Area Forecast Discussion
Issued by NWS Sterling, VA

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173 FXUS61 KLWX 151400 AFDLWX Area Forecast Discussion National Weather Service Baltimore MD/Washington DC 1000 AM EDT Sat Jul 15 2017 .SYNOPSIS... A cold front will push east of the region today, with weak high pressure gaining control of the weather through this weekend. An upper level trough will affect the weather early next week. Another cold front may approach the region by the end of the week. && .NEAR TERM /THROUGH TONIGHT/... Mixing from northwest flow combined with a shortwave moving across the US/Canadian border assisting in driving cold front east of the forecast area this morning. A couple of showers have stubbornly hung on across northeast Maryland. Have had to put 20% PoPs back into the forecast. Still believe that building subsidence and advection of drier air make precip a limited, near-term possibility. While it will be cooler than the past several days, temperatures aloft will only be falling slowly. Thus expect DCA- CHO and southeast will still be AOA 90F. Latest LAMP concurs, and no adjustments made to temperature forecast. Scattered cumulus should also persist through the day. Weak high pressure will be located west of the area tonight and will provide tranquil conditions. There could be some patchy fog, mainly in the western valleys. Lows will be in the 60s for most areas. && .SHORT TERM /SUNDAY THROUGH MONDAY NIGHT/... Forecast has some minor wrinkles for Sunday and Sunday night as the weak high moves offshore and return flow begins (especially by night). Have kept the forecast dry through the daytime hours due to capping and mid level dry air. However, some instability will be returning to the mountains, so that would be the favored area if a stray shower/storm were to form. Theta-e advection begins in earnest overnight, with mid-level lapse rates steepening as well. The question is whether there will be any forcing in this layer. Troughing will be moving into the eastern Great Lakes by that time, so it`s not out of the question. Have allowed for some low end POPs for now. A more defined chance for thunderstorms will come Monday as the upper level trough continues to make slow eastward progression, although it may be filling. Mid-level lapse rates will be sufficiently steep for moderate instability to develop, though deep layer shear will be on the weak side...around 20 kt. A few stronger storms are within the realm of possibility though. The highest chances for rain will be across northern parts of the area. Showers and storms should diminish overnight. Temperatures aren`t expected to stray too far from normal Sunday through Monday as advection remains near neutral. Dew points will rise though, especially on Monday. && .LONG TERM /TUESDAY THROUGH FRIDAY/... An upper level trough will remain over the region Tuesday, with attendant instability due to cooler air aloft and continued warm southerly flow at the surface. This would result in a continued chance of showers and t-storms with seasonable temps. The lower end of the trough looks likely to cut off into a closed low and sink southward away from our region later next week, allowing heights to rise and warm air aloft to return. This would likely reduce the shower/t-storm risk but also increase temps back to above normal readings. By the end of the week, another shortwave trough looks likely to pass the region, possibly bringing a surface cold front with it and another increasing risk of showers/t-storms. && .AVIATION /14Z SATURDAY THROUGH WEDNESDAY/... Front will push south this morning, with NW winds around 10 kt through the day with sct cumulus. There is at least some minor potential for valley fog at MRB tonight. Low probability of any aviation impacts Sunday and Sunday night as weak high pressure moves east. Scattered to numerous thunderstorms are expected Monday with an upper disturbance, especially in the afternoon and evening. Should be VFR through most of the week, except during any brief intervals when showers/t-storms could affect the terminals on Tuesday and again late in the week. && .MARINE... Northwest flow overspreading the waters this morning. The leading edge achieving full mixing, which is able to tap winds close to 20 kt in gusts. Have added a Small Craft Advisory where this threat resides (Maryland portion of the Bay below Sandy Point and tidal Potomac below the Rte 301 bridge). Am using 3pm as an end time for now. Threat will be diminishing through the day (best mixing only at leading edge), so Advisory may be able to be cancelled early. Light winds tonight and Sunday with high pressure, then return southerly flow will begin Sunday night into Monday. Will have to keep an eye on channeling potential, but otherwise confidence is low on SCA conditions being reached. After Monday, winds look likely to stay sub-SCA most of the week. Main concern will be any need for special marine warnings with t-storms, with highest risk being Tuesday and Friday. && .TIDES/COASTAL FLOODING... Southerly flow developing across the region as high pressure shifts offshore will start to build anomalies Sunday, with minor flooding at high tide possible at our sensitive sites (most likely Straits Point) by Sunday aft/eve. This would likely continue into Monday. && .LWX WATCHES/WARNINGS/ADVISORIES... DC...None. MD...None. VA...None. WV...None. MARINE...Small Craft Advisory until 4 PM EDT this afternoon for ANZ532>534-536-537-541. && $$ SYNOPSIS...RCM/HTS NEAR TERM...HTS/ADS SHORT TERM...ADS LONG TERM...RCM AVIATION...ADS/RCM MARINE...HTS/ADS/RCM TIDES/COASTAL FLOODING...RCM

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