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FXUS62 KRAH 060212 AFDRAH

Area Forecast Discussion National Weather Service Raleigh NC 910 PM EST Tue Dec 5 2023

SYNOPSIS
An upper level disturbance and a weak surface low will move east through the Mid Atlantic region tonight through Wednesday morning.
Surface high pressure will then build into the Southeast from Wednesday afternoon through early Friday, before moving offshore Friday night. A storm system will move into the region late in the weekend.

NEAR TERM /THROUGH TONIGHT/
As of 910 PM Tuesday...

Forecast largely on track, with minor tweaks. The anticipated potent shortwave is seen clearly on GOES WV and IR imagery over W NC, with the RAP indicating the 1.5 PVU surface down to 600 mb. As expected and suggested by CAMs, a batch of high-based showers have blossomed ahead of this wave over the W Piedmont, progressing E within the fast cyclonic steering flow. The existing forecast of chance pops crossing the CWA through the next several hours remains valid.
Leaning toward the HRRR, which has been consistent and initialized well, have brought shower chances further south, which aligns with current radar trends. QPF should be limited by the lack of deep moisture and fairly high cloud bases. However, given the presence of decent low level lapse rates and strong, focused forcing for ascent to make the most of the layers of moisture through the column (including within the mixed phase region aloft), as indicated by the 00z soundings, have nudged pops up slightly and expanded them areally. At any rate, most places should see just a few hundredths of an inch. Current temps vary widely across the region (including some uncomfortably low values in the mid 30s, ranging to the mid 40s). Once skies start to clear out over all but the NE sections later tonight, temps areawide should eventually bottom out in the mid 30s to lower 40s, with a few lower 30s in rural areas of the W Piedmont. -GIH

Previous discussion from 1250 PM: Significant amplification of the long wave trough in place across the Eastern US trough will begin late today but especially tonight and Wednesday(H5 anomalies-2 to -3 SD) as an intense upper PV anomaly dives into the SE US. Skies will remain sunny through the mid afternoon will clouds starting to spread/fill in from NW during the late afternoon and early evening. Central NC will experience seasonable to slightly above normal temps, ranging from lower/mid 50s north to lower 60s across southern NC.

The robust synoptic scale H5 falls of 120-130 meters coupled with strengthening upper divergence from the merger of the PJ with the STJ will be able to overcome modest moisture profiles to support some scattered light rain showers and sprinkles, mainly across central and northern portions of central NC, this evening and tonight. The cloud cover and precip will keep it slighter warmer than previously advertised. Lows 35 to 40.

SHORT TERM /WEDNESDAY THROUGH THURSDAY NIGHT/
As of 150 PM Tuesday...

The core of a deep mid/upper trough will be situated over the Mid- Atlantic 12z Wed then shifting east and offshore by 00z Thurs with NW to WNW flow directed over central NC through Thurs night. This feature will provide deep layer forcing for ascent overspreading an area over the northern/central Coastal Plain where a nearly saturated layer up to 700mb will be in place as the 850mb front provides some extra low-level forcing. This pattern will be supportive of light rain and/or sprinkles during the early morning into the early afternoon hours, which is hinted at by 12z CAM guidance. Accumulations will likely be minimal, if any. Expansive cloud cover will slowly collapse to the ESE throughout the day with mostly clear conditions expected Wed night.

At the surface, a strong pressure gradient will be in place Wed as an area of low pressure develops in the western Atlantic and high pressure over the Mid MS Valley continues to build into the eastern CONUS. A deep elevated mixed layer will quickly be tapped into Wed morning as surface heating erodes the very shallow stable layer at the surface. The result will lead to brisk NW winds gusting up to 20- 25 mph beginning shortly after sunrise and persisting throughout Wed afternoon.

High pressure will briefly settle over GA and the Carolinas Thurs morning and provide a few hours of optimal radiational cooling, especially across the western/southern Piedmont, allowing temperatures to dip into the mid 20s to low 30s. High pressure will keep conditions dry and seasonably cool as it traverses across central NC through Thurs afternoon before shifting offshore, returning SW flow at the surface late Thurs into early Fri.

LONG TERM /FRIDAY THROUGH TUESDAY/
As of 150 PM Tuesday...

The extended forecast will start and end on the quiet side, but be rather active in the Sun-Mon period. Ridging at mid-levels will continue to expand eastward from the central US Fri into Sat. In contrast, a strong shortwave will dig into the central/southern Plains and will be our weather maker for Sun-Mon. At the surface, high pressure over the western Atlantic will bring return southerly flow Fri-Sat and bring our highs back to above seasonal normal in the upper 50s/low 60s Fri to the mid/upper 60s to near 70 Sat. Some high clouds will be around Fri into Sat with a southern system tracking into the SE US but we should be dry with limited moisture.

