Area Forecast Discussion
Issued by NWS Sterling, VA

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935 FXUS61 KLWX 210104 AFDLWX Area Forecast Discussion National Weather Service Baltimore MD/Washington DC 904 PM EDT Thu Jul 20 2017 .SYNOPSIS... Weak high pressure will remain over the region through tonight, followed by the potential for weak disturbances Friday through Sunday. An upper level trough of low pressure will move over the northeastern US early in the new week. && .NEAR TERM /THROUGH FRIDAY/...
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Very warm and humid this evening across the Mid-Atlantic region. The 8PM ob at DCA came in with 91/77 with a south wind and heat index of 105. Warm and humid conditions will continue overnight with temps only dropping into the 70s and low 80s in the metros and near the Bay. Showers and thunderstorms have been approaching the region from PA and the Ohio Valley this evening. Activity has been weakening as nocturnal inversions set-up. Showers will likely enter the Potomac Highlands and northern MD tonight but dissipate as they move further south. Most areas will be dry overnight. A heat advisory is in effect for much of the region east of the Blue Ridge from 10am-8pm Friday. Heat indices around 105 are forecast to occur with dewpts climbing into the low 70s. Temps in the mid to upper 90s expected by afternoon. For tomorrow, models don`t indicate much change in MaxT so temperatures will be as hot as they are today. Moisture might be a little higher with 850 mb and sfc dewpoints progged much higher than today. Heat advisories will likely be needed again for most of the area similar to today. Showers and thunderstorms expected to form across the Appalachian Mtns Friday afternoon and move eastward into the evening. Shear is still minimal however instability will be high due to the high dewpts. Isolated thunderstorms may become strong to severe.
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&& .SHORT TERM /FRIDAY NIGHT THROUGH SATURDAY NIGHT/...
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Potentially active convective day Saturday afternoon and evening. Both GFS and ECMWF show an MCS over the lower Great Lks moving sewd across the fcst area Sat afternoon and evening. As always with summertime convection, details especially with respect to timing are very uncertain this far out. Continued hot and humid with heat advisories still likely to be needed.
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&& .LONG TERM /SUNDAY THROUGH THURSDAY/... Upper level energy from the west will set up a trough of low pressure on the leeside of the Appalachians Sunday into Sunday night. Additional showers and thunderstorms could form along this trough but could mainly be isolated to scattered. Highs will reach the middle to upper 90s. A cold front will approach the region Monday before pushing across the region Monday night. Showers and thunderstorms will develop along and ahead of this front. Highs will reach the 90s again. The front could stall just to our south Tuesday, allowing for a couple of showers or a thunderstorm to develop. High pressure will build in from the west and northwest Tuesday night through Thursday. Temperatures will be nearly 10 degrees cooler than previous days with a comfortable relative humidity. && .AVIATION /01Z FRIDAY THROUGH TUESDAY/...
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Hot and hazy next few days. Sct-nmrs t-storms mainly Sat. Mvfr to ifr conditions possible with showers and thunderstorms Sunday through Monday night. Winds southwest becoming west-northwest 5 to 10 knots Sunday into Sunday night. Winds northwest becoming southwest 5 to 10 knots Monday and Monday night.
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&& .MARINE... Potential for showers and thunderstorms Fri night and Saturday and again Sun which may require special marine warnings. No marine hazards expected Sunday through Monday night. Winds southwest 10 knots Sunday and Sunday night. Winds southwest 5 to 10 knots Monday and Monday night. && .CLIMATE...
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Late July is climatologically the warmest time of the year, just like six months from now is the coldest time. How has the heat of 2017 compared with heat waves (up to now) in other years? I chose "consecutive days that reached 90 and above" as the criteria. At DCA there have been two times this summer with stretches of six consecutive days - beginning June 29 and again July 10. In terms of ranking (going back to 1871) these are #47 and 48. #1 (18 consecutive days) occurred 145 years ago - beginning 6/27/1872! For Baltimore there were also six consecutive days beginning June 29, which puts it in 36th place. #1 was 13 consecutive days beginning 7/3/1993. The all-time record for June-Aug: for Baltimore 25 consecutive days beginning 7/12/1995. For DC there have been two periods of 21 consecutive days - beginning 7/29/1988 and 7/25/1980.
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&& .LWX WATCHES/WARNINGS/ADVISORIES...
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DC...Heat Advisory from 10 AM to 8 PM EDT Friday for DCZ001. MD...Heat Advisory from 10 AM to 8 PM EDT Friday for MDZ011-013-014- 016>018-503>506. VA...Heat Advisory from 10 AM to 8 PM EDT Friday for VAZ037>039- 050>057-502-506. WV...None. MARINE...None.
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&& $$ SYNOPSIS...LFR NEAR TERM...HSK/LFR SHORT TERM...LFR LONG TERM...KLW AVIATION...LFR/KLW MARINE...LFR/KLW CLIMATE...WOODY!/HSK

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