Area Forecast Discussion
Issued by NWS Sterling, VA

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495 FXUS61 KLWX 161615 AFDLWX Area Forecast Discussion National Weather Service Baltimore MD/Washington DC 1115 AM EST Tue Jan 16 2018 .SYNOPSIS... A cold front will approach the area today before passing through tonight into Wednesday. Low pressure will develop along this boundary offshore later Wednesday before high pressure settles to our south Thursday. High pressure will remain to our south through the weekend. && .NEAR TERM /THROUGH TONIGHT/... 11 AM UPDATE: A steady light snow continues over northwestern Maryland. 12z model guidance trickling in has raised confidence enough to hoist a Winter Weather Advisory for central Virginia (higher amounts/better forcing) and the metro areas (impactful for the Wednesday morning commute despite lighter amounts around 1 inch). The central Shenandoah Valley appears it will be in a bit of a snow shadow in the lee of the ridges, and this is showed nicely in latest hi-res guidance. Therefore, left those areas out of the advisory for now with generally around an inch expected tonight. Meanwhile, patchy freezing fog is being observed over the western shore of the Chesapeake Bay, but this should abate as temperatures warm above freezing through about noon or so. Patchy fog is holding tough in parts of central Maryland (near BWI to Annapolis) this morning. A few breaks of sun are being observed across central VA at the moment, but any breaks should quickly fill in again as the upper trough/cold front approach from the west. Light snow was observed along the MD/PA border (1/2" in Manchester MD) associated with upper jet dynamics this morning, but that light snow has since lifted north. Snow is now confined to NW MD and is generally light. Cdfnt will cross the Appalachians later today and slowly progress ewd reaching the I-81 corridor by 00Z Wed. Snow today will fall mainly over the upslope areas of the Appalachians behind the front/wind shift. Some snow could also affect nrn MD counties along the Mason- Dixon line but this is not a done deal as some models and ensemble keep this area dry through much of the day. Tonight and the first half of Wed is when the majority of precip will fall as heights fall in response to approaching upper level trof and forcing for upward vertical motion increases. Deterministic models and their respective ensembles depict precip breaking up as it crosses the mtns with a secondary area of snow over the VA piedmont late tonight into Wed. Ensemble QPF is generally 0.1 to 0.2 inches along and south of the I-64 and west of the I-81 corridors. A big minima or hole in the precip is shown over nrn VA and central MD. Even with these small amounts of QPF, the very cold temperatures will result in SLRs in the order of 15 to 1 and even a little bit of snow potentially causing moderate impacts given sfc temps expected in the mid 20s. It is possible that additional advisories may be needed for the Charlottesville and Fredericksburg areas, but since precip is not expected to begin until late second period or perhaps closer to 12Z Wed decided to hold off on any advisories. Also, our snow probs indicate only a 50% prob of snow exceeding 2 inches and 80% confidence is needed. For the metro areas, the prob of exceeding one inch is about 50-60% which would require an advzy given snow is expected to impact the Wed morning commute, but this is again late second or even third period event. && .SHORT TERM /WEDNESDAY THROUGH THURSDAY NIGHT/... Snow will begin to taper off quickly in the afternoon Wed with snow ending by 7 PM Wed if not sooner. Clearing and turning blustery Wed night and Thu. Wind and wind chill advisories may be needed for the mtns. && .LONG TERM /FRIDAY THROUGH MONDAY/... Surface high pressure will build over the southeastern states on Friday while a shortwave trough swings eastward. Very little moisture will be associated with this trough, and thus only some increase in mid-level clouds can be expected. Temperatures will begin to increase above climatology due to warm air advection. The surface high will move off the southeast coast and broader ridging aloft will develop over the southeastern CONUS through the weekend. Meanwhile, low pressure will be developing in the lee of the Rockies and lifting toward the Great Lakes. A second low will be crossing well to the north in Canada. Overall, temperatures should continue their above normal trend. It`s worth noting that the 00Z ECMWF sends a backdoor cold front southward from the northern low and thus prevents the really warm air from working into the area on Sunday. For example, there is a 10 degree spread in the forecast high temperature at Baltimore between the ECMWF and GFS MOS. It looks like the weekend will be dry in most places, though a few showers extending along the warm front could clip northwestern parts of the CWA. As the low lifts north of the Great Lakes Monday, the trailing cold front will approach the area. There`s fairly good model agreement with the front crossing Monday afternoon and evening, though of course timing could change some at this range. Showers will become likely ahead of the front. Rain will be exiting with the front anyway, but even so, flow will be zonal behind the front and cold air limited. Only the upslope areas of the Allegheny Front may see a changeover to snow showers briefly before ending. && .AVIATION /16Z TUESDAY THROUGH SATURDAY/... IFR cigs across eastern terminals this morning lifting to VFR this afternoon. BWI dropped to 1/2 mile in FZFG with CIGS of 100 feet, but believe this will slowly lift through noon. Light snow developing tonight first at KMRB then spreading slowly eastward overnight with MVFR/IFR conditions expected at all terminals. Snow begins to taper off by midday Wed with conditions improving rapidly and winds picking up from the NW. Blustery Wed night and Thu with wind gusts well over 20 kts. No aviation impacts are expected Friday and Saturday as high pressure builds south of the area. && .MARINE... Light and variable winds through this evening, then winds strengthening late tonight into Wed with SCA conditions expected Wed into Thu. Risk of gales appears low at this time for late Wed night and Thu. A weak disturbance will be passing Friday, which could kick up westerly winds a little, but it`s uncertain at this time if they will reach SCA levels. High pressure will slide south of the area Friday night into Saturday with a light southwest flow developing. && .LWX WATCHES/WARNINGS/ADVISORIES... DC...Winter Weather Advisory from 9 PM this evening to 9 AM EST Wednesday for DCZ001. MD...Winter Weather Advisory until 10 PM EST this evening for MDZ003-501-502. Winter Weather Advisory from 9 PM this evening to 9 AM EST Wednesday for MDZ011-013-014-016-503>506-508. Winter Weather Advisory from 4 PM this afternoon to 7 AM EST Wednesday for MDZ004>006-507. VA...Winter Weather Advisory until 10 PM EST this evening for VAZ503. Winter Weather Advisory from 9 PM this evening to 9 AM EST Wednesday for VAZ036>040-050>056-501-502-505>508. WV...Winter Weather Advisory until 10 PM EST this evening for WVZ050>053-055-501>505. MARINE...Small Craft Advisory from 7 AM to 6 PM EST Wednesday for ANZ530>543. && $$ SYNOPSIS...LFR NEAR TERM...LFR/DHOF SHORT TERM...LFR LONG TERM...DHOF AVIATION...ADS/LFR/DHOF MARINE...ADS/LFR is the U.S. government's official web portal to all federal, state and local government web resources and services.