Area Forecast Discussion
Issued by NWS Blacksburg, VA

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000 FXUS61 KRNK 170045 AFDRNK Area Forecast Discussion National Weather Service Blacksburg VA 845 PM EDT Sun Jul 16 2017 .SYNOPSIS...
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High pressure will remain over the region overnight in between a weak stalled front to the south and the next upstream upper level disturbance to the northwest. This upper feature should cross into the area later Monday before slowly edging toward the coast by midweek. High pressure will again bring increasing heat and humidity by late next week.
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&& .NEAR TERM /THROUGH MONDAY/...
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As of 840 PM EDT Sunday... Weak boundary remains stalled across southeast sections this evening with again a range in dewpoints from the drier 50s over the mountains where have mixed out to the lower 70s piedmont. Latest analysis showing lingering decent instability over the far southeast where the combination of weak convergence and outflow has led to isolated convection over Southside Va. However given the degree of capping/dry air aloft seen on evening soundings, and lack of much forcing after loss of heating, appears only an isolated threat over the southeast/southern sections before midnight. Therefore will keep in some isolated mention southeast and may include other sections over northwest NC with residual outflow moving toward the foothills attm. Otherwise should trend toward mainly clear central/north and partly cloudy under debris clouds south overnight. Fog again likely most widespread in the western valleys for a few hours late, and perhaps southeast where moisture is better. Dropped lows a little more out west where again some comfortable 50s possible with mainly 60s elsewhere. Previous discussion as of 341 PM EDT Sunday... Though it remains on the humid side across the southeastern portion of the forecast area, it`s been a seasonably warm and dry Sunday from the foothills of the Blue Ridge into much of southeast WV. Afternoon water vapor imagery shows nicely the dry mid-level air associated with subsidence in place across the central Appalachians and mid-Atlantic which has contributed to the more comfortable dewpoints in the western two-thirds of the forecast area. We`re stuck in relative/weak ridging between two features: (1) feed of Gulf moisture extending from eastern GA into eastern VA associated with a dissipating frontal zone and (2) a digging northern-stream longwave trough in mid-levels over the Great Lakes with embedded shortwave disturbances evident in water vapor imagery north of Lake Huron and another over the U.P. of MI/northern WI. For tonight...Sensible weather conditions overall should be fairly similar to last night, with the only nuance being that we`re starting off drier. With surface ridge in place, we should see good radiational cooling in clear skies and light/calm wind conditions. As temperatures fall to lows in the mid 50s to mid 60s, expect to see patchy radiation fog develop mainly after midnight. Area most favored is in the deeper river valleys but rising dewpoints late should allow for a corridor of patchy fog as well into the foothills. Modest increase in low-level southeast winds should help to tap into the pool of moisture south of the forecast area and return it northward into central and eastern sections late, ahead of approaching 500 mb height falls. For Monday...Areas of patchy fog burns off quickly shortly after sunrise. Mostly sunny skies with background 500 mb height falls should allow for an increase in cumulus as atmosphere becomes conditionally unstable. Both the NAM and the GFS show residual lower- level cap in place through early afternoon, which may stunt convective growth at least initially. Above the cap, modeled 700-500 mb lapse rates are on the order of 6.5-7 C/km on the NAM and GFS, which appear reasonable based on morning RAOBs upstream in the Great Lakes/Ohio Valley region. Capping inversion weakens by mid to late afternoon which should allow for isolated to scattered thunderstorms to develop, especially in the vicinity of the Blue Ridge where zone of mesoscale/upslope lift may develop with light southeast winds. Some question still remains on the degree of instability, but based on the steeper lapse rates aloft, the higher NAM values may not be that unreasonable. Lifted indices of -3 to -6 along with downdraft CAPEs (DCAPE) 1000-1200 J/kg should permit a couple cells to become strong to locally severe. Localized gusty to damaging winds should be the chief hazard given inverted-V low-level profile. Though based on the steeper lapse rates aloft, a conditional hail threat may also ensue in the strongest cores. Coverage should be limited to scattered, however, with stronger synoptic lift being limited and passing to the north. And organized updrafts don`t appear likely either given progged 0-6 km shear values below 25 kts. SPC`s Day-2 outlook for Monday has focused 5% severe probabilities/Marginal Risk to the counties near or just east of the Blue Ridge. Storms should begin to decrease in intensity with sunset. Looking at highs Monday in the 80s, with a couple 90s possible in the upper NC Piedmont, Southside into central VA.
