Area Forecast Discussion
Issued by NWS Blacksburg, VA

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258 FXUS61 KRNK 220746 AFDRNK Area Forecast Discussion National Weather Service Blacksburg VA 346 AM EDT Tue Aug 22 2017 .SYNOPSIS... A Bermuda high off the coast will keep a warm and moist airmass in place across the Appalachians and central mid Atlantic region today. This will support some showers and thunderstorms mainly along and west of the Blue Ridge this afternoon. A cold front will approach the region tonight before slowly moving through the area on Wednesday, bringing a better chance for showers and storms to the region. && .NEAR TERM /THROUGH TONIGHT/... As of 230 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 expect a good amount of patchy dense valley fog from the Blue Ridge west through daybreak. Some patchy fog can be expected to the east across the piedmont as well, especially for locations that received rain on Monday. Fog/stratus will burn off after sunrise, but diurnal heating will cook 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 by early tonight 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. Expect any severe threat from activity ahead of the front will be marginal at best as bulk shear parameters and hodographs are not impressive, and arrival time is late. The end result will be solid chance to likely POPs along the southern Blue Ridge with chance POPs extending back westward through the New River Valley and into the Mountain Empire region, followed by chance to likely POPs increasing further west of the Ridge overnight. Juicy airmass will continue to support storms with heavy downpours which may create localized hydro issues. Highs today will generally be in the low/mid 90s east of the Ridge to middle/upper 80s west. Lows tonight look quite muggy in the east with low/mid 70s, and mid/upper 60s west. && .SHORT TERM /WEDNESDAY THROUGH FRIDAY NIGHT/...
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As of 315 AM EDT Tuesday... Expect strong frontal system to be located from near PIT to TYS around 12Z Wed, with a rapidly diminishing line of showers and thunderstorms near the VA/WV border and into SW VA at daybreak Wed. NAMNest model has been consistent for the past couple of days on the rapid diminishment of showers/thunderstorms as they progress east tonight into Wed morning. However, models have also been consistent on slower motion of frontal system through the region Wed as the upper trough moves through the northeast leaving a flat west-northwest flow aloft in its wake into our region. As a result, expect redevelopment of thunderstorms fairly quickly through the late morning and afternoon Wed generally along and east of the Blue Ridge, although some showers may redevelop even back toward the WV/VA border during the late morning hours before moving east. Models in good agreement on scenario, but have generally followed NAMNest for timing and movement of showers/thunderstorms and associated pops Wed. With regard to the severe threat, SPC continues to carry a marginal threat generally across Southside and the NC Piedmont for Wed afternoon. This certainly appears reasonable. Although shear is weak and the best dynamics remain well north of the area, strong heating, and convergence along morning outflow boundaries, cooling temperatures aloft, and the front should provide the necessary parameters for at least scattered strong to severe thunderstorms in the general area of Wilkesboro, to Mt. Airy, to Danville. This activity will diminish and shift east or southeast during the evening Wed. Thursday, while most of the area will be dry, a secondary upper trough will sweep through the Great Lakes. Several models depict shower development, including the NAMNest through the Alleghanys Thursday afternoon. This seems reasonable given the strength of the upper trough and a pool of cold air aloft. Have introduced a 20% pop for northwest Greenbrier, although the best chance for showers will likely remain just northwest of our CWA toward Snowshoe and Elkins. Friday should be mostly sunny and dry across the entire region behind this secondary upper trough and bring the coolest air of the week and a taste of fall-like temperatures and conditions. Temperatures will still be quite warm across the Piedmont Wednesday afternoon, while a good 10 degrees cooler for most areas west of the Blue Ridge Wednesday. However, given increasing clouds and shower/thunderstorm chances by early afternoon at the latest, feel that most areas east of the Blue Ridge will stop short of the 90F mark. While Thursday will be cooler in all areas with lows dipping well into the 50s west of the Blue Ridge, the coolest and driest air will not arrive until the secondary upper trough passes east of the area late Thursday. By Friday, look for lows in the 50s west to lower 60s east with highs in the 70s west to lower 80s east. Very pleasant for late August. Some 40s in locations like Burkes Garden not out of the question at all, with higher elevations in the western mountains seeing highs in the 60s Friday with a sprawling Canadian high moving southeast from the Midwest.
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&& .LONG TERM /SATURDAY THROUGH MONDAY/...
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As of 330 AM EDT Tuesday... The extended periods present many challenges and have been changing considerably from one model run to the next and from one model to the next. The concerns largely focus around the potential for multiple tropical systems to be affecting perhaps both the western Gulf and southeast Atlantic coastal areas by the weekend or early next week. Most models are coming on board with this, but intensity, track, and timing of such systems very widely from one model to the next and are complicated by the development of a very chaotic upper-level weak pressure pattern across the eastern two-thirds of the U.S.. Of these potential tropical systems, the one slated to move into eastern TX appears at some point to be the one most likely to impact our weather down the line as its remnants could eventually track into the TN Valley or southern/central Appalachians, mainly early next week. Until that point, at least through the weekend, generally looking at a large Canadian high pressure area drifting toward New England setting up a cool northeast flow into the region. Return flow could result in clouds and showers developing along the southern Blue Ridge by Sunday. At any rate, there is a great deal of uncertainty moving from Sunday into Monday based on potential tropical systems on both sides of our region. At this point, most of the impact, if any, from these systems would be beyond this period. Thus, have not made any significant changes to the pops or temperatures during this time frame. Given the pattern of northeast flow and 850mb temperatures dropping toward +8C in the northern parts of the CWA, temperatures will be unseasonably cool for late August with widespread lows in the 50s and highs mainly in the 70s, except around 80 Piedmont. Indeed very pleasant for late August.
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&& .AVIATION /07Z TUESDAY THROUGH SATURDAY/... 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. && .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...MBS NEAR TERM...MBS SHORT TERM...RAB LONG TERM...AMS/RAB AVIATION...AL/MBS EQUIPMENT...RCS

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