Area Forecast Discussion
Issued by NWS Raleigh/Durham, NC

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306 FXUS62 KRAH 230755 AFDRAH Area Forecast Discussion National Weather Service Raleigh NC 355 AM EDT Sun Apr 23 2017 .SYNOPSIS... A cold front will push southward through North Carolina overnight, reaching the South Carolina border by daybreak. A strong low pressure system will slowly cross the region through Tuesday. && .NEAR TERM /Today/...
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As of 355 AM Sunday... A significant period of wet weather starts today. The latest surface map shows that the cold front is now settling over far northern SC and SE NC, where it is projected to linger through tonight. Meanwhile, the potent mid-upper low centered over the MO Bootheel is expected to drift across W TN into N AL today, with a series of weak perturbations in the downstream SW steering flow tracking across NC helping to instigate numerous to widespread showers over our area, where PW values that are already approaching the 80th percentile will rise further today. Given this deep moisture, also observed on experimental GOES-16 water vapor imagery and blended TPW imagery, the waves of weak DPVA, strengthening moist upglide through the 290- 300K depth up and over the stable cool pool nosing in from the north, and increasing low level moisture transport, categorical pops are more than warranted. Model guidance is in fairly good agreement on a scenario of steady light to periodically moderate rain today, and expect rainfall amounts through nightfall of around a third of an inch in the SE CWA to an inch and a third in the N and W CWA. While any flooding today should be localized and mostly minor, this rainfall will help saturate the soils, increasing runoff potential for later rainfall, and a few rises on area creeks are likely. With a trend to thick cloud cover areawide and cooler air wedging in from the north, expect highs no better than the mid 50s NW ranging to the upper 60s SE, a diurnal rise of just around 5-10 degrees in most places. -GIH
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&& .SHORT TERM /SUNDAY THROUGH TUESDAY/... As of 355 PM Saturday... ...Flood Watch from midnight tonight through 800 AM Tuesday... The beginning of a long duration rain event will be underway over the western and central Piedmont by 12z/Sunday. The mid/upper trough is forecast to become negatively tilted during this period as it drifts ESE from the Tennesse Valley region across the southern Mid- Atlantic states. At the surface, a backdoor cold front will also begin Sunday either over far southern NC or near the NC/SC border region. This will place nearly all, if not all of central NC in the cooler stable air. Models suggest that a surface low pressure will be located over NW Georgia around 12z/Sunday, then move east across eastern GA and SC Sunday PM. As the low pressure slowly strengthens as it moves east across SC Sunday night and Monday, the low level boundary should back NW and just inland of the coast Monday. This still leaves central NC on the cool, wet side of the boundary with copious amounts of very moist warm air advection from off the Gulf Stream and western Atlantic expected to ride up and over the frontal zone. This occurs as the mid/upper system becomes more negatively tilted as it essentially crawls eastward, just to our south Sunday night and Monday. The heaviest rain is expected to fall along and north of the frontal zone through the period as the strong dynamic forcing slowly advances east across the area. This will place the Piedmont and Sandhills in the heaviest rain Sunday. The heaviest rain is forecast to spread east with time, covering much of our region Sunday night into Monday. Models are not as robust with the elevated instability Sunday night and Monday as they are on Sunday. However, the influx of very high precipitable waters off the Gulf Stream and western Atlantic should will aid in heavy rainfall at that time. The threat of flooding will be the main hazard with this system since our region will be in the more stable area north of the main frontal zone. QPF storm total for the entire event is on the order of 3-5 inches, with potentially 6 inches locally. The main time frame for the heaviest rain should be from midnight tonight through midnight Monday night. Flash Flood Guidance numbers will lower with time, which are currently running high. 6 hour FFG is on the order of 2.5 to 3 inches, and this will lower to around 1.5 inches by Sunday afternoon. Urban areas have much lower thresholds, and they will become an issue well before the rural locations. The soils, which have recently been dry will become wet and saturated with the