Area Forecast Discussion
Issued by NWS Sterling, VA

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000 FXUS61 KLWX 281905 AFDLWX Area Forecast Discussion National Weather Service Baltimore MD/Washington DC 305 PM EDT Sun May 28 2017 .SYNOPSIS... Weak low pressure will move across the area this evening and move out to sea Monday. Upper level low pressure will drift east across the north side of the Great Lakes Monday through Friday. In the meantime, weak high pressure will prevail over the Mid- Atlantic outside of weak trough passages Tuesday night and Wednesday night. Low pressure approaches from the west this weekend. && .NEAR TERM /UNTIL 6 AM MONDAY MORNING/... As of 2pm, an upper low is over the Manitoba/Ontario border with a surface low over western Lake Superior. A southern stream jet is southeast of the northern stream low with 1006mb surface low over the MI/OH border and a mesolow on the south side of the jet over southern VA. This is apparent in GOES imagery as rotation on the south end of stratiform clouds that are draped across the Mid-Atlantic under the jet. Onshore flow is across the CWA north of this mesolow and under the jet making for a cool late May Sunday. This southern VA low is picked up in the 12Z GFS and drifts east across the VA part of the Chesapeake Bay this evening. Moisture on southeast flow should be cutoff by this low, limiting evening rainfall over the Balt-Wash metro. However, the trough over the eastern Great Lakes may still allow excessive rain along the Appalachian crest where our flash flood watch is in effect, so that will remain in effect until 10pm for now. Later in the night the wind shifts west with some upslope rain is expected on the western slopes of the Allegheny Front, but not at an excessive rate. Mid 60s min temps tonight which are near dewpoints. Should clearing occur behind the low to our south, fog would be expected. However, the jet overhead increases overnight, so at least high clouds should remain and limit fog potential. && .SHORT TERM /6 AM MONDAY MORNING THROUGH TUESDAY NIGHT/... Weak trough moves east across the area in the morning with advance showers ending by noon for the western shore of the Bay. 14 to 15C 850mb temps mix down in light westerly flow making for max temps low to mid 80s. The lack of rain and clouds will make for a marked difference from today! Weak surface high pressure lingers over the area as the parent upper low drifts east across the north side of the Great Lakes. A few shortwaves look to round the low and disrupt the tranquil weather. The first looks to be Tuesday night so low chance PoPs for showers were added then. Otherwise slightly above normal temps persist. && .LONG TERM /WEDNESDAY THROUGH SUNDAY/... A weak front across the region could spawn a couple of showers or a thunderstorm in parts of the region Wednesday into Wednesday evening. Weak high pressure will build in Wednesday night through Thursday night. Dry conditions and seasonable temperatures expected. The high moves east Friday to allow for a leeside trough of low pressure to form east of the Potomac Highlands Friday night. There is a chance for rain showers along this trough both Friday and Friday night. Temperatures will be near normal. There is a better chance for rain showers and perhaps a few thunderstorms Saturday into Saturday night as a disturbance moves into the region to accompany the trough of low pressure. By Sunday, a cold front should work into the region and bring an additional threat for showers and thunderstorms. && .AVIATION /19Z SUNDAY THROUGH FRIDAY/... MVFR cigs/vsbys in rain rest of the afternoon across the DC metros with rain tapering off (except for KMRB and west) this evening as low pressure passes to the south. Fog will develop in any areas that clear, but an increasing jet should keep at least high clouds around. Weak trough crosses from the west late tonight with light west winds and VFR Monday into Tuesday. A weak trough crosses late Tuesday with chances for rain then. VFR conditions at the terminals, except where pop-up showers or a thunderstorm develop Wednesday and Wednesday night. MVFR or IFR conditions in rainfall. Winds west-southwest 10 knots Wednesday becoming northwest 5 to 10 knots Wednesday night. VFR conditions Thursday and Thursday night. Winds northwest 5 to 10 knots Thursday, becoming light and variable Thursday night. && .MARINE... Light SEly flow with low pressure over southern VA. This low crosses the VA portion of the Bay this evening with rain ending as it passes. Weak trough crosses from the west Monday morning with light westerly flow Monday into Tuesday. No marine hazards expected Wednesday through Thursday night. Winds southwest 10 knots Wednesday becoming northwest 10 knots Wednesday night. Winds northwest 5 to 10 knots Thursday then light and variable Thursday night. && .HYDROLOGY... Low pressure moves northeast over the eastern Great Lakes through this evening. An associated trough will approach the central Appalachians this evening with advance showers and thunderstorms. Flash Flood Watch is in effect for Allegany, Mineral, Grant, Pendleton, and Highland counties until 10pm. Breaks in the clouds in the mid afternoon should invigorate late afternoon/early evening activity. && .TIDES/COASTAL FLOODING... Water levels running within a few inches of a foot this afternoon. Light onshore flow should keep the departure similar through tonight, if not rising slightly. Since we`ve just passed the new moon, there is a decent spread in the semi-diurnal cycle...with the daytime cycle preferred. Current guidance indicates that we`ll hit minor flood during this cycle for the next couple of days. However, departures a bit greater at Piney Point, such that each cycle should hit minor. It`ll be a similar story at SW DC, and perhaps Alexandria as well. Have made some Advisory adjustments, based on the above guidance and thinking. Included the next two cycles for the upper Potomac, the next three cycles (through Monday night) for St Marys (ultimately, the threat there will last longer), and the second cycle (Monday morning) in Calvert/Anne Arundel. No additional forecasted flooding at this time. Will need to monitor Dahlgren and Baltimore, both of which will come close to minor on Monday. Ensembles suggest a gradual decrease in water level by midweek. && .LWX WATCHES/WARNINGS/ADVISORIES... DC...Coastal Flood Advisory until 2 PM EDT Monday for DCZ001. MD...Coastal Flood Advisory from 3 AM to 1 PM EDT Monday for MDZ014- 018. Flash Flood Watch until 10 PM EDT this evening for MDZ501-502. Coastal Flood Advisory until 10 PM EDT Monday for MDZ017. VA...Flash Flood Watch until 10 PM EDT this evening for VAZ503-504. Coastal Flood Advisory until 2 PM EDT Monday for VAZ054. WV...Flash Flood Watch until 10 PM EDT this evening for WVZ501>506. MARINE...None. && $$ SYNOPSIS...BAJ NEAR TERM...BAJ SHORT TERM...BAJ LONG TERM...KLW AVIATION...BAJ/KLW MARINE...BAJ/KLW HYDROLOGY...BAJ TIDES/COASTAL FLOODING...HTS

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