Area Forecast Discussion
Issued by NWS Sterling, VA

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000 FXUS61 KLWX 180720 AFDLWX Area Forecast Discussion National Weather Service Baltimore MD/Washington DC 320 AM EDT Fri Oct 18 2019 .SYNOPSIS... Low pressure will depart through the Canadian maritimes today as high pressure builds in from the Ohio Valley. The high pressure center will move overhead tonight, then pull offshore Saturday as low pressure moves across the north-central Gulf coast and into the southeastern United States. This low will move northeastward offshore of North Carolina or Virginia on Sunday, then meander offshore through Tuesday until another cold front approaching from the Ohio Valley pushes across the region. High pressure is expected to return for the second half of next week. && .NEAR TERM /THROUGH TONIGHT/... Low pressure will continue to lift northeast through Nova Scotia today while high pressure builds toward the area from the Great Lakes. A northwest flow is expected between these features, and winds will be gusty, but not nearly as gusty as Thursday. Wind gusts for most areas will be around 20 to 30 mph, but less toward central Virginia. A dry northwest flow will lead to plenty of sunshine along with seasonable temperatures. Highs will be in the upper 50s to lower/middle 60s for most locations. A Stratocu deck will affect the Allegheny Highlands this morning and a few cu will spill east, but these should dissipate throughout the day. High pressure will settle overhead tonight, and winds will diminish quickly around sunset. With light winds and clear skies overhead, this will provide a good setup for radiational cooling. Min temps will range from around 30 in the colder valleys to the middle and upper 30s for most other locations. A Freeze Warning is in effect for western Maryland, eastern WV, and extreme northern VA. These areas will be right underneath the high. A Frost Advisory is in effect for locations northwest of Interstate 95. Areas of frost are expected for these locations. Farther east, patchy frost is possible but confidence is too low for any headline at this time since they will be farther from the center of the high. && .SHORT TERM /SATURDAY THROUGH SUNDAY NIGHT/... High pressure will gradually slide offshore Saturday, but it will remain close enough for more dry and seasonable conditions. Surface low pressure will track through the Gulf Coast states into the southeastern CONUS, and the upper-level low in the southern stream will also track through these areas. High clouds ahead of this system will gradually increase Saturday afternoon into Saturday evening. The low will track toward the North Carolina Coast Saturday night and then off the Mid-Atlantic Coast Sunday. Warm and moist air ahead of this system will overrun the cooler air near the surface, and there will be lift from the upper-level shortwave energy associated with the surface low. Therefore, rain is possible across the area later Saturday night into Sunday. Confidence remains low in regards to how far north and west the rain will make it and also how much rain will fall. This is due to divergence in the guidance that still exists. It does appear that a period rain is increasingly likely across central Virginia to southern Maryland as well as the Washington and Baltimore Metro areas. Less confidence for rain exists farther north and west. The low will move away from the area later Sunday and high pressure will return for Sunday night, bringing a return of dry conditions. Low clouds and fog may develop underneath the nocturnal inversion Sunday night. && .LONG TERM /MONDAY THROUGH THURSDAY/... The long term period will start with broad/sturdy ridging over the eastern North Pacific extending into the western CONUS, with a large and deep trough digging across the central CONUS. Ridging will also be centered over or near Florida and the Bahamas, extending northward along the eastern seaboard. Embedded within this ridge will likely be the remnants of an area of low pressure offshore of the Mid-Atlantic (the low expected to originate over the Gulf of Mexico and possibly become Tropical Storm Nestor as it moves into and then offshore of the southeastern US). At the surface, a strong low pressure and cold front will be pushing east across the Midwest while high pressure retreats across northern New England and the Canadian maritimes. This should result in a surface high pressure wedge locally over the Mid-Atlantic in the lee of the Appalachian crest. Given the ridging building locally, any rain should be gone well prior to daybreak Monday. As the