Area Forecast Discussion
Issued by NWS Blacksburg, VA

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000 FXUS61 KRNK 181904 AFDRNK Area Forecast Discussion National Weather Service Blacksburg VA 304 PM EDT Thu May 18 2017 .SYNOPSIS...
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High pressure east of the Carolinas will continue to produce a warm and humid southwest flow of air through tonight. The increasing moisture will result in a threat for widely scattered showers and storms into tonight, especially across the higher terrain. A weak cold front will approach from the northwest Friday, providing a better focus for showers and thunderstorms.
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&& .NEAR TERM /THROUGH FRIDAY/...
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As of 230 PM EDT Thursday... Convection has been slow to develop this afternoon likely due to mixing of dry air aloft and better focus remaining just west of the ridges along a residual trough axis. MLCAPE values also reflect a bit less instability with the best convergence along the southern Blue Ridge where expect more coverage to develop over the next few hours. However guidance remains inconsistent with the latest RAP showing some broken clusters western third, while the HRRR more of a shotgun pattern mountains into early evening. Therefore other than to cut back on pops initially will keep in chances mainly west until just after sunset for now. Initial shearing shortwave will pass well north tonight allowing for a continued weakening of the upper ridge overnight into Friday. May see a few showers linger due to possible outflow and leftover instability into late this evening, and over the far west for late tonight as perhaps another band of convection arrives ahead of an upstream cold front. This shown by a few models so after decreasing pops this evening, will bump back up espcly northwest by daybreak. Otherwise will call it mainly partly cloudy overnight under residual debris clouds including spots of stratus/fog late. Should be quite muggy with dewpoints keeping lows mostly in the 60s except for a few 50s valleys. Still some uncertainty with convective coverage Friday ahead of a slow moving backdoor front approaching from the north under flat flow aloft. Guidance does show a weak shortwave rotating in ahead of the boundary and may coincide with pre-frontal convergence during time of max heating to spark some deeper bands of storms espcly west in the afternoon. This in conjunction with better lapse rates aloft and decent instability without much cap. However flow/shear remains quite weak within a more westerly flow regime that could result in only scattered clusters of shra/tsra as suggested by the SPC-WRF/Nam, with perhaps a more organized area of convection to the northwest closer to the front. Also early clouds along with far western showers could hinder instability for a bit so running with mainly mid/high chance pops with brief likelys mainly southwest sections where late day convergence looks best. Appears temps similar to today pending the degree of insolation with highs mostly 80-85 mountains and mid/upper 80s east with more humidity around.
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&& .SHORT TERM /FRIDAY NIGHT THROUGH SUNDAY NIGHT/...
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As of 230 PM EDT Thursday... A back-door cold front draped from Ohio to northern Virginia will sink south as a weak surface wave moves off the Virginia coast Friday night. This boundary will become stationary over southern Virginia Saturday morning. Some models are agressive with allowing this front to move south during the day Saturday. However, with a heat ridge across the southeast and no jet overhead, it will take some strong diurnal convection along the boundary to help it move south during the day. Strong convection maybe possible during the mid afternoon, but the boundary may not start moving until after sunset Friday evening. Convection likely Saturday afternoon for area outside of the wedge. Mechanisms will be a combination of diurnal heating, orographical and frontal lift. As storms get going in the mid to late afternoon, outflow boundaries will also help initiate convection over North Carolina. Areas inside the wedge could see a showers with a few rumbles of thunder from elevated instabilities. Tricky temperatures forecast Saturday depending on where the wedge stalls. Inside, temperatures will range from the upper 60s to mid 70s. Outside the wedge. temperatures could warm into the upper 70s to mid 80s. The wedge will sit over the area Saturday night into Sunday morning, then erode during the day Sunday as a front moves over the Ohio- Tennessee Valley. The piedmont may not see the wedge erode until late in the afternoon. Showers from the front will enter the mountains during the late afternoon/evening. Some strong storms are possible especially with an eroding wedge boundary. Majority of the showers will enter the mountains after peak heating, therefore limiting the potential of strong storms to just a few. Showers expected to move east of the Blue Ridge after midnight, then east over the coastal piedmont by noon Monday. Jet dynamics and instabilities remain high enough to keep some mention of thunder overnight. Another tricky temperatures forecast for Sunday with wedge eroding and convection coming in late in the afternoon from the next front. Thinking wedge will be gone by mid afternoon allowing temperatures to warm very quickly before frontal showers move in. Sunday afternoon temperatures should warm into the upper 70s to lower 80s.
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&& .LONG TERM /MONDAY THROUGH THURSDAY/...
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As of 240 PM EDT Thursday... The front and rain will clear the area by Monday afternoon. High pressure will bring a brief period of dry weather from Monday afternoon into Tuesday evening. A strong cold front will move across the region Wednesday. A dry slot will grace the area Wednesday night then the upper level portion of the storm will move across the area Thursday. Temperatures will remain close to normal for early next week, then fall below normal to end the week.
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&& .AVIATION /19Z THURSDAY THROUGH TUESDAY/... As of 1220 PM EDT Thursday... VFR conditions expected to continue through the TAF valid period. The only exceptions will be the development of widely scattered shra/tsra this afternoon between 18z/2PM and 00Z/8PM, and some late night - early morning fog Friday morning espcly where earlier rain occurred. Therefore will continue with the going VCTS mention at all TAF sites along and west of the Blue Ridge for a few hours late this afternoon/evening and include a TEMPO for showers outside of the deeper convection. A period of fog/mist will also be possible espcly in some of the western valleys late tonight espcly KLWB/KBCB so included a low confidence MVFR mention in these spots. SCT-BKN layer of stratus is also possible early Friday morning mainly around KBLF given westerly flow. This could result in local IFR cigs. Medium confidence for the remainder of the TAF period per thunderstorm uncertainties and fog/stratus potential late tonight. Better threat for greater coverage of afternoon showers/thunderstorms Friday with periodic rounds of sub-VFR possible Friday afternoon/evening as a frontal boundary moves southeast toward the central Appalachians. Low clouds from earlier burn off of fog along with possible convection to the west could result in MVFR cigs through midday Friday mountains. Otherwise wont include any shra/tsra mention on Friday as appears most coverage wont occur until near or after the end of the valid TAF period. Extended Aviation Discussion... The front is forecast to stall across the region into this weekend, with best shower/storm chances over the mountains Saturday afternoon/evening. Coverage should then trend more isolated until late Sunday as this residual boundary lifts out to the northeast ahead of another cold front approaching from the west. This front may then bring another round of deeper convection to the mountains by Sunday evening and over much of the region into Sunday night. Front crosses the region Monday morning with possible residual sub-VFR in the east early on and across the mountains due to upslope flow through much of the day. && .RNK WATCHES/WARNINGS/ADVISORIES... VA...None. NC...None. WV...None. && $$ SYNOPSIS...JH/PM NEAR TERM...JH SHORT TERM...RCS LONG TERM...RCS AVIATION...JH/PM

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