Area Forecast Discussion
Issued by NWS Blacksburg, VA

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380 FXUS61 KRNK 271359 AFDRNK Area Forecast Discussion National Weather Service Blacksburg VA 959 AM EDT Tue Jun 27 2017 .SYNOPSIS... An upper-level disturbance moving across the area today will produce partly to mostly cloudy skies and isolated light showers. The passage of the upper disturbance will provide a reinforcing shot of unseasonably cool, dry and pleasant temperatures tonight through Wednesday. Temperatures and humidity levels then begin to rise with continued dry weather on Thursday. Conditions then turn more unsettled Friday and into the upcoming weekend. && .NEAR TERM /THROUGH TONIGHT/... As of 955 AM EDT Tuesday... Have continued the trend of decreasing precipitation chances today. Our latest forecast update removes any potential the remainder of the morning hours, with the first isolated showers developing between noon and 100 PM EDT. Have also adjusted hourly temperature, dew point, wind, and sky cover forecast grids to reflect the latest observations and expected trends into the early afternoon. As of 733 AM EDT Tuesday...Overall no significant changes made to the going forecast. Did adjust sky cover downward slightly through the next couple of hours. A review of incoming 06z guidance and CAM models has shown a trend toward less coverage of daytime showers; though I`ve reduced PoPs somewhat, maintained isolated showers in the forecast for most locations but preferentially within a few miles on either side of the Blue Ridge. As 1000-500 mb thermal trough builds over the area, expect destructive sunshine promoting cumulus development, a few which may pop some showers but dry sub-cloud air mass is a significant hindrance toward measurable rainfall. No changes to temps at this point. Previous near-term discussion issued at 345 AM follows... Skies early this morning are mostly clear to partly cloudy, with scattered mid-level cloudiness affecting areas along and west of I- 81. These clouds lie ahead of a cold front in association with a reinforcing shot of cooler, drier cP air across the upper Midwest. Broad 500 mb longwave troughing/cyclonic flow in place across much of the eastern two-thirds of CONUS this morning. Two shortwave disturbances were evident in morning water vapor imagery: the stronger shortwave over northeastern OH with the weaker feature over southwestern IN. For Today...axis of broad longwave trough progresses across the forecast area through the first half of the day. Shortwave trough now over IN should progress eastward across WV into the VA Piedmont by mid-afternoon. Daytime heating may be slowed to some extent by increased mid-level cloudiness, but expect some cumulus to develop by mid/late morning into the afternoon with progged weak instability. That should be enough to generate isolated to scattered showers along the cold front. Based on BUFKIT forecast soundings off the NAM, RAP and GFS which all depict a dry and well-mixed sub-cloud air mass, it`s questionable if rain will even reach the ground. Could see several Cu today generate a good deal of virga, but did keep QPF less than 0.03" today. Skies should be partly to mostly cloudy through early afternoon, with greater clearing associated with building surface ridge behind the frontal zone. Should be somewhat breezy as well with northwest gusts as high as 25 mph especially during the afternoon when the boundary layer mixes out. Highs today remain unseasonably cool but refreshing given late-June, which range from the mid 60s to lower 70s along and west of the Blue Ridge to the mid and upper 70s in the VA/NC Piedmont and foothills areas. For Tonight...Cold, dry advection regime as broad high pressure area now over the upper Midwest builds southeastward, becoming centered over the forecast area by early Wednesday morning. Any cloudiness erodes quickly and many areas should go clear by sunset. Questions for tonight pertain to how quickly do northwesterly winds ease and on fog development. It seems likely per 00z NWP suite that there should be enough of a gradient to permit slower radiational cooling, but especially around/after midnight, winds should drop off and temperatures stand to plummet pretty quickly given the dry air mass and 850 mb temps around +8 to +10C. I`m not as confident on the potential for radiation fog development for the overnight as we`ll be lowering dewpoints for a good part of the night; areas that do get rainfall do stand a better chance, though. All in all, a comfortable night with lows some 7-10 degrees colder than late-June climo