Area Forecast Discussion
Issued by NWS Blacksburg, VA

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000 FXUS61 KRNK 121057 AFDRNK Area Forecast Discussion National Weather Service Blacksburg VA 557 AM EST Sun Nov 12 2017 .SYNOPSIS... Weak high pressure remains in control through today, though with increased clouds through the day. A upper-level disturbance and cold front moves across the area tonight into Monday morning, bringing light showers and patchy fog. Dry conditions and a moderating trend in temperatures for the midweek, with more unsettled weather into late next week. && .NEAR TERM /THROUGH TONIGHT/... As of 400 AM EST Sunday... Salient surface sea-level pressure features early this morning include a 1034 mb surface high centered over Portland, ME with an extending residual dry wedge across the mid-Atlantic into the Piedmont of VA/NC. An extensive low-level cloud shield tied to southeasterly moisture return around the wedge front was evident in morning infrared imagery across the Upstate SC area into a large part of the Deep South. As inferred by 00z RNK RAOB and veering winds in regional Doppler profiler data, much of the region is in an elongated and elevated warm advection regime that trails back to the MS Valley, associated with a weak mid-level trough in the central Plains. High clouds continue to spill in as well amid a fast stream of 60kt zonal flow aloft. For today, the main forecast challenge is on high temperatures. Aside from our NC mountains and Smyth/Wythe counties, most areas are under a canopy of high clouds. That`ll change as we progress through the morning hours as we start to tap into the cloudiness across the South. Expect there to be some modest warming from current readings to the upper 30s/lower 40s by early afternoon, but by this afternoon it`s expected that cloudiness will be enough to hinder much of any further warming. NAM-based MOS at Lynchburg is only in the low-40s and mid-40s at Greensboro. Did slow down earliest timing of PoPs for light rain back into the afternoon, though even then some of that may evaporate and further slowing temperature rise. Feel best shot at rain isn`t until the evening, but kept limited chances in the far southwest. Used a non-diurnal curve with highs today in the mid 40s in the Piedmont and Blue Ridge, to the upper 40s and near 50 in Mercer and Tazewell counties, closer to what the NAM is indicating. We`ll then await the arrival of the mid-level trough from the Ohio Valley. This morning`s guidance all tend to show similar trends in that there should be two somewhat disjointed areas of light rain: more warm advection precip across much of NC into the Piedmont, and then tied with the upper trough/cold front across the Ohio Valley. First batch of rain likely falls into residual wedge, with saturated isothermal vertical profiles - a regime fostering low clouds and patchy fog in many locations tonight. Second area of precip tied to the cold front moves into southeast WV after midnight which lingers Monday morning. So a rather raw, dreary evening with chance to lower Likely PoPs for light rain (amounts less than a tenth of an inch). Very slow fall in temperatures likely tonight in the upper 30s to low 40s with lots of clouds and fog around. && .SHORT TERM /MONDAY THROUGH WEDNESDAY NIGHT/... As of 405 AM EST Sunday... A general fast, highly kinematic, westerly zonal flow will remain in place through this period. This will keep any bitterly cold Arctic air well north of the area, but will result in a highly variable weather pattern with several chances for synoptic scale systems associated with vigorous short wave troughs to move west to east across the region. The first such system will be moving through the CWA Monday morning. Associated rainfall will generally be tracking east of the forecast area during this period. Although the models are now showing an increase in potential QPF across the Piedmont area, rainfall amounts for most of the CWA should be in the 1/4 to 1/3 inch range with this system, the majority of which will all be before 18Z Mon. Lingering upslope rain showers are possible across the Alleghanys of eastern WV into the evening, but little additional QPF is expected with that. The air mass behind this system is largely of maritime Pacific (mP) origin and as such is actually warmer than the antecedent Canadian (cP) air mass residing over the region today. Depending on amounts of any brief afternoon sunshine, especially east of the Alleghany front, temperatures should warm into the 40s west and 50s east. Tuesday and Wednesday should be mostly dry with seasonable temperatures under the relatively flat zonal flow in place with continental air masses traversing the U.S. The