Area Forecast Discussion
Issued by NWS Raleigh/Durham, NC

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000 FXUS62 KRAH 201949 AFDRAH Area Forecast Discussion National Weather Service Raleigh NC 349 PM EDT Sat May 20 2017 .SYNOPSIS... A backdoor cold front over east-central VA this morning will move southwest through NC late this afternoon through 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. && .NEAR TERM /THROUGH TONIGHT/... As of 1100 AM 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 (between 200-300 mb per 12Z RAOB data) within the ridge --centered over srn NC and nrn SC-- and evident in WV imagery this morning, will move slowly Ewd and offshore by tonight. Meanwhile, visible satellite imagery and model-analyzed mid level vorticity fields depict an MCV over Northampton Co. NC, and this feature and associated mid-high level clouds is forecast to continue moving SEwd, down the E side of the aforementioned ridge axis aloft, to the Outer Banks by mid-late afternoon. While the presence of the mid level ridge, subsident wake of the MCV, and minimal influence by the shallow tropopause disturbance all suggest an unfavorable environment for deep convection, observed proximity RAOBs and model forecast soundings both reveal no evidence of subsident warming/capping; and in fact, instead depict modest mid level lapse rates on the order of around 6.5 C/km. At the surface, sub-tropical ridging continues to extend Wwd into the SErn U.S., while a backdoor cold front was analyzed at 14Z from Nrn WV SEwd across central VA roughly along I-64 (from CHO-RIC-PHF). A pre-frontal trough/wind shift precedes the front by about 60 miles, and was analyzed along an ROA-CXE-ONX arc. Lastly, an outflow boundary and associated undular bore/stable wave clouds were evident in both visible satellite and regional radar imagery over Srn NC this morning, but uniform diurnal heating on both sides of this boundary suggests it will likely be a non-factor relative to convective initiation this afternoon. Instead, scattered convection is likely to focus along the convergence axes along both the aforementioned pre-frontal trough and cold front as both settle SWwd into Nrn and NErn NC, coincident with peak diurnal heating and within an associated moderate instability axis amidst low level moisture pooling, roughly along and N of U.S. Hwy 64, through this afternoon. 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. Since the actual cold front is not expected to make inroads into even the NErn portion of the RAH CWA until near peak heating, high temperatures have been raised to meet, or exceed by a couple of degrees, those of Fri. Tonight: The front, likely by that time augmented by convective outflow, will continue to settle SWwd into Srn NC by around midnight. 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. && .SHORT TERM /SUNDAY AND SUNDAY NIGHT/...
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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.
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&& .LONG TERM /MONDAY THROUGH SATURDAY/...
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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.
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&& .AVIATION /18Z SATURDAY THROUGH THURSDAY/...
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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.
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&& .RAH WATCHES/WARNINGS/ADVISORIES... None. && $$ SYNOPSIS...MWS NEAR TERM...MWS SHORT TERM...MWS LONG TERM..MLM AVIATION...MWS

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