values, running from the mid 40s to the lower/mid 50s. && .SHORT TERM /WEDNESDAY THROUGH THURSDAY NIGHT/... As of 345 AM EDT Tuesday... High pressure centered over West Virginia Wednesday morning will slide east Wednesday afternoon and push into the Atlantic ocean Thursday. A northwest to west flow aloft behind the exiting trough will advective cooler air into our region Wednesday. High temperatures Wednesday afternoon will range from the lower 70s in the mountains to the lower 80s in the Piedmont. The transition to higher humidity levels and warmer temperatures begins Wednesday night into Thursday as higher low level theta-e air lifts north into our area as a deepening west to southwest flow arrives. Low temperatures Wednesday night will vary from the mid 50s in the mountains to the lower 60s in the southeast. A few showers or thunderstorms may be possible Thursday afternoon across southwest portions of the forecast area with the nose of a low level theta-e ridge, increasing PWATs and differiental mountain heating. Do not cancel your outdoor plans because of storms will be quite limited. Highs thursday will be a couple of degrees warmer than Wednesday with readings from the mid 70s in the mountains to the mid 80s in the Piedmont. Under clear to partly cloudy conditions Thursday night, Low temperatures will generally be from the the upper 50s in the mountains to the mid 60s in the Piedmont. && .LONG TERM /FRIDAY THROUGH MONDAY/... As of 345 AM EDT Tuesday... A return to weather conditions more typical for summer are expected during the long term period, as the long range models show good agreement transitioning from an upper level zonal flow early in the period to an upper trof forming somewhere around the Great Lakes and extending south into the eastern U.S. by late in the weekend into Monday. This will result in increasing temperatures and humidity, as well as an increase in storm chances. An upper level disturbance moving out of the western Gulf of Mexico into eastern VA on Friday/Friday night will help lead a surge in higher PWAT air our area. This higher moisture combined with a series of upper level disturbances...with the usual differences in timing shown in the long range models...will help generate scattered showers and storms each day during the long term period, especially during the peak heating in the afternoon and early evening hours. With increasing CAPES, but low shear through the period,a few pulse severe storms and locally heavy rain are possible. Warmest temperatures are expected Sunday and Monday as 850mb forecast temps approach +20C. This may generate heat index values in the low to mid 90s in the east. && .AVIATION /14Z TUESDAY THROUGH SATURDAY/... As of 725 AM EDT Tuesday... Mainly VFR through the 12z TAF forecast period. Cold front continues to slowly progress east out of central WV, and ahead of it lies a plume of SCT-OVC mid-level clouds. A few showers are possible as cumulus clouds develop this afternoon, though dry sub-cloud layer likely leads to any isolated showers producing no restrictions to visibilities. Indicated VCSH for the late AM/early PM period, with threat of isolated showers generally ending at or before 20z. Will see west to northwest winds 6-9 kts with gusts to near 20 kts as the mixed layer deepens this afternoon. Confidence in this period is moderate to high on ceilings, visibilities and winds. Continued VFR currently anticipated for the evening. There is the chance for patchy radiation fog and associated IFR to LIFR visibility restrictions but confidence is not high on occurrence and its areal coverage would depend to a large extent on if any airport can experience a wetting rainfall this afternoon. Building high pressure ridge promotes lightening surface winds and clearing skies. Confidence in the Tuesday evening period is low to medium on visibilities, high for ceilings and winds. Extended Aviation Discussion... VFR conditions then anticipated until Friday, with potential for overnight IFR to LIFR radiation fog Wednesday and Thursday evenings. Friday into Saturday, the potential for sub-VFR conditions increase as showers and storms become more numerous. Saturday appears to feature a greater coverage of showers and storms associated with a cold frontal passage. Likewise, an increase in the boundary layer moisture will also lend itself to better chances of late night/early morning patchy fog. && .RNK WATCHES/WARNINGS/ADVISORIES... VA...None. NC...None. WV...None. && $$ SYNOPSIS...AL NEAR TERM...AL/DS SHORT TERM...KK LONG TERM...KK/PH AVIATION...AL

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