The main story in the extended in the vigorous shortwave trough discussed previously over the central/southern Plains Sat that is forecast in the guidance to track into the OH valley/Great Lakes Sun evening and into eastern Canada Mon. The system becomes negatively tilted as it tracks to the NE. Accompanying this mid-level wave is a surface low that is forecast to track into the Mid-Atlantic, with a trailing cold front moving through sometime late Sun night. Nearly all ensemble guidance indicates a high likelihood of precipitation Sun given anomalous moisture (PW's 200+ percent of normal at 1.5-1.6 inches), strong lift from the trough/jet streak, and even some instability. One area where models still differ is in regards to timing. The latest GFS is the fastest solution, ending precipitation early Sunday evening, while the GEFS/NAEFS/ECMWF are about 6-12 hours slower, ending rainfall late Sun night to early Mon. An ensemble approach would favor these slower solutions, with the GFS an outlier on timing. Precipitation could start as early as late Sat night to early Sun associated with isentropic ascent with a warm front. The the main batch of showers appears favored Sun afternoon into the evening, ending from west to east late Sun night. It could be rather gusty Sun as well with southerly winds gusting to 25-35 mph and highs still above normal in the mid/upper 60s. As for rainfall amounts, the range in expected amounts varies from a quarter inch to three quarters of an inch based on ensemble data, though higher amounts up to an inch is possible. As for storm potential, the dynamics support strong shear but the instability is somewhat lacking. We introduced a chance of thunder on Sun, however, given a strong fetch of moisture with low to mid 60s dewpoints advecting into the area. The CIPS/CSU analog severe probabilities suggest a signal across NC/VA, so the threat of storms will continue to be monitored as we get closer to the event.

Drier and more seasonal weather is forecast Mon and Tue as zonal flow takes over at mid-levels and high pressure builds into the area from the lower Mississippi Valley. Highs should range from the low to mid/upper 50s and lows in the upper 20s to mid 30s.

AVIATION /00Z WEDNESDAY THROUGH SUNDAY/
As of 611 PM Tuesday...

VFR conditions are expected at all terminals to start the 24 hr TAF period. A potent short wave currently moving over the mountains will move east across central NC tonight. As it moves east, it may generate a few showers near KRDU between ~02 and 06Z. If showers do develop, they will likely be light in nature and thus will likely not produce any sub-VFR conditions at KRDU or elsewhere. These showers will dissipate with time overnight. A stratocu deck is expected to develop late Wednesday morning and move from north to south across the Piedmont/northern Coastal Plain tomorrow. Enough guidance is hinting at the potential for a brief period of MVFR ceilings at both KRDU and KRWI to warrant at least some mention.
Thus, have added a TEMPO group for MVFR ceilings at both KRDU and KRWI from ~14 to 18Z Wednesday with this TAF package. Additional scattered showers may accompany the lower cloud deck, but should remain light enough to preclude any sub-VFR reductions in visibility. Any lingering MVFR stratocu should scatter and lift to VFR by mid to late Wednesday afternoon. Lastly, nwly gusts of 20 to 25 kts appear likely at all terminals starting mid to late Wednesday morning, before tapering off near sunset.

Outlook: Dry VFR conditions are expected to dominate through Saturday. A strong cold front crossing the region will bring likely to categorical rain chances, strong gusty winds and associated sub- VFR restrictions to all of central NC late Saturday night and Sunday.


RAH WATCHES/WARNINGS/ADVISORIES
None.

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Conway
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Tue -- 12:51 AM EST     Last Quarter
Tue -- 02:39 AM EST     0.86 feet Low Tide
Tue -- 07:06 AM EST     Sunrise
Tue -- 08:45 AM EST     1.10 feet High Tide
Tue -- 01:01 PM EST     Moonset
Tue -- 02:53 PM EST     0.91 feet Low Tide
Tue -- 05:07 PM EST     Sunset
Tue -- 08:55 PM EST     1.22 feet High Tide
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Conway, RR. bridge, Waccamaw River, South Carolina, Tide feet

Grahamville
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Tue -- 12:51 AM EST     Last Quarter
Tue -- 03:43 AM EST     0.67 feet Low Tide
Tue -- 07:05 AM EST     Sunrise
Tue -- 09:43 AM EST     0.88 feet High Tide
Tue -- 01:01 PM EST     Moonset
Tue -- 03:57 PM EST     0.71 feet Low Tide
Tue -- 05:06 PM EST     Sunset
Tue -- 09:53 PM EST     0.98 feet High Tide
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Grahamville, Waccamaw River, South Carolina, Tide feet

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