Area Forecast Discussion
Issued by NWS Sterling, VA

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000 FXUS61 KLWX 260749 AFDLWX Area Forecast Discussion National Weather Service Baltimore MD/Washington DC 349 AM EDT Sat May 26 2018 .SYNOPSIS... High pressure will remain over the western Atlantic through the weekend. A cold front moving south out of Pennsylvania will cross the Mid-Atlantic late Sunday, then stall to the south until Wednesday of next week. The front will likely return north as a warm front Wednesday into Thursday. The remnants of Alberto may pass to the west of the region late next week. && .NEAR TERM /THROUGH TONIGHT/... A very humid/high PWAT airmass is advecting into the region on southwest flow around high pressure stationed over the western Atlantic. PWATs will likely exceed 1.75" by this afternoon. Convergence along a surface trough coupled with low LFCs in the humid airmass are expected to result in scattered to widespread showers and thunderstorms developing around noon over portions of the central Shenandoah Valley, then moving eastward and congealing into semi-organized clusters through the afternoon into the early evening. Before storms organize into clusters, thinking that slow storm motion and heavy rainfall could result in flash flooding, especially given recent heavy rains over the central Shenandoah Valley and adjacent areas of eastern West Virginia, so have issued a Flash Flood Watch for these areas from noon to 8 PM this afternoon. The severe (large hail/damaging wind) threat today should be relatively limited given marginal lapse rates and shear, but a few strong wind gusts are possible with more robust/water-loaded updrafts. Frequent lightning may be a hazard given large areas of updrafts in the 0 to -15 C region, suggesting lots of suspended small ice particles (graupel) which would be favorable for excess charge separation. This could prove especially hazardous given the holiday weekend and lots of ongoing outdoor activities, so even though storms may technically not be severe, they could prove just as dangerous. Storms should gradually weaken this evening with the loss of heating, but hi-res guidance suggests storms will linger over southern Maryland where the surface trough and residual outflow from daytime convection combine to result in persistent showers and heavy thunderstorms. Heavy rain potential is definitely there, but given uncertainties in magnitude/placement (NAM12 further north over the metros, other guidance further south) and sensitivity to prior convection during the day, did not have confidence to issue a Flash Flood Watch for any areas tonight, though one may be needed. && .SHORT TERM /SUNDAY THROUGH MONDAY NIGHT/... Another round of showers and heavy thunderstorms is likely on Sunday as a cold front drops southward out of Pennsylvania, running into the humid airmass over the Mid-Atlantic. The presence of the front and some increase in shear may lend to a little more storm organization, but the details will rely heavily on prior day`s convection. Training storms seem more likely on Sunday on a more widespread scale, though, given storm motions largely parallel to the low-level boundary. Convection may linger well into Sunday evening as the front will likely be slow to clear the area. The high pressure to the north forcing it through is forecast to only be around 1020 mb, which usually means it gets hung up over southern parts of the area, which could lead to another round of showers and thunderstorms Monday afternoon and evening across central VA to southern MD. && .LONG TERM /TUESDAY THROUGH FRIDAY/... A weak front will be slipping southward through the area on Tuesday with drier air being advected in behind it around high pressure building into Quebec. While a few showers or storms will be possible due to the front in the vicinity, the deeper moisture will be south of our area by this time, which will likely limit coverage when combined with weak convergence. Temperatures will remain above normal (highs in 80s) behind the front with little to no cold advection. The high will move into New England on Wednesday, but moisture advection will likely be commencing as the remnants of Alberto move west of the area (reference the National Hurricane Center for the latest track forecast) and the surface boundary begins lifting northward. Therefore, some showers and storms could develop by afternoon, with increasing chances with southward and westward extent across the area. Thursday and Friday will likely be unsettled as the remnants of Alberto become absorbed in the westerlies and the trough of low pressure advances eastward. Exact details and impacts remain uncertain at this time. Temperatures will likely remain above normal and be accompanied by humid conditions. && .AVIATION /07Z SATURDAY THROUGH WEDNESDAY/... VFR with sct-bkn mid level overcast through this AM. Showers/tstms expected to become widespread along/ahead of a surface trough mainly 18-22Z for metro terminals, about an hour earlier MRB/CHO, and could linger well into the eve near/SE of DCA. IFR vsby possible in +TSRA during this time. Winds generally SW 5-10 kts with a few gusts around 15 kts. A few storms may also be accompanied by isolated strong wind gusts, but more widespread severe convection seems less likely. Clouds should preclude fog formation tonight. Similar conditions expected Sunday as a cold front drops south out of PA. Front likely stalls near CHO to southern MD Monday with more convection possible in this corridor. North of the boundary, winds will become northerly generally AOB 10 kts. The weak front will slip south through the area on Tuesday and remain nearby on Wednesday. While shower and thunderstorm coverage may be limited, they will be possible each day. && .MARINE... SCA today for middle/lower MD Chesapeake and lower tidal Potomac waters on marginal SW gradient. Thunderstorms likely this aft/eve and a few could produce SMW (34+ kt) gusts as well as heavy rain and frequent lightning. Guidance indicates convection could linger over lower MD Chesapeake through much of the overnight. Gradient becomes stronger overnight but low-level inversion brings mixing into question. SCA conditions possible especially over open waters overnight into early Sun AM but confidence too low for a headline at this point. As a cold front approaches the waters from the north Sunday, a renewed round of showers and thunderstorms is likely. The front will probably get hung up near southern MD through Monday with at least scattered thunderstorms possible again Monday aft/eve. Light winds are expected Tuesday as the weak front sags southward through the area, although there could be a few showers and thunderstorms. Southeast winds may near SCA thresholds on Wednesday as the front lifts back to the north and high pressure moves off the New England coast. Additional thunderstorms will be possible. && .HYDROLOGY... Have issued a Flash Flood Watch noon to 8 PM this afternoon for portions of the central Shenandoah Valley in Virginia and adjacent portions of eastern West Virginia. Slow storm motion combined with very heavy rainfall (PWATs 1.75+ inches) may result in flash flooding, especially given recent heavy rains. The threat is more isolated (though certainly non-zero) further east this afternoon and evening. Hi-res guidance indicates persistent convection mainly S of DC overnight, so this may be another area to watch. More widespread training thunderstorms with heavy rain are possible Sunday as a cold front drops south out of Pennsylvania. Additional watches may be needed to account for this threat Sunday into Sunday night. && .TIDES/COASTAL FLOODING... A Coastal Flood Advisory has been issued for Washington DC for this morning`s high tide cycle. Given current anomaly, SW waterfront should crest right around minor threshold. The subsequent tide cycles are forecast to be close to, but just shy of, minor flooding through the weekend. && .LWX WATCHES/WARNINGS/ADVISORIES... DC...Coastal Flood Advisory until 8 AM EDT this morning for DCZ001. MD...None. VA...Flash Flood Watch from noon EDT today through this evening for VAZ026-027-029-030-507. WV...Flash Flood Watch from noon EDT today through this evening for WVZ055-505-506. MARINE...Small Craft Advisory from 10 AM this morning to 8 PM EDT this evening for ANZ532-533-537-540>542. Small Craft Advisory until 8 PM EDT this evening for ANZ534- 543. && $$ SYNOPSIS...DHOF NEAR TERM...DHOF SHORT TERM...DHOF LONG TERM...ADS AVIATION...ADS/DHOF MARINE...ADS/DHOF HYDROLOGY...DHOF TIDES/COASTAL FLOODING...DHOF

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