Area Forecast Discussion
Issued by NWS Sterling, VA

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027 FXUS61 KLWX 151420 AFDLWX Area Forecast Discussion National Weather Service Baltimore MD/Washington DC 920 AM EST Mon Jan 15 2018 .SYNOPSIS... Canadian high pressure over the area will weaken over the next 24 hours as a clipper system brings a cold front through the area Tuesday night into Wednesday. Low pressure will develop offshore Wednesday night into Thursday before high pressure settles to our south late in the week. && .NEAR TERM /THROUGH TONIGHT/... UPDATE 930 AM: Added a slight chance of freezing drizzle this afternoon and evening immediately adjacent to the western shore of the Chesapeake Bay. This is because of shallow moisture (3000-5000 feet) in a layer that is -3 to -5 C which may preclude ice crystal growth at times. It`s a very slight, but non-zero chance. Otherwise, these same areas have a chance of flurries and light snow, with a few flurries possibly extending west through the metros later today. Previous discussion... High pressure over the area this morning will weaken over the next 24-36 hrs. A light onshore flow today could bring areas of low clouds to the counties adjacent to the Chesapeake Bay and as far west as the Rt 15 corridor. Overall, increasing clouds and not as cold as yesterday with highs climbing to near freezing under light and variable winds. Thickening mid-level clouds tonight as moisture advection increases ahead of clipper system. Min temps staying mostly in the 20s as opposed to 10s like the last couple of nights. && .SHORT TERM /TUESDAY THROUGH WEDNESDAY NIGHT/... A clipper system is expected to cross the Appalachians Tue afternoon. Light snow is expected to develop along the trailing front Tue morning and slowly spread ewd but any precip should remain confined west of the I-81 through 00Z Wed. Heights will then begin to fall rapidly Tue night in response to approaching mid-upper level trof. As heights fall and forcing for upward vertical motion increases, expect snow to spread ewd reaching the I-95 corridor by 12Z Wed. There continues to be some differences with the strength and timing of the upper level system with the Canadian and ECMWF slower and stronger indicating a closed low and the GFS faster and much weaker showing an open trof. As a result, the ECMWF and its ensemble mean show higher QPF generally 0.1 to 0.2 inches while the GFS shows 0.1 inches or less. With the very cold air mass in place, expect SLRs to be higher than climatology in the order of 15 to 1 with snow totals in the 1-2 inch range likely with isold higher totals in the south and over the mtns closer to the track of h5 closed low and in the better moisture. Snow will begin to taper off quickly Wed afternoon with most of the measurable precip gone by 00Z Thu. Advisories will probably be needed for areas west of the Blue Ridge either later today or tonight and Tue or Tue night for areas east of the Blue Ridge. && .LONG TERM /THURSDAY THROUGH SUNDAY/... Overall there is good pattern agreement amongst guidance for the long term period. On Thursday morning, the upper level trough will be exiting the east coast, giving way to a shortwave ridge and surface high pressure building across the Mid South. While high temperatures will return closer to seasonal normals, Thursday night will likely be cold (teens/20s) again with light winds and mostly clear skies. The surface high will drift into the southeastern states on Friday while a shortwave trough aloft swings eastward. There`s still some uncertainty in the magnitude of forcing and moisture in the trough, although the bulk of it will likely stay to the north. If any precipitation did make it into the area, it could be wintry depending on timing...although this is a very small (<10%) chance. Otherwise highs will push into the 40s for most areas. The surface high will move off the southeast coast and broader ridging aloft will develop over the southeastern CONUS through the weekend. This pattern will allow southerly flow to develop and temperatures to continue rising to above normal values. Highs in the 50s will become more widespread with each passing day, and lows may not fall below freezing by Saturday night. Meanwhile the next low pressure system will be churning through the Plains. There could be a few rain showers on the warm frontal side of this system by Sunday, mainly across northwestern parts of the CWA. && .AVIATION /14Z MONDAY THROUGH FRIDAY/... MVFR cigs moving into eastern terminals as of mid-morning as ATLC moisture spreads westward. MVFR cigs will likely persist through the evening before lifting somewhat late tonight or early Tue. Light snow Tue afternoon at KMRB will likely result in some flight restrictions. Snow spreads eastward Wed with flight restrictions likely rest of the terminals. Snow ends by 00Z Thu with conditions improving. West winds could gust to 25 kt on Thursday. Otherwise, no ceiling or visibility restrictions are expected Thursday and Friday as high pressure builds toward the area. && .MARINE... Winds will diminish this afternoon and become light and variable tonight through Tue evening. Winds will begin to strengthen late Tue night with SCA conditions likely Wed through Thu. SCA conditions will remain possible through Thursday in northwest- west flow behind the departing low pressure. While winds will subside Thursday night, a weak weather disturbance may increase them again Friday. However, there is still some uncertainty about the strength of this system. && .LWX WATCHES/WARNINGS/ADVISORIES... DC...None. MD...None. VA...None. WV...None. MARINE...Small Craft Advisory until 4 PM EST this afternoon for ANZ534- 537-543. && $$ SYNOPSIS...LFR NEAR TERM...LFR/DHOF SHORT TERM...LFR LONG TERM...ADS AVIATION...ADS/LFR/DHOF MARINE...ADS/LFR

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