Area Forecast Discussion
Issued by NWS Blacksburg, VA

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000 FXUS61 KRNK 181828 AFDRNK Area Forecast Discussion National Weather Service Blacksburg VA 228 PM EDT Sun Jun 18 2017 .SYNOPSIS... High pressure offshore will maintain a warm and humid airmass across the region, continuing our chances for scattered showers and thunderstorms through tonight. A weak cold front will approach the area from the northwest by Monday with a better chance for strong thunderstorms and heavy rainfall. Behind the front, a bit drier weather is expected through the middle of next week, along with a warming trend to above normal temperatures. && .NEAR TERM /THROUGH TONIGHT/... As of 1000 AM EDT Sunday... Satellite imagery shows eroding clouds over the region and earlier bloom of light showers across the NRV have dissipated. Expect increasing sunshine to create considerable instability as modified 12Z RNK sounding indicates potential for CAPE values to meet or exceed 2000J/Kg this afternoon. However, the environment remains weakly forced as the approaching cold front remains well off to our west and indications are that upstream outflow from earlier convection will wash out before reaching our area. Thus, will be relying on orographic effects and subtle mesoscale boundaries to initiate convection. Believe the most likely scenario will be for most of the thunderstorm activity to occur west of the Blue Ridge later in the afternoon. The airmass remains juicy with precipitable water values generally over 1.5 inches so locally heavy rain is expected with storms. Increasing swly flow ahead of the front late in the day will also give storms a boost and help some clusters organize with the potential for gusty winds and hail in the stronger updrafts. Since the better synoptic support will still be off to the west with the cold front, storms this afternoon are expected to exhibit a strong diurnal trend and taper off this evening. Previous discussion... Residual weak upper trough and associated leftover sheared energy aloft will continue to persist over the region today in advance of a much stronger digging 500 mb upper system that will approach later tonight. However despite lingering tropical moisture, forcing remains rather limited this afternoon under better mixing and some subsequent drop in low level moisture. Guidance also remains quite widely scattered with afternoon convection with focus mainly along the Blue Ridge given weak convergence and better instability than yesterday. This should lead to scattered coverage mainly along/west of I-81 early this afternoon per latest HRRR, with perhaps isolated coverage east of the mountains as old outflow translates east later in the day. Appears better concentration of storms may actually take place along/west of the I-77 corridor late in the day or early evening as an outflow band from upstream Midwest convection propagates east ahead of a another shortwave aloft and enhances going coverage. This a bit more uncertain but in line with the latest HiRes-ARW and to some degree the Cam ensembles. Thus after tweaking down pops early on, left in a few swaths of low likely pops southern Blue Ridge west into this evening with low chances for less coverage elsewhere. Will be warmer and turning a bit more breezy this afternoon as southwest flow starts to increase ahead of the upstream cold front. With 850 mb temps back to around +20C under plenty of sun after early fog fades, bumped highs close to the warmer Mav/Euro Mos with 80s to low 90s outside of the higher elevations. Expect evening convection to fade before midnight ahead of another band of showers/storms in advance of the cold front that should arrive in the far northwest by daybreak Monday. Timing and coverage given early morning arrival remains iffy but enough to keep some higher pops mainly west of I-81 late with little east of the Blue Ridge at this point. Otherwise partly to mostly cloudy and quite warm with mixing/clouds keeping lows in the muggy 65-72 range. && .SHORT TERM /MONDAY THROUGH WEDNESDAY NIGHT/... As of 400 AM EDT Sunday... Early Monday, widespread convection is likely to be ongoing across the western parts of the CWA, having spread eastward into the area overnight as a short wave embedded within a broad trough moving slowly through the Great Lakes. As always with this type of situation, it is somewhat unclear as to how much/when/where strong to severe thunderstorms will redevelop. Current thinking is that morning activity will be in a dissipating state, then redevelop mainly along and east of the Blue Ridge during the afternoon. Models have slowed somewhat in the forward progression of this synoptic scale system, so activity may tend to develop a little further west toward the WV/VA border in the afternoon, depending on amount of air mass recovery from morning convection. Do feel that situation is conditional based largely on potential heating, which should be maximized east of the Blue Ridge. Convection will be slow to spread east and south and will likely linger through the evening into the overnight hours across the Piedmont and north central NC counties. While this convection will not be particularly strong, it still poses a heavy rain threat. Will discuss this threat in further detail in the hydro section below. SPC has outlooked much of the area for a slight risk of severe, except the far west, where it is just marginal and morning activity will likely suppress the threat somewhat there. Instability is conditional as noted on amount of heating, but feel that this will occur sufficiently across the Piedmont to support some fairly robust clusters or small line segments during the afternoon east of the Blue Ridge