Area Forecast Discussion
Issued by NWS Sterling, VA

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271 FXUS61 KLWX 161927 AFDLWX Area Forecast Discussion National Weather Service Baltimore MD/Washington DC 227 PM EST Tue Jan 16 2018 .SYNOPSIS... A cold front will cross the area tonight. High pressure will build into the area Wednesday, then shift south and east of the region later this week into the weekend. && .NEAR TERM /THROUGH WEDNESDAY/... Snow continues to gradually increase in coverage and slowly push south and east ahead of an approach cold front/upper trough. Forward progress is slow but steady. Expecting appreciable snow to begin in the I-81 corridor between noon and 5 PM, and the I-95 corridor between 8 PM and midnight. Hi-res guidance has been remarkably consistent on a "snow hole" over much of the Shenandoah Valley in the lee of the Allegheny highlands, with around an inch expected in the valley and around inches on the ridges (the Allegheny Front itself is expected to see 2 to 4 inches). This relative minima in QPF extends east into northern Virginia and central Maryland including the Baltimore/DC metros due to a weakness between better jet forcing to the north and strong PVA with the 500 mb low to the south. Given that the one inch will be falling into the morning commute Wednesday, an advisory has been issued for the Interstate 95 corridor/metros. The advisory has also been expanded to include portions of central Virginia closer to the upper low/better moisture where 1 to 2 inches is expected. Areas adjacent to better jet forcing in northern Maryland likely see 1 to 3 inches. Despite favorable upper level dynamics, low level dry air intrusion on NW flow begins by daybreak across much of the area, and this is expected to eat away at the snow from north to south. Snow should end around midnight across northwestern Maryland, around daybreak from the northern Baltimore suburbs southwest into the Shenandoah Valley and by mid-morning elsewhere, though some light snow may linger across the central VA piedmont until noon as the upper low moves across. If this drier air moves in quicker than currently forecast, snow will end sooner and totals will be lower. On the flip side, if upper level dynamics overcome low level dry air, snow totals could be a little higher, especially considering the strength of the upper jet and colder temperatures resulting in higher ratios toward the end of the event. Overall, a relatively dry and powdery snow is expected. Temperatures will fall as the sun sets and precipitation starts, quickly tumbling below freezing shortly after snow begins in any one location. Temperatures are expected to then remain below freezing through the day Wednesday as another Arctic airmass spills into the region on gusty northwest winds. && .SHORT TERM /WEDNESDAY NIGHT THROUGH THURSDAY NIGHT/... Clearing is expected on brisk northwest winds through the night Wednesday night. Low temperatures will fall into the teens for most areas, single digits at the higher elevations. With a tight pressure gradient and cold air advection in place, wind gusts could approach Wind Advisory criteria over the higher elevations Wednesday into Thursday. The combination of these strong winds and cold temperatures could also result in dangerously low wind chills approaching -10 F over the ridgetops. Winds should be somewhat lighter at the lower elevations precluding both Wind Advisory and significant wind chill threats in the lowlands, though it will be far from mild (wind chills likely in the single digits). As the building high pushes to the south of the area on Thursday, winds become more westerly and temperatures quickly begin to moderate both as warmer air advects in aloft and westerly downsloping component helps boost surface temperatures east of the higher elevations. Highs a few degrees on either side of 40 are expected. If winds go light Thursday night under clear skies, the ambient airmass in place could allow temperatures to drop below the 20s currently forecast. && .LONG TERM /FRIDAY THROUGH MONDAY/... Surface high pressure will cover the southeastern states on Friday as an upper level trough axis slowly moves offshore. Heights build aloft as a broad upper level ridge becomes more established across the central and eastern U.S. Temperatures on Friday continue to moderate with dry weather expected. The weekend will feature high pressure settled over the southeastern and mid-atlantic states with a continued moderation in temperatures, 10 to 15 degrees above normal and continued dry weather. By Monday, the area of low pressure that developed in the central plains on Sunday will move into the western Great Lakes region early Monday. The low level flow will bring an increase in moisture and continued relatively mild temperatures on Monday. Best rain chance (not snow) will be Monday into Monday night. The cold front extending from the Great Lakes surface low will sweep across the region on Tuesday. 85 hPa temperatures behind the cold front on Tuesday finally go back below freezing, with some upslope snow possible in the western mountains. && .AVIATION /20Z TUESDAY THROUGH SUNDAY/... Conditions at most terminals have returned to VFR as of early this afternoon. MTN still has MVFR CIGS and this may continue immediately adjacent to the Chesapeake Bay in light onshore flow. Lower CIGS are still on the doorstep of BWI as well and likely re-develop around sunset. Meanwhile, -SN is expected to move into MRB momentarily, which will start causing VSBY to drop there. -SN expected at all terminals tonight with a period of IFR likely. LIFR can`t be ruled out in any briefly moderately heavy snow bands. Winds will be light and variable tonight as the cold front causing the light snow moves overhead. NW flow behind the front should bring drying and a return to VFR shortly after daybreak Wednesday. Winds could gust 25-30 kts into Wednesday evening before gradually diminishing. Expect VFR conditions through the extended. Due to the cold water temperatures on the Potomac, there could be IFR or LIFR issues at both DCA and MTN due to fog during the morning hours Fri-Sun. && .MARINE... Light/variable winds are expected over the waters as a cold front moves overhead through tonight bringing light snow and lower visiblities. Patchy fog is possible as well. NW winds increase behind the front Wednesday morning into SCA territory, and likely remain there well into Wednesday night. Winds should be lighter Thursday, but SCA gusts are still possible. With surface high pressure established, expect wind conditions on the waters to be below SCA levels Friday-Sunday. With the moderation in temperatures expected this weekend, some breakup in ice will occur by late this weekend. && .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 until 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>057-501-502-505>508. WV...Winter Weather Advisory until 10 PM EST this evening for WVZ050>053-055-501>505. MARINE...Small Craft Advisory from 3 AM Wednesday to midnight EST Wednesday night for ANZ530>533-535-536-538>542. Small Craft Advisory from 3 AM Wednesday to 6 AM EST Thursday for ANZ534-537-543. && $$ SYNOPSIS...DHOF NEAR TERM...DHOF SHORT TERM...DHOF LONG TERM...SMZ AVIATION...SMZ/DHOF MARINE...SMZ/DHOF

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