Area Forecast Discussion
Issued by NWS Blacksburg, VA

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000 FXUS61 KRNK 250000 AFDRNK Area Forecast Discussion National Weather Service Blacksburg VA 800 PM EDT Mon Jul 24 2017 .SYNOPSIS... A weak cold front will slide southeast through the area tonight before stalling over the Carolinas Tuesday into Wednesday. High pressure builds north of the region behind the front bringing a return to more seasonal temperatures later tonight through mid or late week. Another cold front approaches from the northwest on Friday and works slowly through the area by the weekend. && .NEAR TERM /THROUGH TUESDAY/... As of 745 PM EDT Monday... Line of showers and thunderstorms extends very near I-81 this hour from JFZ northeast to DCA. This is associated with last short wave of northeast U.S. trough. This should be the last round of convection for a couple of days of any significance. HRRR indicates that this convection will dissipate quickly with the loss of daytime heating, roughly in the 02Z-03Z time frame. Otherwise, looking for partly cloudy skies overnight, patchy dense fog development near LWB and BCB possible later tonight. Made some minor tweaks to temperatures and dewpoints, mainly cooling areas a tad quicker west of the Blue Ridge, and not quite so quickly to the east. As of 210 PM EDT Monday... Convection continues at a minimum this afternoon as the region remains in a somewhat drier westerly flow between the exiting surface trough to the east and the cold front along the Ohio River. This front should finally slide southeast and across the region later this evening as the upper trough to the north swings by to the northeast. Latest guidance still showing some degree of at least shallow convection just ahead of the boundary with the Nam quite agressive, while other solutions only show widely scattered coverage within the instability axis along our northwest slopes. Since some potential for a band of broken convection to move into the northwest sections, have included low to isolated pops later this afternoon into early evening for locations mainly west of the New River Valley. Otherwise should see clearing take shape overnight as the front sags to the south allowing for a bit more comfortable air to advect into the mountains where expect lows well into the 60s. Moisture may linger across the south/east so kept lows mainly upper 60s to lower 70s there. Also despite good drying today, appears some fog/stratus development possible espcly valleys given heavy rain from last night and better radiational cooling west tonight. High pressure builds in from the north on Tuesday with weak northerly flow resulting in enough dry advection to limit most convection to across far southern sections at best. This seen via model lowering of PWATS from north to south and axis of deeper moisture well to the south closer to the residual surface front. Therefore reduced pops to slight chance mainly across northwest NC where perhaps enough low level moisture/instability may linger to pop a shower or two but iffy. Otherwise enough subsidence for overall mostly sunny skies and not quite as hot/humid with highs low/mid 80s mountains to 87-92 east of the Blue Ridge. && .SHORT TERM /TUESDAY NIGHT THROUGH THURSDAY/... As of 1245 PM EDT Monday... Little change in the weather pattern and no synoptic scale forcing Wednesday and Thursday. Will keep minimal probability of precipitation, mainly in southern Virginia and northern North Carolina each afternoon and evening. Persistence combined with bias corrected MET/MAV guidance will be reasonable for daily highs and lows. && .LONG TERM /THURSDAY NIGHT THROUGH MONDAY/... As of 1245 PM EDT Monday... Upper pattern amplifies starting on Friday with the upper ridge over the western United States slightly retrograding and a trof digging in the east. An upper low may eventually close off somewhere over the Tennessee Valley or Mid Atlantic region by Sunday or Monday. This closed low solution was not supported by a majority of the extended guidance. Probability of showers and thunderstorms will increase on Friday and Saturday with some vorticity advection and lowering heights. Air mass ahead of the trough stays in the +16 to +20 range for temperatures at 850MB and will be a little cooler on Sunday and Monday. && .AVIATION /00Z TUESDAY THROUGH SATURDAY/... As of 745 PM EDT Monday... Line of showers and isolated thunderstorms stretches from DCA to JFZ, very close to the I-81 corridor at this hour. Activity associated with final short wave tracking through the base of the northeastern U.S. upper trough. Latest HRRR has this activity persisting for another couple of hours, especially in the far southwest VA/northwest NC area, then dissipating quickly toward 03Z. Have indicated this trend in the grids and TAFs, with the only TAF to include VCTS at this time BLF due to a nearby TSRA in Tazewell county. Have included VCSH at BCB and LYH because of nearby activity that could affect these sites before they dissipate in the next couple of hours. Ceilings associated with these showers are mostly in the 040-050 range. Concerns overnight mainly focus around fog development and possible low cloud development along the Alleghany front. Will discuss the later first as that possibility seems limited due to decreasing upslope W-NW flow becoming near calm or light and variable, possibly even light northeast. Will include for now as it remains a possibility per model soundings. Fog development at LWB and BCB especially seems a better possibility due to recent rainfall, hence moist ground in the area, clearing skies, and near calm winds. Have continued earlier thinking on this and continued potential for IFR-LIFR FG at BCB and LWB in the 08Z- 12Z time frame. Will hold off on fog elsewhere, although MVFR BR conditions at LYH and DAN do not seem out of the question, just not enough confidence to include at this time. Winds will be light WSW-WNW 4-6kts this evening, becoming calm overnight at most sites. Winds will be light and variable Tuesday, but good potential for light northeast winds east of the Blue Ridge. Medium confidence in ceilings through 13Z, then high confidence in VFR ceilings. Medium to high confidence in visibilities through 13Z, then high confidence in VFR visibilities. Medium to high confidence in wind speed and direction through the TAF valid period. Aviation Extended Discussion... A frontal boundary will remain stalled to our south through Wed, with high pressure wedging southward on Wed from the northeast U.S. Overall VFR, but with flow turning more east, this could bring some MVFR/IFR ceilings, which should quickly dissipate by Thursday. Any convection also likely to remain isolated and south of the TAF sites again later Wednesday. Next best threat for showers and storms arrives late Thursday as a new front approaches from the north with more widespread activity in the area Friday as the front drifts slowly southeast through the region. && .RNK WATCHES/WARNINGS/ADVISORIES... VA...None. NC...None. WV...None. && $$ SYNOPSIS...JH NEAR TERM...JH/RAB SHORT TERM...AMS LONG TERM...AMS AVIATION...JH/RAB/WP

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