Area Forecast Discussion
Issued by NWS Blacksburg, VA

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000 FXUS61 KRNK 251350 AFDRNK Area Forecast Discussion National Weather Service Blacksburg VA 950 AM EDT Sun Jun 25 2017 .SYNOPSIS... Weak high pressure will linger over the region through Monday. An upper trough and associated cold front will sweep through the area Tuesday with a few showers followed by a cooler, drier air mass. Toward the end of the week, high pressure will move east of the region into the western Atlantic, allowing much warmer and more humid air to return to the region along with increasing chances for showers and thunderstorms. && .NEAR TERM /THROUGH TONIGHT/...
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As of 945 AM EDT Sunday... This morning`s limited patchy fog has dissipated. Cloud cover continues to stream southwest to northeast across mainly central and eastern parts of the area. As high pressure continues to build into the area, the extent of this cloud cover will trend eastward as the day progresses. No significant changes to the overall forecast are being made this morning. Have made minor only adjustments in the hourly temperatures, dew points, winds, and sky cover to reflect the latest observations and expected trends into the early afternoon. As of 330 AM EDT Sunday... A diffuse frontal boundary is evident across the extreme southern parts of the CWA or toward the I-40 corridor in North Carolina. There is quite a range in dewpoints this hour from the mid 50s across the Alleghanys and the northwest part of the CWA to near 70 yet across the southeast part of the CWA, say Caswell/Halifax areas toward Yadkin. A broad upper trough will continue to deepen across the Great Lakes today pushing the baroclinic boundary further south, such that by evening, it should be located from east central NC into central SC, then west into central GA. Any showers or thunderstorms today will be near and south of the front, leaving our entire CWA rain free today with lower humidity levels and slightly cooler temperatures, since not only do the dewpoints drop into the 50s north to 60s south by evening, but the 850mb temperatures show a marked drop from +18C of the past few days to around +8C by Monday morning. Some cirrus can be expected as a result of the prevailing southwest flow aloft as we remain on the east side of the broad upper trough anchored over the Great Lakes, but SC/CU development becomes less of a concern with time, even in the upslope areas of eastern WV. As noted above, temperatures will be on a cooling trend thanks to cold advection at 850mb and the deepening upper trough. High temperatures today will range from the 70s in the northwest to lower 80s in the southeast, on average about 5 degrees cooler than what we saw Saturday, with elevations above 3000 ft. likely only seeing high temperatures in the 60s today. Lows tonight will be very pleasant for the end of June with widespread lows in the 50s. A few 40s for lows in places like Lewisburg, Burkes Garden, and similar across the mountain valleys in the west would not be unprecedented at all. However, the coolest air of the week will come after a secondary front moves through the area Tuesday.
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&& .SHORT TERM /MONDAY THROUGH WEDNESDAY NIGHT/... As of 330 AM EDT Sunday... An upper level low over the Great Lakes will bring an increase in clouds Monday afternoon and a chance for rain Tuesday. This low will open into a trough Tuesday night as it continues to track east and off the New England coast. Cool high pressure will settle over the region on Wednesday. Models vary greatly for Tuesday`s rain chances in timing, confidence and rainfall amounts. The most agressive is the ECM which has rain entering the mountains before sunrise Tuesday. The GFS has rain in the mountains during the morning while the NAM holds everything off until late in the afternoon. The ECM is very confident on rain chances Tuesday with PoPs 80-90%. The GFS confidence is 30-50% for the mountains and zero east of the Blue Ridge. The NAM has likely PoPs (60%) for only the mountain in the afternoon. Model rainfall amounts range from half an inch (ECM) to a couple of hundredths (GFS/NAM). We favored the GFS solution for a few reasons. First, as the upper level trough axis pivots over the Ohio Valley and Mid Atlantic region, the associated short waves track across central VA and not down into NC. This pattern and northwest downsloping flow will