Area Forecast Discussion
Issued by NWS Raleigh/Durham, NC

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000 FXUS62 KRAH 231057 AFDRAH Area Forecast Discussion National Weather Service Raleigh NC 655 AM EDT Sun Apr 23 2017 .SYNOPSIS... A cold front will settle near the South Carolina border today. A strong low pressure system will slowly cross the region through Tuesday. && .NEAR TERM /Today/... 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 && .SHORT TERM /Sunday night through Tuesday/... As of 455 AM Sunday... ...Flood Watch remains in effect from midnight tonight through 800 AM Tuesday... Tonight-Mon night: The areal extent and intensity of showers will ramp up beginning tonight, peaking midday Mon through Mon evening, as we see a continued increase in PW (reaching near daily records, around 1.5"), upper divergence (on the east side of the upper cyclone), and low level moisture transport (nearly 200 g/kg, associated with a strengthening 925 mb long-fetch ESE jetlet, originating with a jet near the Bahamas), with falling mid level heights and increasing DPVA. These factors appear to be the most ideally juxtaposed in the vertical over southern NC Mon morning through Mon evening, focusing first on the southern Mountains and Foothills, then shifting to the central and southern Coastal Plain and SE NC. While the models differ somewhat on exact amounts, they all agree on this overall pattern and evolution, including the NCAR ensemble, MPAS, and SREF. Will continue with the flood watch in place, with categorical chances for periods of moderate to heavy rainfall, with the heaviest rainfall potential where we see contributions from convection, most probable near the lingering frontal zone where moist upglide will be strongest and moisture most available. Have raised storm total rainfall amounts to 3.8-4.7", based on latest model guidance and increasing confidence in heavy rainfall totals. While our antecedent conditions are somewhat dry, the potential for deep layer ascent compounded by the anomalously high PW, along with today`s rain to help saturate the soil, will likely cause many of our creeks to rise enough to allow water to spill onto roads, and urban/street flooding is likely to be a big problem through Mon night, irrespective of initial soil conditions and unremarkable flash flood guidance ahead of the event. Regarding the chance for storms, models are suggesting minimal CAPE confined to the SE CWA through Mon, shifting to the Coastal Plain Mon night, although deep layer shear values will be favorable (30-40 kts) with rich low level vorticity near the boundary, where surface-3 km helicity values are projected to exceed 300 m2/s2. So if we can manage to achieve higher potential instability, a few storms could certainly become strong to severe with a risk of damaging winds, hail, and perhaps a brief tornado, focusing Mon evening into the early overnight hours. Expect lows tonight not too far below today`s highs, from around 50 to the lower 50s. Highs Mon from the upper 50s NW to the lower 70s SE, which should get into the warm sector. Lows Mon night 50-60, with decreasing precip chances late from west to east as the low exits. -GIH The upper low begins to lift up the coast early Tuesday, with wrap around rainshowers, mainly in the east, due to strongly convergent low level flow. These showers will be gradually diminishing throughout the day as the low departs, ending over nearly all but the northern coastal plain by sunset. Cloudiness and northerly low level flow will hamper wedge airmass scouring, especially in the west, so temperature moderation will be delayed another day, with highs in the mid 60s across the north to perhaps very low 70s along the southern tier counties. -mlm && .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 Wednesday...then climb into the mid and upper 80s Thursday through Saturday. Mins will be mostly in the 60s mid to late week. && .AVIATION /12Z Sunday through Friday/... As of 655 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 tonight, 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 today through tonight 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 12z 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 isentrope 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 positive 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...Hartfield/MLM LONG TERM...MLM AVIATION...Hartfield HYDROLOGY...MLM is the U.S. government's official web portal to all federal, state and local government web resources and services.