Area Forecast Discussion
Issued by NWS Blacksburg, VA

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885 FXUS61 KRNK 200527 AFDRNK Area Forecast Discussion National Weather Service Blacksburg VA 1227 AM EST Fri Jan 20 2017 .SYNOPSIS... High pressure centered over our region this afternoon will shift east to the coast tonight as a warm front lifts north from the Gulf Coast states into the southern Appalachians. The warm front will swing north across Virginia and into the mid-Atlantic Friday, before stalling. Another storm system moving across the southern U.S. will impact our region Sunday into Monday. && .NEAR TERM /THROUGH TODAY/... As of 1100 PM EST Thursday... Adjustment to the forecast address to main features, overnight low temperatures and timing of arrival of precipitation. Several locations over parts of Southside Virginia and neighboring portions of north central North Carolina are already experiencing temperatures in the upper 30s. Forecast lows were in the lower 40s. Have adjusted forecast lows to reflect readings not too much lower than the ongoing readings, allowing for additional slow cooling into the early morning hours before a slight rise in temperatures before sunrise as weak warm air advection starts. Have slowed the arrival of rain into the region from west to east. This amounted to not a whole lot of changes across the western portions of the area, but for eastern areas, the arrival of the main area of rain looks delayed until a little after sunrise Friday. As of 700 PM EST Thursday... Incoming moisture is hitting a wall of dry air this evening, therefore slowed the arrival of rain entering the forecast area until after midnight. Otherwise no major changes to previous forecast. As of 200 PM EST Thursday... Upper Ridge over our region this afternoon will slide east tonight into Friday. Warm advection high clouds this afternoon will be followed by increasing mid clouds starting to roll into the southwest by dusk. Low pressure over western Tennessee this afternoon will lift northeast into the Ohio Valley tonight and reach Lake Erie by Friday afternoon. A warm front trailing from the low will lift northward tonight into Friday. Isentropic lift increases this evening across the mountains of North Carolina and shift to the rest of the forecast area overnight. With a dry airmass in place, slowed down the onset of the rain this evening. Used a blend of HRRR and HiResW-arw-east for pops this evening, then utilized a mix of NAM and Continuity with pops tonight into Friday. The best chance for rain is in the mountains with lower threat in the Piedmont. Lower dewpoints in the evening combined with increasing moisture may allow for temperature drop once the rain starts to fall tonight. However, it should remain mild with low temperatures ranging from the upper 30s in the Alleghanys and portions of the Greenbrier Valley to upper 40s to about 50 degrees in Mountain Empire. On Friday, the best lift will push quickly northeast through the region. An upper level shortwave ridge will build over the Mid- Atlantic region. The bulk of the rain will have moved northeast of the area by the early to mid afternoon. Added the mention of fog to isc grids. Lowered high temperatures a few degrees especially in the north Friday with clouds and rain. High temperatures Friday will vary from the upper 40s in the mountains to the lower 60s in the south. Rainfall amounts overall will average around a quarter inch with locally higher in the western mountains of Southwest Virginia and North Carolina. && .SHORT TERM /TONIGHT THROUGH SUNDAY/... As of 300 PM EDT Thursday Even with limited sunshine on Saturday the air mass will be unseasonably warm, 15 to 25 degrees above normal. Deep moisture begins to arrive back into the area from the southwest on Saturday afternoon. But Bufkit not showing a saturated sounding until closer to Sunday morning. Models forecasting good lift Sunday with a strong vorticity maximum crossing the central and southern Appalachians along with upper diffluence. Winds at 850MB back to the southeast by 12Z/7AM Sunday with good upslope and increasing inflow off the Atlantic first along the Appalachians in northern North Carolina on the Sunday morning gradually shifting north of Roanoke by the end of the day. More significant differences in the guidance in the amount of instability across southern Virginia and northern North Carolina on Sunday all related to the location of the warm front. 