Area Forecast Discussion
Issued by NWS Raleigh/Durham, NC

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712 FXUS62 KRAH 290017 AFDRAH Area Forecast Discussion National Weather Service Raleigh NC 817 PM EDT Sun May 28 2017 .SYNOPSIS... In southwesterly flow aloft around a broad trough centered over the upper Great Lakes and southern Ontario, a weakening cold front will settle into and stall over the southeastern U.S., and become increasingly diffuse and modulated by convection, through Mon. Another cold front will cross VA and NC late Wed and Wed night. && .NEAR TERM /THROUGH TONIGHT/... As of 815 PM EDT Sunday... An aggregate cold pool/outflow and apparent weak MCV from convection over TN this morning crossed the srn and central Appalachians and supported the development of scattered storms over the NC Foothills and wrn piedmont during the past several hours, which now stretch from near VUJ to FQD. These storms have generally lacked intensity largely owing to a mixing out of surface dewpoints into the lower 60s and an associated instability minimum characterized by MLCAPE of less than 1000 J/kg. More organized and stronger to severe storms, also immediately downstream of the MCV, developed along a surface trough from sern NC WSWwd through central SC, in an environment characterized by much stronger instability (1500-3000 J/kg) owing to both upper 60s-lower 70s surface dewpoints and the presence of the lingering northern periphery of the EML that resided previously over central NC during the past 36 hours. Some of this latter activity will move across srn Sampson county in a strong state for the next couple of hours, while the fate of the former remains more unclear. Earlier HRRR runs indicated continued multi-cell growth would be possible as the those storms tracked east across the srn Piedmont and Sandhills through this evening, while the past couple of runs show complete dissipation of this activity by 01Z. Given that surface dewpoints have begun to nocturnally- recover into the upper 60s to around 70 degrees in this area (and throughout central NC), and since the convection is expected to track along the moisture gradient accompanying a moisture maxima in the low levels over east-central SC per RAP-initialized 925 mb moisture fields, it seems more reasonable that earlier runs of the HRRR may prove more accurate in their depiction of continued Ewd propagation of this activity across the Srn Piedmont and Sandhills through the next few hours. Gusty, sub-severe winds may result. But in the absence stronger instability that appears will not materialize there, updrafts are not expected to become vigorous enough to interact with relatively strong environmental winds characterized by 40-50 kts of 0-6 km bulk shear, so severe storms are not anticipated there. Elsewhere across central NC, a lack of instability and/or subsidence on the nrn and wrn edge of the aforementioned MCV will likely result in dry conditions tonight. Previously forecast lows in the mid-upper 60s north to lower 70s south remain unchanged. && .SHORT TERM /MONDAY THROUGH TUESDAY NIGHT/... As of 300 PM Sunday... Monday and Monday night: The closed low over the Upper Great Lakes will wobble slowly east on Monday. The primary northern stream shortwave trough is expected to weaken as it lifts northeast away from the area, shearing out across the northern Mid-Atlantic and NE U.S., with attendant trailing sfc cold front expected to stall out across the area late Monday into Monday night. The arrival of this front into the area Monday afternoon/night coupled with individual perturbations ejecting NEWD in the broad cyclonic flow, along the southern fringes of the strong belt of westerlies, will bring another round of showers and thunderstorms to the area that could linger well into the overnight hours as the front begins to stall out. Strong daytime heating within the moist and moderately unstable airmass combined with continued strong mid-level flow that will support favorable/strong deep layer shear of 40-45 kts, will result in another slight risk for severe storms across the area with damaging winds and large hail the primary threats. Storm intensity should weaken significantly with loss of daytime heating, so the severe threat should fall between 2 to 10 pm. Highs in the mid 80s north to lower 90s south. Lows Monday night will depend on the location of the front, ranging from lower/mid 60s NW to lower 70s SE. Tuesday and Tuesday night: As the upper low continues to wobble slowly eastward over southern Ontario/Upper Great Lakes, the quasi- stationary frontal zone and attendant axis of deeper moisture bisecting central NC Tuesday morning will inch ever so slightly east/southeastward through Tuesday night/early Wednesday. Any shortwave energy moving through the upper jet streak extending from the Tn Valley into the Mid-Atlantic region could result in a convective flare-up and locally heavy rain near the front. As such, expect areas south of east of the Triangle to see the highest rain chances Tuesday afternoon and evening. While, deep layer shear won`t be as good as in previous days, 30-35 kts will still be sufficient to support a few strong to severe multicell clusters during peak afternoon heating. Highs ranging from lower to mid 80s north to upper 80/near 90 south. Lows in the lower 60s NW to upper 60s/near 70 south. && .LONG TERM /WEDNESDAY THROUGH SUNDAY/... As of 245 PM EDT Sunday... General model consensus for the mid to late week period is that the closed low over Ontario and associated northern stream trough will get kicked east-northeastward late Wednesday, only to get replaced by a reinforcing closed cyclone diving SE out of central Canada. The eastward progression of the lead trough passage through the NE U.S. and Mid-Atlantic States late Wednesday will push another cold front from the northwest late Wednesday afternoon and evening, which will allow for a continued threat of showers and storms area-wide Wednesday. West-northwesterly flow in the wake of the trough passage on Thursday, may be just enough to push the front south of the region with drier more stable air advecting in from the west, potentially keeping central NC convection free on Thursday. However, these dry conditions look to be short-lived with low-level SWLY flow in advanced of a phased shortwave trough approaching from the west, marking the return of higher humidity and instability, along with the daily chance of mostly diurnal convection Friday and into the weekend. Seasonable temperatures Wednesday and Thursday will return into the above normal ranges by the weekend. && .AVIATION /00Z MONDAY THROUGH THURSDAY/... As of 732 PM EDT Sunday... 24-HR TAF Period: VFR conditions will prevail through the period. Isolated convection could impact all sites over the next several hours before diminishing across the area tonight. Potential at any particular site is too low to mention at this time. Winds will remain from the SW at 5-10 kts. Looking ahead: Aside from a potential for isolated/scattered diurnal convection during the afternoon/evening hours each day, VFR conditions are expected to prevail through mid-week. -Vincent && .RAH WATCHES/WARNINGS/ADVISORIES... None. && $$ SYNOPSIS...MWS NEAR TERM...MWS SHORT TERM...CBL LONG TERM...CBL AVIATION...30/Vincent

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