Appalachian Ski Mountain

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Blowing Rock, NC

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Forecast Discussion

Summary

SYNOPSIS

... Widespread rain is expected tonight and Friday as the remnants of tropical system Beta move across the area. A mainly dry weekend is expected before unsettled weather returns with a cold front next week.

NEAR TERM /THROUGH FRIDAY/... As of 215 pm: The latest surface analysis indicates the remnants of TC Beta centered over central Miss early this afternoon, with water vapor imagery depicting moisture plume extending across the TN Valley into the Appalachians. Widespread light rain continues across much of the forecast area at this hour, but remains light...in fact, even though it's been raining for the better part of the past 12 hours across far western NC, total rainfall in those areas is at most about 1/3 inch across the Smokies and vicinity, while precip is amount to little more than sprinkles across a good chunk of the Piedmont. Having said that, with the center of a steadily weakening Beta expected to lift up the Cumberland Plateau late tonight into tomorrow, rain rates will steadily increase from mid/late evening into the overnight, as moisture and deep layer forcing increase... including stout isentropic lift developing as a consequence of a 25-30 kt low level jet overspreading developing in-situ cold air damming. Warm conveyor belt moisture will begin to shift east of the area by around sunrise, taking the widespread rain along with it. Rainfall amounts of 1 to 2 inches are expected across much of the forecast area by 12Z Friday, but localized amounts of 2 to 3 inches will be likely across the southern mountains of NC. This should be sufficient to produce a localized excessive rainfall/flash flood threat, but not enough to warrant Watch consideration at this time.

Although the deeper moisture will be shifting east of the area at the start of the Friday period, a surface boundary will remain just west of the area, with plenty of low level moisture lingering. Short term guidance depicts eroding CAD air mass, with fairly robust instability (sbCAPE of 1500-2000 J/kg) developing in the warm sector, which could retreat as far north as I-85 during the afternoon. With potent northern stream short wave trough digging toward the region during the afternoon, at least scattered deep convection will develop in the warm sector, with shear being adequate for development of organized/isolated severe storms. Storm relative helicity is generally meager, but shear/SRH profiles along the retreating CAD boundary should be sufficient to pose an isolated tornado threat.

SHORT TERM /FRIDAY NIGHT THROUGH SUNDAY/... As of 300 PM EDT Thursday: The remnant low associated with what was once Beta will be positioned around the Upstate of SC and eventually the NW piedmont of NC Friday night into Saturday morning. Low chance severe weather could be in store for the first couple of hours of the short term with a line a decent CAPE (1000-2000 J/kg), moderate effective bulk shear (~30-40 kt), and good low-level helicity (100- 200 m2/s2). The primary threats will be damaging winds and an isolated tornado, which can't be ruled out, mainly south of I-85. This should be the last burst of decent rain rates and isolated thunderstorms before the system fully pushes off to the east late Friday/early Saturday. The merged upper trough will propagate eastward as well with the axis fully leaving the cwa by lunchtime Saturday. A dry NW flow aloft will filter in during the day Saturday, but residual moisture from the remnants of Beta will linger and could spark a couple of showers across the cwa during the afternoon and evening with most locations staying dry. Therefore, isolated PoPs will remain in the forecast for much of the area. During the latter half of the short term, weak mid-level ridging will set up shop over the East Coast on Sunday and should leave the cwa mostly dry with a slight chance for a shower. With the quick uptick in heights, expect Sunday to be the warmest day over the next seven days. An approaching cold front from the west will help downgrade thicknesses and develop some activity late in the day Sunday and early Monday with a deep layer S to SW flow ahead and along the front. This initial frontal boundary will be weak compared to what's coming in the extended. Expect temperatures to be slightly above-normal through the period.

LONG TERM /SUNDAY NIGHT THROUGH THURSDAY/... As of 300 PM EDT Thursday: After the initial cold front pushes through Monday, a deep digging trough with a secondary cold front will be swinging in from the NW during the first half of the medium range. Monday is looking more wet than the weekend with the initial boundary paving the way for the next and much more healthy looking system coming in behind it. By Tuesday when the secondary frontal boundary approaches the area, that's when model guidance begin to diverge a bit. The GFS digs the trough across the Southeast and has the axis of it moving through the cwa by Tuesday evening as it is more progressive with this system. On the other hand, the ECMWF develops a cut off low as the trough digs south and has it approaching the area late Tuesday into Wednesday. The associated frontal boundary would become occluded and impact the cwa for much of Wednesday before pushing off to the east late Wednesday night. This could pack a punch if the ECMWF is the correct model in this situation. Heavy rain and a severe potential would be included in this scenario. The ECMWF has the activity subsiding across the cwa by 12Z Thursday. Temperatures will be at or slightly above-normal Monday and trimming down a few degrees Tuesday and Wednesday before the real cool air filters in after the frontal passage.

A post-frontal environment will be in store for the latter half of the new work week, which will keep the cwa dry and relatively cool Thursday and Friday. Both models have another digging shortwave trough stretching from the central CONUS through the Great Lakes region. This could bring another chance for elevated activity next weekend, but that's beyond day 7 and wouldn't count on it much at this point in the forecast. After the frontal passage late Tuesday, the rest of the extended looks dry and much cooler with temperatures comparable to mid- to late October.

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