Area Forecast Discussion
Issued by NWS Blacksburg, VA

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000 FXUS61 KRNK 231006 AFDRNK Area Forecast Discussion National Weather Service Blacksburg VA 506 AM EST Tue Jan 23 2018 .SYNOPSIS... A strong cold front will continue to move east across the region. It will usher in colder temperatures, gusty northwest winds, and mountain snow showers this afternoon and continue into Wednesday. Temperatures will moderate to above normal again later in the week before the next frontal system arrives during the weekend. && .NEAR TERM /THROUGH TONIGHT/... As of 450 AM EST Tuesday... 997 mb surface low over northern Michigan trails a frontal occlusion/Pacific front across central WV into roughly the I-77 corridor early this morning. This boundary best shows itself as a wind shift from south to west-southwest/west. Parent upper level low currently per water vapor imagery is spiraling east-northeast across IL/IN. This frontal system will lead to a relatively active 24 hour period with ongoing heavy showers early this morning in the Piedmont, falling temperatures today/tonight, a surge of northwest winds along the Blue Ridge this afternoon and tonight and potential minor accumulations of snow tonight continuing into the Wednesday timeframe. Primary cold front and dry slot aloft moves eastward through the morning hours per the GFS. This will lead to a west to east rather rapid decrease in PoPs along with clearing skies except in the western mountains. By early afternoon, most of the eastern two- thirds of the forecast area should see at least partly sunny skies. Falling 850 mb temperatures should also result through today, with most areas west of the foothills having already seen their high temperature occur overnight. Would expect any additional rains amounting up to 0.15" in the far eastern Piedmont counties. By afternoon, temperatures should range from the mid 30s in the far west to the lower 50s in southeastern counties. In terms of wind headlines, have opted to cancel the Wind Advisory for the south/southwest winds that was in effect until 6 AM for our far southwestern mountains. However, have opted to issue a impact based wind advisory in effect from 1 PM today until 10 AM Wednesday from the Blue Ridge from Roanoke to Watauga County. Will see winds also flip to west and eventually northwest by mid to late this morning. These should become at least breezy by early afternoon as cold advection aloft steepens lapse rates. Based on GFS/MAV guidance, local research wind guidance indicates only marginal Wind Advisory criteria for Roanoke and Boone, with the highest pressure rises anticipated to occur around 4 pm to around midnight, while the core of the northwesterly low level jet of around 50 kts looks to occur after 7pm into early Wednesday morning timeframe. Generally think peak wind gusts of around 50-55 mph should occur between 6 pm and 3 am, and continuing to diminish into the Wednesday morning period. Appears too marginal north of Roanoke and have opted to hold on further extension per collaboration with NWS Sterling. And then there`s upslope snow to talk about. Cold advection and enhanced northwest flow will again favor a changeover from rain to mountain upslope snow showers from western Greenbrier County southward into Watauga County. Changeover should occur by late this afternoon in southeast West Virginia and toward early evening from Grayson through Watauga County. Though flurries or light snow showers may at times advance to the New River Valley, simulated reflectivity from most SPC HREF members suggests any accumulating snows would be in southeast West Virginia, Tazewell County VA and Ashe/Watauga Counties in NC. Snow accumulations through 12z Wednesday should add up to between 1-2 inches, greatest in western Greenbrier County. Additional accumulations are expected into Wednesday as well. Low temperatures tonight range from around the mid 30s in the Piedmont/foothills, to the upper 20s/around 30 west of the Blue Ridge. && .SHORT TERM /WEDNESDAY THROUGH FRIDAY NIGHT/... As of 430 AM EST Tuesday... The deep low pressure area moving through the Great Lakes today will be moving rapidly northeast away from the area early Wednesday. At 12Z Wednesday, it is expected to already be in the eastern Canadian provinces. This will allow winds to begin to diminish and thus the Wind Advisory is slated to end at 15Z or 10 AM EST. Unlike the first two weeks of January, the upper flow will remain progressive and essentially zonal as this low pressure area departs. We will see a quick shot of colder air with 850mb temperatures dropping into the -8C to -10C range which will equate to lows in the 20s. Temperatures are not low enough at this point for wind chills to be a concern. Upslope snow showers across the Alleghanys will linger into Wednesday morning, with generally 1-2 inch accumulations western Greenbrier, less elsewhere in the west. Given that the flow is fairly progressive and zonal, the next upper short wave, a fairly weak one, will move into the region let Wed or Wed night, a psuedo Alberta Clipper type system. The main dynamics are across the Great Lakes and moisture is limited. However, it will provide just enough dynamics to enhance upslope snow showers across the Alleghanys where another 1-2 inches of snow is possible by Thursday morning. This short wave will quickly exit the region later Thursday as a