Area Forecast Discussion
Issued by NWS Blacksburg, VA

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000 FXUS61 KRNK 220245 AFDRNK Area Forecast Discussion National Weather Service Blacksburg VA 1045 PM EDT Sun May 21 2017 .SYNOPSIS... Abundant moisture will lift over a stalled front tonight, then a cold front will cross through the region from the west of Monday. Several periods of precipitation are expected through the rest of the week. && .NEAR TERM /THROUGH MONDAY/... As of 1037 PM EDT Sunday... Have cancelled the flash flood watch as hourly rainfall has been generally no more than one quarter to one third of an inch. Should see the rain move slowly north and east ahead of a mesolow over southwest NC that will slide toward the Virginia piedmont by morning. Pops were adjusted to account for radar and high-res trends keeping higher threat along/east of the Blue Ridge, with less on the west side of the I-77 corridor. Both the HRRR and RAP show it drying out in the southwest/west after 08-09z. Staying generally cloudy but seeing opportunity in this weather regime for dense elevation based fog. Will have to monitor obs overnight to see if a dense fog advisory or special wx statement is needed. This will be more likely along the southern Blue Ridge, northwest to the Alleghanys. Previous discussion from early evening... Increased PoPs to 100% for the mountains and foothills counties where rain shield currently resides. A vast majority of the area is only seeing light rain. The heaviest rain remains south of the wedge boundary and closer to the low`s center. This rain shield will move east over the piedmont by midnight, exiting in the morning. Dry slot already moving northeast across the Smoky Mountains and should be entering the Mountain Empire later this evening. Flood threat looking less likely but will keep the watch in place as the low is taking more of a north-northeast track into North Carolina this hour. As of 345 PM EDT Sunday... Flash Flood Watch in effect along the southern Blue Ridge this afternoon and tonight. Regional radar showed elongated axis of showers from western Pennsylvania to South Carolina. Along the southern end of this band of precipitation the air mass was unstable enough to support scattered thunderstorms. Soundings showed the wedge of colder stable air below 850 mb with the boundary at the surface well west of Boone and well south into northern North Carolina. Satellite and radar loops showed a compact vorticity maximum tracking into extreme southwest North Carolina. Models showed good consistency in the timing and location of this feature through Monday, bringing it into southern Virginia after midnight tonight and into Maryland Monday afternoon. As this wave crosses the area southeast upslope winds will be enhanced and that will result in a period of moderate to heavy rainfall. The surface cold front approaching from the west will cross southwest Virginia, northwest North Carolina and southeast West Virginia Monday morning. At 850MB the front may not make much progress southeast as the region will be between the large 500MB low moving east through the Great Lakes and the upper ridge moving east off the coast. Used a non-diurnal trend for both tonight and Monday. Temperatures may have a slight rise as the wedge erodes tonight and with any clearing behind the front on Monday afternoon. && .SHORT TERM /MONDAY NIGHT THROUGH WEDNESDAY NIGHT/... As of 330 PM EDT Sunday... Southwesterly flow aloft ahead of a developing closed low moving out of the midwest will keep us in a wet and unsettled pattern through midweek. A cold front will be pushing off to our east early Monday night, but any break in precipitation will be short lived as the front stalls out just to our east and waves of energy slide northward along the front and push a good chance of precipitation back into the region during the predawn hours of Tuesday morning. With the wave expected to pass by to our southeast, the best isentropic lift and deformation look to bring the greatest precipitation totals to locations east of the Blue Ridge. There is considerable spread in guidance QPF but there is an upward trend and the latest runs indicate a good probability for amounts over one inch, especially across Southside. This will be enough to cause hydro problems with the possibility of river flooding so later model runs will be watched closely to see if the upward trend continues. The wave will move off to our northeast Tuesday night with some high pressure wedging developing behind, but Wednesday looks to be wet once again as another wavy front moves in from the southwest. Deep moisture transport and strengthening low level flow will once again bring widespread precipitation with potentially another good amount of rainfall. If the expected antecedent wet conditions materialize as currently indicated by guidance, the hydro threat will remain elevated Just lingering showers overnight Wednesday night as we get into the dry slot of the large stacked low spinning over the Great Lakes. Given expected clouds, precipitation, and being on the cool side of the boundaries, temperatures will be trending well below normal through Wednesday. && .LONG TERM /THURSDAY THROUGH SUNDAY/... As of 330 PM EDT Sunday... On Thursday, short wave energy and the cold pool associated with the upper low moving through the region will generate showers and thunderstorms with moderately steep mid/upper lapse rates aiding convective instability. Relatively low freezing levels and increasing low level flow make graupel/small hail and gusty winds possible with any showers/storms especially west of the Blue Ridge, but severe storms are not likely. The upper pattern will then be progressive into the weekend as the upper low moves off to the northeast Friday allowing for a brief period of ridging, followed by a transition to a zonal regime by Sunday. This will give us a mostly dry Friday after any lingering upslope showers west of the Blue Ridge dissipate, but another frontal system will push in from the north and stall over the region on Saturday and bring an increasing chance of showers/storms to the forecast for the weekend. Temperatures will be trending from below normal on Thursday to at or slightly above normal levels for the weekend with lower 80s east of the Ridge and mid/upper 70s to the west. && .AVIATION /03Z MONDAY THROUGH FRIDAY/... As of 720 PM EDT Sunday... Stalled front over the southern Appalachians will result in abundant cloud cover into Monday morning. A compact short wave along the North Carolina/South Carolina border will track northeast tonight. This will enhance southeast winds and upslope along the Blue Ridge and into the eastern foothills. For now, a vast majority of the area is only seeing light rain, even in the upslope regions. The heaviest rain remains south of the wedge boundary and closer to the wave`s center. This rain shield will move east over the piedmont by midnight, exiting in the morning. Dry slot already moving northeast across the Smoky Mountains and should be entering the Mountain Empire later this evening. Even if rain should exit early, MVFR-IFR ceilings and/or visibilities likely stay until west winds pick up Monday, behind a cold front. Extended Aviation Discussion... Expecting several periods of precipitation with sub-VFR conditions Monday night through Friday. Any breaks between rainfall events with better flying conditions will likely be less than 12 hours. && .HYDROLOGY... As of 1034 PM EDT Sunday... Not expecting any flooding tonight, as high rainfall rates and deep convection is not expected. Will still need to watch Monday night into midweek as more rainfall is expected, which could lead to small stream and river flooding, with models favoring areas along/east of the Blue Ridge. && .RNK WATCHES/WARNINGS/ADVISORIES... VA...None. NC...None. WV...None. && $$ SYNOPSIS...AMS/PM/WP NEAR TERM...AMS/RCS/WP SHORT TERM...MBS LONG TERM...MBS AVIATION...AMS/RCS HYDROLOGY...WP

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