Area Forecast Discussion
Issued by NWS Sterling, VA

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000 FXUS61 KLWX 141918 AFDLWX Area Forecast Discussion National Weather Service Baltimore MD/Washington DC 318 PM EDT Fri Jul 14 2017 .SYNOPSIS...
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A cold front will drop south across the area this afternoon and evening. Weak high pressure will influence the weather over the weekend, before another cold front moves in from the north and stalls over the area next week.
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&& .NEAR TERM /THROUGH TONIGHT/...
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Latest surface analysis suggests that a cold front is on our doorstep as of 18z. The air mass to the south of the front is characterized by MLCAPE 1500-2500 J/kg, due to sufficient daytime insolation (surface based up to 3500 J/kg). Special 18Z LWX RAOB also have instability in that range. There are weak inversions around 800 mb and 500 mb. Cores have already exceeded those levels. Shear isn`t that strong and mid level lapse rates aren`t impressive...which are mitigating factors. The above factors suggest that activity will be driven by small scale interactions...which has been the case thus far. Severe Thunderstorm Watch remain in place til 9pm for practically the whole forecast area... except for Albemarle, Nelson, Augusta, and Highland Counties and associated independent cities. Regional radar depiction suggests that premise to hold off on Flash Flood Watch still appropriate. Cells are on the smaller side, without much in regard to repeating cells. Precip waters highest (around 2 inches) around metro Baltimore, and that`s where the storm organization most concerning. LWX RAOB 1.9 inches. Will continue to monitor, and issue a short fused watch if that proves necessary. Heat index values in the metro area hovering around 100-105, due more to high dewpoints than excessive temperatures. Midday cloud cover across northern Maryland has been the mitigating factor. Will be keeping Heat Advisory as is spatially. Expiration time 7pm, but likely the end will be marked by passage of thunderstorms. Convective evolution this evening suggests that storms will be passing through DC around 4-5pm, and passing off southern Maryland around 8-9pm. Both RAP and HRRR suggests that there will be sufficient CAPE and ample shear after the initial round. Have chance PoPs lingering into the evening.
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&& .SHORT TERM /SATURDAY THROUGH SUNDAY NIGHT/...
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No changes made to the forecast this weekend at this time. It looks like the front will push far enough south on Saturday that the forecast area should be dry. While it will be cooler, temperatures will still remain near normal for this time of year, in the mid 80s to around 90. Low pressure developing along the front may nudge it back toward southern MD Saturday night, though have kept the forecast dry for now. Winds should back to the south again on Sunday as a shortwave trough aloft swings into the Great Lakes. It looks like the prior front and its deep moisture remain to the south, so potential for convection looks limited. If there is a chance, it would be over the mountains (closer to trough) and southern MD (should front try to pivot north due to the prior low pressure). Temperatures should rebound a few degrees over Saturday.
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&& .LONG TERM /MONDAY THROUGH FRIDAY/...
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A persistent upper level trough of low pressure will be accompanied by a couple of disturbances from the west Monday through Wednesday to enhance the threat of showers and thunderstorms each period. High temperatures will be in the upper 80s to lower 90s. Weak high pressure will build in Wednesday night and Thursday. Drier air but warm temperatures will evolve. The next storm system will enter the region from the west Thursday night and Friday. There is a chance for showers and thunderstorms with this system and its associated cold front. High temperatures will be in the lower 90s.
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&& .AVIATION /19Z FRIDAY THROUGH WEDNESDAY/...
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Scattered thunderstorms lurking north of the metros early this afternoon. Local IFR restrictions likely within any storms. Will be updating TAFs as needed once threat becomes apparent at a specific terminal. Timing will bring the most numerous storms through the metros between 20-00z. Less certain about whether CHO will be affected, but if they are, the window would last through about 02z. No significant aviation impacts are expected Saturday and Sunday with weak high pressure behind the cold front. Local IFR conditions possible Monday through Tuesday night with showers and thunderstorms in the region.
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&& .MARINE...
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Scattered to numerous showers and thunderstorms will be crossing the waters this afternoon and evening. Local strong/gusty winds probable within storms. Monitor for Special Marine Warnings, which will be issued as needed. Northwest flow will overspread the waters behind the cold front for Saturday. Gusts still forecast fall just short of SCA criteria. Have removed that threat from the marine forecast. Winds eventually become southerly again by late Sunday as the weak high pressure moves offshore. No marine hazards expected through Tuesday night.
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&& .LWX WATCHES/WARNINGS/ADVISORIES... DC...Heat Advisory until 7 PM EDT this evening for DCZ001. MD...Heat Advisory until 7 PM EDT this evening for MDZ011-013-014- 016-504-506. VA...Heat Advisory until 7 PM EDT this evening for VAZ052>056. WV...None. MARINE...None. && $$ SYNOPSIS...HTS NEAR TERM...HTS SHORT TERM...HTS/ADS LONG TERM...KLW AVIATION...HTS/ADS/KLW MARINE...HTS/KLW

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