Area Forecast Discussion
Issued by NWS Blacksburg, VA

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000 FXUS61 KRNK 220543 AFDRNK Area Forecast Discussion National Weather Service Blacksburg VA 143 AM EDT Thu Sep 22 2016 .SYNOPSIS... An upper low will meander across the eastern Carolinas through Thursday before finally weakening and shifting offshore by the end of the week. High pressure will gradually build in from the west Friday into Saturday. A backdoor cold front drops south through the area late Saturday into Saturday night followed by cooler high pressure Sunday into early next week. && .NEAR TERM /THROUGH TODAY/... As of 245 PM EDT Wednesday... Closed low over the Southeast Coast will continue to interact with the Post tropical remnants of Julia during the period. As been the case for a couple of runs, the ECMWF and NAM remain slightly west of the GFS. One of the forecast challenges is the western edge of the moisture and the sharp gradient in showers. The HiResw-arw-east, wrf-arw-rnk, RAP and HRRR support the mention of isolated to scattered showers in southeast portions of forecast area this afternoon into tonight. The upper low remains fixed across the coastal Carolinas tonight into Thursday morning. believe that shower coverage may wrap tighter towards the coast tonight with drier air pushing into northwest portions of forecast area. Cloud cover tonight will vary from clear in the northwest to cloudy in the southeast. Lows tonight will range from the mid 50s in the northwest mountains to the mid 60s in the east. The closed upper low will slowly weaken and lift northeast on Thursday. The result of this transition will be even less coverage of isolated showers across the south and southeastern portion of the region. High temperatures Thursday will vary from the mid 70s in the mountains to the lower 80s in the Piedmont. && .SHORT TERM /TONIGHT THROUGH SATURDAY NIGHT/... As of 230 PM EDT Wednesday... Persistent upper low will finally open up and shift northeast Thursday night in response to ridging nudging east from the Midwest. However may still be enough moisture around with the deformation axis on the back of the coastal system to produce a few added showers far southeast so left in an isolated mention early. Otherwise expecting a drier northerly flow to take shape under weak high pressure later Thursday night into early Saturday. This should bring clearing/sunny skies for late week into the weekend. However passing 500 mb trough well to the north by Saturday will propel a backdoor cold front toward the area Saturday afternoon and into parts of the region Saturday night. This a bit slower than previous given strong upper ridging overhead and warmth ahead of the front that should slow it down until Saturday night. Other than perhaps some developing post frontal low level cloudiness by late Saturday per the GFS, deep moisture to remain limited so not including any pops with the boundary for now. Given gradual 850 mb warming Friday and weak compression ahead of the front Saturday, looks like will return to above normal highs with mostly low/mid 80s both days with most surface based cool advection holding off until Saturday night at this point. && .LONG TERM /SUNDAY THROUGH WEDNESDAY/... As of 130 PM EDT Wednesday... High amplitude blocky pattern looks to evolve later in the weekend into next week as potential cut off upper lows develop off New England and across the upper Midwest. This could basically sandwich the area within a sharp shortwave ridge axis between these two features pending exactly where the upper block develops. Differences between models continue in regards to strength of the upper systems and subsequent speed of surface features in this setup. However still appears will see a rather significant backdoor cold front drop south into the region by Saturday night/early Sunday before the boundary fades crossing into the Carolinas. Guidance still suggests some spotty shower potential Sunday with the front aloft nearby and developing onshore flow. Thus thinking more clouds and spotty shower pops Sunday and southern sections Sunday night with a shallow moisture axis lingering but iffy. Lots of uncertainty to take shape early next week as model spread remains large in just how fast a second cold front to the west will be able to slide east given eastern ridging in place. Latest 12Z GFS has continued to make a shift toward the slower ECMWF from overnight in keeping this slow moving front west of the mountains through midweek. However these solutions remain much slower and stronger than the GEFS which looks too weak/fast in lifting support out to to the north, while spilling the front quickly east. Overall the slower scenario would keep the area in more or less a more stable low level wedge formation per high pressure to the north under ridging aloft. Tendency in this setup likely to lead to more clouds and spotty showers/drizzle including much cooler temperatures into midweek. Therefore have trended in this direction with highs below Mos, but still mostly 70s and lows 50s- mid 60s, although highs by early week could be only in the 60s if more widespread low clouds do materialize. && .AVIATION /06Z THURSDAY THROUGH MONDAY/...
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As of 135 AM EDT Thursday... Surface low is east of the GA/FL coast early this morning with the upper low/trough still situated across the coastal Carolinas. We are seeing a bkn/ovc deck of MVFR to at times IFR ceilings pushing toward DAN/LYH. Spotty showers also exists near DAN. Pattern favors in and out periods of lower ceilings at DAN/LYH through the morning and added tempo for -ra at DAN as well. Question will be how far west the sub-VFR ceilings reach. The models hint at potential for BCB/ROA to get a small window of MVFR ceilings this morning so added a tempo group at Roanoke from 09-11z. Am not forecasting it for BCB but should see scattered at 2-4kft with broken cirrostratus. However, fog should form some at BCB with MVFR vsbys while LWB drops to LIFR at times toward 09-12z. Will notice more subsidence in the west today with upper trough weakening. Should be VFR in the west after fog burns off with LYH becoming VFR by mid morning/14z. Danville is forecast to stay socked in a little longer but should go VFR in the afternoon. Precip should be more to our southeast today. Extended aviation discussion... VFR conditions will prevail Thursday night outside of late night/early morning fog/low clouds mainly BCB/LWB. GFS/ECMWF have been consistent in filling and lifting out the low to the northeast on Friday. VFR conditions will prevail on Friday. A back-door cold front will come through the Mid-Atlantic region on Saturday. High pressure will build in for Sunday. The chance of precipitation or any sub VFR conditions will increase through the day Sunday and into Sunday night into Monday as wedge sets up with easterly flow.
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&& .RNK WATCHES/WARNINGS/ADVISORIES... VA...None. NC...None. WV...None. && $$ SYNOPSIS...JH/RAB/WP NEAR TERM...KK SHORT TERM...JH LONG TERM...JH AVIATION...AMS/KK/WP

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