cold front to the west approaches Monday night into Tuesday, increasing clouds and rain chances are expected. The upper trough looks pretty strong, which makes sense given the upstream ridging and amplified flow pattern overall. This may very well lead to another soaking rain event, especially given strong southerly 850 hPa flow advecting moisture into the region ahead of the front. Enhanced synoptic lift may even result in a few heavier downpours or rumbles of thunder as the upper trough goes negatively-tiled. As to be expected several days out, there is some uncertainty as to the exact timing and strength of the front, but overall there appears to be a pretty high chance of rain on Tuesday. A large area of high pressure is expected to follow during the second half of next week accompanied by mainly dry weather until perhaps the very end of the week when another system approaches. && .AVIATION /07Z FRIDAY THROUGH TUESDAY/... VFR conditions are expected through Saturday. Rain is likely along with subVFR conditions later Saturday night into Sunday, especially for the eastern terminals. High pressure will return Sunday night, but low clouds and fog may develop underneath the nocturnal inversion. Could see some patchy sub-VFR fog Monday morning in the wake of Sunday`s rain as a wedge of high pressure builds overhead. Otherwise VFR likely later Monday into Monday night in light northerly flow. Winds should turn southeasterly and increase a bit at the surface late Monday night into Tuesday ahead of a strong approaching cold front. Low-level wind shear and sub-VFR conditions in rain are possible Tuesday, with winds shifting to westerly behind the front Tuesday night into Wednesday (in tandem with a return to VFR). && .MARINE... A Small Craft Advisory remains in effect for the waters today due to northwest winds gusting around 20 to 30 knots. Winds will quickly diminish toward sunset as high pressure builds toward the waters. The high will build overhead tonight before moving offshore Saturday. A return southerly flow will develop Saturday into Saturday evening but winds should remain below SCA criteria for most areas. Low pressure will pass by to the south and east late Saturday night before moving offshore Sunday. An SCA may be needed for portions of the waters during this time. High pressure will build toward the waters Sunday night, but and SCA may still be needed for portions of the waters. Generally light winds are expected Monday as surface high pressure ridges across the waters and low pressure meanders out at sea. A strong cold front will approach from the west Tuesday, and some SCA level gusts are possible in SE flow ahead of the front (along with an increasing potential for soaking rain and lower visibilities). Winds will shift to the west behind the front Tuesday night into Wednesday and may again approach SCA levels. There`s a low chance of gusty winds accompanying heavier downpours immediately ahead of the front Tuesday, as well. && .TIDES/COASTAL FLOODING... Tides are running below normal as of early this morning, with no flooding issues anticipated over the next 24 to 36 hours. Onshore (easterly) flow is expected to increase ahead low pressure approaching from the south Saturday night into Sunday, resulting in increasing water levels. There remains a moderate level of uncertainty as to the extent and duration of onshore flow, as that will be reliant upon the exact strength and track of approaching low pressure. Therefore, the confidence in exact water levels is low, though the confidence in seeing at least minor flooding at the more vulnerable tidal sites seems likely. Light northerly flow should cause water levels to decrease a bit heading into Monday, before onshore flow (southeasterly) flow resumes Monday night into Tuesday, leading to a subsequent rise in water levels once again. && .LWX WATCHES/WARNINGS/ADVISORIES... DC...None. MD...Frost Advisory from 2 AM to 9 AM EDT Saturday for MDZ004>006- 503>507. Freeze Warning from 2 AM to 9 AM EDT Saturday for MDZ003-501- 502. VA...Frost Advisory from 2 AM to 9 AM EDT Saturday for VAZ025>027- 029-030-036>040-050>053-055-501>506. Freeze Warning from 2 AM to 9 AM EDT Saturday for VAZ028-031. WV...Frost Advisory from 2 AM to 9 AM EDT Saturday for WVZ505-506. Freeze Warning from 2 AM to 9 AM EDT Saturday for WVZ050>053- 055-501>504. MARINE...Small Craft Advisory until 6 PM EDT this evening for ANZ530>543. && $$ SYNOPSIS...DHOF NEAR TERM...BJL SHORT TERM...BJL LONG TERM...DHOF AVIATION...BJL/DHOF MARINE...BJL/DHOF TIDES/COASTAL FLOODING...DHOF

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