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FXUS62 KRAH 101408 AFDRAH

Area Forecast Discussion National Weather Service Raleigh NC 907 AM EST Sun Dec 10 2023

SYNOPSIS
Low pressure will consolidate and strengthen along a cold front that will cross NC today, then rapidly deepen while tracking up the coast up the coast of middle Atlantic and Northeast tonight into Monday.
Seasonably chilly high pressure will follow and extend across the Southeast through early to mid next 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 420 AM Sunday...

Lingering troughiness will be positioned over Quebec down through the southern Mid-Atlantic Monday morning, but the potent PV anomaly responsible for the storm system moving through Sun night, will have shifted up into VA/MD by this time. The parent trough will shift over the Northeast through the day allowing for westerly confluent flow aloft to overspread the area through Mon night. At the surface, this translates to a rapidly deepening area of low pressure off the Northeast coast while high pressure strengthens while shifting over the southern Appalachians. Strong and deep CAA behind the cold front will continue to mix down strong winds through the morning hours with NW gusts 25-30 mph. Due to the progressive nature of the surface features, the pressure gradient will quickly begin to relax Sun afternoon with gusts weakening to 15-20 mph then subsiding around sunset.

Highs will be 15-25 degrees cooler than Sun thanks to the cold fropa and strong CAA. Downsloping NW winds may offset the cooling a bit, but highs should only reach into the upper 40s to low 50s. A relaxing pressure gradient and period of calm winds underneath clear skies will lead to radiational cooling allowing lows to fall into the low/mid 20s across the Piedmont with mid/upper 20s in urban areas and the Sandhills and southern/central Coastal Plain.

.

LONG TERM /TUESDAY THROUGH SATURDAY/
As of 420 AM Sunday...

...Dry through the work week with low forecast confidence heading into the weekend...

Surface high pressure centered over the Mid-Atlantic will become reinforced behind a secondary cold front Wed night as an broad upper trough rotates through eastern Canada. The arctic airmass will be pushed through southern Mid-Atlantic as a nearly 1040mb surface high migrates into the OH Valley Wed into Wed night shifting over the Mid- Atlantic through Thurs night. Calm conditions underneath the Arctic high will result in the coldest conditions in the extended Fri morning with statistical guidance supporting lows in the 20s area wide.

Forecast attention shifts to the weekend as a mid/upper level closed low that becomes pinched off from the northern stream flow as upper ridging quickly rides over the top on Thurs. As which is often the case, forecast confidence decreases on the evolution of shortwave that is briefly cut off from the northern stream branch. GEFS and EPS ensemble guidance is understandably very broad with the mean 500mb flow during this time, but does show troughing in the vicinity of the closed low over the Four Corners Region that slowly meanders east into the lower MS Valley Sat into Sat night. Given the directed SW flow over the area as high pressure shifts offshore and broad low pressure develops in the northern GOM and tracking east with time towards the Southeast, support at least a continuation of slight chance PoPs into the area late Sat. Temperatures will trend towards near normal after the moderating Arctic high weakens and moves offshore late week into the weekend.

AVIATION /12Z SUNDAY THROUGH THURSDAY/
As of 735 AM Sunday...

Widespread IFR ceilings now over cntl NC, amid strong and increasingly moist sswly low-level flow and risk of low level wind shear with mainly light surface winds this morning, will gradually lift to MVFR and perhaps scatter to VFR from west to east at FAY, RWI, and RDU and be accompanied by the development of breezy and gusty sswly surface winds between 15-20Z. They will do so ahead of a line of pre-frontal convection now crossing the Triad terminals, which may strengthen as it moves ewd and which may also be accompanied by preceding, scattered showers and storm through the afternoon. A secondary, narrow line of showers and storms with briefly intense rain and wind (squalls) with gusts between 35-45 kts for a couple of hours along and behind an associated strengthening cold front, is forecast to develop and intensify as the front moves across SC and e-cntl through ern NC --including at FAY, RWI, and perhaps as far west as RDU-- early tonight. A large area of post- frontal rain will otherwise blossom and overspread all of cntl NC tonight, during which time IFR-MVFR conditions are expected, with west to east clearing and a return to VFR likely during the last several hours of the 12Z TAF period. Strong and gusty nwly winds behind the cold front will continue through the night and into Monday.

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

RAH WATCHES/WARNINGS/ADVISORIES
None.

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