Area Forecast Discussion
Issued by NWS Raleigh/Durham, NC

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000 FXUS62 KRAH 211126 AFDRAH Area Forecast Discussion National Weather Service Raleigh NC 726 AM EDT Fri Apr 21 2017 .SYNOPSIS... A pre-frontal surface trough will strengthen over west-central NC today, then linger tonight. Meanwhile, a cold front will approach from the northwest and move into NC on Sat. The front will settle slowly south Sat night and Sun, in advance of a slow-moving storm system will cross the Carolinas through Monday. && .NEAR TERM /THROUGH TONIGHT/... As of 350 AM Friday... Generally Wly flow aloft will prevail through tonight, between a sub- tropical ridge over the GOM/FL and a mid-upper low and trailing positive tilt trough axis now centered over the upper Great Lakes and srn Ontario. At the surface, a cold front trails from a surface low over wrn NY this morning, SWwd to WSWwd across the lower OH Valley/Mid-South/srn Plains; and this front will drift into the VA`s by this evening. Meanwhile, a pre-frontal surface trough or two will sharpen over west-central NC this afternoon, then linger through tonight. Weak height falls aloft (10-20 M/12 hr) and low level convergence invof the surface trough(s) will support the development of scattered convection, within a weakly to moderately unstable and uncapped environment over NC, owing in part to very warm temperatures in the 80s (to near 90 degrees over the Sandhills) and surface dewpoints in the upper 50s and lower 60s. Associated 25-30 C surface dewpoint depressions will favor strong to marginally severe wind gusts in mostly multi-cellular convection, as NC will remain on the srn fringe of stronger mid level flow over the OH Valley/Middle Atlantic states (Ie. effective shear values ranging from about 20-30 kts in central NC - strongest near the VA border). Associated outflow will likely drift across the remainder of central NC and provide a focus for a slight chance of showers and storms overnight, while the synoptic frontal zone slows over srn VA. Mild, and somewhat humid, tonight, with lows mainly 60 to 65. && .SHORT TERM /SATURDAY AND SATURDAY NIGHT/... As of 415 AM Friday... A strong NWly mid-upper jet from the Pac NW to the Four Corners will help dig a trough/developing closed low into the central Plains/mid MS Valley vicinity by early Sat. Upstream perturbations embedded within the strong NW flow will pivot around the developing mid-upper low in positive tilt fashion across the Mid-South/lower MS Valley through Sat night. An associated surface cyclone will track slowly ENEwd to Ewd across the TN valley and into the srn Appalachians by Sun morning. The front initially over srn VA will likely sag Swd into at least the nrn NC Piedmont Sat afternoon, with an associated focus for diurnally-enhanced convection along and N of the front. WSWly mid level flow will have increased into the 30-40 kt range over central NC by Sat afternoon, downstream of the aforementioned approaching upper low and trailing positive tilt trough. Mostly moderate instability during the afternoon in the presence of this enhanced mid level flow will favor semi-organized multi-cell storm modes, and perhaps at least transient supercells in the vicinity of the front/outflow --and enhanced low level shear-- axis. It should again be warm south of the front Sat, with temperatures comparable to those of Fri, while cooler 70s are expected over the nrn Piedmont and nrn Coastal Plain. The synoptic surface front is likely to become quasi-stationary and bisect central NC from W to E Sat night, though outflow from earlier convection may serve to shove an associated effective front into srn NC by SUn morning. The focus for showers and storms, and low overcast, will remain along and N of the synoptic/deeper front. Lows in the low to mid 50s N to lower 60s S. && .LONG TERM /SUNDAY THROUGH FRIDAY/... As of 335 AM Friday... While confidence remains low concerning finer details such as the timing and location of the frontal zone and surface low, consensus on the track of the upper low off the South Carolina coast Monday night does provide higher confidence in a widespread long-duration rain event Sunday and Monday with wrap around showers lingering into Tuesday. Storm total rainfall amounts for the entire event... with the bulk of the rain spanning ~48 hours...should easily reach 1 to 1.25 inches, with potential for 2+ inches in the west. Duration and intensity of the rain could cause minor flooding on creeks and streams from Sunday onward, and if we receive much more than 2 inches, we would likely see minor flooding on the larger rivers as early as Sunday night and into early next week. Cool air remains dammed into the region Sunday and Sunday night, with strong isentropic upglide...especially from the Piedmont westward, and will increase PoPs to a minimum of 60% and downplay the chance of thunder...which would be elevated considering the depth of cool air and minimal instability over all but the far southern counties. Guidance highs have been trending downward as well, and there will likely be a strong gradient from upper 50s north to mid or even upper 70s across the south. Mins Sunday night will be in the 50s. The surface and upper system become better aligned south of the area on Monday, and the ensuing deep northeast flow, along with the rain and cloudiness, will produce highs mostly in the mid 60s. Showers will be tapering off in the west Monday night as the upper trof bottoms out south of Cape Lookout and begins to lift up the coast with showers diminishing over all but the northeast by Tuesday afternoon. Highs Tuesday will reach lower 70s in the south, while northeast flow limits highs in the mid 60s across the north. Return flow overspreads the Southeast Wednesday through Friday with dry weather and a warming trend from the mid and upper 70s Wednesday to the low and mid 80s Friday. && .AVIATION /12Z FRIDAY THROUGH TUESDAY/...
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As of 725 AM Friday... Mainly VFR conditions are expected, at least until scattered showers and storms develop in the vicinity of a pre-frontal surface trough centered over the NC Piedmont this afternoon. Brief sub-VFR conditions and gusty winds will result in/near these showers and storms. If storms merge into clusters and form a well-developed rain- cooled/outflow airmass in their wake, sub-VFR ceilings and visibilities may result later tonight. Outlook: A cold front and focus for showers and storms along and to the north of the boundary will settle south into central NC Sat. After that time, Sat night through Sunday night, there remains larger than average uncertainty regarding the location of the front, and continued focus for showers and storms along it, and showers and low overcast to the north of it. An area of low pressure will then develop along the front and move slowly east through, then along the coast of, the Carolinas Sun through Mon night. This complex system will likely result in a prolonged period of rain and low overcast during that time (Sun-Mon night).
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&& .RAH WATCHES/WARNINGS/ADVISORIES... None. && $$ SYNOPSIS...MWS NEAR TERM...MWS SHORT TERM...MWS LONG TERM...mlm AVIATION...MWS

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