Area Forecast Discussion
Issued by NWS Raleigh/Durham, NC

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000 FXUS62 KRAH 242358 AFDRAH Area Forecast Discussion National Weather Service Raleigh NC 758 PM EDT Wed May 24 2017 .SYNOPSIS... A warm front will lifting through North Carolina this afternoon and into Virginia this evening. A cold front will move sweep east across North Carolina late tonight and early Thursday morning. A second cold front will move across the region later Thursday. Fair and dry weather will return Thursday night and Friday. && .NEAR TERM /THROUGH TONIGHT/... As of 1115 AM Wednesday... ...Little change in previous thinking... ...Multiple rounds of severe storms possible this afternoon through this evening, with localized flooding possible through tonight... Widespread severe weather possible today and this evening, with several factors appearing to line up ideally to support vigorous convection, including the possibility of discrete super cells this afternoon and a more organized line favoring bowing segments crossing the area late afternoon into the evening. Additionally, with saturated soil and high water in creeks, the risk of heavy downpours producing localized flooding and high water on creeks and rivers persists through the evening. An upper low over the MS Valley this morning will continue to dig south-southeastward toward the Mid-South this afternoon and evening and then into the SE states overnight. Individual strong vort maxes rotating about the deep cyclonic flow will impact the region through Thursday. However, the upper wave of most immediate concern that will impact central NC today and tonight, is currently over the Lower MS Valley, and is forecast to lift NEWD into the Carolinas by tonight. Deep layer lifting with the approach of the wave into the area coupled with a strengthening deep layer wind field will support the development of rapidly developing and robust convection. Three noteworthy features can be seen on the latest surface analysis: a roughly west-east wedge/CAD frontal zone across the northern Piedmont, a warm front lifting into southeastern NC, and a cold front to our west through the W Ohio Valley and lower Miss Valley. The latest high-res models take the wedge front northward gradually this morning, with expectations that the warm/maritime tropical front will rapidly retreat and potentially merge with the wedge front across the central/western piedmont this afternoon. And finally, the west-to-east passage of the cold/occluded front (featuring a triple point low tracking near or just NW of the Triad) very late evening through the overnight hours. Rapid destabilization as the warm sector spreads north and westward and an improving wind field/effective bulk shear increasing to 50 to 55 kts over NC, expect development of discrete northward-moving cells, starting in the southern CWA, by early to mid afternoon. Such development is suggested by the NAM Nest, HRRR, and SPC`s SSEO. The low stratus (NW CWA) and stratocu (SE CWA) will initially limit insolation and destabilization through the morning hours, thus limiting shower coverage early today over central NC, so will start the day with low pops. But the wind field will improve through a deep layer as afternoon approaches, with a 60-80 kt 500 mb jet spreading from the FL panhandle NE through the Carolinas, and a 30- 40 kt cyclonic 850 mb SSW jet over the Southeast nosing into NC. The GFS brings MUCAPE up to 750-1500 J/kg (NW-SE) this afternoon, likely due in part to a batch of higher mid-level lapse rates now over the north-central/northeast Gulf moving NE through GA/SC/NC later today, as well as the low level heating and resident moist low levels. On top of the increasingly favorable kinematics and thermodynamics is the increase in mid level DPVA and upper divergence, the latter associated with an accelerating southerly jet streak extending from SW GA through the eastern Ohio Valley later today. All of this supports the development and maintenance of strong to severe convection this afternoon into this evening, with large hail possible, along with damaging wind gusts. And, with the retreating N Piedmont frontal zone potentially providing a rich source of low level vorticity and enhanced 0-1km SRH of 200 to 400 ms, there will be a chance for a tornado or two, with the greatest threat over the N and W Piedmont. As the cold front shifts eastward late tonight, expect a gradual tapering down of precip chances, and a trend to mostly post- convection stratiform rain with embedded thunder, from WSW to ENE overnight, before ending late. The risk of high rainfall totals possibly leading to flooding is less certain, as the highest PW values have shifted east of the CWA, and we`re unlikely to see the deep saturation through the column that we saw yesterday. But given the wet ground from recent rains, the high water levels on area creeks, and a decent chance of S-N- oriented convective bands leading to training and streaks of high rainfall totals, isolated to localized flooding threats will persist. Will continue the flood watch as is, through much of tonight, areawide. Expect additional rainfall amounts of 1-2", highest in the NW CWA where moist upglide should be strongest. Thicker stratus across the far N and NW CWA associated with the wedge/CAD airmass may be hard to dislodge and thus may hold temps down significantly for much of the day. However, across the south and along the eastern and southern fringes of the central Piedmont, any breaks will cause a quick rise in temps. Highs ranging from around 70 NW to near 80 SE. Lows tonight 