Area Forecast Discussion
Issued by NWS Blacksburg, VA

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000 FXUS61 KRNK 220624 AFDRNK Area Forecast Discussion National Weather Service Blacksburg VA 224 AM EDT Tue Aug 22 2017 .SYNOPSIS... High pressure will gradually move off the Mid Atlantic coast tonight, allowing winds to turn southerly and bring a return of deeper moisture to the region along with a chance for showers and thunderstorms mainly along and west of the Blue Ridge. A cold front will pass over the area Wednesday, bringing a better chance for showers and storms to the Appalachians and central Mid Atlantic region. && .NEAR TERM /THROUGH TODAY/... As of 716 PM EDT Monday...Surface METARs and SPC mesoanalysis graphics depicts surface convergence boundary/trough which extends essentially along much of the Blue Ridge. This is helping to spark widely scattered thunderstorms in conjunction with moderately unstable air mass (~2000 J/kg MUCAPE) in the Piedmont. Coverage and strength of storms were more limited along the southern Blue Ridge in Wilkes and Watauga Counties in NC. Cells this afternoon have been slow east-southeast movers and a few have produced locally high rainfall rates, particularly in central Bedford County and earlier in northern Amherst County in VA. May continue to see pop-up isolated to widely scattered storms through sunset, a few which could become locally strong but severe is not presently anticipated attm. An expected decrease in coverage and intensity is expected by 10 pm with stabilization of the PBL. Indication from RAP-based RH cross sections shows clearing and, as PBL decouples, that should encourage a good coverage of patchy radiation fog. Outside of the western river valleys, areas which saw rain this afternoon pose the best chance at seeing patchy fog. No changes made to lows. Previous near-term discussion issued at 200 PM follows... Surface high pressure will continue to move off the Virginia and Carolina coast tonight. Return southwesterly flow will maintain an influx of heat and humidity into the region. Under a weakly capped environment this afternoon, upslope flow has generated isolated showers and thunderstorms across the eastern slopes of the North Carolina High Country and southwest Virginia Blue Ridge. Some instability showers have also popped across the piedmont. Some lack in coverage is due to the eclipse as heating is temporarily halted. Some storms may continue into this evening as they track east in zonal flow, but will fade with loss of heating. Skies should be mostly clear through midnight then high clouds will advance over the area through the early morning hours. These high clouds should not be thick enough to deter patchy fog from forming in mountains and river valleys. Overnight conditions will remain muggy with lows in the 60s. Surface high pressure will lose its influence on our weather by Tuesday afternoon. A lee trough and southerly upslope flow may help generate a few storms across the mountains through the afternoon, but coverage may be limited with increase mixing ahead of an approaching front. Any diurnal showers will fade with loss of heating. There is a small chance that prefrontal showers may make it to the western slopes of SE WV and SW VA by evening, however the bulk of the showers will come with the front Wednesday. With limited coverage of showers, abundant sunshine and southerly flow, afternoon temperatures Tuesday will warm into the mid to upper 80s across the mountains to the lower 90s across the foothills and piedmont. && .SHORT TERM /TONIGHT THROUGH THURSDAY NIGHT/... As of 230 PM EDT Monday... Short wave crosses through the base of a broad, somewhat flat upper trough of Monday night. This will result in a chance of showers Tuesday night. Better upper diffluence and coverage of precipitation will be approaching southeast West Virginia by 8AM/12Z Wednesday morning. 00Z ECMWF was bringing a pre-frontal trough and front through southern West Virginia, Virginia and northern North Carolina on Wednesday, with reasonable agreement from the 12Z NAM and GFS. The earlier in the day the front crosses through, the less instability will be available for showers and thunderstorms until the front gets farther east. Cloud cover from ongoing precipitation upstream Wednesday morning will also limit instability in the mountains. Higher probability of showers and thunderstorms Tuesday night and Wednesday will remain over the mountains due to the west wind that will be in place ahead of the front. By Wednesday evening the focus will shift to the piedmont of Virginia and North Carolina. The front and deeper moisture may be slow to exit southern Virginia and northern North Carolina. Clearing in those locations may not occur until late Thursday. && .LONG TERM /FRIDAY THROUGH MONDAY/... As of 130 PM EDT Monday... Deeper upper trof develops in the eastern United States with the Canadian, ECMWF, and GFS showing another strong short wave/potential upper low re-enforcing the trof over the Great Lakes on Monday. For the Mid Atlantic region the primary long wave trof axis will cross through the area on Friday. Then not much recover in 500 MB heights through Monday. By Friday morning 850MB temperatures are down to around +12. At the surface a large high passes through the Great Lakes on Friday, then wedges down the east side of the Appalachians on Saturday through Monday. Surface dew points and precipitable water values also drop by Friday, down under an inch for precipitable water and into the 50s for dew points. Saturday, Sunday and Monday some trapped or return low level moisture in the northwest North Carolina mountains, spreading north, west of the Appalachians each consecutive day. Sunday also has the potential for scattered to broken mid and high clouds along with the passage of a warm front from the west. Will keep the forecast dry Friday and Friday night with below normal temperatures through Sunday. && .AVIATION /06Z TUESDAY THROUGH SATURDAY/...
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As of 215 AM EDT Tuesday... The latest satellite imagery shows just some high clouds moving over the region as high pressure off the coast keeps a warm and moist airmass in place. The high clouds are not hampering radiational cooling much and valley fog is evident in satellite imagery and surface obs. Expect LIFR conditions to develop at KLWB and also at KBCB where earlier showers added to the moist environment. KLYH and KDAN also had precipitation earlier today so expect at least a tempo to IFR through around daybreak. Fog/stratus will burn off after sunrise and allow for a window of VFR conditions before heating cooks up convection by the afternoon. Expect pattern of convection will be similar to what was experienced on Monday though with a bit better coverage as model soundings are not indicating quite the degree of capping. Orographic forcing looks to be the primary trigger so initial activity will get started from the Blue Ridge westward, with differential heating boundaries and outflow interactions allowing cells to spill eastward into the foothills. A cold front will then approach from the northwest late in the TAF period with some convection out ahead of it. However, best dynamics will be north of the area and arrival time well after peak heating/instability, showers and storms look to be breaking up as they start to enter the region. With the lack of discrete features present today and the degree of uncertainty in convection associated with the approaching front late, believe it will be best to use a broad brush and go with VCTS all sites with amendments today as convection declares itself and later updates as guidance gets more precise with the approaching front. Winds will generally be light through the period. Extended Aviation Discussion... The cold front will continue moving through the region on Wednesday. This will lead to more better potential for sub-VFR conditions Wednesday when more widespread showers and storms fire along the sluggish front. Flying conditions should slowly improve behind the passing cold front on Thursday although sub- VFR cigs may linger across southern/western sections a while longer.
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&& .EQUIPMENT... As of 930 AM EDT Monday... KFCX doppler radar is expected to be back in operations as early as Wednesday evening (Aug 23rd). Technicians have replaced the bull gear and are reassembling the radar today and tomorrow with calibration worked on Wednesday. && .RNK WATCHES/WARNINGS/ADVISORIES... VA...None. NC...None. WV...None. && $$ SYNOPSIS...RCS NEAR TERM...AL/RCS SHORT TERM...AMS LONG TERM...AMS AVIATION...MBS/AL EQUIPMENT...RCS

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