Area Forecast Discussion
Issued by NWS Sterling, VA

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000 FXUS61 KLWX 122001 AFDLWX Area Forecast Discussion National Weather Service Baltimore MD/Washington DC 301 PM EST Tue Dec 12 2017 .SYNOPSIS...
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An arctic cold front will move off the coast tonight. Weak high pressure will build overhead Wednesday. An Alberta Clipper will pass nearby Wednesday night through early Thursday. Developing low pressure will pass off the east coast late Friday and high pressure will return for Saturday. Low pressure may impact the area early next week.
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&& .NEAR TERM /THROUGH TONIGHT/...
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As of 1 PM, a strong arctic cold front bisected the CWA fairly close to the I-95 corridor. Since then it has continued eastward and should exit the CWA soon after crossing the Chesapeake Bay. Aloft, a very deep and becoming negatively tilted shortwave with strong vort max is pushing eastward into western West Virginia. It will cross the region tonight. Relatively high mixing thanks to this shortwave combined with the shortwave/vort max itself will likely support continued snow showers along the Allegheny Front, where winter weather advisories remain in effect for 3-6 inches of snow. Further east, while snow showers will diminish in the lower terrain, enough forcing will persist to allow snow showers to reach the I-95 corridor at times. The strongest of these will be this evening, perhaps occurring near rush hour, while they should wind down to flurries after that, once the main vort max has passed. While temps have been warm, strong cold advection will likely bring temps down near freezing during the latter portion of rush hour in the metro area, so will need to watch for any potential problem snow showers over the next few hours. Most likely though, any effects in the metro will be limited to reduced visibility, briefly wet roads and perhaps some motorist surprise at a brief burst of snow flakes out of the sky. Cold advection continues tonight and the winds will remain quite gusty through the night. Again considered potential for wind advisory, but given that it will need to mix the strong winds down several thousand feet during the middle of the night to achieve this, did not issue any advisories. Any advisory level gusts should be limited to the highest mountain ridges and the open waters of the Chesapeake, the latter of which is covered by the gale warning (see marine section). Lows will bottom in the teens and low 20s, and with the wind, it will feel like its below 10 degrees, with some spots feeling more like 0. Wind chills near or below -10 are still expected in the far western VA mountains, the higher eastern WV mountains, and western MD, where Wind Chill Advisories remain in effect. As the flow diminishes a bit, the atmosphere becomes more stable behind the passing trough, and moisture decreases, snow showers along the Allegheny Front should wind down towards morning.
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&& .SHORT TERM /WEDNESDAY THROUGH THURSDAY NIGHT/...
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Weak high pressure noses into the region on Wednesday, so the sun should return, but a steady wind looks likely to remain as the gradient between low pressure to the northeast and high pressure to the southwest remains relatively strong. Cold advection will diminish, but we will be solidly within the cold air mass, and with very cold mid level temps, surface readings will struggle to recover. Most places will end up below freezing all day long, and even the warmest spots probably only reach the mid 30s. Wednesday night into Thursday, a clipper will move across the region. Latest guidance has trended the low track northward, with most models now sending it along or just north of the Mason-Dixon line. Typically this means that any accumulating snow will stay north of our region. There remains some potential for a wiggle back southward, so did not completely remove chance of precip, but did nudge it back down somewhat. It is notable that with the low now expected to pass north, it could conceivably warm up overnight as south winds behind a warm front push temps above freezing in parts of the metro. Highs on Thursday look like most places get above freezing, with 40 looking more likely in the metro. The clipper moves east Thursday afternoon, and we will try to dry out at night, but a weak vort max passing through could cause a few flurries - nothing more, most likely. Lows will drop back into the sub-freezing category.
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&& .LONG TERM /FRIDAY THROUGH TUESDAY/...
