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FXUS62 KRAH 101955 AFDRAH

Area Forecast Discussion National Weather Service Raleigh NC 255 PM EST Sun Dec 10 2023

SYNOPSIS
A strong cold front will move across the region tonight. After the front passes the state, high pressure will build across the Southeast for much of the upcoming week.

NEAR TERM /THROUGH TONIGHT/
As of 900 AM Sunday...

Radar displayed a large band of rain and scattered thunderstorms moving east out of the Triad and heading towards the Triangle and Sandhills. Parts of the Triad have already observed around three- quarters to one and one-quarter of an inch of rain today, with more to come. No major changes were needed with the update this morning, mainly small tweaks to account for the latest trends of timing of precipitation today (slightly slower) and small tweaks in temps (a degree cooler NW and warmer SE). Please see the discussion below for additional details on the forecast through tonight. -JJT

Previous discussion from 515 AM Sunday: Within a progressive longwave trough extending from wrn Hudson Bay sswd through the upr and middle MS Valley and srn Plains, a basal perturbation now over east-cntl TX will assume negative tilt while pivoting ewd into the TN Valley through 00Z Mon and newd across the Carolinas and cntl VA through 12Z Mon. Amid strong deep layer, swly to south southwesterlies and height falls throughout the ern US ahead of that trough, a concentrated area of intense (150-180 m/12hr) 500 mb height falls will precede the vigorous basal perturbation.

At the surface and in a complex pattern, broad and strengthening sly flow will exist across the South Atlantic states, between high pressure near Bermuda and an initially wavy frontal zone now nearing the wrn slopes of the cntl and srn Appalachians, cntl AL, and the Mouth of the MS. Between those synoptic features, a couple of notable sub-synoptic/meso-alpha features have developed: 1) in the past 12-24 hrs a composite outflow boundary and ongoing strong linear convection, elevated from near PSK to FQD in wrn VA/NC and surface-based from near GVL and MGM in GA and AL and 2) in the past 6-12 hrs a pre-frontal trough and confluence axis beneath a low level jet and WAA maximum, and ongoing strong to occasionally severe mixed mode of discrete cells and line segments, from the FL panhandle newd and across n-cntl GA and the wrn Carolinas. All of the above are forecast to move slowly but steadily ewd and across and offshore NC through 06-09Z Mon, culminating in consolidating and strengthening low pressure across the ern Carolinas and up the middle Atlantic coast through 12Z Mon.

The sensible weather forecast related to the pattern described above will feature somewhat higher than average uncertainty for a 12-24 hr forecast given the presence, evolution, and influence of the aforementioned mesoscale features on the downstream environment and warm sector over the cntl and ern Carolinas. The most likely scenario at this time appears to be increasing coverage of elevated showers and storms edging ewd and across the srn and wrn NC Piedmont through midday to early afternoon. This regime will tend to reinforce stable low-levels there, while the warm sector to the east (roughly east of a line from MEB to RDU to IXA) should diurnally warm and destabilize into the lwr-mid 70s and provide a favorable environment for the development of both discrete cells and line segments, and/or intensification from a quasi-linear mode moving into it from the west. A risk of both damaging wind gusts and an isolated tornado will accompany that activity through the afternoon.

A separate line of convection --possibly in the form of a narrow cold frontal rainband (NCFR) and squalls-- will likely accompany the advancing, frontogenetic synoptic cold front and deepening surface low across SC and e-cntl and ern NC early tonight, during which time a large area of post-frontal precipitation will otherwise blossom in a region of strong deformation and overspread all of cntl NC.
Momentum transfer within the shallow but intense NCFR, and also immediately post-frontal where isallobaric forcing along the track of the deepening low and amid steepening lapse rates within a regime of strong CAA and diabatic cooling from stratiform precipitation falling into the increasingly-dry low-levels, will favor a couple of hour period of 35-45 kt wind gusts along and immediately behind the low/front.

Lastly, the far wrn edge of the aforementioned stratiform precipitation shield may be accompanied by a brief (1-2 hr) mix, or shorter-lived changeover to snow, across the nw NC Piedmont between roughly 06-10Z, as low-levels cool and freezing levels lower sufficiently to allow otherwise melting snow aloft to reach the surface and melt on contact owing to an antecedent warm and wet ground, and slightly above freezing surface temperatures.

After the stronger surge of immediately post-frontal surface winds/squall noted above, it will otherwise turn blustery throughout cntl NC amid CAA and temperatures decreasing into the 30s. -MWS

SHORT TERM /MONDAY AND MONDAY NIGHT/
As of 255 PM Sunday...

By Monday morning, low pressure will be over eastern Massachusetts with today's cold front extending to the south. While there could be some lingering showers over coastal North Carolina, all rain should have moved east of the forecast area. Cloud cover will rapidly decrease east of Raleigh through the morning, with clear skies expected in the afternoon and continuing into the overnight hours.
The wind will gust between 15-30 mph in the morning, but drop to 10- 25 mph in the afternoon with a light wind continuing overnight.
After above normal temperatures today, Monday will have below normal temperatures, with highs ranging from the mid 40s to the low 50s.
Lighter winds and clear skies will also bring below normal temperatures for Monday night, with lows from 22-32.

.

LONG TERM /TUESDAY THROUGH SUNDAY/
As of 255 PM Sunday...

High pressure will be over central North Carolina Tuesday morning, and the high will be reinforced from the Great Lakes through the middle of the week. There should be some high clouds Wednesday, but Tuesday and Thursday will be mostly clear. By the weekend, there is much lower confidence in the forecast. The 12Z run of the ECMWF has maintained a dry forecast through the weekend, and the 12Z run of the GFS has dried out compared to previous runs, now not bringing precipitation into central North Carolina until Sunday. Meanwhile the 12Z GEFS has been fairly consistent with its previous run, bringing precipitation into the area as early as Friday night and keeping rain in the forecast through the entire weekend. Considering the 12Z GEFS is a wet outlier, have gone with a dry forecast Friday night, only kept slight chance pops across the south on Saturday, then added chance pops to the forecast for Saturday night and Sunday.

Temperatures will yo-yo through the extended forecast, with the warmest highs a few degrees above climatology over the weekend and the coolest highs on Thursday only reaching the 40s. Thursday night looks like the coldest night with widespread lows in the 20s, and lows will be at or below freezing every night except Saturday night.

AVIATION /18Z SUNDAY THROUGH FRIDAY/
As of 130 PM Sunday...

Frequent SHRA/RA and scattered TSRA will help continue sub-VFR conditions through this evening, with conditions improving from west to east overnight as a few disturbances move across the region.
Besides gusty and erratic winds possible near TSRA, southerly wind gusts to 25 kt are possible near KFAY/KRDU/KRWI through this afternoon, then widespread NW wind gusts of 25 to 40 kt are possible behind a cold front this evening with highest gusts near and east of Hwy-1. Conditions will improve to VFR late tonight, however breezy NW wind gusts of 15 to 20 kt could linger through Mon morning.

Outlook: High pressure will favor VFR conditions through early to mid week.

RAH WATCHES/WARNINGS/ADVISORIES
None.

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