Area Forecast Discussion
Issued by NWS Sterling, VA

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030 FXUS61 KLWX 201448 AAA AFDLWX Area Forecast Discussion...UPDATED National Weather Service Baltimore MD/Washington DC 1048 AM EDT Thu Jul 20 2017 .SYNOPSIS... Weak high pressure will build into the region today, 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 /UNTIL 6 PM THIS EVENING/...
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12Z IAD sounding came in significantly warmer aloft than 24 hrs ago with a 4 deg rise at 500 mb with an inversion evident above that level which should suppress deep convection completely, so have removed the slight chance of thunder that we had. At low levels, global models indicate a 4C deg rise at 850 mb by day`s end and 12Z IAD sounding also showed an increase in 0-6km mean flow (300/09kt) which is double the strength than yesterday. This will result in temperatures hitting the century mark in a few places or perhaps a deg or two above 100F. Dewpoints will mix out to the mid 60s, but still with temps around 100F heat indices will max out around 105. Criteria west of the Blue Ridge is 5 degs lower at 100 this should be met easily at places like Hagerstown, Martinsburg, Cumberland and Winchester. New heat advisories have been issued for these areas.
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&& .SHORT TERM /6 PM THIS EVENING THROUGH SATURDAY NIGHT/... Upward trend in heat should allow heat advisories across lower elevations (especially since the advisory criteria was lowered to 100F west of the Blue Ridge this year) Friday and through the weekend. The only limiting factor is clouds/rain/outflow from ring of fire thunderstorm activity. This is a multi-day heat wave with particularly warm overnights. Repeated days with such heat and humidity takes a toll on the human body - even if we don`t hit excessive heat warning criteria (110F east of the Blue Ridge). Please check in on your neighbors. && .LONG TERM /SUNDAY THROUGH WEDNESDAY/... The heat stays high for Sunday, then slightly cooler temperatures are expected next week. Heat will continue Sunday as subtropical ridge off the coast keeps pumping warm moist air into the Mid Atlantic. Dewpoints still around 70. Highs mainly in the low to mid 90s. With this much energy in the atmosphere there will be the chance for thunderstorms. Monday a shortwave will traverse the northeastern US. This will cause heights to lower slightly into the Mid Atlantic which will drop temperatures a few degrees, although given the amount of heat the surface has absorbed over the past week am still expecting temperatures to top 90 east of the mountains, perhaps mid 90s in the cities. Again, given amount of energy in the atmosphere and approaching short wave have to release the dispersion may lead to thunderstorms, possibly over NY/PA dropping into this forecast area. Tuesday should feel more comfortable as the trough moves offshore. Highs in the 80s. This should continue Wednesday. Between today and the final day of July we lose 18 minutes of daylight. && .AVIATION /15Z THURSDAY THROUGH MONDAY/... VFR prevails under high pressure through the weekend. Organized thunderstorm chances tonight through the weekend. VFR conditions expected Sunday. Scattered thunderstorms may occur Sunday afternoon/evening, then a better chance Monday. && .MARINE... Bay breezes at the shores with south flow in the middle each afternoon through this weekend under high pressure centered over the Mid-South States. Organized thunderstorm chances begin tonight. Winds expected to remain below SCA Sunday with possible scattered afternoon and evening thunderstorms. Better chance for storms Monday. && .CLIMATE... 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 19. 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. && .LWX WATCHES/WARNINGS/ADVISORIES...
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DC...Heat Advisory until 7 PM EDT this evening for DCZ001. MD...Heat Advisory until 7 PM EDT this evening for MDZ003>006-011- 013-014-016>018-502>508. VA...Heat Advisory until 7 PM EDT this evening for VAZ027>031- 036>040-050>057-501-502-505-506. WV...Heat Advisory until 7 PM EDT this evening for WVZ051>053. MARINE...None.
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