Area Forecast Discussion
Issued by NWS Raleigh/Durham, NC

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000 FXUS62 KRAH 210757 AFDRAH Area Forecast Discussion National Weather Service Raleigh NC 350 AM EDT Wed Mar 21 2018 .SYNOPSIS... Cold surface high pressure will build into the region from the north tonight as a storm system develops over southern Appalachians. This storm will move off the coast this morning, and will develop into a Nor`easter as it tracks up the Delmarva coast today. Cold high pressure will build into NC Wednesday night through Friday. && .NEAR TERM /THROUGH TONIGHT/... As of 330 AM Wednesday... Lead shortwave trough/energy is currently lifting off to the northeast, with the trailing closed upper low progged to cross central NC by the early afternoon. Synoptic scale lift/forcing from the attendant height falls will lead to further deepening of the surface low off the NC/Va coast, with yet another Nor`easter expected to develop off then Delmarva coast today and tonight as the upper low catches up with the coastal low. While we are currently in a lull, the wrap-around deformation band is still on track to pivot east across the area between 09 to 18z. As mid-level temperatures continue to cool aloft, eliminating the warm nose, the low-level thermal profile in place across the northern Piedmont will be marginally cold enough to support a transition from rain to a wet snow, with some sleet possibly mixed in at onset. Fcst soundings continue to suggest that lift will be weak/modest within the dendritic growth zone, suggesting that overall precip intensity/rates within the deformation bend will be on the light side. Additionally, without the higher precip rates, any snow that falls after daybreak will have a difficult time accumulating due to tempered insolation above the cloud/precip layer, while also having to overcome boundary layer temps at or above freezing. Given these limitations, areas that stand most likely to see light accumulations up to an 1.0" will be along and north of the I-85 corridor, and from the Triad to Person County. South of I-85, many areas could see some wet snowflakes, but no accumulations are expected with boundary layer temps above freezing. The deformation band looks to be largely east and north of the area by late morning. However, additional shortwave disturbances, diving down the backside of the larger scale trough will support scattered showers through the afternoon and early evening across the far NE zones, mostly in the form of rain as temperatures warm into the upper 30s/near 40 north, to upper 40s/near 50 south. At this time, no changes will be made to the current advisory in place across the northern Piedmont. Tonight: Subsidence and low-level dry cold air advection in the wake of the exiting system will lead to partial clearing overnight. Lows 30 to 35. && .SHORT TERM /THURSDAY THROUGH THURSDAY NIGHT/... As of 220 AM Wednesday... One last wave dives down through NC Thu, and with lingering shallow moisture aloft, we may see a period of broken mid clouds, mainly across the NE. Otherwise, with the long and narrow surface high to our west and the nor`easter lifting up offshore of New England into the Canadian Maritimes, we`ll stay in a dry northerly surface flow with falling PW to around a quarter inch. Despite decent sunshine, thicknesses will be around 40-45 m below normal, supporting highs in the 50s, around 10 degrees below normal areawide. Fair skies expected Thu night with lows from the upper 20s to mid 30s. -GIH && .LONG TERM /FRIDAY THROUGH TUESDAY/... As of 350 AM Wednesday... Fri/Fri night: A small but potent upper low will dive SE over PA and NJ or the Delmarva peninsula Fri, leading to an accelerating northwesterly jet over NC. But moisture will remain scant with PW values sitting near or just over a quarter inch, so the greatest impacts will be a period of broken clouds, a result of residual moisture in the upper reaches of a deep mixed layer. These clouds should be most pronounced over the northeast CWA beneath the greatest DPVA and a pocket of upper divergence. Highs still well below normal, in the 50s. Mid and high clouds will be increasing from the NW Fri night ahead of the next wave traveling through the northern and central Plains. Under fair skies, expect lows again to be in the upper 20s to mid 30s. Sat through Sun: The aforementioned wave will dive SE through our area over the upcoming weekend, before becoming absorbed in a large vortex that will settle over the far NW Atlantic, just SE of the New England and Canadian Maritime coast. At the surface, a warm frontal zone will approach from the SW but stall to our S, while surface low pressure (corresponding to the mid level shortwave trough) tracks along the front, crossing MO to St Louis to E TN Sat/Sat night, before dropping into NE GA and filling as secondary low pressure forms near the SC coast early Sun. With cold mid levels, owing to the NW Atlantic vortex, and cold low levels, courtesy of chilly high pressure nosing in from the N, the chances of non-liquid precip with the shortwave passage are increasing. The GFS and ECMWF are coming into better agreement on timing, focusing precip from late Sat afternoon through mid to late morning Sun, and partial thicknesses from both models support a trend to broad corridors of mixed ptypes, mainly along/north of 64 through Sat night, finishing off as mostly light snow Sun as cold air rushes in on the back side of the secondary low tracking away from the Southeast coast. Given the high uncertainty of specific ptypes at this time range, however, have opted for a rain/snow mix, mainly across the north, with mostly rain south, for the time being. With confidence increasing in precip focusing on Sat night, have raised pops to likely, and have brought highs down several degrees both days, to highs in the upper 40s to lower 50s, and even this may not be cool enough, particularly on Sun when the exiting low will draw even cooler surface air southward into NC. Lows in the low-mid 30s. Mon/Tue: Between the strong NW Atlantic low and western US troughing, a highly amplified mid level ridge will build along the eastern half of NOAM early next week. Meanwhile, at the surface, chilly high pressure centered over SE Quebec will nose SSW through NC Mon, bringing cool temps. This ridge then splits, with the southern portion modifying by Tue as the ridging continues to build, so expect temps to start trending back toward seasonal normals. Dry weather is likely. -GIH && .AVIATION /06Z WEDNESDAY THROUGH SUNDAY/... As of 200 AM Wednesday... A closed upper low will move across the Carolinas through midday. Wrap-around deformation band associated with the upper low will pivot eastward across the area between 09 to 18z. As mid-level temps cool, rain will mix with or change to snow at KINT/KGSO/ and likely KRDU between 09z and 12z. A mix of light snow and rain showers are expected at KRWI between 12 to 15z, while KFAY should remain mostly all rain. Expect IFR to MVFR ceilings to persist with visbys decreasing to IFR with any moderate snow, mostly likely at KINT and KGSO. Additional shortwave disturbances, diving down the backside of the trough will support widely scattered showers, mostly in the form of rain, across the area during the afternoon. However, ceilings are expected to slowly improve through the afternoon with a return to VFR conditions everywhere by the evening. Additionally, gusty NW winds into the upper teens to lower 20s are expected during the afternoon. Outlook: VFR conditions are expected through Friday. Next storm system will likely impact the area Saturday through Sunday, producing widespread sub VFR conditions in rain, with the possibility of a winter mix Saturday night/early Sunday. Stay tuned. && .RAH WATCHES/WARNINGS/ADVISORIES... Winter Weather Advisory from 1 AM to 11 AM EDT Wednesday for NCZ007>009-021>025-038-039. && $$ SYNOPSIS...CBL NEAR TERM...CBL SHORT TERM...Hartfield LONG TERM...Hartfield AVIATION...CBL is the U.S. government's official web portal to all federal, state and local government web resources and services.