showers/storms this evening into Sunday. This will lead to runoff issues with the widespread heavy rain Sunday afternoon through Monday evening. Gusty NE winds will be felt in all zones with sustained winds of 15-25 mph, gusts to 30-35 Sunday into Monday. Less important forecast parameters include the temperature forecast which will be much cooler with the heavy rain and breezy NE winds. Highs Sunday should only be in the 50s/60s NW/SE both Sunday and Monday. Lows will be only about 5-7 degrees lower than daily maxes, if that. The heaviest rain should move offshore on Tuesday or Tuesday evening, with the threat of moderate river flooding on the main stem rivers expected into late week. && .LONG TERM /TUESDAY NIGHT THROUGH SATURDAY/... As of 330 AM EDT Saturday... Lingering showers will be ending in the northeast by around midnight Tuesday night as the upper low lifts northeast and off the Delmarva peninsula by Wednesday morning. The lows will be in the mid 50s. We then see dry weather with a rapid warmup though the end of the week as shortwave ridging and return flow settle along the eastern CONUS. Highs will approach 80 on Wednesdsy...then climb into the mid and upper 80s Thursday through Saturday. Mins will be mostly in the 60s mid to late week. && .AVIATION /06Z Sunday through Friday/... As of 155 AM Sunday... High confidence in poor aviation conditions over the next 24 hours, With IFR conditions common in all locations except mostly MVFR at FAY. A cold front dropped southward into and through NC this past evening, and is now settling near the NC/SC state line. This front will remain stretched across northern SC and southeast NC through the next 24 hours, while surface low pressure drifts slowly from northern AL toward southwest NC, and strong mid level low pressure passes over the Mid South. This will all keep unsettled weather over central NC, with deep moisture leading to low cigs, lowest at INT/GSO, through Sunday night, with periods of MVFR to IFR rain, most widespread at INT/GSO but likely everywhere. Surface winds will be mainly from the NE at 10-15 kts gusting to 15-20 kts infrequently through Sunday night. While winds aloft (4000-5000 ft AGL) will be strengthening from the SE late Sunday through Sunday night to 25-35 kts, however the shift of winds in the vertical from NE to SE through the low levels will not meet low level wind shear criteria, although it may make handling of small aircraft difficult. Looking beyond 06z Mon: Poor aviation conditions will continue to dominate through Monday night as the deep low pressure system slowly crosses the region, with periods of rain through Mon, tapering down from south to north Mon night, and persistent low MVFR/IFR cigs and vsbys. Cigs should rise gradually to MVFR areawide Tue as the mid level low slowly departs the area, then trend to VFR Tue night lasting through Fri with a drier air mass and deep SW flow, although a few afternoon showers are possible Thu/Fri. -GIH && .HYDROLOGY... As of 310 AM Sunday... ...A Flood Watch is in effect through 800 AM Tuesday... Rain has begun to saturate soils in the northwest Piedmont with 1/2 to 1 inch of rain on Saturday in the Triad. Rain will continue to spread across the area as isentropic upglide (looking at the 295K isentope analysis as representative) increases with strengthening southeast low level flow today. Even modest rain rates will produce hefty amounts, and given that we have categorical PoPs for ~48 hrs, our 3-4 inch storm total forecast looks reasonable...perhaps even conservative. Flooding will be likely on smaller creeks and streams since we will have periods of convectively enhanced rain covering smaller areas. Our larger mainstem river basins will take longer to saturate, and will begin to rise more quickly after we prime the basins with the first couple of inches of rain. Thus river flooding will become more likely on Monday and could linger into midweek. Based on our expected rainfall distribution at this time, the Haw river would be particularly prone to minor flooding above Lake Jordan, but any of the mainstem rivers could reach minor flood stage, especially given just a bit more rain than currently forecast. On a postive note, we were on the cusp of drought conditions beginning to have more significant impact across the area due to rainfall deficit over the late winter and spring, so this event will provide at least a couple of weeks` respite. && .RAH WATCHES/WARNINGS/ADVISORIES... Flood Watch through Tuesday morning for NCZ007>011-021>028-038>043- 073>078-083>086-088-089. && $$ SYNOPSIS...Hartfield NEAR TERM...Hartfield SHORT TERM...Badgett LONG TERM...MLM AVIATION...Hartfield HYDROLOGY...MLM

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