Area Forecast Discussion
Issued by NWS Raleigh/Durham, NC

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000 FXUS62 KRAH 221755 AFDRAH Area Forecast Discussion National Weather Service Raleigh NC 155 PM EDT Sat Apr 22 2017 .SYNOPSIS... A cold front will settle south through srn VA and nrn NC this afternoon, then south through central NC tonight. The front will stall over southeastern NC by Sunday, in advance of a slow-moving area of low pressure that will track along the coast of the Carolinas through Mon night. && .NEAR TERM /THROUGH TONIGHT/... As of 1145 AM Saturday... Little change in forecast rationale and expected sensible weather through tonight. Downstream of a mid-upper low that will deepen as it tracks from south-central MO to west-central TN/KY by 12Z Sun, a series of low amplitude disturbances in WSW flow aloft will track ene across the srn Middle Atlantic states and Carolinas, with an associated corridor of steady 20-30m/12 hr mid level height falls, through tonight. At the surface, a synoptic cold front stretched from the mouth of the Chesapeake Bay wwd through srn VA, then swwd across central TN/nwrn MS/nrn LA/sern TX. This front --which will become diabatically-enhanced by convection likely to form along both a differential heating cloud band near the VA/NC state line and other convection forced along the Blue Ridge and outflow from earlier TN Valley convection-- will settle slowly swd today, to the nrn NC Piedmont and nrn Coastal Plain late this afternoon and early this evening (between 4 pm and 9 pm), and likely be accompanied by a weak lee surface wave forecast to develop over swrn VA this afternoon. Warm and breezy conditions will continue to develop through early afternoon. Strong diurnal heating, with temperatures comparable to those of Fri (well into the 80s, to around 90 degrees over the Sandhills), and surface dewpoints in the middle 50s to lower 60s and maximized along the aforementioned cold front, will yield weak, to perhaps moderate instability immediately south of the approaching front, by mid afternoon. A relative minimum in low level moisture and MLCAPE will likely manifest over the srn Piedmont and Sandhills, as a pocket of lower 0-1 km mean mixing ratios sampled by the 12Z FFC RAOB advects newd, and downward. As such, the greatest concentration and intensity of resultant showers and storms will occur along the approaching front, focused initially over southside VA, before drifting swd across the NC state line late this afternoon. However, isolated to more widely scattered activity may also fire along any soil/surface moisture gradients resulting from earlier convection Fri-Fri night. 25-40 kts of WSWly mid level flow, strongest north, will favor multi- cell storm modes, and probably a few supercells in the vicinity of the front and preceding the aforementioned weak lee surface wave, where locally backed low level flow and shear will become maximized and yield perhaps 100-150 m2/s2 effective srh. While mean storm motion will be around 210-220/30kts, right-moving storm motion vectors will be closer to 260-270 degrees and consequently be aligned more parallel to the approaching cold front. So while strong to marginally severe wind gusts would be favored with multi-cellular storms, any supercells --near the VA border-- will also be capable of producing hail, and perhaps a tornado owing to updraft propagation contribution from both shear/dynamical influences and surface boundary ones, which would favor storm motion roughly along the relatively helicity-rich front. Post-frontal low overcast, patchy light rain/showers, and and gusty post-frontal NEly winds, will develop behind the synoptic surface front as it settles south through central NC tonight. Showers will become numerous to widespread late tonight, as a vort max now crossing LA nears from the sw. Cooler, with lows in the upper 40s to around 50 degrees nw to around 60 degrees se. && .SHORT TERM /SUNDAY THROUGH MONDAY NIGHT/... As of 400 AM Saturday... This long duration rain event on tap through the short term forecast has the potential to produce widespread 2-3 inches of rain. This will ultimately be beneficial in alleviating the abnormally dry to moderate drought conditions which have been gradually developing through the late winter/spring across central NC. The heavier rain focus shifts from the west initially Sunday to the east on Monday, which will help mitigate the potential for widespread flooding, but some flooding is possible on creeks and streams, and even our larger rivers may have some minor flooding from Sunday night into early next week. As mentioned earlier, the initial focus for heavier rain will be in the western Piedmont on Sunday as low level isentropic lift will be focused on the frontal/CAD zone. Rain will ramp up over the remainder of the forecast area later in the day and peak Sunday night over the Piedmont as low level flow backs southeasterly as the surface low pressure system moves slowly east across South Carolina. The heavier rain then shifts east as the digging cutoff low phases with the surface system Monday morning and begins to lift slowly up the coast through Tuesday. Categorical PoPs through the period with QPF amounts ranging from slightly >3 inches across the north to 2.5 inches in the southeast. CAD-flavored high temperatures in the Triad will struggle into the mid and upper 50s Sunday and Monday with the southeast reaching low 70s. Scattered non-elevated thunderstorms will be possible around the periphery of the wedge, with lesser potential for elevated convection heading into the deeper cool air north and northwest. && .LONG TERM /TUESDAY THROUGH SATURDAY/... As of 400 AM EDT Saturday... Precip chances diminish rapidly Tuesday, with only slight chances in the northeast by late afternoon. Shortwave ridging is expected in the wake of the upper wave on Wednesday with dry conditions and a warming trend through Saturday as an upper level ridge builds over the southeast CONUS. Highs Tuesday will reach 70 to 75, with a return to mid and upper 80s for Friday and Saturday. && .AVIATION /18Z SATURDAY THROUGH THURSDAY/...
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As of 155 PM Saturday... VFR conditions and a warm and breezy SW wind will prevail outside of scattered thunderstorms that will grow in coverage and intensity, -- both greatest over the nrn Piedmont as storms over swrn VA/nwrn NC propagate east along a cold front drifting south across srn VA-- during the next couple hours. MVFR ceilings, showers, and gusty post- frontal NEly winds will develop behind, and with the passage of, the front this evening through early tonight. Those conditions, which may dip into IFR range by Sun morning at Piedmont sites, where coverage of showers will also be greatest, will then likely continue through the day Sun. Outlook: A prolonged period of rain/showers and sub-VFR ceilings and visibilities --in springtime cold air damming-- can be expected through Tue, as an area of low pressure tracks east along the aforementioned front, then slowly up the coast of the Carolinas. Conditions will be slow to improve as the coastal low drifts only slowly away from our region, with a return to VFR late Tue or early Wed.
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&& .RAH WATCHES/WARNINGS/ADVISORIES... None. && $$ SYNOPSIS...MWS NEAR TERM...MWS SHORT TERM...mlm LONG TERM...mlm AVIATION...MWS is the U.S. government's official web portal to all federal, state and local government web resources and services.