limit rain chances east of the Blue Ridge. Second, while the upper level trough is in play, our flow will be from the northwest. The moisture source will be limited coming from either the Great Lakes or residual soil moisture from TS Cindy`s track across WV/OH. There is no return flow from the Gulf or the Atlantic for rainfall amounts to match that of the ECM. While under the influence of an upper level trough and cool northwest flow, temperatures Monday and Tuesday will run 5F-10F cooler than normal. Wednesday`s temperatures will also be slight cooler than normal but will begin a warming trend for the end of the week. && .LONG TERM /THURSDAY THROUGH SATURDAY/... As of 230 PM EDT Saturday... Trough lifts out midweek with height building across the southern U.S. Surface high will slide east of the region Wednesday night. Digging trough over the upper midwest to start to build a little more of heat ridge in the southeast with 591 heights. Will stay mainly dry but heating up with threat of typical summertime convection as early as Thursday over the WV mountains, with better threat Friday-Saturday. Southwest flow aloft increases by Saturday with a frontal boundary slowly moving toward the Ohio Valley. Should see an uptick in coverage in convection over the weekend across the mountains. Thursday will see moisture increase with highs bumping up another 5 degrees with upper 70s/lower 80s west, mid to upper 80s east. These warmer temperatures which are close to normal for the end of June will stick around into July 1st, but the humidity will be more noticable as well. && .AVIATION /14Z SUNDAY THROUGH THURSDAY/... As of 745 AM EDT Sunday... Moving into an extended period of mostly VFR conditions across the region over the next few days. The weak front has pushed southeast of the area down toward the I-40 corridor taking the showers and thunderstorms with it, along with the deeper moisture. Flow aloft is still broad west-northwest under the influence of a broad upper trough deepening across the Great Lakes. This will continue until Tuesday when a short wave rounds the base of the upper trough. Following the passage of this feature, the flow begins to relax, become more zonal, and eventually southwest as the trough weakens and lifts out. The only potential through the period for any non-VFR conditions would be during the late night/early morning hours when any of the typical spots that decouple and radiate effectively will see fog development. KDAN was on the edge of deeper moisture all night and rainfall late in the day Saturday. As a result, they fogged in just after midnight last night and remained LIFR-VLIFR much of the night in dense fog. A slight northwest wind and increased mid/high clouds have allowed the fog to finally break up there. Given a different synoptic situation Monday morning, do not see a repeat of this. Given the moist ground from recent rainfall and the cooler/drier air mass moving into the region for the first half of the week, fog seems like a reasonable possibility at sites such as KLWB AND KBCB, but less so elsewhere. Not overly confident about such tonight because air mass has not fully transitioned yet and there are some high clouds skirting the area. Will continue to advertise MVFR BR for KBCB and KLWB Monday morning as air mass may more effectively radiate tonight with less clouds and near calm winds. Winds will be WNW-NW at speeds of 3-6kts before 13Z, then increase to 6-11kts with low end gusts at most locations during the 14Z-22Z time frame, diminishing or becoming calm after 00Z. Medium to high confidence in ceilings through the TAF valid period. Medium to high confidence in visibilities through the TAF valid period. Medium to high confidence in wind speed/direction through the TAF valid period. Extended Aviation Discussion... A weak disturbance dropping in from the northwest may bring a period of MVFR cigs along with a few showers to the mountains late Monday night into Tuesday. Otherwise expecting overall VFR for Monday through Thursday with some brief late night/early morning mountain and river valley fog possible. On Friday, the potential will increase for some brief, localized sub-VFR conditions as shower and thunderstorm chances increase. More widespread, still diurnally driven convection expected Saturday. && .RNK WATCHES/WARNINGS/ADVISORIES... VA...None. NC...None. WV...None. && $$ SYNOPSIS...RAB NEAR TERM...DS/RAB SHORT TERM...RCS LONG TERM...RCS/WP AVIATION...RAB

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