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An upper level trough of low pressure will move across the region Friday into Friday night. Snow shower chances will linger due to this trough. Temperatures will remain below normal. A modifying area of high pressure is expected to build eastward behind the upper level trough Saturday and Saturday night. Milder temperatures and a southwest wind are expected through the period. The next threat for rain or snow showers will be Sunday through Monday as a cold front sags southward across the region. For the most part, rain showers should be the story and not so much the snow showers. High pressure will build into the region Tuesday and bring another blast of chilly air to the region.
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&& .AVIATION /20Z TUESDAY THROUGH SUNDAY/...
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Generally VFR thru the period. Any snow shower this afternoon/evening could briefly reduce cigs/vis to IFR, but impacts should be brief. Lasting longer will be the gusty winds from the northwest, which will continue through much of the night and perhaps into the morning Wednesday. Clipper crossing the area Thursday night appears more likely to stay north of the region. Best chance of any impacts will be the northern terminals (MRB/BWI/MTN), but even here, odds seem to be decreasing. Will need to monitor however, as a wiggle in the low track back to the south could make snow more likely in these areas. VFR returns or continues later Thursday and Thursday night behind the clipper. VFR conditions Friday through Saturday night. Snow showers on Friday into Friday night could briefly reduce conditions to MVFR. Winds light and variable Friday into Friday evening, becoming west 5 to 10 knots late Friday night. Winds west becoming southwest 5 to 10 knots Saturday into Saturday night.
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&& .MARINE...
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Cold front crossing the waters now is resulting in rapidly increasing winds, with gales likely to start late this afternoon and continue through Wednesday. Gales are up for all waters tonight and the more open waters through Wednesday due to this expectation. Winds should gradually slacken starting late tonight and then decrease more signficantly Wednesday night as the flow becomes southerly ahead of an approaching clipper, but SCA level gusts still expected, so have added SCA for the open Bay on Wednesday night. No marine hazards Friday through Saturday night. Winds light and variable Friday into Friday evening, becoming west 10 knots later Friday night. Winds west becoming southwest around 10 knots Saturday and Saturday night.
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&& .TIDES/COASTAL FLOODING...
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A strong northwest flow is developing behind a cold front at present which will continue through Wednesday. Tidal blowout conditions are possible late tonight through Wednesday.
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&& .LWX WATCHES/WARNINGS/ADVISORIES...
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DC...None. MD...Wind Chill Advisory from 6 PM this evening to noon EST Wednesday for MDZ501. Winter Weather Advisory until 7 PM EST this evening for MDZ501. VA...Wind Chill Advisory from 6 PM this evening to noon EST Wednesday for VAZ503-504-507-508. Winter Weather Advisory until 7 PM EST this evening for VAZ503. WV...Wind Chill Advisory from 6 PM this evening to noon EST Wednesday for WVZ503-506. Winter Weather Advisory until 7 PM EST this evening for WVZ503. Winter Weather Advisory until 7 AM EST Wednesday for WVZ501- 505. Wind Chill Advisory until noon EST Wednesday for WVZ501-505. MARINE...Small Craft Advisory until 4 PM EST this afternoon for ANZ530>543. Small Craft Advisory from 6 PM Wednesday to 6 AM EST Thursday for ANZ532>534-537-540-541-543. Gale Warning until 6 PM EST Wednesday for ANZ530>534-537- 539>541-543. Small Craft Advisory from 6 AM to 6 PM EST Wednesday for ANZ535-536-538-542. Small Craft Advisory from midnight Wednesday night to 6 AM EST Thursday for ANZ542. Gale Warning until 6 AM EST Wednesday for ANZ535-536-538-542. Small Craft Advisory from 6 PM Wednesday to midnight EST Wednesday night for ANZ530-531-539.
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&& $$ SYNOPSIS...RCM NEAR TERM...RCM SHORT TERM...RCM LONG TERM...KLW AVIATION...RCM/KLW MARINE...RCM/KLW TIDES/COASTAL FLOODING...RCM

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