Area Forecast Discussion
Issued by NWS Sterling, VA

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513 FXUS61 KLWX 170749 AFDLWX Area Forecast Discussion National Weather Service Baltimore MD/Washington DC 349 AM EDT Thu Aug 17 2017 .SYNOPSIS... A warm front will pass through the area today and a cold front will approach from the west Friday before passing through Friday night. Weak high pressure will build into the area for the weekend before moving offshore early next week. A stronger cold front will pass through the area during the middle portion of next week. && .NEAR TERM /UNTIL 6 PM THIS EVENING/... Partly to mostly clear skies and light winds combined with abundant low level moisture has led to areas of fog again early this morning. Due to the presence of some cloud cover and being one day removed from widespread rainfall, density is significantly less than yesterday, however there will still likely be localized areas where dense fog does develop. Warm frontal boundary is also positioned near the region early this morning, and this is continuing to help sustain a few rain showers in central VA and possibly into southern Maryland. There could be additional showers that develop along or south of the boundary into the morning hours. During the day today, warm frontal boundary will gradually lift northward, providing for a warm and humid day. Once any fog lifts, partly sunny skies are expected, but will likely give way to more clouds than sun through the day. Highs today should reach the upper 80s to around 90F. This combined with surface dew points in the low 70s will allow for the development of 1000-1500 J/KG of MLCAPE, especially west of I-95 where warm front will clear first. While CAPE profiles are not that conducive for widespread strong to severe convection with tall/skinny CAPE depicted, 0-6KM shear of near 30 knots and the presence of a warm frontal boundary and its directional shear along it, may be enough for a few isolated strong to locally severe thunderstorms. Convection-allowing models differ on evolution and coverage, but highest threat area appears to be west of the metros. In addition, precipitable water values increase markedly as warm front lifts northward, with values near or exceeding 2" by late in the day. This combined with 11-12K foot warm cloud depths, wet antecedent conditions, and the potential for slow movement/repeated convection sets up the possibility for an isolated flood risk as well. && .SHORT TERM /6 PM THIS EVENING THROUGH SATURDAY NIGHT/... For tonight, warm and muggy conditions will persist. Convection from this afternoon may continue to propagate eastward, with additional scattered development also possible as low level jet moves towards the region overnight. Potential for heavy rain will exist with any showers/storms overnight as well. Lows tonight low to mid 70s. Area of low pressure will push into southern Canada on Friday, with its trailing cold front entering the Mid-Atlantic states during the day. This will serve as another focus for scattered showers/thunderstorms during the day Friday. A severe threat will exist again on Friday, with potentially greater coverage/more intense storms than today. Temperatures and low level moisture will surge out ahead of the front, leading to the development of 1500- 3000 J/KG of MLCAPE, with 30-35 knots of shear moving overhead. In addition to the severe threat, flash flooding is also a concern as precipitable water values surge over 2", possibly exceeding 2.25", and potential for training convection exists. Highs reach the upper 80s to around 90F. Convection will push eastward and gradually wane Friday night. Model guidance has come into better agreement regarding progression of the front for Saturday. It now appears more likely that the front will clear most of the region before stalling out along the Atlantic coastline and into southeastern Virginia. This will provide for a drier day Saturday, although a few isolated- scattered showers/storms are still possible, mainly near the front, as well as in the higher terrain as an upper trough moves across later in the day. Highs will still be in the upper 80s, although dew points will drop back into the 60s behind the front. && .LONG TERM /SUNDAY THROUGH WEDNESDAY/... High pressure will remain overhead Sunday...bringing dry conditions for most areas along with warm conditions. A downsloping low-level flow along with some sunshine will cause max temps to range from the mid to upper 80s in northern Maryland to the lower 90s in central Virginia. High pressure will slowly move offshore Sunday night through Monday. A return flow will develop and this will allow for humidity to increase along with very warm conditions. The increased humidity may trigger some showers and thunderstorms. Coverage should be isolated to scattered since there will not be a strong lifting mechanism. A southerly flow will continue Tuesday with more warm and humid condtions along with isolated to scattered showers/thunderstorms. A cold front will approach the area from the north Wednesday before passing through Wednesday night. Showers and thunderstorms are possible ahead of the boundary...especially during the afternoon and evening hours. More warm and humid conditions will persist ahead of the boundary...but noticeably cooler and less humid conditions are expected behind the boundary. && .AVIATION /07Z THURSDAY THROUGH MONDAY/... Reductions possible again this morning in areas of fog, with greatest risk for IFR at MRB/BWI. DCA has lowest probabilities of seeing fog/br. Once any fog lifts, scattered to broken clouds expected, although mainly VFR. Isolated to scattered showers/thunderstorms then develop this afternoon, possibly lingering into tonight, and some may bring heavy rain/gusty winds. Some patchy fog and low clouds are possible again tonight. Additional showers/thunderstorms are then likely Friday afternoon/evening with potential additional reductions and gusty winds. Conditions improve for Saturday. High pressure will remain over the terminals Sunday. VFR conditions are likely. The high will move offshore Sunday night into Monday. Warm advection may bring an MVFR deck of lower clouds later Sunday night into Monday. A few showers and thunderstorms are possible...but coverage should remain isolated to scattered. && .MARINE... Sub-SCA conditions exist on the waters this morning, but south winds will be on the increase later today, tonight, and through Friday ahead of a frontal system. A Small Craft Advisory is in effect for portions of the central Chesapeake and lower Tidal Potomac from late this afternoon and through Friday. Additionally, isolated to scattered showers and thunderstorms are possible both today and Friday, mainly in the afternoon/evening, and Special Marine Warnings may be necessary. High pressure will remain just to the north and west Sunday while a weak surface trough remains over the waters. Light winds are expected. The high will move offshore Sunday night into Monday and a return southerly flow will develop. The southerly flow will continue through Tuesday. Increased moisture from the southerly flow may trigger a few showers and thunderstorms...but coverage should remain isolated to scattered. && .TIDES/COASTAL FLOODING... Elevated water levels continue early this morning. However...the next high tide will be the lower of the two so no minor flooding is expected today. The high tide cycle tonight into early Friday will be the higher of the two. Also...southerly winds may increase a bit ahead of a cold front over the Ohio Valley. This may cause minor flooding for sensitive areas such as Annapolis...Straits Point and possibly Washington DC. Coastal Flood Advisories may be needed. The southerly flow will continue through Friday evening before turning offshore behind the boundary. More minor flooding is possible with the high tide cycle Friday night into early Saturday for sensitive areas. Anomalies should decrease later Saturday due to the offshore flow. && .LWX WATCHES/WARNINGS/ADVISORIES... DC...None. MD...None. VA...None. WV...None. MARINE...Small Craft Advisory from 4 PM this afternoon to 6 AM EDT Friday for ANZ533-534-537-541-543. && $$ SYNOPSIS...BJL NEAR TERM...MM SHORT TERM...MM LONG TERM...BJL AVIATION...BJL/MM MARINE...BJL/MM TIDES/COASTAL FLOODING...BJL

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