short wave ridge in the central U.S. moves into the eastern U.S. for Friday. Wednesday and Thursday will be on the cool side, but actually really close to normals for this time of year. Look for lows mainly from near 20 west to near 30 east. Maximum temperatures will range from the 40s in the west to the 50s in the east. These are within a few degrees of normal for mid-January, but about 20-30 degrees warmer than we were much of the first half of January! Friday as the upper ridge approaches and 850mb temperatures surge back above zero, to perhaps as warm as +8C, look for surface temperatures to warm into the 50s west and 60s east, about 10 degrees above normal. Other than the upslope snow showers, little meaningful precipitation is expected during this period leaving monthly precipitation totals a bit below normal for this time of year. && .LONG TERM /SATURDAY THROUGH MONDAY/... As of 450 AM EST Tuesday... As we move into the weekend, the next in the series of short waves within a continued progressive upper flow will move from the Midwest into the eastern U.S. The ECMWF suggest that this could be a decent liquid precipitation maker for the forecast area as the system becomes more amplified as it moves into the eastern U.S., and low pressure develops off the northern Mid- Atlantic coast, while the GFS is much weaker, faster and less excited bringing about just brief period of showers. WPC seems to be favoring the slower ECMWF, which brings a general 1-2 inches of rainfall to the area in the Sun-Mon time frame. Little concern about winter weather with the weekend/early next week system as 850mb temperatures remain above 0C for several days, topping out in the +8C to +10C range as the front approaches Sun-Mon. Given the differences in the intensity and timing between the ECMWF and the GFS, the degree of cold air that follows int the wake of this frontal system early next week and the amount of post frontal upslope snow showers will largely depend on which solution verifies. For now, will carry snow showers across just the Alleghanys following frontal passage Sunday night into Monday. 850mb temperatures fall back toward -8C per the ECMWF, but barely reach 0C per GFS with a flatter, more zonal flow. At any rate, temperatures for the remainder of January will hover much closer to normal than the first two weeks did, with essentially equal periods of moderately above and moderately below normal. It could be said that the temperature variability will likely be greater than the degree above/below normal. && .AVIATION /10Z TUESDAY THROUGH SATURDAY/... As of 1222 AM EST Tuesday... Several potential weather-related issues to TAFs and aviation operations in the next 6-8 hrs of the 06z TAF period. This spans mostly low-level wind shear, gusty frontal showers and MVFR ceilings. Initial flight categories should stay MVFR due to low ceilings, but frontal rains expected to begin between 08-12z per recent guidance and will add 4-6 SM SHRA/-SHRA in this period for most TAFs. May see a rumble or two of thunder with the best chance being in the Piedmont/southern Blue Ridge foothills. Low- level jet of up to 60 kts out of the south/southeast will lead to low- level wind shears of 40-45 kts producing mechanical turbulence. Given the strong wind fields, even heavier showers/embedded thunder expected between 08-12z may lead to gusty winds, with best chance again mainly for Lynchburg and Danville but can`t entirely be ruled out elsewhere. Will need to TEMPO if TSRA or stronger convective gusts more imminent at the TAF as confidence in timing and placement is too low to place in TAFs attm. Greater improvement in ceilings/visbys expected post-frontal, with showers/possible thunder moving east into eastern Virginia. Should see rapid clearing of ceilings to VFR along and east of the Blue Ridge after 14z. Post-frontal VFR stratus at Bluefield and Lewisburg will be slower to thin out into the afternoon. Late aftn- evening, ceilings lower for these two TAFs to VFR/MVFR levels as post- frontal northwest upslope becomes enhanced. May see 3-6 SM -SHSN in this period, becoming more likely after 03z. Should see winds steadily veer to west/northwest 8-14 kts. May need to add/increase afternoon wind gusts in later TAF issuances once degree of clearing becomes more apparent. Medium confidence in ceilings, visibilities and winds through the TAF valid period. Extended Discussion... MVFR ceilings with low clouds and potentially -SHSN can be expected across the westernmost sites, and possibly as far east at times as KBCB. High pressure and VFR conditions along with diminishing winds should return to the entire area by Wednesday. Dry weather including good flying conditions can be expected Thursday and Friday. && .EQUIPMENT... As of 355 PM EST Monday... NOAA Weather Radio, WXL60 (Roanoke Transmitter) which broadcasts at 162.475 MHz remains off the air. The phone company reported that the circuit line had been cut and is being worked on. We are told it is a fiber optic line, so it is taking some time to repair. && .RNK WATCHES/WARNINGS/ADVISORIES... VA...Wind Advisory from 1 PM this afternoon to 10 AM EST Wednesday for VAZ015>017-022. NC...Wind Advisory from 1 PM this afternoon to 10 AM EST Wednesday for NCZ001-002-018. WV...None. && $$ SYNOPSIS...AL NEAR TERM...AL SHORT TERM...RAB LONG TERM...RAB AVIATION...AL/DS EQUIPMENT...PM/RAB

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