next in the series of short waves traversing the U.S. will arrive in the Mid- Atlantic/central Appalachians Wed night into Thu. This system is weaker than the Monday system, but will push a weak cold front and some associated light precipitation into the region from the northwest. The front will likely stall just to our south and return north as a warm front later in the week. As noted above, dynamics are weak with this system and tracking mainly to our north, so only light rain showers are expected, mainly across the western mountains with amounts mostly 1/4 inch or less with this event. Very little if any rain will make it east of the Blue Ridge, so there is a notable gradient in pops from good chance west to slight chance or less west. Look for near normal temperatures through the period with lows mostly in the 30s and highs mainly in the 50s, a category cooler in the western mountains and possibly some lower 60s in the Piedmont. && .LONG TERM /THURSDAY THROUGH SATURDAY/... As of 430 AM EST Sunday... Highly kinematic flow continues at the beginning of the period, but potential to transition to a deeper polar vortex across the eastern Canadian provinces/northeast U.S. is evident during the later periods. The weak front to our south will return north as a warm front Friday as the next in the series of strong upper-level troughs moves from the Midwest into the eastern U.S. Some warm frontal precipitation can be expected Thu night into Friday, but a strongly focused line of showers appears likely with the front/upper trough Friday night into Saturday. There is a strong jet associated with this system and thus some strong gusty winds may translate down to the surface with the convective showers along the front, but instability suggests thunderstorms will not occur. Precipitation will be focused mainly within a narrow window late Friday or Saturday morning. Of greater concern will be the northwest winds behind this system with a 50-60kt jet evident at 850mb and notable pressure rises, strong gusty northwest winds are likely, especially along the higher terrain of the Blue Ridge from northwest NC into southwest VA. This has been highlighted in the HWO extended portion and eHWO, but it is quite likely that a Wind Advisory and possibly even a High Wind Warning will be needed across the higher terrain in these areas by Saturday. Slightly above normal temperatures are anticipated until the cold front moves through Saturday when temperatures will drop to levels about 10 degrees below normal. For the most part, temperatures will remain above freezing through Saturday. Some snow showers will be possible across the Alleghanys of eastern WV in the strong cold advection behind the front, but it does not look very significant at this point in time. && .AVIATION /12Z SUNDAY THROUGH THURSDAY/...
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As of 556 AM EST Sunday... Generally VFR, though some MVFR stratus initially along the southern Blue Ridge in NC and into the Mountain Empire. Many areas experiencing some high cloudiness, though ceilings will gradually lower to an OVC VFR low cloud deck through the early afternoon. Initial burst of light rain associated with lead upper disturbance will advance northeastward later this afternoon, and while it`s not likely to influence visibilities much, it will lower ceilings to an MVFR. Winds should be light and variable today, though becomes southwesterly above 3000 ft. Degradation to poor flight conditions late this afternoon and especially this evening from low stratus, intervals of patchy fog and light rain showers at most TAFs. Ceilings likely MVFR though may drop to IFR at times overnight, with IFR mist or fog especially along or east of the Blue Ridge. Though winds should stay light to calm, a shift to west from 06-12z Monday anticipated with frontal passage. Medium to high confidence in ceilings, visibilities and winds through this afternoon, with low to medium confidence in aviation elements for the evening. Extended Discussion... May see continued sub-VFR conditions in the Piedmont as frontal rains come to an end Monday, though post-frontal sub-VFR stratus may linger into the afternoon. Should trend VFR by Monday evening. VFR conditions then anticipated Tuesday into early Wednesday. Next chance for possible sub-VFR conditions is with a cold front later Wednesday/Wednesday night time frame. Returning VFR conditions behind the front on Thursday.
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&& .RNK WATCHES/WARNINGS/ADVISORIES... VA...None. NC...None. WV...None. && $$ SYNOPSIS...AL NEAR TERM...AL SHORT TERM...RAB LONG TERM...RAB/WP AVIATION...AL/JH

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