Area Forecast Discussion
Issued by NWS Raleigh/Durham, NC

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814 FXUS62 KRAH 211933 AFDRAH Area Forecast Discussion National Weather Service Raleigh NC 230 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/... As of 400 AM Saturday... Unseasonably rich moisture will surge N/NE from the GOMEX into the Carolinas on Sunday as low-level flow backs/strengthens in response to height falls in advance of a potent upper level low progressing east from the lower MS river valley into the Deep South. If an MCS develops over the Deep South tonight, it is possible that convection could affect portions of central NC Sunday morning, though confidence remains low with regard to the evolution/track of upstream convection that may develop later tonight. Otherwise, expect chances for convection to increase from SW-NE Sunday afternoon as small amplitude waves rounding the base of the upper low eject NE from the Deep South into the Carolinas amidst diffluent flow aloft and increasingly favorable thermodynamics as rich low-level moisture advects poleward underneath a modified elevated mixed layer characterized by H7-H5 lapse rates on the order of 7-8 C/km. If sufficient forcing is available to break the cap at the base of the EML and/or discrete convection develops upstream and propagates into central NC during the early/mid afternoon, the thermodynamic/kinematic environment would support supercells capable of producing all severe weather hazards. Otherwise, the primary severe weather threat will not unfold until late Sunday afternoon/evening when a strongly forced squall line is expected to propagate into central NC. Expect highs ranging from the lower 60s (NW) to the mid 70s (SE). -Vincent && .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 /18Z 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...VINCENT LONG TERM...SMITH AVIATION...MWS is the U.S. government's official web portal to all federal, state and local government web resources and services.