Area Forecast Discussion
Issued by NWS Sterling, VA

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714 FXUS61 KLWX 171443 AFDLWX Area Forecast Discussion National Weather Service Baltimore MD/Washington DC 1043 AM EDT Sat Mar 17 2018 .SYNOPSIS... Weak low pressure will pass by to the south this afternoon into this evening. High pressure will return for Sunday and Sunday night. Stronger low pressure will track into the Tennessee Valley Monday and it will transfer its energy to a coastal low off the Mid-Atlantic Coast Monday night. The low will track to the northeast Tuesday but before another area of low pressure passes by to our south Wednesday. High pressure will return for late next week. && .NEAR TERM /THROUGH TONIGHT/... Low pressure over southern Ohio will move east the remainder of this morning. Its upper level support will catch up to it this afternoon and bring a light wintry mix to parts of the region this afternoon, but not before a light wintry mix affects the Appalachian Mountains late this morning. Current radar shows multiple bands of precipitation but most of this is not reaching the ground considering dewpoint temperatures are in the teens across much of the northern half of our zones. There is currently a Winter Weather Advisory in effect for the Appalachian Mountains and the Blue Ridge Mountains through late afternoon. As for precipitation type overall, uncertainty remains high considering the amount of dry air in the lower levels and the steady rise in temperatures this morning into the afternoon. Warm advection continues to increase temperatures. As of looks like a wintry mix is likely across the Potomac Highlands and the Blue Ridge Mountains. The primary threat will be a light glaze of ice from freezing rain on elevated surfaces. However...there may be enough cooling and forcing for a band of snow to develop with localized accumulations around 1-2 inches. There could be enough mid level forcing and cooling with banding precipitation for a period of snow around midday...and slushy accumulations are possible. However...most accumulation should be on grassy surfaces and it appears too localized for an advisory across the entire area. For the metro areas...a period of light rain or rain/snow mix is possible this afternoon. No accumulation is expected since precipitation rates will be less intense as the system continues to weaken. Should the banding precipitation hold longer than expected it is not out of the question for a coating on grassy surfaces. High pressure will return for tonight, bringing dry and chilly conditions across most areas. However...a weakening shortwave that will be remnants from the closed upper-level low this morning will pass by to the south. A few showers are possible across the central Shenandoah Valley into the Potomac Highlands. && .SHORT TERM /SUNDAY THROUGH MONDAY/... High pressure will build overhead Sunday through Sunday night. Dry and seasonable conditions are expected. Low pressure will track through the Tennessee Valley Monday and it is expected to transfer its energy to coastal low pressure off the Mid-Atlantic Coast Monday night. A confluence zone around upper-level low pressure over the Canadian Maritimes and ridging over the north-central CONUS will pump high pressure to our north over New England. This should cause the primary low to transfer its energy to the coastal low south of our latitude...putting our area on the cold side of the storm. Having that been is the middle of March so there will be issues with warmer air near the surface. Precipitation is expected to gradually overspread the area Monday into Monday evening. Exact timing is still a bit uncertain...but temps should be well above freezing for most areas Monday due to warmer air near the surface. This means that the main p-type should be rain Monday into Monday evening. As the low transfers its energy to the coastal low Monday night...north to northeast winds will increase and this will draw colder air down from the north and east. Rain may mix with or change to snow...especially across higher elevations and climatologically favored areas (northern Maryland and near/west of Blue Ridge and Catoctin Mountains). Uncertainty remains high because if the low tracks far enough to the south then colder air could work its way into the Washington/Baltimore Metro areas causing rain to change to snow. For now, the latest forecast takes a blend of the latest guidance which continues to keep the best chance for snow north and west of the metro areas. However...confidence remains low at this time. && .LONG TERM /MONDAY NIGHT THROUGH FRIDAY/... Tuesday morning, low pressure will be emerging off the VA/NC coastline as strong high pressure resides north of the Great Lakes region. Precipitation will be observed areawide Tuesday morning, with differing of opinions between model guidance as to how thermal profiles shake out aloft. As always, this will be a major factor in determining