Area Forecast Discussion
Issued by NWS Sterling, VA

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929 FXUS61 KLWX 260135 AFDLWX Area Forecast Discussion National Weather Service Baltimore MD/Washington DC 935 PM EDT Sun Sep 25 2016 .SYNOPSIS... A frontal boundary will remain stalled to our south tonight while high pressure passes to our north. The high will move off the coast Monday. A cold front will pass through the area overnight Monday into Tuesday. An upper-level low will likely impact the area during the middle portion of next week. && .NEAR TERM /UNTIL 6 AM MONDAY MORNING/... A stationary front remains over the Carolinas, with high pressure extending from Quebec down the eastern side of the Appalachians. While the CAD-induced cloud deck developed some breaks, it never fully dissipated and currently resides south of a Woodstock- Fredericksburg line. Tonight, a cold front currently in the Mississippi Valley will push eastward and the high will shift east as well. THis will cause the flow to gradually become more southeasterly and increase moisture flow into the region. The general consensus is that the cloud deck will spread back northward, and perhaps eastward late. The lowest clouds (with reduced ridgetop visibility and perhaps lighter low elevation fog) are expected over the southwestern CWA. A little drizzle or perhaps some areas of light rain may occur in west-central Virginia. The NAM is the most robust with its QPF field, but have confined mentionable precip to near and west of the Blue Ridge. The clouds will have an effect on low temperatures, but overall it will be another seasonable night. Lows will be closer to 60F where it stays cloudy and in the urban centers. To the north and east where it stays less cloudy, radiational cooling should allow temps to drop well into the 50s and perhaps some 40s. && .SHORT TERM /6 AM MONDAY MORNING THROUGH WEDNESDAY/... Cold front will cross the region from west to east during the period followed by potential cyclogenesis at the end of the period as a strong upper low drops southeastward towards the area. Monday morning will start fairly cloudy especially towards the southwest (west-central VA) where upslope flow will be ongoing. Some drizzle or showers will be possible here. As we head through the day, the front will approach and clouds at higher levels will become more widespread as well. Think highs manage the low 70s before the clouds get too dense to block the sun completely but its not impossible most places get stuck in the 60s, especially where the lower cloud deck doesn`t erode. Showers will start to push east from the west as we head through the afternoon but thunder odds are not high given we stay fairly stable on the north side of the front over the Carolinas. The Potomac Highlands will have the highest chance of thunder during the early evening. Main fgen band with front moves through Monday night. It may slow down as it crosses the area in response to the upper low dropping down from the Great Lakes but think it should generally clear the CWA by later Tuesday. Instability will be minimal since warm sector will have trouble coming back north ahead of the cold front (which really will likely pass our area as more of an occluded front) but forcing aloft with high PW`s may be enough to squeeze out some thunder. Rainfall amounts very uncertain but a decent rain on the order of a half inch is expected...perhaps more if the forcing is strong enough and the front slows down enough. Lows will stay relatively mild with the clouds and rain Monday night...50s and 60s...while temps should try to rebound into the 70s on Tuesday after clouds break. Things get very uncertain as we head into Wednesday. Tuesday night should start out dry but as the upper low approaches from the west cyclogenesis may begin in our area by midday Wednesday which would result in the redevelopment of showers. Still lots of uncertainty here so have kept highs in the 70s...but if the more pessimistic scenarios happen we may be stuck in the 60s Wednesday. && .LONG TERM /WEDNESDAY NIGHT THROUGH SUNDAY/... A cutoff upper level low over the Great Lakes will push south and east towards our area Wednesday night and into Thursday... and possibly Friday. There is uncertainty on the exact details of the forecast as guidance is in disagreement with location timing of the upper level low. If it pushes further south, a coastal low pressure could develop over the Mid-Atlantic region. With this scenario the chance of showers over our CWA extends from Wednesday night and into Friday, and dry conditions into the weekend. If the upper level low lingers to our west and north then the chance of showers could extend from Wednesday night into the weekend. Basically, expect a chance of showers Wednesday night into Friday and uncertain into the weekend... depending on position of the upper level low. && .AVIATION /01Z MONDAY THROUGH FRIDAY/... While the clouds never fully eroded across central VA, they are currently in the VFR category. Would expect these to lower again during the late evening or overnight. Guidance indicates IFR is possible, but have not gone that low in the TAF yet. These lower clouds may encroach northward late tonight so MVFR cigs are not impossible...especially at IAD and MRB. Whatever low cigs develop may have a difficult time lifting on Monday, although dry weather is expected through the day outside of drizzle/sprinkles. Sub-VFR and quite possibly IFR cigs/vis expected Monday night as front moves through with rain. Improving conditions by Tuesday afternoon then perhaps worsening again later Wednesday as another storm approaches. Sub-VFR conditions possible Wednesday night and maybe into Saturday as upper level disturbance could bring showers into the region. Details with this forecast are uncertain as guidance is in disagreement at this moment. && .MARINE... Light winds tonight with high pressure, then SCA takes effect midday tomorrow through tomorrow night with front approaching and then crossing area with rain. May need to extend into Tuesday depending on winds behind front. SCA may again develop Wednesday as another system approaches. Small craft advisory possible Wednesday night and into Friday as coastal low could develop off of the Mid-Atlantic coast. Winds could be diminishing into Saturday but it is uncertain at the moment. && .TIDES/COASTAL FLOODING... The high tide this evening/overnight is the greater of the two today. Anomalies have decreased this evening into the 0.6-0.9 ft range. This departure may allow many locations to fall short of flood thresholds, but will certainly need monitoring if the anomalies recover. Have issued a Coastal Flood Advisory for Annapolis where flooding looks more certain. DC SW Waterfront, Straits Pt, Solomons Island, and Baltimore will be close. Onshore flow will increase and turn to the southeast for Monday through Monday night. Minor flooding is likely during this time...especially during the high tide cycle Monday night which will be the higher of the two astronomical norms. Should the SEly winds be a little stronger, a coastal flood watch may be necessary for Monday night. && .CLIMATE... Unusually warm weather has persisted over the area for quite some time. Autumn-like temperatures are finally making an appearance over the area, and DCA has finally fallen below 60 degrees this morning for the first time since June 9th (total of 107 days). The record most consecutive days at or above 60 degrees in the Washington DC area is 112 set in 2012. && .LWX WATCHES/WARNINGS/ADVISORIES... DC...None. MD...Coastal Flood Advisory until 5 AM EDT Monday for MDZ014. VA...None. WV...None. MARINE...Small Craft Advisory from noon Monday to 6 AM EDT Tuesday for ANZ530>543. && $$ SYNOPSIS...RCM NEAR TERM...ADS/RCM SHORT TERM...RCM/ADS LONG TERM...IMR AVIATION...ADS/IMR/RCM MARINE...IMR/RCM TIDES/COASTAL FLOODING...BAJ/ADS CLIMATE...LWX is the U.S. government's official web portal to all federal, state and local government web resources and services.