saturday. Some cirrus from the northern stream jet may pass by at times, but that`s about it weather-wise. The main story will be temperatures which will continue to warm. The only limiting factor will be how much mixing can occur under a strong subsidence inversion. High temperatures in the 50s will be limited south of I-66 on Friday, but will encompass more of the area by Saturday. Overnight lows will be in the mid 20s to lower 30s Friday night and lower to mid 30s Saturday night. By Saturday night, isentropic ascent increases west of the Appalachians ahead of a developing low pressure system in the Plains. Any light precipitation is expected to remain west of the divide. Forecast soundings would likely support drizzle/light rain, so any isolated pockets of below-freezing temperatures could pose a concern. Will limit this mention to this discussion for now due to isolated nature/low probability combination. && .LONG TERM /SUNDAY THROUGH WEDNESDAY/... High pressure will be located along the South Carolina coast on Sunday, with a warm front lifting northward to our west as a low pressure system develops over the central Plains. Isentropic lift is not strong and frontogenesis is weak, but there may be just enough moisture in the return flow combined with the meager forcing to cause some light rain or drizzle along and west of the Allegheny Front. This could be a concern if temps hang near freezing, especially early in the day, but this concern should diminish later in the day as temps moderate. Further east, drier air and less forcing should prevail to keep most of the CWA dry on Sunday. Southerly flow will prevail Sunday and Monday as the high shifts east off the coast and the storm in the Plains moves northeast towards the Great Lakes, but enough of a weak wedge may get stuck on the eastern side of the mountains to prevent a strong warmup. Nevertheless, temps will be above normal for late January, with highs likely reaching the 50s both days, with Monday the warmest day. It is possible temps pop into the 60s on Monday just ahead of the cold front, but uncertainty is high on this due to the timing of the front during the overnight. Showers don`t look likely across most of the CWA until later Monday night, with showers likely to linger early Tuesday as the storm from the Plains passes north and drags its cold front across the area. Mild temps early may drop later in the day, though this is dependent on timing. Rainfall doesn`t look particularly heavy and a couple days of warmer weather should allow most soils to thaw, so not expecting a big hydro concern with this system. We still need rain. Dry weather returns as high pressure builds in from the west on Wednesday, but as this high has more of a Pacific origin, temps may only drop to near normal. && .AVIATION /09Z THURSDAY THROUGH MONDAY/... VFR conditions expected today as a low pressure system moves offshore. However, the gradient between this system and high pressure in the deep south will result in gusty west winds around 20 kt. Winds will subside tonight. No further impactful weather is expected through Saturday, with west winds gradually backing to the southwest. VFR likely Sunday and Monday, though will need to watch for possible low cigs as warmer, moist air moves north over our region ahead of the next cold front. && .MARINE... While there was a bit of a lull on some of the waters earlier, gustiness has increased again as temperatures continue to fall further below water temperatures and low pressure strengthens as it moves off the Carolina coast. Therefore, an SCA will be in effect for all waters until 4 PM this afternoon. It`s possible winds subside in some areas before that as air temperatures rise through the 30s to near 40. Westerly winds will continue through Friday as high pressure builds south of the area, becoming more southwesterly by Saturday. A strong subsidence inversion and warm temperatures should keep winds below SCA thresholds. Winds likely sub-SCA Sunday and Monday as high pressure slides east off the coast ahead of the next cold front. && .LWX WATCHES/WARNINGS/ADVISORIES... DC...None. MD...Wind Chill Advisory until 9 AM EST this morning for MDZ501. VA...Wind Chill Advisory until 9 AM EST this morning for VAZ503-504- 507-508. WV...Wind Chill Advisory until 9 AM EST this morning for WVZ501-503- 505-506. MARINE...Small Craft Advisory until 4 PM EST this afternoon for ANZ530>543. && $$ SYNOPSIS...RCM NEAR TERM...ADS SHORT TERM...ADS LONG TERM...RCM AVIATION...ADS/RCM MARINE...ADS/RCM is the U.S. government's official web portal to all federal, state and local government web resources and services.