Area Forecast Discussion
Issued by NWS Raleigh/Durham, NC

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000 FXUS62 KRAH 230912 AFDRAH Area Forecast Discussion National Weather Service Raleigh NC 412 AM EST Tue Jan 23 2018 .SYNOPSIS... A lead cold front and markedly drier air will cross the Carolinas and Virginia early today, followed by a secondary cold front, with noticeably cooler conditions, this evening. Pacific high pressure will then build across the region through mid to late week. && .NEAR TERM /TODAY/... As of 400 AM Tuesday... Modified discussion due to unfolding low, but non-zero risk of high shear low CAPE severe convective episode. Current convection has organized along a surface confluence axis resulting in part due to strong low level jet-driven WAA --and associated underlying height falls-- beneath strengthening sswly/ swly deep tropospheric flow that includes 50 kts as lows as 1500- 2000 ft, per regional VWP and model analysis data this morning. Associated shear, both in the lowest few kilometers and above, is extreme. However, generally moist adiabatic/neutral thermodynamic profiles, above a still-slightly stable near surface layer, are supportive of only weak instability this morning; though this instability will likely increase into the 200-400 J/kg range and become increasingly surface-based as lwr 60s surface dewpoints over SC continue to advect nwd, immediately in advance of a lead cold front, analyzed at 09Z from the cntl FL panhandle and far sern AL newd across nern GA and far swrn NC, and which will cross cntl NC later this morning, through the midday to early afternoon hours over the Coastal Plain. Weak diabatic diurnal heating potential will also exist beforehand, though limited by multi-layered overcast. Both an isolated tornado and strong to locally damaging wind gusts will be possible with this lead activity this morning, when the character of the shear will favor cyclonically-curved hodographs. The (marginal) tornado potential will decrease with subsequent convection that will probably organize into a broken line along the cold front, as the llj axis peels away and surface winds veer to a more sswly component, thereby straightening forecast hodographs. The passage of this cold front, one that may be better characterized as a dry line, will result in clearing; drying in the low levels; and the deepening of a diurnal mixed layer that will promote the development of strong sswly winds and associated gusts that will peak in the 30s kts for several hours immediately behind the front, with winds then lessening thereafter. Any appreciable cooling will lag this feature and not occur until a secondary front, one analyzed at 09Z from wrn PA swd across cntl WV, settles sewd across cntl NC after 21Z. High temperatures will consequently again climb into the 60s, to lwr 70s over the ern CWFA. && .SHORT TERM /TONIGHT/... As of 410 AM Tuesday... Mostly CAA-driven lows, though with brief radiational cooling potential over the wrn Piedmont, are expected to dip into the lwr to mid 30s for most. Skies will be mainly clear, aside from srn stream upr jet-induced cirrus that will nose nwd across SC, and into at least srn NC, after midnight. && .LONG TERM /WEDNESDAY THROUGH TUESDAY/... As of 145 AM Tuesday... Wednesday through Saturday: Expect dry weather through Saturday. An upper level shortwave and a reinforcing dry cold frontal passage is expected Wednesday night/Thursday. Surface high pressure is expected to build into the area in the wake of the secondary cold front and remain over the area into Saturday. Meanwhile, the upper level ridge will move eastward over the Carolinas through Friday night. Highs will be generally in the upper 40s north to mid 50s south through Wednesday and Thursday, with moderation beginning Friday. Lows in the upper 20s to low 30s Wednesday and Thursday nights, increasing Friday night. Saturday Night through Tuesday: Another upper level trough and surface cold front will develop over the Midwest and progress eastward toward the Mid-Atlantic over the weekend. As southwesterly flow increases, so will the warm, moist advection into the region. As a result, cloud cover and chances for precipitation will increase late Saturday into Sunday as the cold front approaches. Some differences in the evolution of the upper level trough and the timing and duration of convection as it moves into/through Central NC have developed between the medium-range models. The ECMWF is more amplified, slower, and wetter than the GFS. As a result of the latest forecast cycle model differences, confidence in the timing of the front and convection associated with it is low. && .AVIATION /06Z TUESDAY THROUGH SATURDAY/... As of 130 AM Tuesday... Generally IFR-MVFR ceilings, and MVFR visibility restrictions in showers and isolated thunder, will become VFR with the passage of a lead cold front from west to east between 12-14Z at wrn sites and 15- 17Z at ern ones. A strong sswly low level jet, on the order of 40-50 kts as low as 1500-2000 ft, with progressively stronger winds above that level, will continue to migrate newd across cntl NC this morning. The presence of this feature will support the continued newd development of the aforementioned ceilings and convection, and the maintenance of low level wind shear conditions this morning. While some of this stronger flow and momentum aloft will mix to the surface and manifest as a breezy sly surface wind, the magnitude of the flow aloft suggests llws nonetheless remains warranted until the aforementioned cold front crosses the region later this morning. Behind that front, abrupt clearing will occur, with a veering of surface winds to swly and similarly abrupt increase into the 15-25 kt range (sustained), with gusts to around 35 kts for several hours immediately following the frontal passage. These winds will then lessen and continue to veer to wly or nwly later this afternoon and evening. Outlook: MVFR ceilings possible Sat, with showers Sat night, ahead of, and in association with, a weekend frontal system. && .RAH WATCHES/WARNINGS/ADVISORIES... None. && $$ SYNOPSIS...MWS NEAR TERM...MWS SHORT TERM...MWS LONG TERM...KC AVIATION...MWS

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