Area Forecast Discussion
Issued by NWS Sterling, VA

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000 FXUS61 KLWX 261918 AFDLWX Area Forecast Discussion National Weather Service Baltimore MD/Washington DC 318 PM EDT Sat May 26 2018 .SYNOPSIS...
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High pressure will remain over the western Atlantic through the weekend. A cold front 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.
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&& .NEAR TERM /UNTIL 6 AM SUNDAY MORNING/...
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Showers and a couple of thunderstorms are sliding eastward across the western and northern half of our CWA. The main threats since around 1230 pm have been very heavy rainfall. A few thunderstorms over Augusta, Rockingham, Rappahannock, and Fauquier Counties in Virginia produced rain rates as little as 1 inch per hour to as much as 4 inches per hour. A few Flood Warnings and one Flash Flood Warning has been issued so far in terms of this afternoon convection. Additional showers and thunderstorms are expected to develop farther to the east and south the remainder of this afternoon into this evening. This activity will be slow-moving and is expected to produce very heavy rainfall. A Flash Flood Watch remains in effect for parts of the Potomac Highlands and Shenandoah Valley through 8 pm this evening. 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 will have to investigate this further later this afternoon and evening in order to decide if we are going to issue any additional Flash Flood Watches farther to the east.
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&& .SHORT TERM /6 AM SUNDAY MORNING 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 SATURDAY/...
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To start out the long term period on Tuesday, a front will be located off to our south. A stray shower or two can`t be ruled out (especially over southern portions of the forecast area), but it appears as though most locations should stay dry. The main feature of interest throughout the long term period will be the remnants of Tropical Storm Alberto. The official NHC forecast has Alberto making landfall late Monday Night along the Gulf Coast. After landfall, Alberto will weaken, but it`s remnants will still have a large influence on our weather through the week. Also of interest are a trough currently located over the western CONUS and a stronger trough currently located in the Gulf of Alaska. Over the next couple days, the western CONUS trough will become largely cutoff from the polar jet and progress slowly eastward. Meanwhile, the stronger trough over the Gulf of Alaska will dive into the Pacific Northwest by Tuesday. Alberto is expected to track north on Tuesday into the lower Mississippi/Tennessee Valleys beneath an anomalously strong upper ridge. As Alberto progresses further north, the aforementioned western CONUS trough will be kicked out onto the Upper Plains by the stronger trough moving into the Pacific Northwest. That will place Alberto within the steering influence of the ejecting trough, causing it to initially continue north, before turning more to the northeast as the weakening trough tracks to it`s north. These complex interactions between the two troughs and remnants of Alberto suggest that there is a fair amount of uncertainty as to where the remnants will track. However, most model guidance (deterministic and ensemble) moves the system from the Tennessee Valley into the Ohio Valley by Thursday. Regardless of the exact details of the track, it appears as though we will remain to the east of the track. Southerly flow to the east of the system will transport a very tropical airmass into the region. The deterministic GFS and Euro, as well as their respective ensembles suggest that precipitable water values will approach or exceed two inches by late Wednesday into Thursday. Recent model runs have the best forcing for ascent associated with Alberto`s remnant circulation located off to the north of our area. However, this far out there is a fair amount of track uncertainty. At the very least, showers and thunderstorms will be possible on both Wednesday and Thursday. Given the near record precipitable water values, any storms that do form could produce heavy rainfall. In the wake of Alberto`s remnants, model guidance pushes a cold front through the region. There are considerable differences between the models with regard to the timing and sensible impacts of this frontal passage.
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&& .AVIATION /19Z SATURDAY THROUGH THURSDAY/...
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VFR conditions with a broken mid level deck this afternoon. One exception will be brief MVFR conditions due to very heavy rainfall from showers and thunderstorms through 22z this evening near the Metropolitan terminals. IFR vsby possible in the heaviest thunderstorms. 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 dense fog formation tonight, though fog still hinted at strongly in guidance wherever clouds break. 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.
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&& .MARINE...
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SCA the remainder of the day for middle/lower MD Chesapeake and lower tidal Potomac waters on marginal SW gradient. Thunderstorms likely to continue late this afternoon into this evening 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.
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&& .HYDROLOGY...
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A Flash Flood Watch remains in effect through 8 pm this evening 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. Some HREF members also hint at NE MD late this afternoon. 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.
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&& .TIDES/COASTAL FLOODING... Subsequent high tides at DC SW waterfront through the weekend should fall just short of minor flood as well. && .LWX WATCHES/WARNINGS/ADVISORIES... DC...None. MD...None. VA...Flash Flood Watch until 8 PM EDT this evening for VAZ026-027- 029-030-507. WV...Flash Flood Watch until 8 PM EDT this evening for WVZ055-505- 506. MARINE...Small Craft Advisory until 8 PM EDT this evening for ANZ532>534-537-540>543. && $$ SYNOPSIS...KLW NEAR TERM...KLW SHORT TERM...KLW LONG TERM...KJP AVIATION...ADS/KLW MARINE...ADS/KLW HYDROLOGY...ADS/KLW TIDES/COASTAL FLOODING...ADS/KLW

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