that mainly pose a damaging wind threat. Tuesday through Wednesday, a baroclinic boundary/frontal zone will linger near or across the southeast part of the CWA. Am not confident based on lack of southward push of the upper trough that the front will move far enough south/east of the CWA to provide any bonafide drying to areas east-south of the Blue Ridge, especially Southside VA and the NC Piedmont. Will retain slight chance to chance pops through the period, more so Tuesday than Wednesday, but until there is better consensus among the models as to how far southeast the baroclinic zone will go before stalling, am leaning toward the pessimistic side across the southeast part of the CWA. The northwest part of the CWA should see less convection than in recent days, along with slightly lower dewpoints. Temperatures will remain warm through the period, but some relief from the high humidity levels of recent days can be expected across the northwest. 850mb temps hovering around +20C Monday will drop back toward +15C by Tue-Wed. Look for highs mainly in the 80s, except 70s mountains and lows in the 60s. && .LONG TERM /THURSDAY THROUGH SATURDAY/... As of 415 AM EDT Sunday... The extended portion of the forecast is high problematic and uncertain focusing around a tropical system that is advertised by all models to track through the Gulf early next week. The problem is that there is no consensus yet as to where this system will track inland, how long it will linger in a certain area, and where the remnant moisture will remain. Solutions vary widely from the GFS which stalls the system with massive amounts of rainfall across southern GA/AL early in the week, then take it northeast across the Carolinas, to most other model solutions that take it much further west toward TX/LA, but then the remnant moisture eventually gets caught up in the mean flow bringing the deep tropical moisture back into VA/NC during the mid to later part of next week. Entirely too much uncertainty to key on any one solution at all. The GFS exhibits the greatest deviation from the mean and has in general been thrown out by WPC as well as by me at this point. Thus, the tropical system will likely not influence our weather until the later part of next week at the earliest. Instead, we can likely expect an increase in showers and thunderstorms after a slightly drier Tue/Wed period as upper troughing persists across the Great Lakes and keeps our region subject to northwest flow disturbances. Above normal temperatures will be the rule during much of this period, especially into the weekend as 850mb temps hover into the +20C to +22C levels across VA/NC. && .AVIATION /18Z SUNDAY THROUGH FRIDAY/...
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As of 225 PM EDT Sunday... Expect another day of generally VFR flight conditions with VCTS this TAF period. Will use VCTS for prevailing conditions and issue short term amendments when radar trends show near term convection approaching an airfield. Winds will be increasing a bit as swly flow amplifies ahead of approaching cold front, but expect gusts to remain in the ball park of 20kts. Higher gusts should be expected near thunderstorms. Expect convection to largely dissipate this evening with loss of heating, but expect more in the way of lingering cloud cover. This may limit fog/stratus development compared to previous nights, but still expect at least a tempo to MVFR/IFR toward daybreak, especially if clouds disperse more than anticipated and if an airfield receives a good dose of rain this evening. The cold front will slowly move in from the west tomorrow with more thunderstorms expected. Storms tomorrow have the potential to become severe with gusty winds, hail, and heavy downpours. Timing is a bit varied between guidance solutions so will not get too detailed so close to the end of the period. Extended Aviation Discussion... Appears will see more in the way of widespread VFR conditions develop by midweek given arrival of post frontal dry air with only brief periods of fog resulting in brief sub-VFR during the late night/early morning hours. However given uncertainty in just how far south the deeper moisture may or may not make it, the threat for isolated showers could still linger Tuesday through Thursday at this point.
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&& .HYDROLOGY... As of 420 AM EDT Sunday... While there have been localized issues of flooding and flash flooding over the past 3-4 days, mainly across Southside VA, the Roanoke area, NC Piedmont, as well as a few other spots, there are other areas that have had very little rainfall during the past two weeks. Do not feel that rainfall will be widespread enough or heavy enough over a large enough area to warrant a flood or flash flood watch at this point in time. If any hydro watch were issued, a flash flood would probably be more appropriate given the localized nature of summertime convective thunderstorms and past localized rainfall. A flood watch would be more appropriate for widespread rainfall and runoff into area rivers when evapotransporation is low, which is certainly not the case as we near the summer solstice. In addition, projected average rainfall amounts around an inch or so do not warrant a flash flood watch. Problems can be handled via localized flood advisories, warnings, or flash flood warnings in the worst case scenarios, just as have been done during the past week. Later shifts will continue to monitor the situation. && .RNK WATCHES/WARNINGS/ADVISORIES... VA...None. NC...None. WV...None. && $$ SYNOPSIS...MBS/JH NEAR TERM...MBS/JH SHORT TERM...RAB LONG TERM...DS/RAB AVIATION...MBS/JH HYDROLOGY...RAB

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