Area Forecast Discussion
Issued by NWS Blacksburg, VA

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000 FXUS61 KRNK 240342 AFDRNK Area Forecast Discussion National Weather Service Blacksburg VA 1142 PM EDT Fri Sep 23 2016 .SYNOPSIS... Low pressure along the coast of the Carolinas will weaken and head east tonight. A back door cold front enters the area by Saturday night. As it moves south of us Sunday the flow turns east which brings in more clouds and a threat of rain into early next week. && .NEAR TERM /THROUGH SATURDAY/... As of 1100 PM EDT Friday... No significant changes needed at this time. Forecast appears on track with clear skies overnight. Fog development will be late and spotty given overall dryness of low-level air mass. As of 745 PM EDT Friday... Temperatures were running quite a bit warmer than forecast, so had to raise them a few degrees over the next few hours. Otherwise, forecast on track with no cloud cover and zero pops through much of the night. As of 310 PM EDT Friday... An upper level trough across the southeast coast will move offshore tonight. Heights will build behind this trough from an upper level ridge centered over the southern Mississippi Valley. Heights will begin to lower again as a back door cold front approaches the area. This front may bring in low clouds and light rain late in the afternoon to the southern Shenadoah Valley. Even though the atmosphere remains stable, weak convergence over the foothills and along the Blue Ridge may also pop a few shallow showers in the afternoon. For now, the best chance for any rain will be north of I-64. Since this is a slow moving front, Saturday afternoon temperatures will once again be warmer than normal with widespread 80s, near 90F toward the NC/VA border. && .SHORT TERM /SATURDAY NIGHT THROUGH MONDAY NIGHT/... As of 250 PM EDT Friday... A upper level ridge will be building over the eastern part of the country through Sunday, then it flattens Monday into Monday night. Surface high pressure over central Canada will build southward and wedge down the east side of the Appalachians. A back door cold front drops south across the area Saturday night into Sunday. Initial, shaped the pops for the light upslope with the HiResW-ARW-east and NAM for Saturday evening into Saturday night. Low temperatures Saturday night will range from the mid 50s in the mountains to the mid 60s in the Piedmont. Look for much cooler conditions behind the front Sunday with a good deal of low level moisture and isentropic lift over the wedge. The wedge looks to be solid and wet bulb temperatures are in the 60s so any precipitation into the wedge will be effective in holding temperatures down. Lowered high temperatures Sunday to the upper 60s in the north with wedge to near 80 degrees in far west. Sunday night will feature plenty of clouds and potential for some light upslope rain showers. Low temperatures Sunday night into Monday morning will generally be from the mid 50s in the northern mountains to the lower 60s in the southeast. By Monday evening, the axis of the upper ridge will move to our east and allow a large closed low over the Great Lakes region to push a cold front toward the region from the west. Southwest flow ahead of the front will continue to generate lift over the wedge, while the western and southern portions of the region on the fringes of the wedge may get into enough unstable air to support a slight chance of thunderstorms. Highs on Monday will range from the around 70 in the northeast to the upper 70s in the western mountains. Monday night will cloudy with chance of showers. Elected to increase pops to allow for the mention of likely in northwest Greenbrier county in Southeast West Virginia. Low temperatures Monday night will vary from the lower 50s in the northwest mountains to the mid 60s in the Piedmont. && .LONG TERM /TUESDAY THROUGH FRIDAY/... As of 230 PM EDT Friday... A deep upper level low over the Great Lakes will slowly push a cold front into the region Tuesday. The front and a secondary trof axis should continue to move east Tuesday night into Wednesday and push further off the coast Wednesday night into Thursday. (The 12Z GFS is on the slow end of the guidance envelope, while the 00Z ECMWF shows a more progressive solution with the surface cold front passage.) Decided to used Superblend with ensemble solution approach for the timing with the front. High pressure will build east across the region Thursday and Friday, then slide into the Atlantic ocean on Saturday. && .AVIATION /04Z SATURDAY THROUGH WEDNESDAY/... As of 745 PM EDT Friday... VFR conditions expected throughout most of the TAF valid period, with the exception of late night/early morning fog in the usual locations. Synoptic pattern features a west-east weak cold front sagging southward through NY/PA this evening. This front will reach the I-64 corridor by Saturday afternoon, then move toward the NC/VA border by Saturday evening. It appears the atmosphere will be too stable and dry in the lower levels for any precipitation with the front, which is unusual in the east. However, as currently noted, an extensive area of mid clouds will accompany the front. It appears that these will mostly be in the 050-080 range, depending on elevation. So have introduced bkn clouds in this range beginning Saturday afternoon, but no precipitation. Confidence is lower than normal in fog development tonight because of abnormally warm temperatures this afternoon and a dry boundary layer. Model cross sections and BUFKIT soundings are not favoring fog development tonight. For this reason, have backed off advertising LIFR FG at LWB/BCB from a FM group to a TEMPO. Winds will be light and variable or calm overnight, then becoming NW 6-8kts after 14Z Saturday. Medium to high confidence in cigs through the TAF valid period. Medium confidence in vsbys through 14Z, then high confidence through the remainder of the TAF valid period. Medium to high confidence in wind speed/direction through the TAF valid period. Extended aviation discussion... As the weekend progresses and the backdoor front stalls just to our south allowing easterly flow to become established across the region, low-level moisture increases and a wedge evolves. Expect cigs to lower into the MVFR category or worse and likely remain in place through Monday with areas of fog and drizzle or light rain developing with time as well, especially along the east facing slopes of the Blue Ridge. A strong, but slow moving, cold front will be approaching the area from the west on Tuesday. Models have significant differences on the timing and location of this front, but rain chances during the first half of next week will be on the increase. && .RNK WATCHES/WARNINGS/ADVISORIES... VA...None. NC...None. WV...None. && $$ SYNOPSIS...RCS NEAR TERM...RAB/RCS SHORT TERM...KK LONG TERM...KK AVIATION...RAB/RCS

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