Area Forecast Discussion
Issued by NWS Blacksburg, VA

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105 FXUS61 KRNK 200845 AFDRNK Area Forecast Discussion National Weather Service Blacksburg VA 445 AM EDT Thu Jul 20 2017 .SYNOPSIS... High pressure building eastward across the area will provide hotter temperatures today through Saturday. At the surface, a frontal boundary will slowly track from the lower Great Lakes today, to the Northern Virginia area by Friday evening, before stalling. A cold front will move into the area early next week. && .NEAR TERM /THROUGH TONIGHT/...
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As of 310 AM EDT Thursday... Upper ridge over the southern Plains and Mid-Mississippi Valley this morning extends eastward today to the mountains, while the upper trough along the coast of the Carolinas, weakens and actually starts to slide back southwest toward the Florida Panhandle tonight. 8h temps reach +22C today and with a lack of cloud cover somewhat of a westerly downslope component, though not strong, temperatures should heat up into the mid 90s over the piedmont this afternoon as well as urban areas further west, like Roanoke and Lexington. Subsidence behind the upper trough and westerly flow should keep dewpoints somewhat lower to offset heat indices, but some areas could exceed 100 degrees for an hour in the Buckingham to South Boston area around 3-4pm this afternoon. Threat of convection looks very low, so have removed isolated threat along the Blue Ridge. May see some storms nearby over the Alleghanys west and north of Hot Springs after 4pm, but appears better coverage will be further north over northern WV/PA. Tonight, 8h flow turns more northwest and a couple of models have a piece of vort energy/possible convective vort moving from Indiana/Ohio around midnight, to the central Appalachians by 8am Friday. Some hint in the previous forecast for this possibility so keeping slight chance of thunderstorms late tonight northwest Lewisburg/Hot Springs. Otherwise, muggy with lows from the mid to upper 60s mountains, to lower to mid 70s east.
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&& .SHORT TERM /FRIDAY THROUGH SUNDAY NIGHT/...
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As of 400 AM EDT Thursday... Appearing to be an active to potentially very active period for convection, thanks largely to a warm/hot, very unstable, and moist air mass that will be in place acted upon by an active northwest flow on the northeast side of a strong subtropical ridge. We begin the period with a strong 594dm upper ridge anchored just to our southwest across the TN Valley. An active northwest flow will be established from the Upper Midwest toward the Ohio Valley and into the Central Appalachians and Mid-Atlantic region. Several ripples are embedded within the flow that will trigger MCSs across the Upper Midwest/Ohio Valley and which will propagate east to southeast into the warm/unstable air mass present across our region. The upper ridge will not be located sufficient far enough northeast to suppress the convection much if at all. As we move toward the Sunday, the upper ridge will break down further and retrograde back into the South Central and Southwest U.S. This will open the door for even more MCSs to track into our region from the northwest. Timing of individual short waves that will trigger convection is difficult and varies among the various models. Initial thinking is that Friday afternoon could be quite active as most models show a short wave moving southeast from OH into WV during peak heating. The next short wave may track further north across the northern Mid-Atlantic, but the first short wave and associated convection will leave an outflow boundary in the region Saturday that will most likely be the focusing mechanism for additional convection, especially west of the Blue Ridge. On Sunday, a stronger short wave will push a cold front southeast through the area that will likely stall just to our southeast early next week, a common theme this summer so far. Another round of strong convection is likely Sunday as a result. SPC has already outlook roughly the northern 2/3rd of the CWA for a marginal severe threat Saturday/Day 3, with a marginal threat just to our north Day 2. This certainly seems reasonable given the pattern that will be in place. The other concern over the weekend will be the warm/hot temperatures and accompanying high humidity levels. With the center of the upper ridge remaining to our southwest and 850mb temps topping out around +22C, don`t see anything dramatically worse than what we have already experienced so far this summer. High temperatures over the next few days will range from the mid to a few upper 90s east of the Blue Ridge to the mid 80s in the west, near upper 70s to 80 in the higher elevations. Past heat waves of , e.g., late June/early July 2012, have seen 850mb temps closer to +25C and over a larger area than even what is expected with the +22C area this time. Dewpoints across the Piedmont will hover in the lower 70s much of the next several days with mid to upper 60s western areas. This will net heat indices below 105 criteria for the most part Friday and just a brief period in a few of the far eastern Piedmont counties Saturday around the 105 criteria. Will continue to highlight in the HWO, but will not be issuing any Heat Advisory headlines with this package. This is not to say that it won`t be hot and humid because it will, just about everywhere. So, just keep that in mind as you go out and about outside the next few days. For what it is worth, am still not convinced that BCB will see 90, as we have not yet this summer. With 850mb temps topping out at +22C or less, am reluctant to forecast 90 at BCB, so held the max there at 89 for both Fri and Sat.
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&& .LONG TERM /MONDAY THROUGH WEDNESDAY/...
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As of 315 PM EDT Wednesday... Broad troughing will develop across the Eastern U.S. through the first half of the next week, similar to what we saw earlier this week. The upper ridge will relocate to the southwest U.S. leaving us in an active northwest flow aloft, at least at times. A frontal boundary should be pushed southeast through the CWA Sunday into Monday. The GFS suggests that the boundary will be pushed far enough south to remove our CWA from the convective threat for much of the period after Monday. However, the ECMWF is less excited about this possibility and that would be more consistent with what we have seen repeatedly this summer. Have retained the chance to high chance pops previously advertised for this period. With the upper ridge weakening and shifting way from the away, a notable drop in 850mb temps is expected from +20C on Sunday to around +15C or so by the mid part of next week. High temperatures will drop back into the 80s, even in the Piedmont, with 70s for highs in the mountains for the first half of next week. Low temperatures will drop back to the 50s mountains and 60s elsewhere. Very pleasant temperatures for late July!
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&& .AVIATION /08Z THURSDAY THROUGH MONDAY/... As of 110 AM EDT Thursday... Fairly quiet aviation wise this period with high pressure aloft building in from the west. The exceptions will be a period of sub-VFR vsbys and some cigs with fog early this morning, especially LWB/BCB, but also LYH/DAN. LWB should be favorable for 1/2sm after 08-09z. Most of the sub-VFR conditions will improve to VFR by 14Z/10AM Thursday. it is unlikely any showers or thunderstorms will develop Thursday under a warm ridge, but if they do they will be extremely isolated and likely confined to near the higher ridges. Winds through the forecast period will mainly be light westerly or calm. Confidence levels are high regarding the wind through the 24 hour TAF period. Confidence levels regarding ceiling and visibilities are high during the daylight and evening hours. Confidence is moderate into early Thursday morning. Aviation Extended Discussion... Mainly VFR conditions are expected through Sunday as upper level ridging allows for at best isolated to scattered afternoon and evening showers and storms, most likely in the mountains and less likely to impact farther east such as KLYH and KDAN. Brief, sub- VFR conditions will be possible with the heavier showers and storms. Late night/early morning sub- VFR fog will be possible, mainly in the mountain and river valleys. A cold front will move into the region Sunday night into Monday. Look for an increase in convection, and better chances of sub- VFR conditions in association with convection along the front. Overnight fog will be more likely Sunday night into Monday morning. Confidence of all weather parameters is moderate during the extended portion of the forecast. && .RNK WATCHES/WARNINGS/ADVISORIES... VA...None. NC...None. WV...None. && $$ SYNOPSIS...WP NEAR TERM...WP SHORT TERM...RAB LONG TERM...RAB AVIATION...SK/WP

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