Area Forecast Discussion
Issued by NWS Raleigh/Durham, NC

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000 FXUS62 KRAH 271432 AFDRAH Area Forecast Discussion National Weather Service Raleigh NC 1030 AM EDT Thu Apr 27 2017 .SYNOPSIS... A cold front will approach the mountains from the west late this afternoon and evening, then stall out and dissipate over the Mid- Atlantic tonight. A moist southerly return flow on the western periphery of a strengthening Bermuda high will prevail Friday through the weekend. && .NEAR TERM /through Tonight/... As of 1030 AM Thursday... The 12Z/27th GSO raob. indicated a strong, dry capping inversion near 780mb, while visible imagery, and a glance out the window reveals mostly clear skies east of the Foothills. Big question for this afternoon will be destablization, with RAP soundings and SPC mesoanalysis showing sufficient moistening from the south between 700mb and 925mb to remove the cap, and despite the approach of debris cloudiness from the southern Appalachians, insolation and southerly winds should allow heating to yield at least 500 j/kg MLCAPE. The richest theta e air looks to be immediately ahead of the prefrontal trough and ongoing convection over eastern Alabama, and guidance suggest the mid 60 dewpoint air will make it into thew Piedmont this evening. Could envision a scenario where mid-afternoon convection cross the mountains makes it into the western and northern Piedmont areas as the better jet forcing associated with the shortwave lifting into the Great Lakes passes by, and then some additional showers or isolated storms pop up in the south withing the axis of higher theta e. Thus, there is still some uncertainty in how much of the area will see convection, but the overall trend should be downward in coverage and intensity late this evening and into the overnight. Accordingly, SPC has western NC in a marginal risk for damaging winds. Highs today will be in the upper 70s to mid 80s based on a 1380m thickness at GSO this morning. With increasing low-level moisture, expect overnight lows in the mid/upper 60s. Severe Weather: The MCS progressing through eastern TN this morning is progged to cross the mountains into the western Piedmont invof peak heating this afternoon. The potential for severe weather is difficult to ascertain for several reasons, mainly 1) uncertainty in the evolution of the MCS as it crosses the southern Appalachians and 2) environmental uncertainty associated with rapid modification of thermal profiles progged to occur in advance of the system. With the above in mind, an isolated instance of damaging winds cannot be ruled out in the western Piedmont late this afternoon and early this evening. -Vincent && .SHORT TERM /Friday and Friday Night/... As of 400 AM EDT Thursday... Moderate to extreme destabilization (~3000 J/Kg MLCAPE) is expected during the day Friday in assoc/w strong insolation and seasonable low-level moisture (Td lower 60s) beneath steep mid-level lapse rates /a modified elevated mixed layer/. Despite an anomalously favorable thermodynamic environment, weak forcing and a strong ~800 mb cap may preclude convective development over central NC. Highs will range from the mid 80s (NW) to ~90F (SE). As such, a gradient in CINH (highest west / lowest east) will exist across the state. Unless a well-timed shortwave/MCV in SW flow aloft progresses over western NC or upstream convection propagates downstream into western NC (neither appear likely at this time), dry conditions will prevail. In eastern NC where afternoon temps will reach ~90F and CINH erosion will be maximized, isolated convection cannot be entirely ruled out if sufficient convergence is present (e.g. a robust seabreeze penetrates inland). If the cap breaks and deep convection develops (e.g. updrafts survive entrainment) in the SE Coastal Plain 21-00Z, thermodynamic/kinematic profiles would support robust supercells capable of producing destructive hail/microbursts. If the cap holds, dry conditions will prevail through tonight. Expect lows in the mid 60s to lower 70s. -Vincent && .LONG TERM /Saturday through Wednesday/... As of 345 AM EDT Thursday... The Bermuda high`s influence will be strongest over the area on Saturday with low-level thicknesses progged to peak between 1420m(40m above normal) with H8 temps warming to around 18C. Highs Saturday in the upper 80s to lower 90s on Saturday, when we could challenge the record high max of 91 at RDU. Heights are forecast to lower slightly on Sunday, with highs Sunday similar to Friday`s temps. Rain/convective chances through the weekend will be limited by the dominate ridge with best chance across western NC, where convection is most apt to fire where owing to differential terrain heating. Will retain a slight/small chance pops across the western Piedmont. If storms do develop or propagate east into the area on Saturday, strong instability ~3000 J/Kg aided by steep lapse rates aloft associated with a modified EML, coupled with deep layer shear of ~25kts could support some vigorous updrafts capable of producing large hail and damaging thunderstorm winds. Models have slowed down with the timing of the front on Monday, delaying fropa until after 00z Tuesday. This unfavorable nocturnal timing will not bode well for severe potential, with models suggesting only weak instability across central NC Monday afternoon/evening. When coupled with the better trough dynamics lifting off to the north, the potential for severe storms is low at this time. Cooler/near normal temps will follow Tuesday and Wednesday in the wake of the cold frontal passage Monday night. Late next week, medium range models are actually in good agreement in showing the development of a deep trough east of the MS, with unsettled/active weather expected to accompany this dynamic system late Wednesday night through Friday as it progresses east into the region. && .AVIATION /12Z Thursday through Tuesday/... As of 630 AM EDT Thursday... 24 Hour TAF Period: VFR conditions associated with a dry/capped airmass will prevail through mid to late afternoon. Scattered convection (remnants of a squall line expected to cross the mountains this afternoon) will be possible 20-00Z at INT/GSO terminals and 00-04Z at eastern terminals, with the relative best chance at INT/GSO terminals given an increasingly dry/hostile airmass with eastern extent. SW/WSW wind gusts as high as 20G35KT will be possible in assoc/w any convection. Otherwise, calm or light SSW winds early this morning will increase to 10-15 knots a few hours after sunrise, with gusts as high as 20-25 knots this afternoon (after ~15Z), highest at the INT/GSO terminals. Outlook: Though VFR conditions will prevail through the majority of the upcoming weekend, adverse aviation conditions are periodically expected. Examples include IFR/LIFR ceilings expected to develop each morning Sat/Sun/Mon between 08-12Z, lifting/scattering out to VFR by 12-15Z. Additionally, isolated to scattered afternoon/evening (21-03Z) convection will be possible Fri/Sat. Should convection develop and/or propagate into the region Fri/Sat, environmental conditions would strongly support severe weather. Otherwise, expect the best overall potential for convection Monday evening/night in assoc/w a cold front /squall line/ progged to track eastward through the Carolinas. -Vincent && .RAH WATCHES/WARNINGS/ADVISORIES... None. && $$ SYNOPSIS...Vincent NEAR TERM...Vincent/Smith SHORT TERM...Vincent LONG TERM...CBL AVIATION...Vincent

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