Area Forecast Discussion
Issued by NWS Blacksburg, VA

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000 FXUS61 KRNK 250717 AFDRNK Area Forecast Discussion National Weather Service Blacksburg VA 317 AM EDT Thu May 25 2017 .SYNOPSIS... A weak cold front will pass east of the region this morning. An area of upper level low pressure will then swing across the area this afternoon before exiting tonight. Weak high pressure spills in on Friday with a brief return to drier weather before more showers and storms arrive this weekend. && .NEAR TERM /THROUGH TONIGHT/... As of 245 AM EDT Thursday... Ongoing showers producing heavy rainfall slow to exit as waiting for the upstream dry slot seen on water vapor to kick through which guidance shows taking shape by daybreak. Thus will continue high pops north-east early on before seeing a brief lull as we slide in between deep moisture with the front and the upper low to the west. Core of this cold pool aloft then looks to pivot east through the far western zones around midday with the associated vort lobe swinging across the east during the afternoon. This should bring another round of showers to the western mountains by early afternoon before clusters of deeper convection develop on the leading edge and points east during the afternoon. Secondary wave sliding around the base should aid lift with perhaps some organization into a broken line over the piedmont by late in the day as hinted at by a couple short term solutions. Main aspect will be with cooling aloft and subsequent steep lapses capable of producing hail and even some downburst potential given lingering shear aloft. Also given a bit slower scenario, have pushed thunder chances back to at least the Blue Ridge where will mention small hail per latest SWODY1 with all seeing a period of likely pops this afternoon. Other issue with possible heavy rain and added flooding given saturated conditions and high water levels. However at this point appears far western coverage may be more showery with lower rates while stronger storms to the east will be more capable of producing at least isolated flash flooding. Since still dealing with the current event and given uncertainty with coverage this afternoon, opting not to include another flood watch headline at this point and give the dayshift a chance to issue a short term FFA if needed. Otherwise thinking will see some sun espcly Blue Ridge east as the wedge breaks down and downsloping kicks in. This should help push highs above 70 out east while 60s prevail elsewhere over the west. Upper low finally exits to the northeast this evening with subsidence spilling in on a rather strong 850 mb jet at 30-40 kts that will make for a gusty evening/overnight. However latest soundings suggest speeds to stay below advisory levels over the ridges at this point. Otherwise appears any residual pops will be confined mainly to upslope areas across the northwest where some wrap around showers may persist. Thus keeping in chance pops northwest third with a gradual decrease in coverage late. Elsewhere expecting downslope drying to limit much in the way of added showers with clearing skies east. Lows on the cool side with most seeing 50s overnight if not a few 40s in the valleys. && .SHORT TERM /FRIDAY THROUGH SUNDAY/... As of 305 AM EDT Thursday... On Friday, we will gradually lose the upslope scattered showers, and associated cloud cover, as low level winds start to back more westerly through the day with the approach of an upper level shortwave ridge. The arrival of drier air will also yield a decreasing trend in coverage. The best chances will still be over parts of southeast West Virginia, mainly during the morning hours. The vast majority of the forecast area will experience limited cloud cover and weak winds. Our period of drier weather will be short lived. Friday night into Saturday we will be watching a cold front approach and then enter the region. Isolated showers may develop by sunrise Saturday in the northwest portions of the area, but the better chances will be during the day Saturday with the arrival of the front combined with daytime heating. Guidance varies on what extent the front makes it through the region before stalling. Model are leaning towards a thunderstorm complex potentially riding along this stalled front late Saturday night and entering the area by Sunday morning. This supported by the 00z ECMWF which has multiple complexes riding the boundary from late Saturday through Sunday. Temperatures during this portion of the forecast will start near normal, but trend to a little over five degrees above normal by Saturday. && .LONG TERM /SUNDAY NIGHT THROUGH WEDNESDAY/... As of 205 PM EDT Wednesday... During this part of the forecast, a broad, slow moving upper low will transition from the Great Lakes region to over southeast Canada. This will keep our region in a pattern dominated by a persistent trough of low pressure across the eastern portion of the U.S. Within this trough pattern a series of cold fronts are expected to enter, and then move through the forecast region. Guidance offers the first of these two to have a Sunday night into Monday time frame, with a second during late Tuesday night into Wednesday. Both of these time frames will be periods with a greater potential for showers and storms, with the first being more robust than the second. However, while the area is within the overall trough pattern, daytime heating will be enough to produce at least isolated showers during other parts of this time period. Temperatures during this portion of the forecast will start the period about ten degrees above normal. By Wednesday, readings within a degree or two of normal are forecast. && .AVIATION /07Z THURSDAY THROUGH MONDAY/... As of 1245 AM EDT Thursday... Poor flying conditions to prevail overnight as a complex low pressure system and associated cold front swing through from the west. Expecting showers to slowly wane through the early morning hours as deeper moisture lifts northeast. However still expecting sub-VFR cigs overnight with MVFR to IFR vsbys in fog. High confidence in timing of the models bringing the cold front through the region after midnight. Downslope winds on the east side of the Blue Ridge will erode clouds and fog once mixing begins Thursday morning. Low level moisture and VFR stratocumulus will remain over the mountains. Scattered showers and isolated thunderstorms will develop in the afternoon. Some of the stronger thunderstorms may have small hail and MVFR visibilities along with heavy rain. Extended Aviation Discussion... Friday will be drier with a better chance of VFR ceilings and visibilities. Another front reaches the area for Saturday with more showers and thunderstorms including sub-VFR conditions into Monday. && .HYDROLOGY... As of 255 AM EDT Thursday... Showers continue in bands across the region early this morning leading to high creek and stream levels with minor flooding in spots. As rainfall rates slowly decrease expecting some improvement although may take until midday for levels to start receding while continuing to rise on mainstem rivers. Significant rises will also occur on mainstem rivers as well with both the New and Greenbrier likely reaching bankfull overnight. Minor Flooding is expected to continue on the Dan River at South Boston and is forecast on the Dan River at Paces and on the Roanoke River at Randolph. Another round of showers and storms will occur again this afternoon which given low flash flood guidance values could lead to added flooding issues. At this point will hold off on any added watch headlines given uncertainty in locations of heavier rainfall including rates later today. && .RNK WATCHES/WARNINGS/ADVISORIES... VA...None. NC...None. WV...None. && $$ SYNOPSIS...JH NEAR TERM...JH SHORT TERM...DS/JH LONG TERM...DS AVIATION...AMS/JH HYDROLOGY...JH/AMS

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