Area Forecast Discussion
Issued by NWS Sterling, VA

Current Version | Previous Version | Graphics & Text | Print | Product List | Glossary Off
Versions: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30
-- Highlight Changed Discussion --
-- Discussion containing changed information from previous version are highlighted. --
000 FXUS61 KLWX 150145 AFDLWX Area Forecast Discussion National Weather Service Baltimore MD/Washington DC 945 PM EDT Sat Apr 14 2018 .SYNOPSIS...
-- Changed Discussion --
High pressure will remain across the western Atlantic overnight. A large area of low pressure over the Central Plains will move northeastward into the eastern Great Lakes by Sunday, dragging a powerful cold front across the Mid-Atlantic Sunday night and early Monday. High pressure will return for the middle portion of next week.
-- End Changed Discussion --
&& .NEAR TERM /THROUGH SUNDAY/...
-- Changed Discussion --
A backdoor cold front is presently draped across central and southeast Pennsylvania, stretching west to low pressure over Missouri. Over the past couple of hours, since early afternoon, this backdoor cold front has been showing signs of progressing southwestward...similar to the NAM model indication. The observations near Philadelphia clearly reveal a northeast wind within the last hour or so. Clouds will be increasing, especially after midnight, but the area should remain nearly dry. Keeping lows in the 50s, except for northeast of Baltimore. The challenge of the backdoor front will play a big role in Sunday`s forecast. The differences are significant enough that for now we will take the middle road with the backdoor front`s progression, similar to the ECMWF. Highs 70-75 in Central VA vs 50-55 northeast MD. We knocked down our high temperatures in northeast Maryland by a couple of degrees for Sunday, for now. Should the progression of the backdoor cold front persist, we may be knocking high temperatures down further southwest in our zones. Elsewhere, we are staying on the high side of guidance. There are other ramifications too. With the continued influx of moisture, some instability will be able to develop. The overrunning will lead to light rain/drizzle north of the front. Showers and perhaps a couple of thunderstorms will develop to the south. Regardless, PoPs will be increasing through the day. have lowered them a little, but will still have a 70-80% chance by day`s end.
-- End Changed Discussion --
&& .SHORT TERM /SUNDAY NIGHT THROUGH MONDAY NIGHT/...
-- Changed Discussion --
A deep trough axis will become negatively tilted as it drives cold front through Sunday night. It still is up for debate whether the backdoor front will head north again or not. If it doesn`t it will provide another lifting mechanism for an increasingly moist atmosphere, enhancing the potential for heavy rainfall. Would like to have more confidence in this aspect of the forecast. Surface low pressure will be traversing the stalled frontal zone overnight. The atmosphere south of the low will hang onto marginal instability. Regardless, an increased wind field will provide plenty of shear to drive a convective threat. QLCS has been a consideration for a couple of days, and the signals continue to point in that direction, in spite of the late hour of frontal passage (midnight-dawn). After coordination with SPC, the Day 2 outlook has upgraded to Slight Risk CHO-EZF and Marginal Risk for the rest except for northern Maryland. No changes of consequence made to the Monday forecast. We should be drying out through the day, although the presence of a trough axis suggests it won`t be outright dry. Then colder air returns Monday night, with maybe some upslope snow showers in the mountains.
-- End Changed Discussion --
&& .LONG TERM /TUESDAY THROUGH SATURDAY/... A closed upper level low will continue to spin near the Ontario/Quebec border on Tuesday. A chilly west-northwest flow will remain in place, and a few upslope snow showers will remain possible until drier air arrives later in the day. As surface ridging builds in from the south Tuesday night, some localized frost/freeze concerns could arise (given the growing season has started now). Shortwave ridging aloft will quickly move toward the east coast on Wednesday, with surface flow becoming southerly and temperatures rising closer to normal. A low pressure system will move into the Great Lakes later on Wednesday and eventually drag a cold front across the area by Thursday. There is still some model spread on the timing and location of this system. However, overall consensus is that moisture will be limited with this system. Rain chances, at least east of the Appalachains, will remain low. There will be a closed upper low associated with this system too, which will be slow to dislodge from New England. So while dry weather is likely Friday and Saturday (outside of another possible round of upslope snow showers), temperatures will remain below normal. && .AVIATION /01Z SUNDAY THROUGH THURSDAY/...
