Area Forecast Discussion
Issued by NWS Blacksburg, VA

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218 FXUS61 KRNK 220056 AFDRNK Area Forecast Discussion National Weather Service Blacksburg VA 856 PM EDT Tue Mar 21 2017 .SYNOPSIS... A weak cold front will push south of the area overnight into early Wednesday. A large area of high pressure builds in over the eastern United States behind this front for Wednesday and Thursday before passing offshore by Friday. A warmer southerly flow will develop Friday into the weekend ahead of a slow moving cold front that should approach the region on Sunday. && .NEAR TERM /THROUGH WEDNESDAY/...
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As of 845 PM EDT Tuesday... Area remains in dry air this evening behind an initial surface front with deeper moisture/instability to the south and colder air still lagging to the north along the 850 mb boundary. Once the convective system to the southwest passes over the next few hours should see the colder/drier air to the north start to surge in after midnight into Wednesday. Latest short term guidance also wanting to shift at least a few showers into southern/southeast sections by midnight, although looking iffy given some degree of capping and dry low levels heading north per latest RNK sounding. Therefore reduced pops again overnight while leaving in an isolated mention over the far southeast into the early morning hours per latest HRRR. Also cant totally rule out a sprinkle or flurry far northwest as the secondary front drifts southeast but again little more than isolated pops at best. Otherwise likely to see a period of cloudy skies espcly south and west as debris high clouds spill east from the MCS with perhaps some low clouds filling in once this feature passes as well. Low temps appear on track as lowered a little far northwest where likely to touch the colder air late, while bumping up a little south given clouds and less cooling aloft. Previous discussion as of 318 PM EDT Tuesday... A cold front is draped across the Tennessee Valley with a stationary front situated along the VA/NC border. A disturbance is expected to track along the cold front this afternoon and track east-southeast into North-South Carolina this evening. CAPE is increasing across northern North Carolina this afternoon, but so is northwest flow. Convection from this disturbance should clip the North Carolina High Country late this afternoon/evening, but the bulk of the stronger storms will stay well south. A theta-E boundary is situated over the southern Ohio Valley this afternoon. This boundary may generate a few showers across the region between midnight and sunrise, but mainly looking at overcast conditions. This boundary moves south into the Carolinas by mid morning with mostly clear skies to follow. With clouds and a slow moving theta-E boundary overnight, temperatures will remain mild with readings in the mid to upper 30s across the mountains to mid to upper 40s east. These temperatures are around 5F to 10F warmer than normal. Cool high pressure builds in from the north during the day Wednesday. Afternoon temperatures will be around 5F cooler than normal with highs in the 40s across the mountains to 50s east. With a warm start in the upper 40s, Southside VA and northwestern North Carolina foothills and piedmont could warm into the lower 60s.
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&& .SHORT TERM /WEDNESDAY NIGHT THROUGH FRIDAY NIGHT/... As of 318 PM EDT Tuesday... Relatively uneventful period of weather Wednesday evening through Thursday, dominated by large surface anticyclone now centered over the northern Plains and southern Canadian Prairies. As reflected in 12z GFS and 15z SREF mean model MSLP fields, the high stands to be positioned over eastern OH/western PA Wednesday night and then building off the mid-Atlantic seaboard on Thursday. As it builds off the coast Thursday, surface pattern transitions to one a cold-air damming profile with colder 850 mb temperatures (0 to -4C) and lower 1000-500 mb thicknesses in the Virginia and North Carolina Piedmont areas. Despite easterly flow, there does not appear to be much of any indication of lower clouds associated with the CAD, so have maintained a generally clear sky cover. Thus, sensible weather conditions Wednesday night through Thursday should be about the same - that being generally clear skies with light winds Wednesday night trending light easterly on Thursday. Strong radiational cooling Wednesday night supports lows in the upper teens to low 20s for most of southeast West Virginia and the Alleghany Highlands to the low 30s along the foothills of North Carolina. Relatively uniform Thursday high temps in the upper 40s in the lower valleys to low 50s in the eastern Piedmont and foothills areas, but mid 40s along the Blue Ridge and Appalachians in West Virginia. Still looking at relatively