Area Forecast Discussion
Issued by NWS Blacksburg, VA

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000 FXUS61 KRNK 120527 AFDRNK Area Forecast Discussion National Weather Service Blacksburg VA 1227 AM EST Tue Dec 12 2017 .SYNOPSIS... High pressure remains in control through tonight. An Alberta Clipper system will pass over the region on Tuesday, ushering in periods of snow and blowing snow, cold temperatures, and gusty northwest winds through Tuesday night. Chilly but dry conditions return for Wednesday before temperatures begin to slowly moderate on Thursday. Another surge of cold Arctic air is expected by Friday. && .NEAR TERM /THROUGH TODAY/... As if 1045 PM EST Monday... T/Td readings are so far pretty much on track. Some minor changes were needed in areas such as LWB, where decoupling has allowed strong radiational cooling. Delayed the arrival of snow showers a bit per latest NamNest/HRRR, which show very little arriving much before 12Z. Currently, only some very light echoes showing up on radar in central WV, with slightly more solid activity in the northern part of the state. However, latest observations are all CLR BLO 120, so it does not appear that any of this is yet reaching the ground. As of 319 PM EST Monday... Modest warm advection regime underway across the Appalachians into the central mid-Atlantic region, ahead of a potent Alberta Clipper system and lead parent mid- level trough. Another shortwave is evident in water vapor imagery north of the Great Lakes associated with a strong shot of Arctic air. Each of these features will go to produce a period of wintry weather, cold temperatures and gusty northwest winds, especially along and west of the Blue Ridge Tuesday. Several hazard headlines have been issued with this package. Winter Weather Advisories have been hoisted for western Greenbrier County in West Virginia, and in Ashe and Watauga County in western North Carolina for accumulating snow and low wind chills (low wind chills primarily for Tuesday night). A Wind Advisory has also been hoisted along the southern Blue Ridge from Roanoke down to Boone. Each of these products run until 12z Wednesday. Quiet weather anticipated for the first part of the evening. Will start to build clouds in progressively with time from the west as the clipper system approaches. Should see increasing chances for snow showers into the pre-dawn hours of Tuesday from the mountains of NC northward into the central Appalachians and into western Greenbrier County. Low temperatures tonight should be in the mid 20s to near freezing. Guidance generally agrees on Arctic frontal passage tied to the clipper system from 12-16z Tuesday. By most indications, it should be a well defined and strong cold frontal passage as indicated in simulated reflectivity progs. Noticed on the NAM-12km that steep low- level lapse rates, CAPEs of 50-100 J/kg and Snow Squall Parameter values > 1 unit support an environment conducive to short-duration snow squalls with the Arctic frontal passage, especially along the I- 81 and I-77 corridors during the morning commute. As the air mass behind the front is cold, snow in western counties should be powdery in nature with snow-to-liquid ratios likely greater than 16:1. Pattern then transitions to one of typical northwest flow snow setups, with a Great Lakes moisture connection to Lake Michigan indicated in the higher-resolution NWP. I`ve left open the possibility for light accumulating snow to make it into the New River and Roanoke Valley, but the most persistent snows will be confined to the NC Mountains and in southeast West Virginia (particularly in western Greenbrier County). Including continuing snows for tomorrow night, forecast accumulations run from 3-5" in western Greenbrier County, 1-4" across the rest of southeast West Virginia, 1-3" in Ashe and Watauga Counties in NC, and a coating to an inch in more intermittent snow showers roughly along the I-81 corridor. Specific to winds, these will tend to peak immediately behind the frontal passage. While breezy conditions will exist in most locations west of the Blue Ridge, the strongest winds capable of causing minor damage appears more likely along the southern Blue Ridge with gusts up to 55 mph possible. With the powdery nature of the snowfall and breezy conditions, this will lead to periods of blowing/drifting snow. For temperatures Tuesday, opted for a non- diurnal temperature trend with early high temperatures in the 30s to mid 40s before sharply falling to the mid 20s to upper 30s by late afternoon under strong cold advection. && .SHORT TERM /TONIGHT THROUGH THURSDAY/... As of 319 PM EST Monday... Strong cold advection will persist Tuesday night as the center of the cold pocket aloft passes to the north. This should continue to provide a favorable fetch of upslope driven moisture to result in bands of snow showers overnight espcly western slopes but also with some possible spillover out to the Blue Ridge per strength of the jet. Expect coverage to slowly fade later Tuesday night as soundings show moisture depth decreasing but still likely another 1-3 inches far northwest slopes to an inch elsewhere with only light