Area Forecast Discussion
Issued by NWS Raleigh/Durham, NC

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000 FXUS62 KRAH 221902 AFDRAH Area Forecast Discussion National Weather Service Raleigh NC 302 PM EDT Mon May 22 2017 .SYNOPSIS... 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. && .NEAR TERM /UNTIL 6 AM TUESDAY MORNING/... 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 && .SHORT TERM /6 AM TUESDAY MORNING THROUGH TUESDAY NIGHT/...
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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
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&& .LONG TERM /WEDNESDAY THROUGH MONDAY/...
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As of 300 PM Monday... The upper-air pattern at the beginning of the long term period features a high amplitude trough extending southwesterly from the Great Lakes into the MS Valley with an embedded close low centered near MO/IL. The upper trough axis will slide east on Thursday and take on an increasingly negative tilt with both the GFS and EC moving the long wave trough axis off the mid- Atlantic coast by 12 UTC Friday. The southwesterly flow ahead of the trough strengthens with 75 to 85kts progged at 500 mb over the eastern Carolinas early Thursday. While NWP guidance is similar in the large scale pattern there are differences in some smaller scale features that will play an important role in the forecast details. At the surface, the pattern is rather complex with an existing weak CAD wedge in place across central NC on Wednesday morning with dew points in the upper 50s near the VA border and the upper 60s in the southern Coastal Plain. Guidance is probably too aggressive in driving the warm sector northward to the VA border by mid-afternoon, a more reasonable approach would lift the wedge front more slowly, eventually reaching the VA Wednesday evening as a surface low develops along the front in upstate SC Wednesday evening and moves to southwestern VA Thursday morning. A trailing cold front will then sweep east across the Carolinas and Virginia Thursday morning. Given the strong forcing for ascent ahead of the upper trough, low- level warm advection, localized ascent with the wedge front and then the cold front along with precipitable water values climbing in excess of 1.75 inches, another robust precipitation event is expected Wednesday morning through midday Thursday with general rainfall amounts of 1 to 1.5 inches with localized amounts in excess of 2 inches. Currently SPC has our region outlooked in general thunder and a marginal severe weather risk for the Wednesday and Wednesday night period. Initially, the severe weather risk should be confined to the southeast portion of the area but as the warm sector expands north and with the strong low to mid level flow producing 0-6km bulk shear values in excess of 40-50kts with a weak to perhaps borderline moderately unstable air mass. An expanding severe weather threat is expected, initially in proximity of the wedge front and then ahead of the advancingcold front. The significant precipitation should be over by Thursday but scattered showers and possibly some storms are expected ahead of the upper trough that will be located near the Appalachians. The combination of a relatively moist environment combined with steep low- level lapse rates should support scattered convection and fairly widespread cumulus field. All of the precipitation should end late Thursday night as the air mass stabilizes and larger scale subsidence develops. Dry weather is expected on Friday and Friday night. The pattern becomes less clear over the weekend as troughiness begins to develop over the mid and upper MS valley and a warm front lifts north across the region on Saturday and then stalls near or just north of NC for late Saturday and Sunday. A limited threat of convection persists into Monday but confidence in the forecast over the end of the holiday weekend is limited. Highs on Wednesday and Thursday will be muted by cloud cover and precipitation and modulated by the placement of the warm front. Highs should range around 70 in the Triad to around 80 across the southeast. Highs should moderate into the upper 70s to lower 80s on Friday and the mid to possibly upper 80s on Saturday through Monday. -Blaes
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&& .AVIATION /19Z MONDAY THROUGH SATURDAY/... As of 150 PM Monday... Scattered convection will cross central NC west-to-east this afternoon through this evening. In proximity of the convection expect MVFR/IFR ceilings and sfc winds gusting between 25-35 kts. The convection will be most probable in vicinity of the Triad terminals between 19Z-22Z, and elsewhere between 22Z and 03Z. Away from the convection, expect scattered-broken ceilings in the MVFR/low end VFR range. There is a high likelihood of adverse aviation conditions across central NC through Wednesday night as a series of upper level disturbances crosses the moist air mass in place across central NC. Each disturbance will kick-off scattered showers and thunderstorms with associated MVFR/IFR ceilings and visibilities. The first disturbance will cross central NC late tonight through Tuesday. Another disturbance is expected Wednesday into Wednesday night. Expect IFR/MVFR parameters in proximity of the showers and storms. In between the periods of showers and storms, MVFR ceilings will be common due to the available moisture in the atmosphere. An improving weather pattern is anticipated Thursday with VFR parameters highly probable Friday and Saturday. && .RAH WATCHES/WARNINGS/ADVISORIES... None. && $$ SYNOPSIS...WSS NEAR TERM...WSS SHORT TERM...WSS LONG TERM...BLAES AVIATION...WSS

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