quiet wx for Thursday night, but the mid- level regime transitions to one of height rises with increasing southerly flow associated with a surface warm front. Greater isentropic ascent is primarily focused to our north and west, and that should translate to an increase in clouds and moisture/dewpoints with western extent. That said, locations in the central Virginia Piedmont, Southside Virginia and the North Carolina Piedmont should still remain governed by clear skies and colder thermal structure associated with the CAD through Thursday night. Shown coldest temperatures near freezing out in these eastern locales and in the New River Valley to the mid 30s in Mercer and Tazewell Counties. Used a modified non-diurnal curve after midnight from the Mountain Empire up through the Alleghany Highlands with lows occuring around early morning before hovering/slowly rising. Aforementioned warm front surges north-northeastward early in the day on Friday. Prospects for warm-frontal precip appear nil, as the best lower-level warm advective/isentropic forcing is shunted well to our north into southern Ohio and northern Pennsylvania. Only the 12z ECMWF shows any measurable QPF. Though I haven`t included any showers at this point, I did increase sky cover for our northern counties. Other challenge for Friday is the rate at which the CAD is eroded/weakens. It is a substantial increase in 850 mb temperatures through the day, to values near +10C and south-southwest flow in the wake of the warm front. I`m not sure how much that fully mixes, so while I`ve trended several degrees warmer to values in the upper 50s to low 60s, if greater mixing is realized temperatures probably are still 2-5 degrees too cold. Relatively mild warm sector regime for Friday night with lows in the 40s, some 5-10 degrees warmer than 1980-2010 climo normal lows. && .LONG TERM /SATURDAY THROUGH TUESDAY/... As of 318 PM EDT Tuesday... The outlook for the weekend into early next week is a relatively uncertain one. Source of this uncertainty is due largely to timing differences between the operational 12z GFS, CMC and ECMWF guidance on at least one but potentially two initially potent but deamplifying 500 mb disturbances emanating from the Desert Southwest into the Southern Plains and lower Mississippi Valley. As the air mass out ahead of both of these shortwaves stands to be rather warm and potentially unstable, opening the door for convective potential well to our west in the center of the US. It is somewhat clear that this period probably will trend more active, and that is probably the message to take away for this period. However at this point it is difficult to pinpoint specific details as this is contingent upon greater confidence in shortwave timing and how (the likely progged) upstream convective environment evolves. At this point, it appears that Saturday night into Sunday would be the earliest opportunity for rain showers. Temperatures generally trend near to above climatology. && .AVIATION /00Z WEDNESDAY THROUGH SUNDAY/... As of 650 PM EDT Tuesday... A cold front will slip south of the area overnight into early Wednesday. A weak disturbance riding along the boundary will remain the focus for showers and storms, mainly south of the area into tonight. However with the arrival of the front aloft from the north and possible moisture edging back north toward the VA/NC border, kept in a VCSH mention at KDAN later this evening, although low confidence given dry air and loss of heating. Increasing northwest flow behind the front should make for the possible development of upslope lower clouds at KBLF/KLWB overnight with some of this perhaps spilling east to KBCB for a few hours overnight. This could bring back a period of MVFR cigs over the mountains with VFR expected elsewhere. Dry air under high pressure should win out Wednesday given northerly flow so overall clear/VFR into Wednesday evening. By Thursday the high will wedge in from the northeast, so will remain mainly VFR. It is possible that residual lower ceilings may develop trapped especially along and east of the Blue Ridge Wednesday night into Thursday morning, resulting in possible MVFR ceilings. Extended Aviation Discussion... Good flying weather is expected to linger into Friday before moisture starts to increase over the weekend ahead of a cold front approaching from the west. Appears most of the showers ahead of the front will hold off until Sunday when sub-VFR could occur mainly over the mountains. Otherwise looking at overall VFR to start the weekend with most lower cigs remaining west of the area until later Sunday at this point. && .RNK WATCHES/WARNINGS/ADVISORIES... VA...None. NC...None. WV...None. && $$ SYNOPSIS...JH NEAR TERM...JH/RCS SHORT TERM...AL LONG TERM...AL AVIATION...JH is the U.S. government's official web portal to all federal, state and local government web resources and services.