Area Forecast Discussion
Issued by NWS Sterling, VA

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000 FXUS61 KLWX 151506 AAA AFDLWX Area Forecast Discussion...UPDATED National Weather Service Baltimore MD/Washington DC 1006 AM EST Thu Feb 15 2018 .SYNOPSIS... Low pressure will pass north of the region tonight, dragging a cold front across the area Friday. High pressure will briefly pass north of the area Friday night, then low pressure will pass to the south Saturday. High pressure will return for Sunday before another frontal system affects the area early next week. && .NEAR TERM /THROUGH TONIGHT/... Plenty of clouds this morning, but guidance suggest significant breaks in the clouds will develop this afternoon which should help boost highs today aided by a southerly flow. 12Z IAD sounding showed a strong inversion between sfc to 970 mb with very warm temperatures (18C) in the low levels. Mid 60s look like a slam dunk with 70s a real good possibility if we`re able to mix out completely. Showers today should be confined only to the Appalachians with showers for the rest of the area coming in well after dark. Previous afd... As low pressure develops near Chicago by this evening, lift will increase on the nose of a developing low level jet and result in showers redeveloping by evening across northwestern zones. These showers should focus along the MD/PA border most of the night in concert with best lift. Lows will be very mild, in the 50s and 60s. && .SHORT TERM /FRIDAY THROUGH SATURDAY NIGHT/... As aforementioned low pressure scoots into New England, it will drop a cold front south through the area Friday. Deep layer westerly flow results in a drying downslope component, so showers should tend to become more sparse as they cross the ridges and head into the I-95 corridor. At least scattered showers are still likely, though. Temperatures plummet quickly Friday evening behind the cold front. Gusty northwest winds ahead of a building high pressure should send temperatures below freezing by or shortly after midnight for much of the area. Winds will abate pretty quickly towards dawn as high pressure briefly passes overhead. An upper shortwave diving towards the Gulf of Mexico will be deflected by a strong subtropical ridge, causing it to veer instead towards the Mid-Atlantic. Surface low pressure likely develops over the Tennessee Valley and heads offshore of the Outer Banks of North Carolina by day`s end. Cold air will be in place only briefly, so expect precipitation to start as snow or a snow/sleet mix from SW to NE Saturday midday/afternoon, then change to a mix of rain/sleet and possibly freezing rain. Snow and sleet accumulations should be limited with poor snow-to-liquid ratios (generally an inch or less, except 1-3 for higher elevations and along the Mason-Dixon line). However, this combined with the potential for light icing from patchy freezing rain Saturday evening could still result in slippery travel. The mitigating factor will be warm ground temperatures from several mild days preceding this event. Precipitation should end quickly after midnight as low pressure speeds out to sea. Winds look pretty light late Saturday night, and with temperatures below freezing any residual moisture on the ground will likely turn to ice on untreated surfaces. && .LONG TERM /SUNDAY THROUGH WEDNESDAY/... High pressure will be over the area on Sunday providing sunny, dry, and seasonable weather. However, thats likely to be short-lived as low pressure in the center of the country heads northeast toward the Great Lakes. The attendant warm front will be passing through Sunday night or Monday morning. The isentropic lift from the warm advection the very least...increase cloud cover. Thats likely by Sunday night. Odds are by Monday it will bring a period of showers as well. If any precip were to arrive sooner, then precip type problems would result until the cooler/drier air erodes. This system will become elongated to our northwest. This positioning, while subject to change, would be sufficiently nearby to keep a chance of precipitation in the extended forecast... especially for the Potomac Highlands. Temperatures will be warm enough to preclude any further ptype issues-- it would all be rain as the forecast area would be on the warm side of the system. And warm means that 70 degrees possible Tue-Wed. && .AVIATION /15Z THURSDAY THROUGH MONDAY/... Patchy fog is possible near waterways with mild air over relatively cooler water. Light S flow at the surface, but ~45 kts W flow around 2-3kft results in LLWS. VFR expected today into tonight with lots of mid and high level cloudiness. SW flow 10 kts G 20 kts. -SHRA mainly N of MRB/MTN this evening, pushing south through Friday with occasional MVFR possible. LLWS expected to develop again tonight with strong LLJ. Gusty NW winds Fri-Fri night 10-20 G 30 kts. Sct`d -SHRA but mainly VFR. Wintry mix expected Sat aft/eve likely resulting in IFR. VFR conditions will prevail on Sunday...and potentially into Monday as well. A warm front will approach by early Monday. Associated flight restrictions possible. && .MARINE... SCA gusts expected through Friday night with a strong SW gradient ahead of a cold front, flipping to NW late by late Friday behind the front. Gusts subside late Friday night as high pressure builds overhead. Winds should remain light Saturday as a wave of low pressure passes to the south, but doesn`t deepen enough to result in a tight gradient or strong surface winds. High pressure will be over the waters Sunday, resulting in light winds. Flow will increase from the south/southwest Monday, but it is unlikely to exceed 10-15 kt. && .LWX WATCHES/WARNINGS/ADVISORIES... DC...None. MD...None. VA...None. WV...None. MARINE...Small Craft Advisory until 7 PM EST Friday for ANZ530>532-535- 536-538>541. Small Craft Advisory until 7 PM EST Friday for ANZ533-534-537- 542-543. && $$ SYNOPSIS...DHOF NEAR TERM...LFR SHORT TERM...DHOF LONG TERM...HTS AVIATION...HTS/DHOF MARINE...HTS/DHOF is the U.S. government's official web portal to all federal, state and local government web resources and services.