Area Forecast Discussion
Issued by NWS Raleigh/Durham, NC

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000 FXUS62 KRAH 202006 AFDRAH Area Forecast Discussion National Weather Service Raleigh NC 406 PM EDT Sat May 20 2017 .SYNOPSIS...
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A backdoor cold front over northeastern NC this afternoon will move southwest through central NC through early tonight. Following high pressure will ridge south from New England through Sunday. The front will retreat north across our area as a warm front late Sunday through early Monday.
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&& .NEAR TERM /THROUGH TONIGHT/...
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As of 400 PM Saturday... Mid-upper longwave ridging extends from the SW N. Atlantic NWwd across the OH Valley, Great Lakes, and south-central Canada, and will generally remain there through tonight. A shallow tropopause disturbance (b/w 200-300 mb per 12Z RAOB data) within the ridge, now centered near MEB per WV imagery will move slowly Ewd and offshore by tonight. Meanwhile, the MCV referenced earlier has reached the nrn Outer Banks, and will to continue to move SEwd and offshore through the next several hours. At the surface, sub-tropical ridging continues to extend Wwd into the SErn U.S., while a backdoor cold front was analyzed at 20Z from Nrn WV SEwd across west-central VA and nern NC, roughly from LYH-IXA- HSE. A pre-frontal trough/wind shift precedes the front, now only by about 30 miles, and was analyzed along an ROA-TDF-OCW arc. Scattered to numerous storms have already developed along the far ern portion of the pre-frontal trough, likely aided by the aforementioned passing MCV. While hi-res CAM guidance provides mixed signals with respect to subsequent development over central NC through this evening, it seems likely that at least scattered storms will develop within the convergence axis, and area of deepening cumulus, provided by the pre-frontal trough, which will become enhanced by outflow from ongoing convection. Weak tropospheric-deep flow suggests a dis-organized/pulse storm mode will result, with a primary threat of strong to marginally severe downburst winds resulting from a steep low level lapse rate and 20-25 degree sfc dewpoint depression --and related high DCAPE (aoa 1000 J/kg)-- environment. Up to three quarter inch hail will also be possible owing to the aforementioned modest mid level lapse rates of near 6.5 C/km. Tonight: The front, likely by that time augmented by convective outflow, will continue to settle SWwd into Srn NC by around 06Z. Post-frontal low overcast will develop within a few hours of frontal passage, such that a widespread blanket of low clouds will likely be firmly in place by the time the sun rises on Sunday. Lows in the 60s, coolest NE.
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&& .SHORT TERM /SUNDAY AND SUNDAY NIGHT/... As of 345 PM Saturday... The cold front is forecast to have settled to a position just to the SW of the RAH CWA, or perhaps over the far srn Piedmont, by 12Z Sunday, with a following surface ridge axis extending SWwd from a parent 1030 mb high along the nrn Middle Atlantic coast. Associated CAD and widespread low overcast trapped beneath a strong inversion should consequently be firmly entrenched across central NC through early Sunday, with a gradual Nwd retreat of the front, and associated lifting and scattering of the low overcast through the afternoon. There is a relative high degree of confidence that the warm sector will consequently expand across srn NC Sun afternoon, where temperatures will likely warm well into the (mid-upper) 80s; and there is a similarly high degree of confidence that the low overcast and continued cool conditions in the 60s to near 70 degrees will linger over the far nrn Piedmont. In between, however, in or just south of the larger metropolitan areas, there is large temperature bust potential of 15-20 degrees. Precipitation, in the form of light rain/showers, will be favored along and north of the front early Sun, then become favored along it over srn NC, and with an increasingly deep convective character, by Sun afternoon. A series of disturbances in SW flow aloft, including some convectively-generated or enhanced ones from upstream convection over the lower MS Valley, will track across the Carolinas later Sun and Sun night; and these will promote the development and slow Ewd progression of a band or bands of showers and storms into the western