Area Forecast Discussion
Issued by NWS Sterling, VA

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000 FXUS61 KLWX 182035 AFDLWX Area Forecast Discussion National Weather Service Baltimore MD/Washington DC 435 PM EDT Fri May 18 2018 .SYNOPSIS... A front over southern Virginia will begin to lift north through the area late tonight and move into Pennsylvania Saturday afternoon. A cold front will then move through the area late Sunday before lifting again north during the middle of next week. && .NEAR TERM /UNTIL 6 AM SATURDAY MORNING/... Rainfall so far today has been mainly stratiform in nature with convective cells firing along the stationary front located over southern VA. As pressure gradient tightens and southerlies strengthen, the front will be on the move late tonight with a new round of rain/showers expected tonight into Sat morning. Heaviest amounts are expected east of the Blue Ridge mountains especially over the VA Piedmont and southern MD. Additional rainfall amounts of 1 to 3 inches are likely east of the Blue Ridge with up to an inch west of I-81 corridor. Concern for renewd flooding tonight into Sat morning especially in southern areas with mostly minor flooding occurring/expected along the Potomac river. && .SHORT TERM /6 AM SATURDAY MORNING THROUGH SUNDAY NIGHT/... Early morning showers will shift east of the area during the afternoon. Warm front will lift north into PA in the afternoon with a few breaks in the clouds possible in the west allowing for weak destabilization and perhaps a couple of t-storms. The front will drop south again on Sunday keeping the risk of showers. && .LONG TERM /MONDAY THROUGH FRIDAY/... A chance of showers and afternoon/evening thunderstorms will continue into Monday and Tuesday, as a cold front moves just to our south and stalls out. Both GEFS and EPS ensembles show a potential for 0.25-0.50 inches of rainfall on throughout this period, primarily in the evening. The highest amounts will generally be along the eastern slopes of the Blue Ridge. Temperatures on both days will be in the upper 70`s to low 80`s. The forecast for Wednesday into Thursday is very uncertain at this point. What is known is that a stronger low pressure system will pass well to our north, dragging a cold front through our area in its wake. The timing of this front is highly variable at this time, so will keep chance PoPs in the forecast for Wednesday, moving to slight chance for Thursday. The key thing to note here is that, upon passage, this front should bring in some much drier air behind it. Temperature forecasts for this scenario will almost certainly change, but for now, highs should be in the low 80`s on Wednesday, followed by a slight cool down on Thursday in the wake of the aforementioned cold front, with temps in the upper 70`s. && .AVIATION /20Z FRIDAY THROUGH WEDNESDAY/... IFR/LIFR conditions tonight in rain/showers, then improving to MVFR Sat afternoon. VFR conditions expected during the first half of next week. Mainly VFR conditions forecast Monday and Tuesday, with the only threat of sub-VFR conditions being a passing shower over the terminals. Winds will be light and favor a southerly trajectory. A frontal boundary will drop southward through the terminals late Monday, before stalling south of the terminals through mid week. A moist easterly flow north of the aforementioned boundary will bring the potential for sub VFR VIS/CIGs through mid week. && .MARINE... SCA conditions expected tonight through Sunday, then winds diminishing Sun night as front drops southward. Generally sub-SCA conditions expected Monday through Tuesday, with winds remaining light. A frontal boundary will cross the waters late Monday which will help increase shower activity, resulting in the potential for periodic gusty winds. && .TIDES/COASTAL FLOODING... Water levels are running around a foot to 1.5 ft above astronomical predictions on the Bay and lower tidal Potomac. On the upper tidal Potomac, however, those departures increase to 2 ft or more. For the upcoming tide cycle, that should equate to minor flooding on both sides of the river. Coastal Flood Advisories remain in place. By the day on Saturday, it gets more complicated as there will be freshwater input as well. This will affect Georgetown more than southwest DC or Alexandria. -For Alexandria: have extended the Advisory for another two tide cycles (late Saturday night). It could be necessary for a third cycle, but there is lesser confidence in the duration. -For Washington DC: Georgetown will be the driver due tot he freshwater contribution, where moderate flooding is forecast for three cycles (with levels not dropping below minor). At the southwest waterfront, best confidence for moderate flooding will be during the day Saturday; subsequent cycles may only be minor flooding. Similar to Alexandria, have duration uncertainties. Therefore, have issued a Warning for the Saturday day and night tide cycles, and left the Watch in place for Sunday. Elsewhere on the Bay, minor flooding will be a concern this weekend during the times of high tides. Will be assessing upcoming guidance and could be making Advisory decisions as soon as this evening. && .LWX WATCHES/WARNINGS/ADVISORIES... DC...Flood Watch through Saturday morning for DCZ001. Coastal Flood Warning from 6 AM Saturday to 8 AM EDT Sunday for DCZ001. Coastal Flood Watch from Sunday morning through Sunday afternoon for DCZ001. Coastal Flood Advisory until 6 AM EDT Saturday for DCZ001. MD...Flood Watch through Saturday morning for MDZ003>006-011-013- 014-016>018-501>508. VA...Flood Watch through Saturday morning for VAZ025>031-036>040- 050>057-501>508. Coastal Flood Advisory until 8 AM EDT Sunday for VAZ054. WV...Flood Watch through Saturday morning for WVZ050>053-055- 501>506. MARINE...Small Craft Advisory until 6 AM EDT Sunday for ANZ530>543. && $$ SYNOPSIS...LFR NEAR TERM...LFR SHORT TERM...LFR LONG TERM...CJL AVIATION...LFR/CJL MARINE...LFR/CJL HYDROLOGY... TIDES/COASTAL FLOODING...HTS is the U.S. government's official web portal to all federal, state and local government web resources and services.