Area Forecast Discussion
Issued by NWS Sterling, VA

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000 FXUS61 KLWX 221031 AFDLWX Area Forecast Discussion National Weather Service Baltimore MD/Washington DC 631 AM EDT Fri Jul 22 2016 .SYNOPSIS... High pressure has moved off the coast. A system will pass north of the region today, followed by a weak front moving in on Saturday. High pressure will then be in control of the area`s weather Sunday before another front enters the area Monday into Tuesday. && .NEAR TERM /THROUGH TONIGHT/...
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Large upper ridge remains centered across the Central Plains this morning, with the region sitting in north/northwest flow aloft on the northeastern periphery of the ridge. As is typical in these patterns, a large convective system (MCS) is ongoing early this morning across the Midwest, but is expected to remain west of our area as it weakens this morning. That being said, will likely see some debris clouds in the form of cirrus streaming overhead this morning. Through today, will see an upper level disturbance over southern Canada race eastward and into the northeastern US at the same time a weak frontal boundary pushes southward into the eastern Great Lakes and Northeast. This will be the primary focus for any additional shower/thunderstorm development today and into this evening, and actually already seeing showers/thunderstorms blossom near Lake Erie this morning. While the majority of the convective activity for much of today should remain north of the Mason Dixon line where greater forcing and instability lies, could see a pop- up shower or thunderstorm develop over the higher terrain. Late in the day and into the first half of tonight, any ongoing convection over PA/OH will tend to move southeastward and into the area (mainly DC metro northward) on NNW flow aloft and as MLCAPE increases to around 1500 J/KG. It should be noted that if the activity near Lake Erie this morning can become more organized, its arrival would be more towards mid-late afternoon. Shear profiles are not impressive, with around 20 knots of 0-6KM shear, but any storm during the day has the potential to be pulse severe and any ongoing cold-pool driven convection later this evening has potential to become strong to locally severe. Otherwise...partly to mostly sunny and hot. Highs should max out in the low to mid 90s. With dew points in the mid/upper 60s, heat indices peak around 100F, highest east of the Blue Ridge Mountains. Any convective activity will wane after midnight. Lows tonight from the upper 60s to upper 70s.
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&& .SHORT TERM /SATURDAY THROUGH SUNDAY NIGHT/... Remnant frontal boundary will then push southward into the Mid- Atlantic on Saturday. By this time, will be primarily a moisture/dew point boundary. Current indications are that the front will move to about the DC metro by midday Saturday, separating mid 60s dew points to the north across PA/northern MD from low 70s DC southward. With MLCAPE values progged to reach 1500-2000+ J/KG and with 25-30 knots of 0-6KM shear, any convective activity that can fire Saturday afternoon along this boundary has the potential to become strong to locally severe. In addition, highs will range from about 90-97F, and when combined with increasing humidity, especially on southern side of boundary, heat indices are expected to reach from 100-105F. Thus a Heat Advisory may be needed for Saturday. Convection will once again wane Saturday night with lows in the 70s. Boundary will wash out for Sunday, but dew points not expected to recover past the mid 60s. Thus, should be looking at a mostly sunny and mainly dry day, although the region will still be entrenched in NW flow aloft, so will still need to monitor for any MCS activity upstream. Highs will reach 90-97F again, but with lower dewpoints, heat indices will be a little lower than Saturday, generally maxing out about 100F. Temperatures still warm Sunday night again with lows in the 70s. && .LONG TERM /MONDAY THROUGH THURSDAY/... Monday may end up being both the hottest and the stormiest day of the stretch. Low pressure tracking across southern Canada will drop a weak frontal boundary toward the area. Higher dewpoints will be drawn north in advance of the front, while 850 mb temps remain in the 20-22C range. This temperature/dewpoint combo may yield heat index values approaching 105. The inherent instability, lower heights, and focusing mechanism would be the necessary ingredients for thunderstorm development. It remains to be seen how it all comes together. The synoptic pattern doesn`t look much different for the middle of next week. The westerlies will remain in retreat up in Canada, permitting broad ridging to set up shop across the continental US. The core of this ridge will remain west of the Appalachians, though, meaning the local area won`t be seeing the worst of the heat. Daytime temperatures should still be in the 90s. However, drier air may attempt to filter in; that will be dictated by where the front stalls out. At this point, its not a forgone conclusion that the front will clear the area, so precip chances will linger in the extended...with reduced confidence. && .AVIATION /10Z FRIDAY THROUGH TUESDAY/... Primarily VFR through Sunday. Isolated to scattered showers/thunderstorms around both this evening and Saturday, but coverage/confidence not high enough to warrant mention at this time. Some patchy light fog also possible each morning, but again coverage not expected to be widespread. Southwest winds 5-10 knots today turn west-northwesterly Saturday before becoming light and variable Sunday. Scattered thunderstorm chances Monday, especially afternoon and evening. This looks to be the most likely day to see coverage, but specific impacts still questionable. Front will either be over or south of the terminals by Tuesday. Another round of storms possible, but confidence lower than Monday due to positional uncertainties. && .MARINE... Generally sub-SCA S/SW winds through this afternoon. A few 20 knot gusts possible in the early morning hours across the central Chesapeake and lower tidal Potomac. Later this evening, will see an increase in speeds across the central Chesapeake and lower tidal Potomac again, and a Small Craft Advisory has been issued. These winds will dissipate Saturday morning with sub-SCA winds expected Saturday into Sunday. Gradient winds should increase Monday ahead of front. It is unclear how significant that will be. Of greater...but not high...certainty will be the development of thunderstorms, which may contain local strong wind gusts. That front will either be over or south of the mid Bay and lower Potomac by Tuesday. && .CLIMATE... Record highs and warm lows for the upcoming weekend... Today... DCA...103 in 1926...83 in 2011 BWI...106 in 2011...81 in 2011 IAD...105 in 2011...76 in 2011 Saturday... DCA...102 in 2011...84 in 2011 BWI...102 in 2011...79 in 1978 IAD... 99 in 2011...76 in 2011 Sunday... DCA...101 in 2010...84 in 2011 BWI...101 in 2010...82 in 2010 IAD... 99 in 2010...78 in 2010 Monday... DCA...100 in 1930...79 in 1965 BWI...100 in 2010...77 in 1887 IAD... 98 in 2010...76 in 1965 && .LWX WATCHES/WARNINGS/ADVISORIES... DC...None. MD...None. VA...None. WV...None. MARINE...Small Craft Advisory from 6 PM this evening to 6 AM EDT Saturday for ANZ534-537-543. && $$ SYNOPSIS...MM NEAR TERM...MM SHORT TERM...MM LONG TERM...HTS AVIATION...MM/HTS MARINE...MM/HTS CLIMATE...HTS

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