half of the RAH CWA overnight, with a half to one inch of rain probable there by 12Z Mon, and with much lower to nil amounts east of U.S. Hwy 1. Otherwise, areas of low overcast will redevelop, or lower where they remain throughout the day Sunday, by late evening. Near steady overnight temperatures in the middle to upper 60s are expected. && .LONG TERM /MONDAY THROUGH SATURDAY/... As of 245 PM EDT Saturday... A long wave trof amplifying over the central CONUS will provide us with active weather featuring a couple of frontal passages with below-normal temperatures and potentially 2+ inches of rain across central NC through mid week. An initial front will be moving across the mountains on Monday, with prefrontal convection increasing across the western piedmont during the morning and shifting to the east by mid afternoon. Thunder will accompany the system, but severe potential is low with only modest shear (30Kts of low level flow) and instability (mid level lapse rates <6K/km in warm southwest flow limiting CAPE to ~500J/kg... mainly in the east). Will have 80% PoPs transitioning across the area with the front and rainfall ranging from 0.5 to 1.0 inches through Monday night, although localized heavier showers accompanying storms could double that amount. Highs will range from the mid 70s west to some very low 80s in the east where frontal passage will be later. Even as the front moves offshore, a mid level wave will lift out of the Gulf in deep southwest flow and ride northeast into the area quickly on Tuesday. Will lean towards higher chance PoPs through Tuesday night considering that both ECMWF and GFS are converging on some variation of this scenario. Post-fropa and with continued heavy cloudiness, expect highs mainly in the mid 70s. A cutoff low digs south into the Tennessee Valley, pushing another front towards the area approaching on Wednesday. Drying and cooling in the mid levels will produce a thermal profile more conducive to strong storms Wednesday into Wednesday night, will bear watching and will bump PoPs up a bit through the period. We`ll see proper dry slotting ahead of the cutoff low Thursday, with potential for wrap around showers/isolated thunder...mainly across the north...as the low lifts into the mid Atlantic Thursday night. Then, finally, a dry forecast for Friday and Saturday as high pressure builds in from the west. Highs Wednesday through Friday will be mainly in the mid to upper 70s...with some airmass moderation on Saturday edging us back towards normal...low to mid 80s. && .AVIATION /18Z SATURDAY THROUGH THURSDAY/... As of 155 PM Saturday... Scattered showers and storms are expected to soon develop along a pre-frontal surface trough axis now draped along an ROA-TDF-RWI-OCW arc; and this convection and associated outflow boundary should then settle southwestward toward RDU by 22Z. Probability of shower/storm occurrence is less at INT/GSO/FAY, since the convection is likely to weaken with time owing mostly to nocturnal cooling. A surface cold front lags the aforementioned pre-frontal trough by about 60 miles; and this boundary and associated leading edge of more marked ENE surface winds will settle southwestward through central NC through around midnight. Of greater operationally significant concern, post-frontal IFR-MVFR ceilings will develop within a few hours of the frontal passage - first around 02-03Z at RWI and last around 07-08Z at FAY. Those ceilings will generally persist through the end of the 18Z TAf period. However, the front will retreat gradually northward as a warm front and result in ceilings lifting to above 2000 thousand feet and/or a scattering to VFR in the FAY vicinity by around 18Z. A similar trend toward high end MVFR ceilings and/or (at least short-lived) scattering to VFR will be possible as the front continues a slow retreat toward the Piedmont TAF sites throughout the afternoon Sunday, but with a redevelopment of IFR ceilings and MVFR-IFR visibility restrictions area-wide shortly after the onset of nocturnal cooling Sunday night. Outlook: Unsettled weather, including periods of rain/convection, and sub-VFR conditions are expected as an upper level trough and preceding moist SWly flow aloft approach from the west, combined with waves of surface low pressure that are forecast to track across the Carolinas early to mid next week. && .RAH WATCHES/WARNINGS/ADVISORIES... None. && $$ SYNOPSIS...MWS NEAR TERM...MWS SHORT TERM...MWS LONG TERM..MLM AVIATION...MWS

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