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FXUS62 KRAH 090920 AFDRAH

Area Forecast Discussion National Weather Service Raleigh NC 420 AM EST Sat Dec 9 2023

SYNOPSIS
High pressure will extend from near Bermuda to the South Atlantic states ahead of a powerful low pressure and frontal system that will move across the region Sunday and Sunday night.

NEAR TERM /THROUGH TONIGHT/
As of 420 AM Saturday...

A srn stream perturbation and accompanying high-level OVC and virga will continue across ern NC and offshore through 12Z, while a following mid-level trough now over middle TN will lift newd and across the cntl Appalachians and middle Atlantic today. Associated height falls and forcing for ascent will glance primarily the wrn and nrn NC Piedmont during the morning through mid-afternoon.
Meanwhile, a larger-scale/longwave positive tilt trough will pivot ewd and extend from the Great Lakes through the mid and lwr MS Valley and srn Plains by 12Z Sunday, through which time the basal portion of that trough will be amplified considerably into the ArkLaTex vicinity by a ~150 kt nnwly upper-level jet now digging across the Intermountain West. Swly deep layer flow, and an associated low-level WAA regime, will consequently strengthen and be accompanied by weakly-modestly falling mid-level heights (30-60 meters/12 hrs) east of the Appalachians/across the wrn Carolinas later tonight.

At the surface, high pressure off the South Atlantic coast will drift to near and north of Bermuda while continuing to extend wwd and across the South Atlantic states. On the wrn periphery of the ridge, a warm front now extending across the FL panhandle, swrn AL, and wrn TN, will retreat nwd across GA and the cntl-ern Carolinas, then meet resistance and become quasi-stationary over the Foothills and wrn Piedmont, where an in-situ cold air damming airmass will become established. A cold front will meanwhile move east and across the OH and TN Valleys today and reach the wrn slopes of the srn/cntl Appalachians by 12Z Sun. The cold front will be preceded by a band of pre-frontal convection and outflow that will likely reach the nw NC Piedmont by 12Z Sun.

The sensible weather related to the pattern described above will feature mostly cloudy conditions over the srn and wrn NC Piedmont today, ranging to partly sunny elsewhere. Those clouds over the Piedmont may be accompanied by patchy light rain, as the aforementioned weakening mid-level trough glances those areas from mid-morning through mid-afternoon. Sly flow and WAA will otherwise result in continued moderating and unseasonably mild-warm temperatures into the lwr 60s over the nw Piedmont to lwr-mid 70s through the Coastal Plain. That WAA regime, surface stirring, and increasing multi-layered clouds ahead of the amplifying, positive tilt trough and swly flow tonight, will favor well above average temperatures (20-25 F) in the 50s. Rain chances will also increase from west to east and range from slight in the Coastal Plain to likely/low categorical over the wrn Piedmont, as the band of pre- frontal convection reaches the latter areas around daybreak Sun.

SHORT TERM /SUNDAY THROUGH SUNDAY NIGHT/
As of 343 AM Saturday...

A vigorous and quasi-negatively tilted trough will strengthen and lift across the deep south/Appalachians Sunday into Monday. At the sfc, a cold front will approach the Appalachians early Sunday before moving across central NC late Sunday night into early Monday morning. Ahead of these features, strong sly flow will flux anomalous moisture into the southeast promoting increasing rain chances from west to east into the evening hours. Rain will clear from west to east by sunrise Monday morning.

Severe Weather: A pre-frontal band of showers will be ongoing along the Carolina foothills/western Piedmont early Sunday morning. Stable conditions should largely inhibit thunderstorm chances through at least mid to late Sunday morning (although some lingering MUCAPE may generate some thunder in the Triad early). However, as this initial band of pre-frontal showers moves eastward, diabatic heating and strengthening low-level moisture flux could generate enough MLCAPE (~200 to 500 J/kg) to promote convective conditions. The best combination of shear and instability will likely maximize along and east of US-1. These locations would have the best chance for an isolated damaging wind gust or brief tornado late Sunday morning/early afternoon. Majority of the 00Z HRRR members were on board with this scenario, and a few members did simulate some low- level helicity swaths in this vicinity through early Sunday afternoon.

Beyond this initial band of pre-frontal showers, the severe threat for the remainder of the event is more uncertain. While low-level sly flow will strengthen further through Sunday evening, the thermodynamic environment may be overturned by this time. As expected, CAMs are in a bit of disagreement on whether a second line of convection can race just ahead of the cold front through our area, or if it'll mostly develop east of our area. If some instability can linger, there may be another brief window for the potential for a damaging wind gust or isolated tornado for eastern locations early Sunday evening.

Outside of any convectively-induced wind gusts, strengthening sly flow will likely mix down to the sfc at times during the day Sunday.
Gusts of 20 to 25 mph will be possible as a result. Additionally, forecast soundings suggest we could see a brief post-frontal surge in nwly sfc flow reaching as high as 35 mph in the east late Sunday night.

Flash Flooding: WPC continues to highlight a marginal risk for excessive rainfall over our area Sunday. While available moisture will surely be anomalous, this system appears to be rather progressive which should keep rainfall totals down some (QPF ~1 to 2 inches) and limit true flash flooding potential. However, isolated instances of areal flood impacts may be possible over low-lying and urban areas if higher rain rates can develop.

LONG TERM /MONDAY THROUGH FRIDAY/
As of 343 AM Saturday...

Largely quiet for the extended period as dry air and high pressure settles in behind the exiting cold front. Post- frontal nwly gusts of 15 to 25 mph early Monday will subside through the mid afternoon period. However, associated CAA will trend temperatures downward Monday and Tuesday as highs struggle to reach the upper 40s/lower 50s (lows in the 20s). Temperatures will then remain in the mid 50s Wednesday through Friday with overnight lows in the upper 20s/lower 30s accompanied by dry weather.

AVIATION /06Z SATURDAY THROUGH WEDNESDAY/
As of 105 AM Saturday...

Swly low-level flow will cause 2500-5000 ft AGL stratocumulus now extending across GA and SC to stream newd and across cntl NC this morning. Associated cloud bases are expected to be generally around 3500 ft and scattered in coverage over cntl NC later this morning, though occasional ceilings around that height will be possible over the NC Piedmont between 11-16Z. That swly flow will also strengthen through the morning and may contribute to the development of marginal low level wind shear mainly at GSO/INT, before daytime/ diurnal heating promotes mixing and the development of gusty swly surface winds from late morning through this afternoon. While patchy light rain/showers may also overspread the wrn NC Piedmont ahead of a weakening mid-level trough today, it will be generated in mid- level ceilings (aoa 6500 ft AGL) and be sufficiently light to not produce any flight restrictions.

Outlook: The approach and passage of a powerful frontal system will result in the development of widespread flight restrictions and rain over the wrn NC Piedmont Saturday night, then ewd and throughout cntl NC Sunday into Sunday night. Strong and gusty swly winds and a couple of lines of heavy showers and isolated storms will also result ahead of and along the cold front Sunday, followed by similarly strong and gusty nwly winds behind the front Sunday night- Monday.

RAH WATCHES/WARNINGS/ADVISORIES
None.

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Petersburg
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Sat -- 12:48 AM EST     2.47 feet High Tide
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Sat -- 07:11 AM EST     Sunrise
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Sat -- 02:29 PM EST     Moonset
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