Area Forecast Discussion
Issued by NWS Blacksburg, VA

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000 FXUS61 KRNK 210545 AFDRNK Area Forecast Discussion National Weather Service Blacksburg VA 145 AM EDT Thu Jul 21 2016 .SYNOPSIS... High pressure was over the mid Atlantic region today and will remain over the area through Thursday night. Two weak cold fronts will approach the area later this week into early next week. The first front crosses the Ohio valley and northeast United States on Friday night and Saturday, and the next on Monday night and Tuesday. && .NEAR TERM /THROUGH TODAY/... As of 1115 PM EDT Wednesday... Light rain showers persist in the Augusta/Rockbridge/Amherst/Appomattox/Nelson county area, but as the HRRR and NAM have suggested, but to a much less extent than advertised by the HRRR. Since this has persisted now for several hours and was drifting slowly south more into our CWA, have adjust pops across the region from Amherst down to Henry/Pittsylvania over the next few hours up to 15%. Otherwise, for the remainder of the CWA, have left them below 15%. Air mass too stable to support thunder, so just advertised light rain showers. T/Td/Sky grids pretty much on track. Just a very minor tweak to the T/Td grids to bring into line better with current obs. As of 800 PM EDT Wednesday... Not many changes needed to the grids at this time. Minor tweaks to the T/Td grids to bring them in better line with current observations. So far, a very quiet evening. However, the HRRR and to a lesser extent the NAM suggest that showers and perhaps even thunderstorms could develop across the southern Shenandoah Valley down into the Piedmont of VA/NC over the next few hours. Although there is weak wind convergence in these areas and better moisture, this does not seem very realistic. The RAP and the larger synoptic scale models do not suggest much at all and the low-level environment seems a tad dry and stable for much. Nonetheless, there is a small shower in Nelson county VA at this hour, so will monitor. Have not adjusted pops yet until this appears that it will become more of a reality. As of 345 PM EDT Wednesday... Good mixing has lowered dew points into the 50s over the mountains this afternoon. Water vapor loop showed a pocket of mid level dry air over the Carolinas today. Environment is dry and stable enough to keep precipitation out of the forecast until Thursday afternoon. Hi-res NMM guidance had a reasonable placement along the southern Blue Ridge after noon for the most likely area where isolated thunderstorms will develop. Temperatures will be near normal tonight and Thursday. && .SHORT TERM /TONIGHT THROUGH SATURDAY NIGHT/... As of 330 PM EDT Wednesday... Will be increasing maximum temperature on Friday based on rising 500mb heights and forecast 850mb temperatures around +22C. Heat index values will be at or just above 105 in the far eastern county warning area on Saturday afternoon. A cold front will move gradually south through Ohio and Pennsylvania on Friday night and may continue to drift into Virginia on Saturday. While the front may not be much of a change of air mass it will be an area of better low level convergence and deeper moisture and when combined with the upper support will result in an increases probability of showers and thunderstorms. Even without the boundary passing south of the forecast area, it will be close enough that convergence along the boundary or along outflow boundaries will aide in storm initiation. Precipitable water values are back up over 1.0 inch on Friday afternoon then closer to 1.75 inches on Saturday. && .LONG TERM /SUNDAY THROUGH WEDNESDAY/... As of 230 PM EDT Wednesday... Upper ridge will extend from coast to coast Sunday through Wednesday. In the northern stream the next synoptic scale wave crosses through the northeast United States on Tuesday. Models were similar bringing moisture back into the region on Monday and Tuesday ahead of the front. Therefore this will be the time frame with the highest probability of showers and thunderstorms. && .AVIATION /06Z THURSDAY THROUGH MONDAY/...
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As of 145 AM EDT Thursday... VFR conditions expected to prevail through much of the TAF valid period. Radar at 0530Z showing an isolated shoer or two in vicinity of KLYH and some hi resolution models show isolated showers and perhaps even a thunderstorm developing through some of the overnight hours in the Piedmont and working southeastward toward KDAN. Other models are completely dry. With lingering instability cannot rule the idea of at least a few showers continuing but for now have included on VCSH at KLYH and no mention at KDAN due to low confidence. During the afternoon expect scattered cumulus to develop but most likely not producing any ceilings. There could be an isolated shower or thunderstorm along the Blue Ridge well south of the TAF sites. Winds will remain light and variable through much of the TAF valid period, favoring a S-SE direction at 4-6kts Thursday...with more variable and possibly westerly at times on the west side of the Appalachians. High confidence in cigs through the TAF valid period. Medium to high confidence in vsbys through the TAF valid period. Medium confidence in wind direction, high confidence in wind speed through the TAF valid period. Extended aviation discussion... Strong high pressure aloft centered across the Midwest will attempt to build eastward into the weekend, then retreat back to the west early next week. Our region will remain on the eastern periphery of the upper high and thus subject to weak disturbances in northwest flow aloft tracking around the Great Lakes and into the Mid-Atlantic. Moisture and instability will be sufficient for isolated pop up thunderstorms, but a slightly better coverage will be possible Friday into Saturday with a weak front/short wave moving into the Mid-Atlantic with a better threat for convection early next week as another front moves into the area, stalls, and lingers over the region for several days. Hot and humid conditions east of the Blue Ridge this weekend which may impact density altitude. Late night and early morning fog will be possible at the usual sites almost every day.
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&& .RNK WATCHES/WARNINGS/ADVISORIES... VA...None. NC...None. WV...None. && $$ SYNOPSIS...AMS NEAR TERM...AMS/MBS/RAB SHORT TERM...AMS LONG TERM...AMS AVIATION...MBS/RAB/SK

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