Area Forecast Discussion
Issued by NWS Raleigh/Durham, NC

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000 FXUS62 KRAH 221417 AFDRAH Area Forecast Discussion National Weather Service Raleigh NC 1020 AM EDT Mon May 22 2017 .SYNOPSIS...
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A series of low pressure systems will cross central NC through mid- week, causing periods of showers and thunderstorms. Improving weather conditions are anticipated by late in the week.
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&& .NEAR TERM /today through tonight/...
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As of 1020 AM Monday... An area of low pressure was exiting the northern Piedmont at 14Z, ushering the band of showers associated with it into far northeast NC and southeast VA. Westerly flow and subsidence in the wake of the system was aiding to temporarily clearing the overcast skies. As this system pulls farther away from central NC, expect improving sky conditions across most of the region. The partly cloudy/partly sunny skies and a steady westerly sfc wind will aid to boost temperatures to near 80 across the far northern Piedmont, and the low-mid 80s elsewhere. The heating this afternoon will destabilize the atmosphere enough to support the development of scattered showers and thunderstorms, primarily across the southern and western sections of the forecast area, mainly after 20Z. ~WSS Later tonight, the chance for showers and storms will increase once again from the SW, as a more potent mid level wave (now over E TX) approaches, accompanied by a strengthening surface low with a 35-45 kt low level jet on its SE edge. PW will rise further to over 1.75", and an impressive broad upper divergence maximum will also move overhead from the SW, in the right entrance region of a jet streaking from TN/KY into S Quebec. The deep moisture combined with what is likely to be strong stacked forcing for ascent will support a large area of rain and embedded storms, beginning overnight in the SW CWA then spreading NE. Will trend pops up to categorical after midnight SW to NE. (See below for assessment of heavy rain threat.) With a warm and muggy air mass holding in place, expect highs today from the upper 70s NW to the low-mid 80s SE, with a period of partial sunshine likely. Lows tonight from the lower 60s NW to the upper 60s SE, as the frontal zone bisects the forecast area. -GIH
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&& .SHORT TERM /Tuesday and Tuesday night/... As of 410 AM Monday... ...Flood watch may be needed late tonight through much of Tue... Models agree fairly well on a swath of moderate to heavy rain and embedded storms spreading across central NC late tonight through at least early Tue afternoon, with abundant deep moisture (PW near 2" and a deep warm layer of 3.7-4.0 km, favoring warm rain processes) and the aforementioned strong and vertically stacked forcing features including mid level DPVA, vigorous upper divergence, and low level mass convergence prompted by the strong 850-925 mb jet feeding into the strengthening surface frontal zone. This setup certainly suggests a risk of steady moderate to heavy rainfall, especially along the frontal zone, and this added rain along with this morning`s totals presents a risk of flooding particularly in urban and low lying areas. Would prefer to wait for another couple of model runs, however, to get a better handle on QPF (max amounts and location) before diving into a flood watch. There is also a risk of a few strong to severe storms late tonight but especially Tue, considering the dynamic forcing for ascent and model predictions of marginal to moderate CAPE along/SE of the frontal zone, and strong/shifting winds with height near/NW of the frontal zone, yielding long looping hodographs. Will have categorical pops Tue morning, trending down during the afternoon as the forcing heads to our east and northeast. Some drying through the column is evident in forecast soundings Tue evening/night, as the front heads to our SE with a weak meso-high settling over the region, so expect just small chances of light rain or drizzle overnight. Highs Tue from around 70 NW to near 80 SE. Lows around 60 to the mid 60s. -GIH && .LONG TERM /WEDNESDAY THROUGH SUNDAY/... As of 400 AM Monday... An initially (slightly) positively-tilted longwave trough from ern Canada to the srn Plains, including an embedded mid-upper low over the mid MS Valley at 12Z Wed, will migrate ewd and assume an increasingly negative-tilt as it pivots across the Carolinas and middle Atlantic states by Thu and Thu night. Perturbed and moist SSWly to Sly flow aloft will precede the trough, while height rises and ridging aloft will follow and expand ewd across most of the ern U.S. through the weekend. A complex pattern will result at the surface through the middle of the week, and one that will likely be somewhat similar to the one over the Carolinas during the past 24 hours. That is, a frontal zone will likely be draped SWwd from a series of waves passing across and offshore the Middle Atlantic coast, marking the srn periphery of weak ridging/CAD/low ovc extending across interior VA/NC, then extending NWwd as a warm front to a low over the wrn OH Valley. That front, edge of the CAD regime, and focus for convection, will then likely retreat Nwd with a narrow wedge of warm sector through the srn and wrn NC Piedmont during the day Wed, then through the remainder of central NC early Wed night, all immediately ahead of, and associated with, a triple point low that will develop and migrate across the wrn Carolinas (also similar to this morning). The trailing cold front and preceding warm conveyor belt, and associated axis of continued likely showers and chance of storms, will sweep ewd and across the Carolinas later Wed night, with brief drying and clearing probable through early Thu. Clouds will redevelop with heating, with an additional round of "instability-type" convection -- in cyclonic and CAA-bearing flow aloft-- and result in a chance showers and/or storms Thu afternoon, despite surface dewpoints that will likely have fallen into the 50s behind the aforementioned cold front/warm conveyor belt. Dry conditions will return by Thu night, as the upper trough lifts away and weak high pressure expands across the sern states through early Sat. A warm front will develop newd across the central Appalachians later Sat and across the Middle Atlantic states by Sun; with a return to warmer and more humid conditions, and a chance of convection invof the front late Sat afternoon through Sun. && .AVIATION /12Z Monday through Friday/... As of 650 AM Monday... Poor aviation conditions are expected during the first 3-6 hrs of the forecast period, as a band of showers and isolated storms crosses the area. IFR/LIFR conditions will persist at INT/GSO, with mainly MVFR conditions elsewhere through this morning, with a period of MVFR to IFR vsbys in showers lasting for a few hours at each site. Once this band of showers and storms passes east of the area by noon, cigs will gradually lift to VFR with only isolated showers expected after 18z today, and possibly a few storms near FAY/RWI this afternoon into early evening. Rain coverage will begin to increase again from the SW after 06z, with a trend toward prevailing MVFR/IFR conditions with showers and storms. Looking beyond 12z Tue morning, cigs and vsbys will remain MVFR/IFR at all TAF sites as a low pressure system approaches from the SW, bringing widespread showers and storms. These poor aviation conditions will dominate through much of Tue. A trend to VFR is expected Tue night as the low pushes to our NE and offshore, however sub-VFR conditions will return Wed into Wed night as a cold front approaches from the west. -GIH && .RAH WATCHES/WARNINGS/ADVISORIES... None. && $$ SYNOPSIS...WSS NEAR TERM...WSS/Hartfield SHORT TERM...Hartfield LONG TERM..MWS AVIATION...Hartfield

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