Area Forecast Discussion
Issued by NWS Sterling, VA

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000 FXUS61 KLWX 240124 AFDLWX Area Forecast Discussion National Weather Service Baltimore MD/Washington DC 924 PM EDT Mon Apr 23 2018 .SYNOPSIS... A low pressure system will bring unsettled weather to the region tonight through Wednesday. A brief ridge of high pressure will build in Thursday, ahead of the next low pressure system that may affect the region Friday into Saturday. && .NEAR TERM /UNTIL 6 AM TUESDAY MORNING/...
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A closed low will slowly move eastward from the Tennessee Valley tonight. South to southeast flow will be on the increase through the night along with increasing moisture and warm air advection. Clouds will continue to thicken and lower tonight. The rainfall, however, will be slow to arrive. Incorporating a time- lagged ensemble approach continues to delay the arrival. Not only are dewpoint depressions still 20 degrees or greater, but there is a significant dry layer noted from the 00Z IAD RAOB between 700-500 mb to overcome. Overnight tonight rain probabilities will continue to increase from southwest to northeast. Rain should be falling by sunrise from about Winchester-Fredericksburg south/west. Further northeast, locations will likely remain dry through the night. Do have a buffering band of chance PoPs that includes Hagerstown/Martinsburg-metro DC-southern Maryland. Lows tonight will settle mainly in the mid/upper 40s to around 50F.
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&& .SHORT TERM /6 AM TUESDAY MORNING THROUGH WEDNESDAY NIGHT/... The area of low pressure will continue its slow eastward trek through the day Tuesday, reaching western North Carolina by late in the day. A track northeastward to the Virginia Tidewater region is then forecast for Tuesday night, and into the Philadelphia/New York City area by late Wednesday. Periods of rain are expected during the day Tuesday and into Tuesday night, tapering off to showers on Wednesday. While widespread hydrological concerns are not currently anticipated, with precipitable water values topping off around 1-1.25", there will likely be some areas of locally heavy rain. Total rainfall amounts are expected to range from 1-2" over a 24-36 hour period. With low level southeast flow into the higher elevations of the Blue Ridge and nearby highlands, some localized amounts of up to 3" are possible. 6-hour flash flood guidance ranges from generally 1-3", so any flash flood threat is quite low, however if some locations do manage to see 3", there may be some minor hydrologic issues. WPC has outlined the region in a Marginal Risk for flooding, and this continues to look reasonable. High temperatures on Tuesday will range from the 50s to near 60F, with lows Tuesday night in the upper 40s and low 50s. With precipitation of a more showery nature on Wednesday, temperatures will likely rise a few more degrees, with highs in the 60s area-wide. Drying will take place Wednesday night as the system pulls northeastward. Lows in the 40s to around 50F. && .LONG TERM /THURSDAY THROUGH MONDAY/... Following the passage of the low pressure system from Tuesday and Wednesday, a weak high pressure moves into the region, bringing a period of clear weather on Thursday. With little evidence of cold advection from this system, temperatures can be expected to remain around average, with values in the upper 60`s possible. On Friday, the forecast is a lot less certain at this time. Since this time yesterday, there has been a bit of a shift in model guidance, leaning towards the possibility of a weak coastal low impacting our area, bringing a chance for some light rain. However, the GFS is the only model really showing this at this point. Another point to note is that only a few of the GEFS ensemble members show any rain from a system on Friday. The European and Canadian both try to develop a low across the southeast late Thursday/early Friday, but neither bring this up into our area. For now, keeping the chance for precipitation on Friday for the entire area due to the high degree of uncertainty at this point in the forecast. Temperatures on Friday look to remain near to slightly below average, and could vary greatly depending on the evolution of this potential system. On Saturday a weak cold front, associated with a low pressure system to our north, will pass through the area sometime during the morning. This front doesn`t look to bring a large chance for precipitation, but a few showers can not be ruled out. Temperatures don`t look to be affected to much either, and look to remain pretty consistent with averages for this time of year. High pressure builds in on Sunday, which should keep things dry, with temperatures slightly warmer than on Saturday. && .AVIATION /02Z TUESDAY THROUGH SATURDAY/...
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VFR conditions expected through tonight with a gradually thickening and lowering cloud deck. MVFR conditions then become possible at CHO by 1100 UTC Tuesday morning as rain and lower ceilings approach from the southwest. MVFR is then expected to spread across the remainder of the region during the morning and into the early afternoon Tuesday, with IFR becoming likely by later in the day as well. IFR in rain and low ceilings will likely continue Tuesday night. Clouds and showers will persist for much of the day on Wednesday with potential for sub- VFR conditions continuing. Expect VFR conditions for much of Thursday, and early Friday, as we enter a relatively calm period as high pressure builds into the area. Latest guidance still suggests a chance for some impact from a weak coastal low on Friday, bringing about the chance for some low clouds in the area. Sub-VFR conditions are possible during this timeframe.
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&& .MARINE...
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Winds have eased under Small Craft Advisory conditions. Do not believe gusty winds will return until Tuesday, as low pressure approaches the waters. Have therefore cancelled the Advisory for tonight, and implemented a staggered return on Tuesday. The strongest period of winds is expected Tuesday afternoon and evening. Have reoriented the SCA to have a staggered end as well, first on the mid-upper Tidal Potomac Tuesday evening, then for the rest of the waters predawn Wednesday. Another period of SCA- criteria winds will likely develop Wednesday afternoon and Wednesday night as the system departs. An exiting low pressure system early on Thursday could bring gusty winds to the area, so some SCA criteria winds are possible during this time. Friday is highly variable dependent on the development of the potential coastal low on Friday.
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&& .HYDROLOGY...
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Widespread rainfall amounts from 1 to 2 inches are forecast from tonight through Wednesday, with locally higher amounts likely along the central Blue Ridge and adjacent foothills to the east. Due to relatively dry antecedent conditions and extended time period over which the rain falls, widespread hydrological concerns are not anticipated. However, with increasing southeast flow, localized amounts of up to 3" are possible, again mainly near the higher elevations of the Blue Ridge and near-by highlands which could lead to some minor hydrology issues.
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&& .TIDES/COASTAL FLOODING... As low pressure approaches from the southeastern U.S through Tuesday night, a persistent onshore flow will develop. This will yield increasing tidal anomalies, and the possibility of coastal flooding toward Tuesday and Wednesday. && .LWX WATCHES/WARNINGS/ADVISORIES...
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DC...None. MD...None. VA...None. WV...None. MARINE...Small Craft Advisory from 11 AM Tuesday to 4 AM EDT Wednesday for ANZ530-531-538>540-542. Small Craft Advisory from 7 AM Tuesday to 4 AM EDT Wednesday for ANZ532>534-537-541-543. Small Craft Advisory from 11 AM to 10 PM EDT Tuesday for ANZ535-536.
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&& $$ SYNOPSIS...MM NEAR TERM...MM/HTS SHORT TERM...MM LONG TERM...CJL AVIATION...MM/HTS/CJL MARINE...HTS/MM/CJL HYDROLOGY...MM/HTS TIDES/COASTAL FLOODING...MM

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