Area Forecast Discussion
Issued by NWS Blacksburg, VA

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000 FXUS61 KRNK 160616 AFDRNK Area Forecast Discussion National Weather Service Blacksburg VA 216 AM EDT Sun Jul 16 2017 .SYNOPSIS... Weak high pressure will build east across the area overnight into Sunday. Showers and storms quickly return to the forecast Monday into Tuesday as disturbances move through the area from northwest to southeast. A more summer like pattern returns to the region by Wednesday into Thursday. && .NEAR TERM /THROUGH TODAY/... As of 850 PM EDT Saturday... Surface front has slipped south of the region this evening while the actual dewpoint boundary lingers near the VA/NC border. This seen well via evening soundings with the RNK raob very dry/capped aloft with PWATS half than those seen from GSO where moisture through the column remains deeper. However best support and lingering instability has continued to dip to the south of the region this evening, with a dry pocket along/west of the Blue Ridge per better residual mixing of dry air from aloft this afternoon. Latest short term guidance including the HRRR continues to hint at trying to develop some added showers far south under weak evening convergence and perhaps old outflow but iffy. Therefore may leave in an isolated shower mention over the next few hours before fading to mainly clear skies with some patchy fog around late. Should be a bit more comfortable tonight across central/western sections where dewpoints have dropped enough to allow lows to slide back into the low/mid 60s, with even a few 50s possible deep valleys across the northwest. Will still be muggy over southern sections, with lows closer to 70, given lack of much push of weak dry advection overnight. Previous discussion as of 230 PM EDT Saturday... A cold front was positioned just to the south of the forecast area. Isolated to scattered showers along this front will just barely clip the far southern sections of the area heading into the evening hours. Overnight, skies will trend to mostly clear across the north, and partly cloud south. Light winds, limited cloud cover, and a surface moisture will allow for the development of radiation fog across mainly the river and mountain valleys. This fog should burn off not long after sunrise Sunday. Low temperatures will average five to ten degrees lower than those realized this morning. Expect a mix of the mid 50s to near 60 degrees across the mountains with mid to upper 60s across the Piedmont. On Sunday, expect mostly sunny skies, calm or light and variable winds for the vast majority of the region. The exception may be the Northern Mountains of North Carolina and neighboring sections of the Grayson Highlands of Virginia. This area may experience enough low level convergence and differential heating where winds become south and advect back into the area some slightly higher dew point readings by the late afternoon. The result will be some isolated showers and storms that originate near ridgetop and then move slowly. High temperatures on Sunday will range from around 80 to the lower 80s across the mountains to the mid to upper 80s across the Piedmont. && .SHORT TERM /TONIGHT THROUGH TUESDAY NIGHT/... As of 255 PM EDT Saturday... Dry conditions expected Sunday evening under weak/relative high pressure. Increased low-level southerly flow should allow for a modest uptick in dewpoints along with some mid-level clouds east of the foothills of the Blue Ridge, but skies generally clear west. Lows range from the upper 50s to the mid/upper 60s. Sensible weather conditions Monday and Tuesday should be governed by falling 500 mb geopotential heights associated with northern stream closed low located north of Toronto by 12z Monday. Due to southerly low-level flow transporting higher dewpoints northward, air mass across the forecast area should be sufficiently enough unstable Monday to promote showers and thunderstorms; though it does begin the day with a convective cap around 800 mb. Secondly, I`ve noted a slower and slightly more westward shift in the best dynamic forcing Monday as well as a poorer-defined surface front in today`s guidance. These trends may reflect a condition where the best forcing aloft doesn`t necessarily line up with the time of peak heating and greatest convective instability. I`ve reduced PoPs areawide by around 10% to account for this, still in the high-Chance to borderline Likely range. What I believe unfolds is isolated to scattered showers/storms develop in the daytime heating along the mountains, becoming more numerous in coverage with the best height falls late afternoon into early evening but also weaker toward sunset. A few strong to locally severe thunderstorms producing gusty to at times damaging winds are possible due to lifted index values of -3 to -6 and relatively high downdraft CAPE values around 900 J/kg. With deep-layer wind shear values at or under 30 kts, storms may be loosely organized at best so any instances of severe weather should be more localized/transient. The overnight Day-3 Convective Outlook valid for Monday shows a Marginal Risk/5% severe probability for our central VA Piedmont counties, with general thunder indicated elsewhere. A general decrease in spatial coverage (slight chance level PoPs) are shown after midnight into the Tuesday early morning hours. Looking at highs topping out from the mid 70s at elevation, the lower/mid 80s west of the Blue Ridge to the mid/upper 80s elsewhere. By Tuesday, digging 500 mb vort from Monday over the Ohio Valley digs into the Carolinas - a