12Z NAM/GFS and latest RAP indicating enough of a secondary low developing over eastern North Carolina that with the precipitation there should be strong in-situ wedging. Large spread in location of heaviest rain also so will lean toward WPC guidance. Wedge and precipitation will keep temperatures on the cool side/or below guidance for Sunday. Have added slight chance of thunder from extreme southern Virginia into northern North Carolina but confidence is low. && .LONG TERM /SUNDAY NIGHT THROUGH THURSDAY/... As of 200 PM EDT Thursday Precipitation will remain in the area through Monday with the highest rain amounts on Sunday night and Monday, mainly along and east of the Blue Ridge. Experimental hydrologic ensembles showed at least a small potential for 1 to 3 inches of rain. Localized minor flooding is possible if this amount of precipitation is realized. Once the surface low is off the coast on Monday night and Tuesday, precipitation will be confined to favored western upslope areas. Enough colder air comes in Monday night that we will have snow in the forecast from southeast West Virginia into northwest North Carolina. Winds will also increase Monday night and Tuesday. There may be enough of a low level jet along with cold air advection and pressure rises for wind gusts in the 40 to 50 mph range. By Wednesday and Thursday a long wave positively tilted upper trof will extend from the Great Lakes across the central United States. Prevailing deep southwest flow over the Mid Atlantic region during this time frame results in differences among the models in how fast any front will progress east. For now WPC has surface front crossing the region Wednesday and Wednesday night. Behind this system is a much colder air mass. More upslope snow showers in the mountains are possible. At this time the models are showing snow showers for Thursday night. && .AVIATION /05Z FRIDAY THROUGH TUESDAY/... As of 1210 AM EST Thursday... Fast moving negatively tilted short wave will rotate through the region late tonight/early Friday, followed by an extended period of southwest flow preceding another short wave during the weekend and a deep low pressure area slated to move through the southeast U.S. early next week. This system will be in/out fairly quickly, but will leave low clouds, drizzle, fog in its wake given the persistence of southwest flow aloft remaining in place. It appears that the timing of this system is still even slower than thought at 00Z, with the main wave of rain coming through in the 13Z-17Z time frame as opposed to 08Z-14Z time frame earlier considered. HRRR/GFS used for timing. HRRR also shows redevelopment of convection during the afternoon, which was not indicated in previous runs. Have not included that scenario with this package, but if future runs continue to show this, it may need to be considered. For now, planning on evolution of rain to fog/drizzle/low clouds during the evening. Current VFR ceilings will quickly deteriorate to MVFR/IFR after the rain spreads into the region, then settle into an IFR/LIFR condition during the afternoon in fog, drizzle, low clouds. Visibility will drop into the MVFR category, occasionally IFR during the rainfall, then settle mainly into IFR-LIFR category during the afternoon/evening in fog and drizzle. Winds will be mostly east-southeast 3-5kts tonight, becoming southeast 4-8kts as the rain moves into the region. Some low end gusts are possible at KBLF with the southeast flow. Winds will become light and variable or calm east of the Blue Ridge after the rain moves out of the region, but become SSW-SW 4-8kts with again some low end gusts at KBLF during the afternoon Friday. Winds will trend back toward light and variable Friday evening, expect KBLF which should keep Medium confidence in ceilings and visibilities through the TAF valid period. Medium confidence in wind speed/direction through the TAF valid period. Extended Aviation Discussion... As noted above southwest flow aloft will persist ahead of a deep upper low that will develop across the southeast states over the weekend, then move off the southeast U.S. coast Monday. Low clouds, drizzle, and fog are likely to persist Friday night with additional waves of rain moving into the area late Saturday and persisting into Monday. At this point it appears that the only period of potential VFR cigs would be Saturday afternoon. Otherwise, much of the period will be characterized by MVFR or worse ceilings and visibilities. Winds will become strong and gusty from the northwest Monday night into Tuesday as the major weather system moves up the east coast. Ceilings and visibilities should finally improve east of the Blue Ridge after Tuesday, but likely persist with MVFR and upslope flow west of the Blue Ridge. && .CLIMATE... As of 305 PM EST Thursday... Record warm Mins for January 21 Blacksburg....42 in 1954 Bluefield.....47 in 1999 Danville......56 in 1954 Lynchburg.....51 in 1927 Roanoke.......51 in 1959 Record highs for January 21 Blacksburg....59 in 1954 Bluefield.....60 in 1999 Danville......68 in 1959 Lynchburg.....72 in 1932 Roanoke.......74 in 1932 && .RNK WATCHES/WARNINGS/ADVISORIES... VA...None. NC...None. WV...None. && $$ SYNOPSIS...KK NEAR TERM...DS/KK/RCS SHORT TERM...AMS LONG TERM...AMS AVIATION...RAB CLIMATE...AMS

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