59-68 W-E. && .SHORT TERM /THURSDAY THROUGH THURSDAY NIGHT/... As of 115 PM Wednesday... Another convectively active day expected on Thursday although with a different character and a more limited coverage and threat. A vigorous closed mid and upper-level low and upper-level trough axis over the eastern OH and TN valleys on Thursday morning will become negatively tilted and cross central NC between 18 and 00 UTC. At the surface, an initial cold front will extend north to south across the eastern Piedmont of NC at 12 UTC and then push east reaching the coast by 18 UTC as a second cold front reaches the Foothills of NC by 18 UTC and then the coastal region toward 00 UTC. The radar should be relatively quiet during the morning rush hour although some scattered showers are possible along the initial cold front that will be moving from the Piedmont into the Coastal Plain. Ahead of the front, dew points will range in the mid and even upper 60s supportive of convection. As the front exits, dew points will fall into the mid and upper 50s. Ahead of the approaching upper trough, large scale forcing for ascent will increase during the day Thursday. This will be supported by increasingly cold air aloft with 500 MB temperatures of -18 to -20 deg C which combined with steep low level lapse rates will lead to widespread cloudiness and the development of scattered showers and thunderstorms very late in the morning and into the afternoon. Cold air aloft resulting in suppressed freezing levels and an evolving but still robust wind field supporting shallow supercells, will support numerous cells with small hail and a few storms producing larger hail. As the previous shift noted, long and relatively straight forecast hodographs would lead to some splitting supercells and the potential for severe hail and damaging straight line winds. Limiting factor for severe weather will be the amount of instability while high cloud bases will reduce the tornado threat, they will support enhanced downburst winds. The greatest severe weather threat will be from 16 to 22Z, especially across the northern and western Piedmont. Convection will decrease from west to east during the late afternoon as the secondary cold front moves through and the air mass stabilizes. High temperatures will range in the lower 70s northwest to upper 70s southeast. Fair and dry conditions are expected Thursday night as heights rise and a surface high pressure extends into the region from the south. Low temperatures will range in the mid 50s to around 60. -Blaes && .LONG TERM /FRIDAY THROUGH WEDNESDAY/... As of 125 PM Wednesday... In the wake of the strong, negatively-tilted trough axis forecast to exit the Middle Atlantic coast late Thu and Thu night, height rises aloft/ridging and associated subsidence, will sweep to the East Coast and yield dry and mild-warm conditions on Fri, as underlying surface high pressure settles across the southeastern U.S. A warm front will develop ewd across the central Appalachians and into the srn Middle Atlantic states (VA/NC) this weekend, though there remains uncertainty with respect to the precise location, particularly given probable modulation by convection/outflow. Nonetheless, there has been a consistent signal in model guidance in recent days that would suggest a couple of episodes of convection will likely develop along and north of the boundary Sat afternoon through Sun, before the front likely retreats nwd in advance of a weakening cold front forecast to settle into NC on Mon. Some storms may be severe, given relatively strong wind fields, and steep lapse rates related to a large plume of EML that will likely have overspread much of the sern U.S. and srn Middle Atlantic states, around the top of a progressive sub-tropical ridge along the Gulf coast. The next trough aloft will amplify and progress slowly across the central U.S. early-mid next week, with associated surface lows likely to lift across, and occlude over, the Great Lakes. Trailing fronts will consequently weaken as they drift east of the Appalachians and perhaps serve as a focus for primarily diurnal convection. Continued warm, in the 80s. && .AVIATION /00Z THURSDAY THROUGH TUESDAY/... As of 755 PM Wednesday... 24-Hour TAF period: Showers and storms continue to move through Central NC as of the 00Z issuance, lingering longest north and east of KRDU, which includes KRWI and surrounding areas. Light rain continues in and around KINT, KGSO, and KRDU. A lot of uncertainty still exists for the TAF period with respect to cigs and visbys. Largely expect visbys in the MVFR or VFR range with the showers, VFR elsewhere. Cigs are generally VFR, although they may fluctuate between VFR and IFR going from scattered to broken around 15 kft and 25 kft. Winds will be southerly, backing from SSE to SW through the period. Winds may briefly decrease to around 5 kts or so, but otherwise will be 8-12kts with gusts picking up Thursday morning. Additional showers and thunderstorms are possible during the day Thursday, accompanied by the usual reduced cigs and visbys. Looking ahead: Skies will clear out late Thursday night with a return to VFR conditions Friday and Saturday. There will be a chance for more convection late Saturday into early next week as a cold front approaches, however VFR conditions should dominate through Tuesday. && .RAH WATCHES/WARNINGS/ADVISORIES... Flood Watch through late tonight for NCZ007>011-021>028- 038>043-073>078-083>086-088-089. && $$ SYNOPSIS...Blaes NEAR TERM...CBL/HARTFIELD SHORT TERM...BLAES LONG TERM...MWS AVIATION...KC

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