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Forecast Discussion

Summary

SYNOPSIS

... Dry but cold high pressure will persist through Thursday. A brief period of freezing rain is possible early Friday as a coastal storm develops, but the majority of the precipitation will fall as a cold rain and persist through midday Saturday. High pressure briefly returns late Saturday through early Monday, but the next storm system will begin to impact our area late Monday.

NEAR TERM /THROUGH THURSDAY/... As of 945 PM: With clear skies and relatively light winds, outside of the higher ridges, temperatures have fallen just a bit faster than guidance. Still anticipate slightly improving mixing on light northeasterly flow overnight as the surface high pressure center strengthens to the north of the forecast area. Will thus continue to skew the min temp forecast to the lower edge of guidance, but not reduce further at present. This will put most of the area well into the 20s overnight, with lower 30s mainly in the upper Savannah River area.

Otherwise, the center of the sfc high to the north will drift east Thursday and is expected to be centered just off the New England Coast by early Thursday evening. This will establish a wedge-like pressure pattern over the Carolinas, though with no associated moist upglide yet, and help keep high temps well-below normal for mid- December. Low-lvl winds will remain NELY through the day. The only exception will be the mountains, where winds will become more SELY for tomorrow. Expect a dry day, with dewpoints mixing out a fair amount again by the afternoon.

SHORT TERM /THURSDAY NIGHT THROUGH SATURDAY/... As of 245 pm Wednesday: The period begins Thursday night with moisture beginning to advect northward in response to a shortwave embedded in a broader low-amplitude longwave trough and an attendant GoMex coastal low. As long-advertised, this moisture will encounter a cold-air wedge across our area associated with a strong (1040mb) but transient high pressure system which is already moving off the New England coast at the beginning of the period. Light precipitation is expected to break out in response to upglide over this near-surface airmass after midnight Friday while we lose a direct connection to the cold airmass associated with the parent high. With our source of low-level air originating from the central Atlantic coast, we will need to rely on nocturnal cooling and evaporative cooling from the developing precipitation to reinforce the cold-air damming (in-situ or in-place) and bring low temperatures close to or just below freezing, as indicated by current wet-bulb temperature forecasts.

Meanwhile, guidance continues to advertise, though to varying degrees, a pronounced deep "nose" of warm air aloft (generally +3 to +7C from about 950mb to 700 mb) developing overnight Thursday. For those cold enough, some sleet is possible at precip onset when the warm nose is weakest and the low-level sub-freezing air is deepest, but we still expect a quick transition to freezing rain during the early morning hours on Friday. Note that it will take some time for the column to saturate and the coldest areas (i.e., furthest north) will be the last to see precipitation reach the surface, so this will narrow the "window of opportunity" and work to further limit accumulations. However, further south along the Escarpment, saturation will occur earlier and overnight precipitation forecasts are higher. Therefore, there is a chance, reflected a bit in the current ice accumulation forecast, that the southern NC Escarpment may actually see some of the highest accums.

Nevertheless, the release of heat caused by rain freezing on (primarily elevated) surfaces should limit the duration and significance of any accumulation as we continue to expect surface temperatures to quickly rise above freezing for most locations by mid-morning on Friday absent any cold-air advection. Therefore, we continue to advertise accumulations less than 0.10" across the western NC Piedmont, Foothills, Blue Ridge Escarpment, and central and northern mountains. If anything, the latest model guidance has actually trended a tad warmer in the near-surface profile which, at a minimum, suggests confidence is increasing that this event will be of the nuisance variety. At this time, we believe the impacts will generally be to elevated surfaces (including trees, powerlines, and bridges) since ground temperatures will start off fairly warm; however, plan on altering your commute in the areas of concern on Friday morning to account for patchy slick spots. Though we will moderate above freezing on Friday morning and any ice will quickly melt, expect much of the area to struggle getting into the lower to mid-40s.

Unfortunately, the fun is only just beginning with this system as model guidance remains confident that multiple shortwaves will propagate across the Deep South Friday into early Saturday and this portends multiple rounds of light to moderate rainfall during this time. There remains a non-zero chance that some of our area will see totals in excess of 2" by the time all is said and done by early Saturday afternoon, and general consensus is for the heaviest totals to align closer to any remnant wedge boundary across the southern NC and SC Piedmont. We continue to expect the best chances for heavier rainfall to occur Friday afternoon and early evening in response to strong forcing associated with a negatively-titled shortwave moving across the TN Valley. The operational 06Z, 12Z, and 18Z NAM reverted back to a solution of heavy upslope-induced rainfall from the upper Savannah River Valley north and east along the Blue Ridge Escarpment during this time and while this should be monitored, this solution remains an outlier even amongst all other SREF members. Regardless, Saturday's high temperatures will be a solid 10-12 degrees warmer than on Friday with westerly flow eroding the wedge and perhaps inducing some downslope warming.

LONG TERM /SATURDAY NIGHT THROUGH WEDNESDAY/... As of 245 pm EST Wednesday: By the beginning of the period Saturday evening, conditions will be improving rapidly across much of the area as the previously mentioned negatively-tilted shortwave pivots over and east of the area. However, there will be some moisture associated with the trough axis as it pivots overhead Saturday evening and with short-lived northwest flow behind the trough, we continue to forecast continued chance PoPs along the NC/TN state line resulting from upslope flow. Without much CAA behind this system, the forecast continues to call for mostly rain with perhaps some insignificant light snow along the highest elevations. Thereafter, generally zonal (west to east) flow aloft will set the stage for a benign Sunday across the area while the next upper-level trough begins to eject out of the Desert Southwest.

The guidance begins to activate the warm front associated with the next system as early as Sunday evening from our western most counties and points east across the TN Valley. The ECMWF is the most eastward with this warm front and induces modest upslope showers in the southern mountains beginning Sunday evening whereas the GFS is dry through Monday afternoon. The forecast therefore reflects increasing PoPs heading into Monday peaking with likely to categorical PoPs overnight Monday into Tuesday morning as a cold front enters the area. This system will not present a p-type concern for our area, save perhaps the highest elevations at precip onset Monday night, as we are well south and east of the surface low track and well within the deep-layer warm sector. Guidance is still in good agreement that this front will clear the area fairly quickly by Tuesday night setting up a quiet Wednesday. As a result of the overall pattern, expect high temperatures to be 5-8 degrees above normal area-wide Sunday through Tuesday. Expect a return to near-normal temperatures behind the front for Wednesday.

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