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&& .SHORT TERM /MONDAY NIGHT THROUGH WEDNESDAY NIGHT/... As of 341 PM EDT Sunday... Expect an upper trough over the area Monday night, to shift slowly east to off the coast by Wednesday night. Several shortwaves in the trough will bring a daily threat of showers/storms. For Tuesday, model forecast soundings showing drier airmass under the trough with about 1-2kft layer in the 8h-7h level with higher rh. Soundings are unstable with decent inverted V but lack of upper support for wind or downbursts. SPC just showing general thunder risk for Tuesday. Better support in the low levels per wind fields showing the Blue Ridge and just west, but only 40ish. By Wednesday, the upper heights build somewhat so keeping pops low chance to slight chance, as main energy nudges east of our forecast area. Temperatures this period should be at or a few degrees above normal, with lows ranging from lower to mid 60s mountains, to upper 60s to lower 70s foothills/piedmont. && .LONG TERM /THURSDAY THROUGH SUNDAY/... As of 140 PM EDT Sunday... Not too much change in the forecast this period. Sweltering heat returns with center of 594 DM ridge moving eastward from the southern Plains into the southeast. Temperatures will be about 3 to 6 degrees above normal, so record setting heat not expected. Going to see potential for storms, as our area will be on the southern fringe of the westerlies, and more of a northwest flow Friday- Saturday. At the moment models are hinting out potential convective systems moving out of the midwest into the southern/central Appalachians/mid-Atlantic, but overall too far out to deal with timing of these features so keeping pops in the chance range, mainly during the daytime hours. Still a chance of storms Sunday, but less coverage as heights build a little more and upper flow shifts more westerly. && .AVIATION /01Z MONDAY THROUGH FRIDAY/... As of 736 PM EDT Sunday... SCT to BKN VFR fair wx cumulus will dissipate with the loss of solar heating this evening. Looking at generally mostly clear and calm conditions tonight with development of patchy IFR/LIFR fog mainly after 08z. Best chances for dense fog at Lewisburg and Blacksburg along the Greenbrier and New Rivers, though should also see a potential for MVFR visibility at Roanoke, Lynchburg and Danville as low-level air mass moistens overnight along and east of the Blue Ridge. Light easterly winds in the Piedmont and light northwest along/west of the mountains will becoming light to calm. Any overnight fog should burn off readily with sunshine Monday. Expect to see deepening cumulus field with bases around 050. Scattered MVFR showers and thunderstorms anticipated to develop near the Blue Ridge spine, a couple which may pose a gusty wind threat. Kept the VCTS at Blacksburg, Roanoke and Lewisburg for Monday afternoon after 17z with some timing uncertainties with convection. Potential for thunderstorms also should exist in the southern Blue Ridge for flight routes through the southern mountains. Localized turbulence due to erratic outflow gusts, brief visibility reduction and lightning all potential hazards in thunderstorms. Winds through Monday afternoon generally light southeast east of the Blue Ridge, more of a light west to west- southwest for Bluefield. Medium confidence in ceilings, visibilities and winds during the Taf period. Aviation Extended Discussion... Greater thunderstorm coverage and their associated lightning, outflow wind and IFR visibility risks expected Monday late afternoon into early evening near the Blue Ridge. More limited potential east of the Blue Ridge into the Piedmont. A few may be on the strong side during the mid to late afternoon. Storms should begin to diminish in coverage and strength with sunset with VFR conditions anticipated Monday night. VFR conditions with isolated thunderstorms Tuesday confined to the daylight hours and the mountains. VFR dry weather expected Wednesday before threat of scattered thunderstorms increases again for mid to late in the week. Potential for nighttime IFR to LIFR radiation fog increases each evening during this period as moisture levels increase. && .RNK WATCHES/WARNINGS/ADVISORIES... VA...None. NC...None. WV...None. && $$ SYNOPSIS...JH NEAR TERM...AL/JH SHORT TERM...WP LONG TERM...WP AVIATION...AL/KK

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