Area Forecast Discussion
Issued by NWS Raleigh/Durham, NC

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000 FXUS62 KRAH 260823 AFDRAH Area Forecast Discussion National Weather Service Raleigh NC 422 AM EDT Fri May 26 2017 .SYNOPSIS... High pressure will extend across the Southeast states today into Saturday, as a weak trough extends down through western and central North Carolina. An upper level disturbance will cross the region late Saturday through Saturday night, followed by another disturbance Sunday afternoon. && .NEAR TERM /THROUGH TONIGHT/... As of 400 AM Friday... Lingering cyclonic flow aloft over the ern U.S., behind a powerful upper low now lifting up the Middle Atlantic coast, will flatten and become more zonal/Wly as a shortwave ridge upstream over the MS Valley dampens. Before it does so, a brief period of orographic cirrus will be possible mainly over the nwrn Piedmont between 12- 18Z, as a shield of thin, high level moisture now over the mid-South and upper Midwest streams east of the Appalachians. Skies will otherwise be mostly sunny, with just a smattering of flat cu at the base of a stout subsidence inversion forecast to develop down to about 6-7 thousand ft. A swly breeze will become gusty into the 20- 25 mph range through early afternoon, in the MSL pressure gradient between departing, sub-1000 mb surface low pressure accompanying the aforementioned upper low, and a weak 1017 mb ridge centered over the ern GOM and FL. Warm, with highs mostly 80 to 85 degrees, but with low humidity values courtesy of dewpoints in the 50s. Tonight: An MCV now approaching nwrn MO will likely help initiate, amidst a broad low-mid level WAA regime, another cluster of showers and storms across the upper Midwest and OH Valley today; and some of this activity will drift in a weakening state into the VA`s overnight. Associated mid-high level debris cloudiness may overspread central NC after midnight, with lows generally in the lower 60s. && .SHORT TERM /SATURDAY AND SATURDAY NIGHT/...
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As of 420 AM Friday... In generally Wly flow aloft, around sub-tropical high pressure forecast to migrate across the Gulf coast states, a large plume of hot and dry air (EML), well-sampled by the 00Z RAObs over the ern Rockies and central-srn Plains (see Albuquerque and Amarillo), will have overspread the southeastern U.S. by 12Z Sat. While this will result in mainly hot and dry conditions over central NC, it will also prove a favorable thermodynamic environment for severe storms where moisture streaming in WSWly 850 mb flow beneath it is lifted into it, in a WAA regime and across a weak frontal zone across the central Appalachians. That frontal zone will have already been convectively-modulated by overnight convection that may have drifted into the VA`s, as noted above. This pattern, similar to that of a couple of weeks ago on May 11, will be supportive of the development of a cluster or two of storms to develop and propagate across the central Appalachians and (probably) VA and nrn NC Sat afternoon into Sat night. 35-45 kts of bulk shear, and 150-250 0-3 km SRH will be supportive of an organized convective mode that may include a small forward propagating MCS with embedded supercells. A risk of all severe weather hazards would such a storm mode, but with tornado potential focused along and immediately north of the effective frontal zone that will likely extend just to the north of the RAH forecast area, or perhaps settle into the nrn Piedmont and nrn Coastal Plain. The precise frontal location remains of low predictability, however, given 1) uncertainties in the amplitude of upstream MCV`s and how they may dampen/amplify the flow and 2) probable convective/diabatic modulation. The storms will likely weaken with time and srn extent as they move away from the frontal zone/low level moist axis and into the hot and dry, capped EML. That dry air will favor strong evaporational cooling potential and relatively significant outflow winds (35-45 mph) surging well ahead of the dying storms, however. Hot, with highs in the upper 80s to lower 90s. Lows, in an increasingly humid regime, in the middle to upper 60s.
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&& .LONG TERM /Sunday through Thursday/... As of 315 AM Friday... Sun/Sun night: Tough forecast behind the MCV shifting off the Mid Atlantic coast late Sat night. Models indicate a weak shortwave ridge shifting over the area from the west, with a warm 850-700 mb layer, high clouds to limit insolation (likely convective debris clouds resulting from late-Sat storm complex over the mid Miss Valley), and weak mid level lapse rates during the first half of the day. Models show an eventual recovery of this air mass through the day, with low level warming and mixing allowing for late-day destabilization to 500-2000 J/kg of MUCAPE (highest SW, lowest NE) and deep layer shear of 45-50 kts. And an expected cluster of convection over TN early Sat may work with this destabilization to prompt organized storms over central NC late Sun, a development shared by the NAM/NAM Nest/GFS/ECMWF. After a mostly dry morning, will bring in scattered to numerous showers/storms late Sun into Sun evening. Highs 84-90. Convection chances should dwindle Sun night with models showing a batch of drier air between the exiting storms and well out ahead of the approaching mid level trough, which should feature strongly forced convection from the eastern Great Lakes through the mid/lower Miss Valley to TX. Warm lows of 66-72. Mon/Mon night: A mid level shear axis and accompanying cold front and convective band will approach from the W and NW Mon, as our steering flow gradually becomes more cyclonic as a deep mid level low wobbles eastward over southern Ontario. Deep layer shear and CAPE values are both expected to improve through Mon, especially along and east of Highway 1 where the GFS depicts 1500-2500 J/kg of MUCAPE and 40-50 kts of 0-6 km shear Mon afternoon. Will bring in high pops, especially central/east, during the day, trailing off late Mon night as the surface front pushes to coastal and SE NC. Thicknesses remain elevated above normals, favoring highs of 84-91. Lows 63-70 with some cooling in the NW behind the front. Tue-Thu: Mid level flow decreases and flattens a bit as heights rise over the St Lawrence Valley region, and with the diffuse surface front having settled near the eastern and southern edges of the CWA Tue and much lower PW advecting into the NW, will carry chance pops in the SE CWA only Tue. As the mid level trough axis shifts eastward through the Mid Atlantic and Northeast by early Wed, the below- normal PW and weak mid level lapse rates should continue to spread in from the west, and will have a dry forecast for Wed/Thu as weak surface high pressure builds over the region with rising heights and mid level subsidence dominating. Highs mostly in the lower to mid 80s, near seasonal normals. -GIH && .AVIATION /00Z FRIDAY THROUGH TUESDAY/... As of 220 AM Friday... Strong subsidence, behind the upper level trough and accompanying hail storms that crossed central NC on Thu, will result in VFR conditions through the 24 hour TAF period. SWly surface winds will be the primary aviation concern as they increase into the low to mid teens kts, with gusts around 20 kts, mainly between 14-20Z - highest east/at RWI. Outlook: At least a couple of clusters of showers and storms, some severe, will return to portions of central NC this weekend - one mainly over the northern half of the forecast area but with gusty outflow winds surging well away from them late Sat and early Sat night, then another on Sun. The approach and stall of a couple of cold fronts east of the Appalachians will result in a small chance of diurnal convection Mon and Tue afternoon. && .RAH WATCHES/WARNINGS/ADVISORIES... None. && $$ SYNOPSIS...Hartfield NEAR TERM...MWS SHORT TERM...MWS LONG TERM...Hartfield AVIATION...MWS

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