Area Forecast Discussion
Issued by NWS Raleigh/Durham, NC

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000 FXUS62 KRAH 201849 AFDRAH Area Forecast Discussion National Weather Service Raleigh NC 245 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/... As of 400 AM Saturday... Strengthening southerly moist warm air advection spreading in from the west late Sunday afternoon and into the evening as the upper ridge shifts east of the area in response to the upstream closed low moving NE into the Upper Great Lakes and southern Ontario, will lead to increasing shower chances and possibly even some thunder across the far western/southern zones, concurrent with the scouring of weak cool air damming as the stalled front to our south retreats northward as a warm front. The better rain chances for our appear to hold off until after 00z Monday, so the daylight hours on Sunday should be mostly dry for much of central NC. The evolution/demise of warm season CAD event will make for another challenging temperature forecast for Sunday, with the GFS and NAM much slower than the EC in lifting the warm front northward Sunday afternoon/evening. Difference in forecast highs between the US and EC guidance is as much as 10 to 15 degrees Fahrenheit across the Piedmont and NE coastal plain, with Nam and GFS suggesting temperatures will struggle to warm out of the 60s across the NC Piedmont and northern coastal plain. Will split the difference for now, with highs ranging from lower 70s north to lower 80s south. Lows Sunday night 65 to 70. && .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 Wedneday 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 /12Z SATURDAY THROUGH WEDNESDAY/...
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As of 745 AM Saturday... Fog/stratus across the far SE zones has already dissipated with VFR conditions expected through the early afternoon. A back-door cold front will sag south into the area late this afternoon and into the evening. Scattered showers and storms will accompany the front, impacting RWI around 21Z, and KRDU, KGSO and KINT around 00z. Expect a brief period of post-frontal NELY wind gusts, with ceilings deteriorate rapidly, lowering to IFR-MVFR between 00 to 03z across the north to 06-09z at KFAY. Outlook: The back-door cold front will stall just south of the area on Sunday, before retreating NEwd as a warm front Sunday night. Ceilings are expected to remain sub-VFR through much of the day on Sunday with showers overspreading the area Sunday night as the warm front lifts north across the area. Another cold front, numerous showers and storms, and sub-VFR conditions, will move east across central NC Mon through early Mon night, followed by similar conditions in association with another frontal system on Wed.
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&& .RAH WATCHES/WARNINGS/ADVISORIES... None. && $$ SYNOPSIS...MWS NEAR TERM...MWS SHORT TERM...cbl LONG TERM..MLM AVIATION...CBL

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