accumulations farther east. Strong northwest winds will also still be an issue with another round of subsidence overnight when the coldest 850 mb air should be helping lower the inversion. Therefore will need to keep or expand going wind headlines into early Wednesday morning at this point. This combined with lows in the teens to around 20 likely to spell wind chill issues from the foothills west so keeping mention in the HWO for now. Otherwise should see clearing take shape out east Tuesday night and elsewhere by Wednesday with this lingering into Wednesday night. Next weak clipper will then arrive from the west late Wednesday night and moreso Thursday as this impulse shears east and offshore Thursday afternoon. System appears lacking moisture per lack of digging so only including more clouds and slight pops northwest late Wednesday night and mainly low chance pops Thursday before lift gives way to weak upslope. Otherwise more clouds north and less south with highs 30s north to 40s south. && .LONG TERM /THURSDAY NIGHT THROUGH MONDAY/... As of 319 PM EST Monday... Upper flow will again undergo a quick transition from zonal flow to start Thursday night back to a deeper eastern trough regime on Friday before again flattening over the weekend into early next week. This in response to development of split flow ahead of the last in a series of digging clipper systems that will attempt to phase with energy dropping out of the Rockies. However latest models show quite a spread with this system, from basically dry with a split between the clipper passing farther north and low pressure offshore, to somewhat better linkage with low pressure closer to the Carolinas and thus at least some light precip per the GFS. Boundary layer appears rather marginal for snow outside of the mountains but could arrive early Friday so something to watch. Otherwise will include some low rain/snow pops on Friday most sections for now. Lingering upslope snow showers again likely Friday night before heights build allowing for some temperature moderation on Saturday under warmer westerly flow which should push most into the 40s. This in advance of yet another system swinging northeast out of the southern plains that will link with the next upstream cold front and bring rain showers into the area late Sunday into Monday before colder drier air follows later on Day 7. Appears most precip will be liquid given warming aloft ahead of the system Sunday night with only perhaps some snow showers at the end far west Monday morning. && .AVIATION /05Z TUESDAY THROUGH SATURDAY/...
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As of 1215 AM EST Tuesday... VFR conditions will continue for a few more hours through the evening and likely until around 08Z or 10Z. Ceilings will lower from west to east during the overnight as a strong clipper system approaches from the northwest. For much of the night east of the Blue Ridge it will remain VFR. Ahead of this clipper, winds are expected to be mostly SSE- SSW 4-8kts. As the Alberta Clipper moves into the region early Tuesday, expect periods of mostly MVFR-IFR ceilings and MVFR visibilities in -SHSN are expected west of the Blue Ridge, with VFR conditions continuing to the east of the Blue ridge. A snow begins to accumulate in the west, some locations such as KBLF, may see BLSN even after the snow ends, thus reducing visibility into the MVFR category. The most likely area for SHSN will be across the mountains from SE WV and SW VA into northwest NC. May see a period of short duration snow squalls associated with the Arctic frontal passage between 11-14z at KBLF, KLWB and perhaps even KBCB. Gusty post frontal winds are expected as the strong clipper moves east of the area. Look for WSW-WNW sustained winds of 10-15 kts with gusts up to 30 kts at KROA, with even stronger winds possible at higher mountain peaks west of the Blue Ridge. Winds will not decrease much until late on Tuesday night into Wednesday morning. Medium to high confidence in ceilings throughout the TAF valid period. Medium to high confidence in visibilities throughout the TAF valid period. Medium to high confidence in wind speed/direction throughout the TAF valid period. Extended Discussion... Weak high pressure builds in Wednesday followed by another weaker clipper Thursday that passes by mostly to our north. Another more significant clipper system will arrive by Friday into Saturday, with gusty winds and mountain showers, thus the potential for periods of sub-VFR conditions.
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VA...Wind Advisory from 4 AM early this morning to 7 AM EST Wednesday for VAZ015>017-022. NC...Wind Advisory from 4 AM early this morning to 7 AM EST Wednesday for NCZ001-002-018. Winter Weather Advisory from 4 AM early this morning to 7 AM EST Wednesday for NCZ001-018. WV...Winter Weather Advisory from 7 AM this morning to 7 AM EST Wednesday for WVZ508.
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&& $$ SYNOPSIS...AL NEAR TERM...AL/RAB SHORT TERM...JH LONG TERM...JH AVIATION...RAB is the U.S. government's official web portal to all federal, state and local government web resources and services.