Appalachian Ski Mountain

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Blowing Rock, NC

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Forecast Discussion

Summary

SYNOPSIS

... Warm conditions will linger today, south of a weak stationary front and ahead of an approaching strong cold front. The cold front will cross our area from the northwest on Thursday morning, with cooler high pressure spreading over the region for Friday and Saturday. Low pressure with abundant moisture is expected to affect our region on Sunday and Monday.

NEAR TERM /THROUGH TONIGHT/... As of 1045 AM Wednesday: Isolated showers over the mountains have dissipated this morning. Now we await the potential convection for the afternoon. 12Z KGSO RAOB showed a very steep mid level lapse rate above a very warm low level temp. This lapse rate is shown by meso analysis to exist across much of the CWFA. The analysis also shows the cap across much of the area as well. Expect the nearly full sunshine this morning and early afternoon to create strong warming which should erode the cap as seen in forecast soundings. This should lower the LFC and help create moderate instability across the area. Strong deep layer shear is already in place. The question is the trigger. Visible sat pix shows the outflow boundary from the overnight MCS dropping south toward the I-77 corridor. If this can remain in tact, this would be sufficient to activate convection and aid in organization. As a result, SPC has expanded the Slight risk area across the NW piedmont. The original Slight risk area is still mainly for the overnight convection expected tonight. So the rest of the forecast area is in a Marginal risk. This is due to the moderate instability, strong deep layer shear, but lack of organization. The latest CAM guidance agrees with this scenario, and isn't a significant change from the previous expectations. Have updated PoP timing to go along with this latest thinking. Breezy conditions still expected for the afternoon. High temps look on track with near record high at GSP.

Otherwise, broad upper ridging will remain over the region today and tonight with heights beginning to lower as broad upper trofing digs down over the Great Lakes and makes its way towards our area by the end of the near-term period early Thursday. At the surface, subtropical high pressure will remain off the SE Coast and keep warm, SWLY low-lvl flow over the area. A vigorous cold front will approach the fcst area from the NW by the end of the period early Thursday. Today appears to be one of those scenarios where we may end up getting very few showers and/or thunderstorms, but those that do form could become strong to severe pretty quickly. The main threats from any storms will be straight-line winds and hail.

SHORT TERM /THURSDAY THROUGH SATURDAY/... As of 226 AM EDT Wednesday: A number of concerns make this a busy fcst for the rest of the week. Primary concern will be the passage of a convective line just ahead of a strongly forced cold front Thursday morning. The synoptic scale models continue to show the mountains forming a barrier that a convective line will not cross, but the convection-allowing models that go out that far show the line intact leaving the GSP fcst area around 12Z Thursday. As this is roughly 24 hrs in the future, the timing could easily be +/- a few hours, so think it wise to include a chance of thunderstorms over the Piedmont of the Carolinas first thing Thursday morning. The sfc cold front will move this activity quickly eastward by mid/late morning, so it should be long-gone by noon. Some lingering upslope could keep a chance of showers along the TN border during that same time, but it too should dry up by 18Z Thursday. We should dry out quickly Thursday afternoon with winds becoming gusty from the WNW as be mix deep enough to tap into the faster flow seen around 850mb. We could approach Red Flag Fire Wx criteria in some places if we fail to get a soaking rain from the frontal passage early in the day. The upper trof axis is expected to swing past Thursday night, with moisture caught up in the WNW flow that should miss our fcst area just to the north. The bigger story will be the wind. Fcst soundings show that we may retain a relatively deeply mixed boundary layer overnight, with the base of the subsidence inversion high enough to tap into some strong winds aloft. This could keep allowing wind gusts to mix down all night and into Friday morning, especially at the higher terrain, with Wind Advisory criteria a distinct possibility. High pressure continues to build in on Friday with very dry air. Sky will be bright and sunny with below normal temps, but afternoon RH will crater down perhaps into the teens east of the mtns. Thankfully, the wind should steadily diminish in the morning as the pressure gradient relaxes, but perhaps not fast enough if the timing is off. By that time, we will have dried out considerably, meaning that we may very well push Red Flag criteria in some places again. As if that isn't enough to worry about, on Friday night the sfc high will continue to build in, with the model guidance in agreement that the center of the sfc high will be smack dab over the mtns at sunrise Saturday. That means excellent radiational cooling conditions are possible, which raises the possibility of well below normal low temps and concern for areas of frost east of the mtns, maybe even some pockets of freezing temps over the NC foothills and NW Piedmont. Whew. This is probably not yet worth mentioning in the HWO just yet, while we deal with the severe storms thru Thursday morning. Finally, Saturday should be a spectacular springtime day with more bright sunshine and temps just below normal.

LONG TERM /SATURDAY NIGHT THROUGH TUESDAY/... As of 327 AM EDT Wednesday: As if our plate wasn't full enough already, the medium range part of the fcst serves up the potential for heavy rain and threat for flooding, albeit highly uncertain at this point, for Sunday into Monday. The model guidance remains in general agreement with a srn stream system getting picked up over the srn Plains and being lifted out over the mid-MS Valley and Midwest/Great Lakes, the main problem/disagreement is the timing. The new GFS has this beginning Saturday night and evolving through Sunday into Sunday night, while the new ECMWF has the same general scenario but is 12 hours slower. Fortunately, in spite of these differences, both develop the moisture return and spread the precip up from the SW after midnight Saturday night, with precip breaking out by daybreak Sunday across a good chunk of the fcst area. Both have a primary sfc low lifting past well to our northwest, but there are some differences in terms of a warm sector lifting into our region as the low passes. Still think the GFS has a slightly better chance of keeping a weak in-situ wedge/cool pool over much of the area east of the mtns, which would keep the stronger storm potential and hopefully heavier rain to our south/east. That being said, cannot deny the potential for a soaking rain out of this system, with QPF on the order of a few inches from start (early Sunday) to finish some time on Monday. Whether or not this raises any flash flood threat depends on details that are not clear at this time. Guidance indicates some potential for lingering/re-developing light precip from follow-up forcing rotating around the deep upper low that lifts up into ern Canada Tuesday into Wednesday. The guidance warrants keeping a low precip prob over at least the higher terrain during that period. Temps will fall back below normal late in the period, with enough cooler air aloft to suggest that some of the precip could fall as wet snow at the peaks above 6k feet. So, yes, we even have to talk about a small chance of winter weather after all we have to deal with thru the first six days of this fcst. No dull moments over the next week.

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