Area Forecast Discussion
Issued by NWS Blacksburg, VA

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000 FXUS61 KRNK 300952 AFDRNK Area Forecast Discussion National Weather Service Blacksburg VA 452 AM EST Wed Nov 30 2016 .SYNOPSIS... A cold front slides east to the mountains this afternoon, and east of the piedmont this evening. High pressure will track from the lower Mississippi Valley Thursday to the Southern Appalachians Friday morning. Expect drier and cooler weather for the end of the week. && .NEAR TERM /THROUGH TONIGHT/... As of 340 am EST Wednesday... Radar has been pretty active this morning, with elevated instability leading to thunderstorms moving across the New River Valley/Mountain Empire and Alleghany Highlands, as well as portions of the piedmont. The latest RAP showed this elevated instability lingering into mid morning. There will be a break in the showers across the southeast CWA with main axis of lift and moisture transport staying with the front and upper dynamics across the TN Valley into the Central Appalachians. Therefore will have higher pops in the mountains this morning then models shift the axis east into the piedmont by late afternoon. There is a decent amount of lift and wind energy to give most of not all the forecast area a good rain. Unstable air is limited by clouds and showers but with increase in the jet should be enough dynamical forcing to string some thunder in the forecast, especially in the foothills/piedmont. The Storm Prediction Center shows our eastern 2/3rds of the forecast area in a marginal risk, mainly concerned about winds mixing down with line of showers and isolated storms. Will mention this in the HWO, but more along/east of the Blue Ridge as think the west will be stable enough to keep stronger winds from mixing down past 4000 ft. Some of the higher terrain this morning could see some gusts to 45 mph, although not enough to have a wind advisory. Temperatures despite the clouds and showers will warm into the lower to mid 70s east to mid to upper 60s west. Tonight, the front moves east of the forecast area by midnight with showers ending. There will be clearing skies as we head into late tonight in the east while upslope keeps clouds in the mountains, with slight chance of showers over western Greenbrier County. Colder air will star to track in overnight west of the Blue Ridge with lows in the upper 30s to lower 40s, while the foothills/piedmont slide into the mid 40s to around 50. There will be a gradient behind the front tonight but 8h winds are around 30 kts or less, so winds should be under 30 mph in the gust dept, except possible gusts to 35 mpg across the Alleghany Highlands, higher ridges. && .SHORT TERM /THURSDAY THROUGH FRIDAY NIGHT/...
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As of 400 AM EST Wednesday... The final and most significant short wave of the slowly filling eastern U.S. low will be exiting to the east at the beginning of this period. The pattern will transition back to a cooler/seasonal pattern. Upslope trajectories are not overly favorable, generally a WSW-W flow across the Alleghanys. This in combination with the lifting out/filling upper low and the gradual return to southwest flow aloft does not lead to a very favorable setup for snow. In general, the GFS is the only model indicating much threat for any upslope rain/snow showers. Will hold onto some slight chance -SHSN across western Greenbrier through Thu morning, then drop. For Friday, there is a very weak clipper evident along the bottom of the departing upper low. This may induce a brief period of snow showers across the northern Alleghanys again, mainly north of the RNK CWA, but again the GFS remains one of the few models to depict any QPF with this feature as far south as western Greenbrier. Will carry a low slight chance pop to account for this feature. Both events are barely worth mentioning from my analysis. Otherwise, the main story this period will be much cooler temperatures. 850mb temps will fall back below 0C across most of the CWA during the day Thu and remain in this range well into the weekend, bottoming out Saturday in the -6C range north to around 0C south. Temperatures will trend back toward seasonal levels with lows in the 20s and 30s and highs in the 40s and 50s, colder mountains.
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As of 430 AM EST Wednesday... The focus during this period will be on a southern latitude system originating from a closed upper low over Mexico. There remain significant differences in how this system will evolve with the GFS still slow to bring the parent system out of Mexico, but now on board with what the ECMWF has been showing for several days, namely a northern stream system traversing the region Sunday into Monday. Models have been trending toward a weaker closed low and greater potential for northern stream energy to pick up the system sooner, thus forecast trends lean toward the faster ECMWF solution. These changes will result in chance pops being introduced in the southwest part of the CWA Saturday night/early Sunday, increasing to likely Sunday, but then decreasing quicker back to chance or less by Monday. P-type may be a concern early Sunday with surface temps hovering in the 29F to 32F range and 850mb temps struggling to rise above zero. This would especially be true for areas where the precipitation arrives late Saturday night/early Sunday. At any rate, especially per ECMWF, warm air will begin to surge northward during the day changing any frozen/freezing precipitation to liquid. The caveat may be the potential for an "in-situ" wedge to develop and keep the threat for winter precipitation in the area longer, quite common at this time of year, and something the models are poor at resolving. Uncertainty continues for the remainder of the period as the GFS still has the parent upper low coming out of Mexico and bringing another swath of precipitation across the southeast/Mid-Atlantic Monday into Tuesday, while the ECMWF has moved the entire system out of the region, leaving dry weather in place beyond Monday night. Have nudged the forecast toward the preferred ECMWF. After a seasonably chilly/slightly below normal period through the weekend, expect a return to milder temperatures early next week as heights build aloft across the southeast U.S. in the wake of the weekend storm system.
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&& .AVIATION /09Z WEDNESDAY THROUGH SUNDAY/... As of 1145 PM EST Tuesday... Expecting MVFR to IFR clouds to fill in over southwest Virginia, northwest North Carolina and southeast West Virginia tonight. LIFR fog developed at locations that had rain Tuesday afternoon and stay cloud free longer. Winds will pick up overnight, especially at higher elevations as the low level increases from the southwest. Low confidence on the timing for when visibilities will improve as fog erodes. Area of upper diffluence that was providing lift for the showers and thunderstorms over Mississippi and Tennessee will move into the region late tonight. Guidance was in good agreement keeping much of the area dry until after 06Z/1AM. Some warm air advection also in the region tonight and Wednesday morning which will aide in lift. Next band of heavier showers and will cross the region from west to east Wednesday afternoon allowing for widespread sub-VFR cigs and vsbys until the front crosses Wednesday evening. Shallow convection may enhance the winds. High confidence on the passage of the front and associated moderate showers but lower confidence on the exact timing. Extended aviation discussion... Improving conditions expected after the upper low and final short wave move east of the area later Wednesday night. Conditions should become VFR east of the Blue Ridge into the weekend. Upslope clouds and potentially a weak clipper system could bring a low chance of rain or snow showers into the western sites Thursday and Friday along with lingering MVFR cigs before conditions improve there as well over the weekend. && .RNK WATCHES/WARNINGS/ADVISORIES... VA...None. NC...None. WV...None. && $$ SYNOPSIS...JH/WP NEAR TERM...WP SHORT TERM...RAB LONG TERM...RAB/RCS AVIATION...AMS/JH is the U.S. government's official web portal to all federal, state and local government web resources and services.