p-type across the area. The ECMWF is the colder solution, but also indicates a more suppressed track southward with a stronger high pressure area to our north, and keeping the axis of heaviest moisture just to our south. GFS favors a more northward extent of precipitation, but also not as cold aloft, thus p-type issues result. Regardless, odds of developing coastal low pressure to our south and east are increasing, and sufficient cold air does look like it will be in place for at least portion of our CWA, favoring north and west of the metros at this time. As precip continues during the day on Tuesday, we will be fighting the higher March sun angle which will play a role in boundary layer temperatures and any accumulation potential. Low pressure intensifies off the coast and moves quickly out in to the western Atlantic Tuesday evening. The pattern remains unsettled through Wednesday as mid to upper level troughing over the Mid Atlantic region takes on a negative tilt, and another area of low pressure looks to develop just east of the Outer Banks. GFS depicts a bit more moisture over the region on Wednesday with weak low pressure along the Appalachians, while the ECMWF highlights an even weaker area of low pressure, thus less precipitation. Global guidance does agree on low pressure emerging off the Carolinas early Wednesday, rapidly intensifying and pressing eastward, bringing an end to any precipitation overnight Wednesday. Temperatures Tuesday through Wednesday will remain well below normal for mid-March, with highs in the 30s to near 40, and lows in the 20s to near 30 degrees around the cities. A surface ridge of high pressure extending southward from Canada will stretch over the region Thursday and Friday, as the mid to upper level trough starts to exit the eastern seaboard and ridging aloft takes hold. This will bring dry and unseasonably cold yet moderating temperatures to end the work week with highs in the 40s, and lows in the 20s to near freezing. && .AVIATION /15Z SATURDAY THROUGH WEDNESDAY/... A period of light rain or rain/snow mix is possible across the terminals late this morning into this afternoon. Precipitation amounts should be light but brief reductions to vsbys toward MVFR levels are possible. High pressure will return for tonight through Sunday night. Low pressure will track through the Tennessee Valley before transferring its energy to a coastal low off the Mid-Atlantic Coast Monday night. Rain will likely overspread the terminals later Monday into Monday night. Rain could mix with or change to snow Monday night as colder air works its way into the area. SubVFR cigs/vsbys are likely later Monday through Monday night. Periods of MVFR/IFR conditions expected to persist Tuesday through Wednesday as an unsettled weather pattern affects the terminals with a range of precipitation types. Low pressure passing just south of the area early Tuesday will redevelop off the VA/NC coast, increasing chances of VIS/CIG reductions at all terminals. A secondary area of low pressure looks poised to form off the Outer Banks early Wednesday, favoring potential flight restrictions at DCA/CHO but could have impacts further north depending on how much precipitation overspreads the area and the track of the low. North to northwest winds will remain at 10 knots or less through the period. && .MARINE... Low pressure will weaken as it passes south of the waters this afternoon into tonight. A Small Craft Advisory is in effect for the northern Chesapeake Bay this afternoon, but it will be marginal. High pressure will build over the waters tonight into Sunday. The pressure surge may cause a period of SCA wind gusts late tonight into early Sunday across middle portions of the Bay but confidence was too low for an SCA at this time. Low pressure will track through the Tennessee Valley before transferring its energy to a coastal low off the Mid-Atlantic Coast Monday night. A Small Craft Advisory will likely be needed for the waters Monday night. SCA conditions appear likely Tuesday through Wednesday night for at least a portion if not all of our waters as a series of low pressure areas bring precipitation, potentially of the winter variety to the region. Winds will favor a north northwest trajectory during this period, gusting upwards of 20 knots, with gusts in excess of 20 knots over the lower Chesapeake Bay. && .LWX WATCHES/WARNINGS/ADVISORIES... DC...None. MD...Winter Weather Advisory until 5 PM EDT this afternoon for MDZ501-502. VA...Winter Weather Advisory until 5 PM EDT this afternoon for VAZ503-504-507-508. WV...Winter Weather Advisory until 5 PM EDT this afternoon for WVZ050-055-501>506. MARINE...Small Craft Advisory until 6 PM EDT this evening for ANZ530- 531-538-539. && $$ SYNOPSIS...BJL/KLW NEAR TERM...KLW SHORT TERM...KLW LONG TERM...BKF AVIATION...BKF/KLW MARINE...BKF/KLW is the U.S. government's official web portal to all federal, state and local government web resources and services.