Area Forecast Discussion
Issued by NWS Sterling, VA

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000 FXUS61 KLWX 270120 AAA AFDLWX Area Forecast Discussion...UPDATED National Weather Service Baltimore MD/Washington DC 920 PM EDT Mon Sep 26 2016 .SYNOPSIS... A cold front will pass through the area overnight into Tuesday. An upper-level low will impact the area during the middle to latter portion of the week. && .NEAR TERM /THROUGH TUESDAY/... Cdfnt is currently moving through the Appalachians with numerous showers along and well ahead of it. Hourly rain rates have been around a quarter of an inch in some spots with up to half inch total rainfall since this afternoon. Showers will continue through the night and begin to taper off from west to east after midnight. Showers should end by daybreak in the District but persist over St. Marys until midday. Total rainfall amounts will likely range between a quarter to half inch except up to an inch in St. Marys where cdfnt will slow down. The front slows to a crawl on Tuesday as it becomes oriented parallel to the upper level flow just to our southeast. Thus...while our northwestern zones should dry out...the risk of showers lingering into much of Tuesday increases as one heads southeastward...with showers likely most of the day in southern Maryland. Where the sun breaks out...highs should rebound into the 70s. && .SHORT TERM /TUESDAY NIGHT THROUGH THURSDAY/... A great deal of uncertainty regarding the details of the weather during the middle portion of the week as a large closed low settles southward into the eastern United States and induces cyclogenesis on the stalled frontal boundary near our region. Tuesday night should still be mostly dry as the influence of the upper low is yet to be very significant. By Wednesday, the low is likely to start inducing the development of a low pressure system somewhere in our region to the east or south. A position further east would likely result in a drier day Wednesday into Wednesday night, while if the low develops further south along the front, rain becomes more likely as we head towards Wednesday night, especially across eastern portions of the CWA. Temps are uncertain given the uncertain rain/cloud forecast, but more 70s would seem probable. Guidance is more solid on the low pressure that develops either redeveloping further west or being pulled west by the upper low on Thursday, with most guidance showing at least some rain across the region. It should be noted that while the GFS is fairly dry with more showery weather, the more southwesterly storm development depicted by the ECMWF nows shows several inches of rain. Flash flood guidance being very high thanks to the recent period of dry weather (many areas having seen less than half an inch of rain in the last month), the ground is likely to be able to absorb a fair amount of water. We will have to monitor any potential for excessive rainfall in a relatively short period of time however. Highs on Thursday are more uncertain still but may be stuck in the 60s. && .LONG TERM /THURSDAY NIGHT THROUGH MONDAY/... Looks like guidance is starting to agree on the evolution of the upper level low. GFS and ECMWF have been consistent on its solution - the upper level low is located to over the Great Lakes - tracking slowly south and later retrograding. In general, GEFS members agree on this solution as well, with some members moving it faster north and away from our region. Location of this system will have impacts on the forecast, but for now it seems that there will be a chance of showers Thursday night into Saturday as moisture is advected into our region... and then drying out sometime on Saturday and into Monday with high pressure building in. && .AVIATION /01Z TUESDAY THROUGH SATURDAY/... MVFR to IFR cigs tonight in showers as cdfnt crosses the area. Some improvement likely on Tuesday as the front shifts southeastward. Low pressure development Wednesday-Thursday brings potential for SUB-VFR conditions again, but uncertainty with timing and exact conditions is high at this time. Sub-VFR conditions possible Thursday night and maybe into Saturday as upper level disturbance could bring showers into the region. Conditions becoming VFR Saturday into Sunday as high pressure builds in. && .MARINE... Southerly channeling ahead of the approaching cold front is bringing small craft conditions to most of the waters. These will continue into the evening before gradually winding down from northwest to southeast after midnight as the front moves through. Lighter winds likely Tuesday-Wednesday then new storm developing across the region may result in a return of SCA conditions late Wednesday into Thursday. Small craft advisory possible Thursday night and into Friday night with near or above the threshold. Winds will be diminishing Saturday and into Sunday below the small craft advisory threshold. && .TIDES/COASTAL FLOODING... Have issued Coastal flood advisory for Anne Arundel. All others are expected to remain below flood stage. Tidal Anomalies should drop later Tuesday behind the cold front. && .LWX WATCHES/WARNINGS/ADVISORIES... DC...None. MD...Coastal Flood Advisory from 1 AM to 8 AM EDT Tuesday for MDZ014. VA...None. WV...None. MARINE...Small Craft Advisory until 6 AM EDT Tuesday for ANZ530>543. && $$ SYNOPSIS...RCM NEAR TERM...LFR SHORT TERM...RCM LONG TERM...IMR AVIATION...IMR/LFR MARINE...IMR/LFR TIDES/COASTAL FLOODING...LFR

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