Area Forecast Discussion
Issued by NWS Sterling, VA

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000 FXUS61 KLWX 140135 AFDLWX Area Forecast Discussion National Weather Service Baltimore MD/Washington DC 935 PM EDT Fri Apr 13 2018 .SYNOPSIS... High pressure will remain across the western Atlantic through Saturday. 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. && .NEAR TERM /THROUGH SATURDAY/...
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Gusty southwest winds will diminish overnight, but persistent gradient should keep these winds sustained around 10 knots. The flow will be southerly around high pressure in the western Atlantic. Subsidence will be sufficient to keep skies nearly clear. Lows should only drop into the 50s in most areas with a few 60s downtown DC and Baltimore. There will be little change in the synoptic pattern through the day Saturday. As south winds continue to advect warm air north, 850 mb temps may jump up to 14 C. Going on the high side of temperatures guidance seems prudent, which means that mid 80s are within reach. Instability should be minimal, perhaps enough for cumulus development.
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&& .SHORT TERM /SATURDAY NIGHT THROUGH SUNDAY NIGHT/...
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Clouds will become more prevalent Saturday night as isentropic/upglide lift continues and a mature low pressure system marches across the CONUS. Do not believe that there will be enough forcing for precipitation to develop locally. If there were QPF, would think it would be confined to the mountains (nearer the frontal system) or along the Maryland-Pennsylvania border (close to a backdoor front). Have a slight chance for these locations after midnight. Lows will be unseasonably warm...near 60 degrees. We should continue in the warm sector Sunday, but with increased instability. The question mark though will be the progress of the aforementioned backdoor cold front. The recent 18z NAM continues to push the backdoor front south and west across most of our CWA, perhaps pulling up short of reaching Harrisonburg, VA and Charlottesville, VA. Nonetheless, it remains the colder solution. The 18z GFS pushes the backdoor cold front south and west to zones nearly parallel the I-70 corridor between Hagerstown, MD and Baltimore, MD. The previous run of the ECMWF offers a bit of a compromise, encompassing Baltimore and touching DC. This will have a BIG impact on high temperatures, as evidenced by MOS. Thus, there is a larger than typical uncertainty. Have sought middle ground, siding a pinch toward continuity, at this point. Thus, highs near 60 for northeast Maryland vs mid 70s for central Virginia. The instability and presence of a boundary should be sufficient to encourage showers to develop in the afternoon...with perhaps a few developing prior to that. South of the cold front, believe a rumble of thunder will be possible as well. Granted, if the NAM solution verifies, then the character of precipitation may be more toward light rain, drizzle, and maybe even patchy fog. Hope to iron out those details in the cycles to come. There are hints that the backdoor could (should?) shift northward again as a 50 kt low level jet heads up the coast ahead of the approaching cold front. CAPE will be meager due to the nighttime timing, but still present. Regardless of that, there will be plenty of shear and moisture transport. With precipitable water reaching 1.5 inches, a period of heavy rainfall likely will result. In addition, could not rule out a well-defined axis with gusty winds (a la QLCS environment). Will be focusing on the enhanced wording for heavy rainfall, and keep thunder chances at slight at this point. Certainly an active period that should be monitored.
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&& .LONG TERM /MONDAY THROUGH FRIDAY/... By Monday morning, low level jet induced rainfall will be exiting to the east. A low pressure center tracking northeast along the advancing surface front will likely be located nearby. There`s a possibility some destabilization occurs in the narrow window between the moderate rainfall and the surface front and vort max aloft. Ample deep layer shear will be present, so a squall line may develop. Temperatures will be highly dependent on the warm and cold frontal progression. Drying will take place by later in the afternoon, except along the Allegheny Front, where cold/moist WNW flow will result in upslope snow showers which will persist into Tuesday. Light accumulations will be possible. Temperatures will return below normal areawide Tuesday into Tuesday night as the vertically stacked low pivots toward eastern Canada and the thermal trough passes overhead. Shortwave ridging 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 Wednesday night and drag a cold front across the area early Thursday. Moisture will be limited with this system, with only a small chance of showers east of the mountains. The upper level trough will linger along the east coast on Friday as surface high pressure builds eastward from the Mississippi Valley. Dry weather will result with slightly below normal temperatures. && .AVIATION /02Z SATURDAY THROUGH WEDNESDAY/... VFR should prevail through Sat, with gusty winds (up to 20-25 kt) in the afternoon. The forecast for Sunday more in doubt, as there is a potential for a backdoor front to drop south. If that does occur, then IFR conditions would result. There is high uncertainty in that solution, Nonetheless, flight restrictions likely. Rainfall and gusty winds will become the issue Sunday night in advance of a cold front, along with an isolated thunder concern. A 40-50 kt low level jet may also create LLWS. An area of low pressure and associated cold front will be moving across the area during the first part of Monday. Sub-VFR cigs are possible before the front crosses. There also may be a line of gusty showers (possible thunderstorms) along the front. WNW winds gusting in excess of 25 kt will be possible behind the front. Gusty west winds likely return for Tuesday but with VFR conditions. High pressure brings VFR and light winds for Wednesday. && .MARINE...
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Small craft advisory conditions continue for more than half of the Chesapeake Bay and Potomac River due to the gusty southwest winds. Winds will diminish some overnight, but should become gusty once again tomorrow afternoon with diurnal heating. Winds may be affected by a backdoor front Sunday. If this were to occur, then the winds would back to the east. High uncertainty in this solution as there is a significant spread in guidance. A cold front will cross the waters Sunday night into Monday morning. Strong wind gusts could reside along the front, but full mixing is uncertain. Small Craft Advisory conditions will likely continue into Monday as a cold front passes and turns winds to the west. A line of gusty showers and possibly thunderstorms may develop along the front as well. Gusty winds continue into Tuesday as the low plods to the northeast, and SCA conditions remain possible. High pressure will bring lighter winds Tuesday night into Wednesday.
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&& .FIRE WEATHER...
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RH is expected to increase through the day Saturday, conditions should relax. Widespread rainfall is expected to greatly reduce fire danger Sunday night.
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&& .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, though the flooding threat may be mitigated by fast rain/storm motions and antecedent dry conditions. WPC is placing the western portions of the CWA in a slight risk for excessive rainfall, with marginal risk further east. && .TIDES/COASTAL FLOODING...
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Anticipate water levels to increase slightly this weekend due to southerly flow. The water levels will increase more markedly Sunday night in strong flow in advance of a cold front. Minor inundations would be possible for the Sunday night and Monday tide cycles. However, uncertainty also increases in the specifics.
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&& .LWX WATCHES/WARNINGS/ADVISORIES... DC...None. MD...None. VA...None. WV...None. MARINE...Small Craft Advisory until 6 PM EDT Saturday for ANZ530>534- 537-539>543. Small Craft Advisory until midnight EDT tonight for ANZ535-536- 538. Small Craft Advisory from noon to 6 PM EDT Saturday for ANZ535- 536-538. && $$ SYNOPSIS...KLW NEAR TERM...KLW SHORT TERM...KLW LONG TERM...CJL AVIATION...KLW/CJL MARINE...KLW/CJL FIRE WEATHER...KLW HYDROLOGY...KLW TIDES/COASTAL FLOODING...KLW

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