-- Changed Discussion --
VFR conditions will continue overnight. There is a chance that low clouds will sneak down the coast with a backdoor cold front toward dawn. Although some guidance is suggesting aob IFR, there is low confidence in this outcome, and have capped ceilings at 1000 ft. The backdoor will continue south invof MTN/BWI and maybe MRB. In the most aggressive solution, if could make it to DCA/IAD, but don`t believe that is likely. Regardless, south winds will turn southeast or east, with 10-15 kt winds perhaps gusting to 20 kt in the afternoon. Ceilings will slowly rise, but MVFR should prevail. There is an increasing chance at either light rain/drizzle or showers as the day progresses. Greater uncertainty exists Sunday night. However, this much is clear: a strong cold front will move west to east after midnight, driving a swath of heavy rain and perhaps strong gusty winds. This activity should clear the terminals prior to the Monday morning push. Brief IFR plausible within this period. VFR conditions will return Monday and are expected Tuesday, but gusty west winds in excess of 20 kt will remain possible as low pressure lingers to our north. High pressure to our south will bring lighter winds with VFR conditions Tuesday night into Wednesday. A low pressure system passing to the north may bring some lower clouds and light rain late Wednesday into Thursday, with a small chance of sub-VFR conditions.
-- End Changed Discussion --
&& .MARINE...
-- Changed Discussion --
Winds will continue to diminish the rest of the overnight, dropping below SCA criteria. Sunday`s forecast remains tricky, as a backdoor front will drop south across the waters-- the extent of which is still up for debate after the 12Z forecast cycle. Kept Small Craft Advisories for Sunday afternoon, assuming that the waters will be behind the backdoor, when east winds will provide a push of cooler air thus improving the mixed profile. The SCA will be extended into Sunday evening. We will likely be issuing a Gale Warning for the waters soon for late Sunday evening and overnight due to increased threat for showers and thunderstorms ahead of and along a strong cold front. Winds drop back to SCA on Monday behind the front. Gusty WNW winds will continue into Tuesday as the low pressure lingers to the north, and SCA conditions will remain possible. High pressure will bring lighter winds Tuesday night into Wednesday. A low pressure system will pass by to the north sometime later Wednesday into Thursday, although winds remain uncertain at this time due to the range of possibilities in the low`s strength and location.
-- End Changed Discussion --
&& .HYDROLOGY... A strong upper trough and attendant surface front will barrel through the area Sunday night. PWATs ahead of the front are progged to be around 1.5 inches, near record highs for the month of April. Unidirectional flow and the parent upper low cutting off suggests training of heavy rain is possible. The presence of a stalled backdoor front stretched east-west across the area also suggests that there will be added forcing for heavy rain. Believe the area will be within the path of a warm conveyor, tapping both Gulf of Mexico and Atlantic moisture. These factors all lead to flooding concerns. On the other hand, fast storm motions and antecedent dry conditions may be mitigating factors. WPC placed the western portions of the CWA in a slight risk for excessive rainfall, with marginal risk further east. The location of the backdoor and transport paths will be key players. Have held off on any Watches at this time, but they may become necessary in the next forecast cycle or two. && .TIDES/COASTAL FLOODING...
-- Changed Discussion --
Water levels near astronomical normals overnight through Sunday. Onshore flow followed by a sharp southerly push Sunday night may lead to an influx of water. Minor flooding will be possible beginning Monday. Its unclear at this time how may cycles the threat will last, but it could be multiple.
-- End Changed Discussion --
&& .LWX WATCHES/WARNINGS/ADVISORIES...
-- Changed Discussion --
DC...None. MD...None. VA...None. WV...None. MARINE...Small Craft Advisory from noon Sunday to 2 AM EDT Monday for ANZ530>543. Small Craft Advisory until midnight EDT tonight for ANZ532>534- 537-540-541-543.
-- End Changed Discussion --
&& $$ SYNOPSIS...KLW NEAR TERM...KLW SHORT TERM...KLW LONG TERM...CJL AVIATION...KLW/CJL MARINE...KLW/CJL HYDROLOGY...KLW TIDES/COASTAL FLOODING...KLW

USA.gov is the U.S. government's official web portal to all federal, state and local government web resources and services.