Area Forecast Discussion
Issued by NWS Raleigh/Durham, NC

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000 FXUS62 KRAH 221919 AFDRAH Area Forecast Discussion National Weather Service Raleigh NC 319 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/...
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As of 315 PM Saturday... Severe Thunderstorm Watch #157 in effect for the nrn Piedmont and Coastal Plain of central NC until 9 pm EDT. A well-defined mid level vort max/low now over east-central MO will deepen and reform sewd/across the Mid-South by 12Z Sun, as a trough axis with embedded perturbations that trails wwd to the central Rockies pivots and amplifies to the srn Plains and lower MS Valley. On the se fringe of this larger-scale trough, a series of low amplitude disturbances in WSW to SW flow aloft will track across the srn Middle Atlantic states and Carolinas, with an associated corridor of 20-30m/12 hr mid level height falls and periods of enhanced forcing for ascent, through tonight. Three notable such disturbances were located 1) near the Tri-Cities, 2) over nern GA, and 3) over central MS - the first two of which will enhance ascent over west and north-central NC through early this evening, and the last later tonight. At the surface, a synoptic cold front stretched from the mouth of the Chesapeake Bay wwd through srn VA, to a 1008 mb Appalachian-lee low near South Boston VA (where not coincidentally a TOR-warned storm has recently developed), then w and swwd across central TN/nwrn MS/n-central LA/sern TX. This front --which will become diabatically-enhanced by convection now forming 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 swd to the nrn NC Piedmont and nrn Coastal Plain late this afternoon and early this evening (between 5 pm and 9 pm). Strong diurnal/diabatic 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 MLCAPE immediately south of the approaching front for the next several hours, despite only modest mid-level lapse rates. 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. Indeed, surface dewpoints have mixed into the middle 50s there during the past couple of hours. As such, the greatest convective concentration and intensity will occur along and just ahead of the approaching front, focused at least initially near the VA/NC state line this afternoon. Elsewhere, isolated to widely scattered activity will probably continue to percolate, but the aforementioned mixing out of low level moisture and a lack of any notable surface trigger, will tend limit coverage and intensity. 40-50 kts of WSWly effective bulk shear and locally backed flow/enhanced low level shear in the vicinity of the front and preceding the aforementioned weak lee surface wave will yield 100- 150 m2/s2 effective SRH; and the combination of both will favor supercells amidst the aforementioned 1000-1500 J/kg MLCAPE. 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 cold front. All severe weather hazards, including damaging straight line winds, hail up to one and a half inches in diameter, and perhaps a tornado or two owing to updraft propagation contribution from both shear/dynamical influences and surface boundary ones, which would favor storm motion and relatively long residence times along the corridor of relatively helicity-rich front. Post-frontal low overcast, showers, 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, particularly over the nwrn half of the CWFA, where both influence from the aforementioned central MS disturbance and 925-850 mb frontogenesis will become maximized. Cooler, with lows in the upper 40s to around 50 degrees nw to around 60 degrees se.
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&& .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/... 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. && .RAH WATCHES/WARNINGS/ADVISORIES... None. && $$ SYNOPSIS...MWS NEAR TERM...MWS SHORT TERM...mlm LONG TERM...mlm AVIATION...MWS

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