feature which closes off in the 12z GFS over far western NC. It appears for Tuesday that the focus for showers and thunderstorms would be mainly east of the WV/VA border. Questions on the degree of instability in this area, with up to 2500 J/kg of surface-based CAPE, while the GFS is closer to 1000-1200 J/kg. NAM solution may be too warm and too moist, but even the GFS instability parameters should be sufficient enough to generate scattered thunderstorms, a few could be strong if the NAM instability values are correct. Storms should pose a localized heavy rain threat at least, given their slow moving nature (surface to 200 mb winds are under 20 kts) and high PWATs around 1.8". Storms should weaken and diminish with sunset Tuesday evening. Highs in the mid 80s to low 90s, warmest in the VA/NC Piedmont and Southside, with temps at higher elevations in the upper 70s with lows running in the 60s. && .LONG TERM /WEDNESDAY THROUGH SATURDAY/... As of 255 PM EDT Saturday... A bit less clarity on synoptic-scale details for the mid to late week period, due mainly to the newest wrinkle in the global guidance: the closed off mid-level low the GFS and the prior ECMWF run depict to be near North Carolina to start the period. Earlier solutions had showed the forecast area on the northeastern periphery of an amplified heat ridge over the center of the country, a pattern which may have been more favorable for convective complexes to develop in the Midwest and roll into the Ohio Valley/central Appalachians. Today`s runs show the mid-level low/trough on Wednesday shifting westward, weakening the heat ridge as it builds over the Ohio Valley and keeping the zone of faster mid-level winds well north of our area. Really two potential scenarios during this period: (1) warm to hot, humid and with potential opportunities for thunderstorms or thunderstorm clusters dependent on mesoscale details, or (2) hot, humid and largely dry. Due to the changes in the guidance today, it`s hard to predict which one may play out at this point. For that reason, will stay closer to blended solutions to minimize errors. However potential exists for change here, mainly on PoPs/Wx. Nonetheless, it appears that temperatures should trend above normal at least for late in the week. Indications are at some point into the weekend we do get into somewhat of a reprieve from the warm/humid conditions by the upcoming weekend with more northerly trajectories. && .AVIATION /06Z SUNDAY THROUGH THURSDAY/...
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As of 150 AM EDT Sunday... Generally VFR conditions expected through the TAF valid period. Boundary separating deeper moisture from drier air to the north was near a UKF to DAN line at this hour. Models suggest it will continue to drift slowly south through the period. Thus, this will keep any convection south of the TAF sites through the TAF valid period. Main concern for any non-VFR conditions through the TAF valid period would be from potential fog development at the usual spots in the 08Z to 13Z time frame this morning. Confidence is low at LWB and BCB because of drier air than would normally be in place at this time of year. However, air is cooling efficiently in this areas with clear skies and dry air, so it remains a possibility and thus have included as did previous TAF set. After 13Z, expect VFR conditions for the remainder of the TAF valid period. Model soundings and cross sections show the a moist layer near the inversion level around 040-050, so feel good potential exists for SCT to possibly BKN clouds in this layer during the daytime. In addition, models also hinting at moisture creeping southwest from central VA. Confidence in this is low, but satellite does suggest this is possible. Have melded this into clouds as indicated in above paragraph. Winds will be light and variable at most if not all sites through the TAF valid period. Medium to high confidence in ceilings through the TAF valid period. Medium confidence in visibilities through 14Z, then high confidence in VFR visibilities through the remainder of the TAF valid period. High confidence in light and variable winds through the TAF valid period. Aviation Extended Discussion... Models are trending drier during the first half of the week with the upper trough axis generally over or just east of the CWA. This pattern would favor most of the convection occurring east of the CWA rather than over the area. Models still suggesting afternoon convection across mainly the mountains. VFR conditions will be favored much of the time outside of early morning fog at the usual spots. Wednesday into Thursday, the upper trough lifts northeast of the area placing us back into more of a summertime Bermuda High scenario. Anticipate a return to primarily diurnally based isolated showers and storms. Again, any sub-VFR conditions will be in association with the heavier showers and storms, and late night/early morning patchy river/valley fog. A strong subtropical ridge across the Midwest appears destined to drift closer to the CWA during the later half of the week. This puts the region in an active northwest flow with the potential for any convective complexes developing in the upper Midwest/Ohio Valley to drift our way. Otherwise, conditions should be mostly VFR outside of the late night/early morning fog.
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&& .RNK WATCHES/WARNINGS/ADVISORIES... VA...None. NC...None. WV...None. && $$ SYNOPSIS...DS NEAR TERM...JH/RAB SHORT TERM...AL LONG TERM...AL AVIATION...DS/KK

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