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Forecast Discussion

Summary

SYNOPSIS

... An upper level disturbance moving across the region today will result in accumulating snow at high mountain elevations, and a chance of rain showers elsewhere. Expect dry conditions Tuesday, followed by another weak low pressure area moving by to our south late Wednesday, with another chance for precipitation. Brief drying occurs again Thursday.

NEAR TERM /THROUGH TONIGHT/... As of 1005 AM...Ongoing shower activity associated with upper energy moving through the region will likely peak between now and mid to late afternoon. Webcams and other observations confirm that snow is falling down to around 4000 feet with accumulating snow along and especially above that elevation. Weak cold air advection and diurnal cooling should allow snow levels to fall to around 3000 feet this evening before possible making it to valley floors by around midnight. Model guidance continues to show shower activity becoming confined to near the Tennessee border during this time with a brief period of northwest flow shower activity. All in all the forecast is in good shape with the current advisory seemingly well placed, especially for the northern mountains where there was a report of snow covered roads earlier this morning.

As of 630 AM...On going snowfall across the higher mtn elevations noted on 88d and area webcams. Advisory area looks good expect accum snowfall arnd 1-2 inches over the next couple hrs...mainly abv 4 Kft. A lowering snow level still looks plausible with stg vort lobe and steepening mlvl LRs poised to push into the mtns, which may increase SF rates and dynamical cooling.

Light returns are seen on the 88D crossing over the Smokies and down the Balsams. This is producing -sn abv 4 Kft and expect a couple inches of accum thru daybreak. The best forcing remains to the SW of the FA right now associated with an h5 s/w, thus topo lift will be the main snow generator thru the early morning hrs.

The atmos becomes more dynamic arnd 12z as dPVA increases ahead of an elongated area of channeled h5 vorticity. Loosely accompanying the vort lobe will be increasing mlvl lapse rates and fairly deep coupled omega. Low level moisture will remain high and temps will remain at or below freezing abv 4 Kft...however will anticipate a lowering of snow levels thru mid day as precip rates increase within deeper dyno/thermal forcing. The better storm total snowfall amts will remain abv 4 Kft, where 2-3 inches are probable with 3-5+ inches abv 5 Kft likely. Could see some snow make it to the valley floors with a trace to a tenth of an inch possible arnd and aft daybreak before the sfc layer warms.

Kinda on the edge...but went with a WW.Y for snow abv 3.5 Kft as the potential is high enuf for 2 inches over 12 hrs with increasing precip rates due to the aforementioned synoptic forcing. Snowfall will become more limited once the upper trof moves east and downward motion reduces deep moisture levels. Expect a fairly light NW flow period to continue this evening and overnight with most accum occurring over the nrn mtns. There is even a chance of -sn making it outside the mtns into the NC fthills/piedmont and the Upstate of SC...yet this would be limited in time and sfc temps will be too warm for any accum concerns. Overall...max and min temps will remain right arnd normal thru the period.

SHORT TERM /TUESDAY THROUGH THURSDAY/... As of 200 AM Mon: Dry high pressure will extend from the Midwest into the Southeast Tuesday and Wednesday, beneath a low-amplitude southern stream ridge sandwiched between troughs. The trailing trough will carry a sfc low along the Gulf Coast; models differ in whether they shear off the base of the trough into a separate shortwave tracking across the Deep South. This sort of development was also noted on previous model cycles. The differences mainly lower confidence in the duration of the resulting precip. For our area, the main forcing resulting from this system will be from weak isentropic lift on its warm side, and from divergence and/or deformation north of the shearing shortwave. Precip chances increase from SW to NE beginning Wednesday aftn, with the best chances fcst for Wednesday evening and early overnight. The GFS and EC depict the trailing northern shortwave bringing another shot of precip to the Tennessee border areas early Thursday morning, but this is not well agreed upon by ensemble members and the Canadian.

The near normal max temps Tuesday drop back to a category or so below normal for Wed and Thu, with clouds Wed and a fresher continental airmass for Thu. Given thermal profiles, wherever sfc temp is cold enough in this period, snow is forecast. Model QPF is generally light, but gentle southeast flow into the Escarpment with the passing system Wednesday night likely will result in some terrain enhancement. Taking this into account, can't say we won't end up with a high-elevation Winter Wx Advisory, though with the differences in the models and the near term Advisory, will not mention in the HWO at this time.

LONG TERM /THURSDAY NIGHT THROUGH SUNDAY/... As of 300 AM Mon: Progs for early part of the medium range bear some similarity to the midweek pattern, with weak high pressure and a low-amplitude ridge in place for our area ahead of a trough. However, models still show some disagreement in the evolution of that trough, which reduces low confidence to the fcst for Friday onward. The GFS trough is substantially deeper and broader, which makes it initially wetter across the Mississippi Valley and Southeast. The GFS/EC/Canadian-GDPS solutions depict coastal cyclogenesis Saturday, keeping the really wet weather to our south and east (though the Canadian shows a front developing Friday with the "trailing shortwave" described in the Short Term discussion). The models all show phasing of the trough with a northern-stream shortwave late in the weekend, but still with enough difference in timing and QPF response to keep confidence low.

PoPs have been limited to the Chance range through the period, with the best overall chances early Saturday when the depicted coastal system makes its nearest pass. A lingering PoP persists thru the weekend along the Tennessee border, given the assumption of NW Flow in the wake of the deep trough. Diurnal temp range will be smaller than climo with the expectation of some cloud cover much of the period, but temps warm Sunday as ridge builds. Temps permit a minor snow mention for the mountains and NW NC Piedmont early Saturday, with additional NW Flow snow early Sunday.

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