Area Forecast Discussion
Issued by NWS Raleigh/Durham, NC

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000 FXUS62 KRAH 201925 AFDRAH Area Forecast Discussion National Weather Service Raleigh NC 325 PM EDT Mon Mar 20 2017 .SYNOPSIS... High pressure centered over central NC this morning will shift offshore the Southeast coast this afternoon, in advance of a broad low pressure system and associated cold front approaching the mountains from the WNW. The aforementioned low pressure system will push southward through the Carolinas as a backdoor cold front Tuesday evening into Tuesday night. Canadian high pressure will build into the Carolinas from the northwest Wednesday into Thursday. && .NEAR TERM /Through Tonight/... As of 100 PM Monday... Today: High pressure centered over the area this morning will move offshore this afternoon as a broad surface low and associated cold front approaches the mountains from the west. As the sfc high shifts offshore, warm advection will commence from west-east this aft/eve as initially calm or light/variable winds become W/WSW at ~10 knots. Clear skies this morning will become mostly cloudy this afternoon in the Western Piedmont (and at least partly cloudy in the east) as a thick veil of cirrus emanating from a mesoscale convective complex (MCC) over IN/OH/KY advects downstream into the Carolinas via NW flow aloft. Thickening mid/upper level ceilings could affect afternoon temps by a few degrees in the W/NW Piedmont. With the above in mind, expect highs 5-10F warmer than yesterday, ranging from the lower 60s (NE Coastal Plain) to mid/upper 60s elsewhere, warmest in the Sandhills/SW Piedmont. Tonight: Low-level moisture will increase from the W/WNW tonight as the aforementioned front progresses slowly south/east into TN/WV/VA and low-level flow strengthens downstream in the Carolinas. Model guidance is fairly unanimous in showing measurable precipitation in the N/NW Piedmont late tonight (06-12Z Tue). Model guidance is most likely picking up on remnants of the MCC, e.g. the eastern portion of the complex as it progresses S/SE through WV into southwest VA overnight. Alternately, strengthening low-level warm advection may result in marginal/elevated destabilization and a potential for elevated showers at the leading edge of a 925 mb warm front lifting NE from Upstate SC through the Foothills/Western Piedmont of NC between 06-12Z Tue morning. Given the dry airmass (PWAT 0.20-0.40") currently in place east of the mountains, confidence remains low with regard to whether or not sufficient moistening/elevated destabilization and/or sufficient forcing will be present to maintain lingering MCC remnants or support the development of elevated showers late tonight. At this time, will continue to indicate a ~20% of showers after midnight across the W/NW Piedmont. Given broken/overcast cloud cover and a SW breeze, expect lows Tue morning to be much warmer than this morning, in the mid to upper 40s, coldest in the E/NE Coastal Plain. -Vincent && .SHORT TERM /Tuesday and Tuesday Night/... As of 216 PM Monday... A broad surface low and associated cold front progressing SE into the Appalachians tonight will track slowly southward through central NC as a backdoor cold front late Tue aft/eve into Tue night, in the presence of small amplitude shortwave energy traversing the region in NW flow aloft. Aside from elevated convection that may develop over or propagate into portions of central NC early Tue morning, expect dry conditions to otherwise prevail through mid Tue afternoon given weak forcing and a strong capping inversion associated with a pronounced elevated mixed layer (H7-H5 lapse rates ~8c/km) advecting into the Carolinas from the west. Expect highs Tue afternoon in the mid/upper 70s to lower 80s. By late Tue afternoon, convection is expected to develop over and/or propagate into central NC from the west as the cap weakens via diurnal heating and forcing strengthens as the low/cold front progresses southward into central NC and shortwave energy traverses the region in NW flow aloft. The latest guidance suggests the potential for convection will be greatest along/south of Hwy 64 between 21Z Tue afternoon and 06Z Wed morning. Lows Tue night will be driven by cold advection in the wake of the front, ranging from the mid 40s (N) to lower 50s (S). Severe Potential: Very steep mid-level lapse rates, Spring insolation and seasonable low-level moisture (dewpoints rising into the mid/upper 50s) will yield as much as ~1000 J/kg of MLCAPE late Tue aft/eve, in the presence of deep layer shear sufficient for supercellular organization. With the above in mind, deep convection that develops and/or propagates downstream into central NC Tue aft/eve will have the potential to become severe, with a primary threat of large hail and damaging winds. The threat for severe weather will be greatest (relatively speaking) south of the Highway 64 corridor between 5pm and Midnight. -Vincent && .LONG TERM /WEDNESDAY THROUGH MONDAY/...
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As of 325 PM Monday... Surface cold front and attendant axis of deeper moisture will push south of the area by daybreak. Rain/showers should be exiting southern portions of the forecast area by the morning rush hour commute, with NW-SE post-frontal clearing during the late morning/early afternoon, as the leading edge of the strong Canadian high builds in from the north. Highs Wednesday a good 15 to 20 degrees cooler than Tuesday, ranging from lower to mid 50s NE to lower 60s SW. With the cP airmass centered over the area Wednesday night through Thursday night, temperatures through the period will average a good 10 to 15 degrees below normal. Thursday morning looks to be the coldest morning, with most area at or below freezing, lows in the upper 20s to lower 30s. Highs in the 50 to 55. The Nam is a wet outlier in the depiction of spreading precip into the area Wednesday night and into the day on Thursday, in response to shortwave disturbances moving through the area. The remainder of the NWP guidance keeps this overrunning precip suppress south of the area and thus will keep forecast dry. The modified Canadian parent high will shift offshore on Friday, with southerly return flow allowing for a quick moderation to 70 degree readings by Saturday with ridging aloft keeping it dry through Friday night and possibly through much of the day on Saturday. Strengthening warm moist air advection on the eastern periphery of the closed mid/upper level cyclone moving east-northeast into the Ohio Valley will result in increasing chance of showers and possibly a few thunderstorms late Saturday and into the day on Sunday. The closed cyclone is forecast to gradually weaken as it moves east- northeast, and thus the threat for severe weather is minimal at this time. Continued mild with highs in the 60s and 70s. Lows in the upper 40s and 50s.
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&& .AVIATION /18Z Monday through Saturday/... As of 100 PM Monday... 24-hr TAF Period: VFR conditions will rule through the majority if not all of the TAF period, with light/variable winds becoming SSW/SW at 5-10 knots late this afternoon and evening. MVFR/IFR ceilings will be possible at northern terminals (INT/GSO/RDU/RWI) late tonight, however, confidence is too low to warrant mention at this time. Looking Ahead: Expect a good chance for sub-VFR conditions associated with showers/storms in assoc/w a backdoor cold frontal passage late Tue afternoon into Tue night. VFR conditions will return in the wake of the front on Wed. Winds will become N/NE at 10- 15 knots with gusts up to 25 knots in the wake of the front late Tue night. Gust potential will decrease during the day Wed. VFR conditions will prevail Wed/Thu as Canadian high pressure builds into the Carolinas from the north. Sub-VFR ceilings will be possible Friday when southerly return flow develops as high pressure moves offshore the Southeast coast. Widespread adverse aviation conditions will be possible late this weekend in association with a potent upper level disturbance, attendant low pressure system and associated cold front approaching from the west. -Vincent && .RAH WATCHES/WARNINGS/ADVISORIES... None. && $$ SYNOPSIS...Vincent NEAR TERM...Vincent SHORT TERM...Vincent LONG TERM...CBL AVIATION...Vincent

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