Area Forecast Discussion
Issued by NWS Sterling, VA

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000 FXUS61 KLWX 181350 AFDLWX Area Forecast Discussion National Weather Service Baltimore MD/Washington DC 950 AM EDT Sat Mar 18 2017 .SYNOPSIS... Low pressure and associated cold front will move through the area late this afternoon. High pressure then builds over the area briefly, before another cold front crosses the area Monday night. Stronger high pressure will take hold across the area for most of the remainder of the upcoming work week. && .NEAR TERM /THROUGH TONIGHT/... An upper low is diving across the Great Lakes region this morning, and will slowly traverse across the mid-Atlantic through tonight. The associated surface reflection is rather complex, with low pressure below the upper low, a second low likely to develop near the triple point at the surface (over our area), and then this evening the bulk of that energy will translate off the coast and develop another surface low offshore. For this afternoon, the initial shortwave will push across the region. This turns on the upslope machine out west and also serves to spark convective showers further east. South of the triple point, the southern portion of our forecast area (Charlottesville, Harrisonburg, Fredericksburg) will get into the warm sector and see temperatures rise into the 60s. There will be just enough instability that perhaps a rumble of thunder or two will occur, and this has been added to the forecast for just a few hours this afternoon. As the shortwave pulls east, and the energy transfer occurs to the offshore low, an inverted trough will be left behind and that will serve as a focus for additional rain, especially in the Frederick/DC/Pax River corridor and points east. Went with likely PoPs for the morning update but if model agreement persists with the 12z cycle, I may raise to categorical for the afternoon update. Total rain could exceed 1/2 inch. Some mixing with or changing to snow is likely late tonight at the very end of the precipitation. However, surface temperatures remain at or above freezing except right along the Mason-Dixon line and at higher elevations. I kept the upslope areas in the 1-2 inch range (similar to the previous forecast) but I also added about 1/2 inch of snow in northern Carroll / Baltimore / Harford Counties and around an inch on Catoctin Mountain. Can`t rule out a dusting atop Skyline Drive either. All totals look sub-advisory at this time. && .SHORT TERM /SUNDAY THROUGH MONDAY NIGHT/... Models now in agreement that precip ends by mid-morning Sunday, with temperatures at least slightly cooler. Rather breezy though so wind chills will be in the 30s to lower 40s all day. High pressure shifts south Mon afternoon with scattered rain showers expected with next frontal passage. Cold front clears the area by 12Z Tue. && .LONG TERM /TUESDAY THROUGH FRIDAY/... Computer model guidance is in good agreement about the extended portion of the forecast. Aloft, troughing will persist through the middle of the week, giving way to a ridge by Friday. At the surface, a cold front will be settling south of the area Tuesday morning. A wave of low pressure will ride along the front Tuesday night, while a secondary front pushes south into the area by Wednesday morning. Canadian high pressure will then settle southeast, crossing the area Thursday and continuing offshore Friday. In terms of precipitation, the only notable chance comes with the Tuesday night low pressure. While there is some spread in QPF fields, the consensus at the moment has the best opportunity for the southern quarter of the forecast area. Deterministic runs suggest profiles will be warm enough for rain, but a few GEFS members crash in enough cold air with the secondary front for a changeover to snow before ending. At the moment, it doesn`t appear to be an impactful system regardless of p-type. The coldest period of the week will be Wednesday into Thursday behind the second cold front, with highs in the 40s to lower 50s and lows below freezing. The rest of the period will be fairly close to seasonal normals. && .AVIATION /14Z SATURDAY THROUGH WEDNESDAY/... Aviation forecast is pretty tricky. Rain showers could bring some temporary vsby restrictions, but as precip mixes with or turns to snow late tonight, both cigs/vsby could drop to MVFR or lower for a few hours. Gusty N or NNW winds on Sunday. A period of sub-VFR conditions is possible in rain and low clouds, mainly at CHO, Tuesday into Tuesday night as low pressure moves south of the area. Otherwise VFR prevails through Wednesday. NW winds could gust to 20 kt both Tuesday and Wednesday. && .MARINE... Winds remain fairly light, but will strengthen this evening. SCA begins at 11pm, which might be a touch early. Will assess that further before the afternoon update. Model guidance and blends show wind gusts that could get fairly close to gales on Sunday. Definitely a solid 25-30 knots though. Although winds start to slacken Sunday night, SCA gusts could persist into Monday for portions of the waters. After a cold front pushes south, NW flow will dominate Tuesday into Wednesday, although it could become NE-N for a period as low pressure passes to the south Tuesday night. SCA conditions will be possible both days. && .LWX WATCHES/WARNINGS/ADVISORIES... DC...None. MD...None. VA...None. WV...None. MARINE...Small Craft Advisory from 11 PM this evening to 6 PM EDT Sunday for ANZ530>543. && $$ SYNOPSIS...JE NEAR TERM...JE SHORT TERM...JE/LFR LONG TERM...ADS AVIATION...JE/LFR/ADS MARINE...JE/LFR/ADS

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