Area Forecast Discussion
Issued by NWS Raleigh/Durham, NC

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000 FXUS62 KRAH 212023 AFDRAH Area Forecast Discussion National Weather Service Raleigh NC 323 PM EST Sat Jan 21 2017 .SYNOPSIS... A mesoscale convective system and associated convectively- enhanced surface low will track across the Carolinas this afternoon and early tonight, before lifting away from the Middle Atlantic coast late tonight. A potent upper level low and attendant low pressure system will track northeast from the Deep South into the Carolinas on Sunday, linger over the area on Monday, then gradually track offshore the Mid- Atlantic coast on Tuesday. && .NEAR TERM /THROUGH TONIGHT/... As of 1125 AM Saturday... Visibility in fog will continue to gradually improve as the earlier widespread dense fog lifts into a very low overcast through early this afternoon. Given this low overcast that is expected to persist, temperatures will not recover more than 4-8 degrees from 11 AM readings. Mainly dry conditions are expected through mid afternoon in central NC. A large MCS (currently in GA) is expected to track NEwd through the upstate of SC early this afternoon, then track NE/ENE across central NC this evening. Warm advection, DPVA associated with a robust MCV and above normal moisture (PWAT ~1.50") will help maintain the MCS as it tracks into/across central NC. At the same time, a predominately stable airmass characterized by weak low/mid-level lapse rates and a near-sfc stable layer should temper the intensity of the system and keep convection elevated, precluding a severe potential, though lightning cannot be ruled out. Precipitation chances will be greatest between ~20Z this afternoon and midnight (~05Z) tonight, when warm-frontal precipitation --downstream of the approaching convectively-enhanced surface low related to the MCS-- and rainfall amounts of ~0.5-1.00", with locally higher amounts, should be maximized. The low overcast will likely settle toward the surface and result in (possibly dense) fog this evening, especially N of U.S. Hwy 64, where an in-situ wedge airmass is apt to linger. Lows in the mid 50s tonight. && .SHORT TERM /SUNDAY THROUGH SUNDAY NIGHT/...
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As of 320 PM Saturday... ...Severe storms possible, especially over southern and eastern NC, late Sunday.... There should be ongoing showers and possibly a few thunderstorms Sunday morning, associated with increasing upper diffluence ahead of a deepening upper trough over the Deep South and possibly an MCV from Lower Miss Valley convection tonight. Given the stable airmass over central NC today will hold tonight, the convection should be elevated and not pose a severe threat. The biggest concern will be how the early morning precip impacts destablization during the afternoon, as the upper trough closes off and a surface low deepens toward the southern Appalachians. The primary surface low will initially be over the mid-Miss Valley region, while a secondary low develops over northern Georgia. The two lows are then forecast to consolidate and deepen sub-990mb (-4 to -5 STD) somewhere in the vicinity of the TN/NC/VA border area Sunday night, an fairly typical pattern. Thus, confidence is still below average in the details of potential severe weather on Sunday evening. Strong upper level forcing will arrive after 20Z under increasing diffluence and 100+ meter height falls, not to mention the couple left exit/right entrance regions of the upper jet. Convection will increase from the south during the evening as low-level warm advection increases and a 50kt LLJ crosses SC and southeast NC. It seems pretty certain that a QLCS will cross northern FL, GA and SC, and the question continues to be how far north the line will extend as the warm front penetrates inland in the lingering wedge airmass over central NC. The deepening of the surface low just west of the mountains and the isallobaric response should help to draw the low/mid 60 dewpoint air into the the Piedmont, at least as far as US-1, resulting MLCAPE that could approach 1000 J/KG. This combined with a 100kt flow at 500mb and strong backed low-level flow would support some discrete supercells and a straight line wind/tornado threat, especially in the vicinity of the retreating warm front. It is also possible that the QLCS to our south disrupts moisture transport and mass fields to the north, and renders convection less robust. The main takeaway is that there is a lot in place for severe weather, but tonight`s precip and the mesoscale evolution to our south still leads to a fair amount of uncertainty. QPF on Sunday and Sunday night will be heavy in spots given the unseasonably moist airmass, but widespread flooding is not expected. One area to watch may be the northwest Piedmont Sunday night as the upper low pivots over the mountains and moderate is is potentially sustained for several hours. otherwise, the bulk of the deep convection is expected to depart to the east by 06Z Monday
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&& .LONG TERM /MONDAY THROUGH SATURDAY/... As of 230 PM Saturday... The departing upper low will give way to height rises on Tuesday, with a brief stint of high pressure at the surface. The ridging aloft will quickly give way to increasingly cyclonic flow aloft as a broad trough develops over the eastern US through the end of the week. A shortwave moving out of the Southwest is forecast to lift and shear across the Midwest States Wednesday/Thursday, resulting in limited moisture/lift over our region as the cold front crosses the area early Thursday. The GFS and ECWMF are in pretty good agreement on the timing of the front, but the GFS brings a secondary wave through the base of the trough, resulting in anafrontal precip on Thursday. Confidence in the longwave pattern and trend from above normal temps Tuesday/Wednesday toward below normal is high, but the details of the midweek precip are still uncertain. && .AVIATION /20Z SATURDAY THROUGH THURSDAY/... As of 125 PM Saturday... Poor aviation conditions will continue through the 18Z TAF period, and beyond through the remainder of the weekend... Widespread LIFR ceilings and mostly IFR-MVFR visibility restrictions may briefly improve by a category this afternoon, as a warm front now stretching from SErn NC to central SC to a surface low pressure over central GA --and following lifting and scattering of low clouds- - briefly attempts to retreat Nwd this afternoon. However, an area of warm frontal showers downstream of the surface low will lift NEwd into central NC between 20-23Z, and this will generally result in lowering ceilings and visibilities again through the late afternoon and early evening hours. These showers are expected to continue NEwd and out of central NC, along with the aforementioned surface low, between 02-04Z, with mostly LIFR-IFR conditions expected to remain throughout the night and first half of Sunday. There may also be a brief period of low level wind shear early tonight (02-06Z) at RDU/FAY/RWI, as a generally Sly low level jet surges Nwd ahead of the surface low, atop lingering light surface winds. Outlook: Generally sub-VFR conditions will persist through Sunday night, as a powerful upper level low and deepening sfc low pressure system track across the TN Valley and Srn Middle Atlantic states. However, a brief lifting to MVFR or scattering to VFR will be possible south of the Triad terminals early Sunday afternoon, when a warm front is expected to retreat farther Nwd into central NC. There will also be episodes of convection, focused around both midday Sunday and again Sunday evening, the latter of which could become severe and produce damaging sfc wind gusts and perhaps an isolated tornado or two. The powerful upper level low and attendant sfc low will linger over the region Monday and result sub-VFR ceilings and scattered convection. Improvement should begin by Tuesday as the system tracks toward the Mid-Atlantic coast, though gusty NW winds are likely Tue afternoon. && .RAH WATCHES/WARNINGS/ADVISORIES... None. && $$ SYNOPSIS...MWS/VINCENT NEAR TERM...MWS/VINCENT SHORT TERM...SMITH LONG TERM...SMITH AVIATION...MWS

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