Area Forecast Discussion
Issued by NWS Sterling, VA

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626 FXUS61 KLWX 230134 AFDLWX Area Forecast Discussion National Weather Service Baltimore MD/Washington DC 934 PM EDT Mon Jul 22 2019 .SYNOPSIS... A cold front will slowly move southeastward across the Mid- Atlantic through Tuesday morning. High pressure will build north of the region through the remainder of the week. && .NEAR TERM /UNTIL 6 AM TUESDAY MORNING/... Severe weather threat is winding down as two separate clusters of storms have effectively overturned the airmass for most of the area. Some of the storms in southeast Virginia could be on the strong side if they make a run at southern Maryland in a few hours, where it is still hot and humid. However, the remaining Watch (only along the Bay now) should be able to expire at 10 PM. Cold front is slowly sagging into northwestern parts of the CWA this evening. There has been an uptick in convection as the forcing reaches some elevated instability where the airmass wasn`t turned over. With low level stability, the main threat here will be heavy rain and potential for flooding. Overall the flash flood threat is reduced a bit overnight as regional radar shows convection upstream isn`t particularly intense and the heaviest elements have been progressive. However, the front will be encountering some elevated instability and precipitable water values are approaching two inches. So as the front slows, it`s still possible there could be training moderate to heavy rain. && .SHORT TERM /6 AM TUESDAY MORNING THROUGH WEDNESDAY NIGHT/... The cold front will push southeastward on Tuesday. Showers will likely linger into the morning for much of the region, but clearing is expected to commence in the northwest by afternoon and low- level dry air spills into the region. Did increase PoPs based on latest guidance, as it may turn out to be rainy at times much of the day southeast of DC. In addition, as clouds break in the northwest and the mid level trough axis crosses, instability showers and perhaps a thunderstorm could develop. High temperatures will likely struggle to each 80 degrees, nearly 25 degrees cooler than Sunday, due to cloud cover and expected precipitation. Near to slightly below normal temperatures and noticeably lower humidity are expected through the middle of the week as high pressure builds to the north. && .LONG TERM /THURSDAY THROUGH MONDAY/... The long term period of the forecast will feature an extended period of pleasant weather, with dry conditions, ample sunshine, low humidity levels, and below to near normal temperatures for late July. As the mid-upper level trough responsible for today`s active weather departs off to the east on Thursday, heights will rise aloft, and surface high pressure will start to build in from the west. The area of high pressure will move overhead Friday and Saturday, before sliding offshore Sunday. Other than a few passing fair weather clouds, each day should feature near wall to wall sunshine. Temperatures will gradually moderate from below normal, to near normal, with highs in the low to mid 80s on Thursday and highs in the upper 80s to near 90 by Sunday. Low temperatures will start off in the 50s and 60s on Thursday Morning, and moderate to the 60s to near 70 by Sunday Morning. Humidity levels will remain relatively low, with dewpoints starting in the upper 50s to near 60 on Thursday, then gradually climbing into the mid 60s by Sunday. && .AVIATION /01Z TUESDAY THROUGH SATURDAY/... Prevailing MVFR in ceilings will likely develop around or after 06Z (already occurring at MRB) as a cold front and areas of rain move into the area. There could be some embedded thunderstorms, but chances will decrease overnight. Some periods of IFR aren`t out of the question, but not likely to be extended. VFR expected to return from NW to SE as Tuesday progresses. Scattered instability showers may develop in the afternoon. VFR continues through Wednesday in NW flow. VFR conditions will persist through the long term period, as high pressure dominates our weather. && .MARINE... Occasional SCA gusts likely ahead of a cold front in WSW flow through this evening, with much higher gusts still possible in and near any thunderstorms. SCA in NW flow again for parts of the waters (especially on the Bay, but could in theory occur anywhere) behind the front late tonight through Tuesday morning, then sub-SCA and dry through mid to late week. A prolonged stretch of precipitation free conditions and sub-SCA level winds is expected through the middle to latter portions of the upcoming week. && .HYDROLOGY... High PWATs of 2 inches are expected tonight ahead of a slowly approaching cold front. Steering flow will be parallel to the boundary, and with mid-level height falls moving overtop of persistent and excessively humid low- levels, re-generative thunderstorms capable of very heavy rain seem likely. The exact placement is still somewhat uncertain, but it seems most likely that these factors will all come together across northern/central MD, the eastern WV panhandle and portions of northern Virginia into the Shenandoah Valley and Potomac Highlands where synoptic lift from the surface front/trough approaching from the north will overlay higher instability airmass to the south. Areal average rainfall of 1-1.5 inches is expected for much of the Mid-Atlantic, but isolated totals in excess of 3 or 4 inches are possible. The higher end solutions are beginning to appear less likely though. Due to the convective nature of this system, much of this rain may fall in a short amount of time, resulting in the risk for flash flooding. && .LWX WATCHES/WARNINGS/ADVISORIES... DC...Flash Flood Watch until 6 AM EDT Tuesday for DCZ001. MD...Flash Flood Watch until 2 AM EDT Tuesday for MDZ501-502. Flash Flood Watch until 6 AM EDT Tuesday for MDZ003>006-011- 013-014-503>508. VA...Flash Flood Watch until 2 AM EDT Tuesday for VAZ025>027-029- 030-038>040-051-501>504-507-508. Flash Flood Watch until 6 AM EDT Tuesday for VAZ028-031- 052>054-505-506. WV...Flash Flood Watch until 2 AM EDT Tuesday for WVZ050-055- 501>506. Flash Flood Watch until 6 AM EDT Tuesday for WVZ051>053. MARINE...Small Craft Advisory until noon EDT Tuesday for ANZ531>534-537- 539>543. Small Craft Advisory until 10 PM EDT this evening for ANZ530- 535-536-538. && $$ SYNOPSIS...DHOF NEAR TERM...ADS/DHOF SHORT TERM...ADS/DHOF LONG TERM...KJP AVIATION...ADS/DHOF/KJP MARINE...ADS/DHOF/KJP HYDROLOGY...ADS/DHOF

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