Area Forecast Discussion
Issued by NWS Raleigh/Durham, NC

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000 FXUS62 KRAH 242021 AFDRAH Area Forecast Discussion National Weather Service Raleigh NC 420 PM EDT Mon Apr 24 2017 .SYNOPSIS... A strong and deep low pressure system will slowly cross the region through Tuesday, producing wet and unsettled weather. Behind this low, a ridge of high pressure will extend into the region for mid to late week. && .NEAR TERM /through Tonight/... As of 320 PM Monday... With a few exceptions, rainfall so far has been steady and moderate, with numerous but minor flooding confined mostly to streets and high water in creeks. This should change over the next 9-12 hours as we increasingly tap into a greater depth of Atlantic-source moisture penetrating further inland, north of a surface low that will move northward over southeastern NC tonight (although models disagree on the exact track of this low). Low level easterly flow is expected to strengthen this evening into tonight north of the approaching surface low, with the strongest flow of 30-40 kts over northern sections of central NC, mainly along and north of the Highway 64 corridor. This will continue to draw anomalously high PW values into the area through tonight, as well as allowing the modified wedge boundary to shift further inland. The resulting enhancement of deep isentropic upglide will be vertically juxtaposed with focused upper divergence to the ENE of the mid level low, fostering efficient rain production with a warm layer nearing 3.5 km. And the additional lift generate via elevated convective elements, most likely over our SE and far S sections through this evening, may result in locally heavier rain rates and higher totals. Will leave the flood watch in place, as an additional 1.5-3.0" of rain is possible through tonight, with higher amounts focused along bands with embedded convection and particularly near the pivot point of any such bands as they become more W-E oriented with time tonight as the mid level low shifts eastward into SC. Temps should waver only a category or so through tonight at most, with many spots seeing steady temps through the night or a minor rise as warmer air shifts inland. -GIH && .SHORT TERM /Tuesday and Tuesday night/... As of 340 PM Monday... The mid level low is expected to track from NE SC across E NC Tue, before moving NNE just off the Delmarva/NJ coast Tue night. This low will be nearly vertically aligned with the surface low (or series of lows rotating about one another) over E NC. The strong low level jetting north of the surface low will shift northward into VA Tue morning, along with the upper divergence maximum, as the mid level dry slot punches in from the south, and this drying aloft will lead to diminishing chances for heavy rainfall. But the surface winds will remain from the N and be fairly light with the low to our NE, which should help lock in the cool/damp low level stable pool a bit longer, especially in the western Piedmont. PW values will have dropped but remain above normal, with both low and mid level lapse rates near or over 6 C/km, mainly over the eastern two-thirds of the forecast area. Have retained a chance of showers and isolated storms, higher east than west, through Tue, decreasing Tue evening as the mid level low exits and the column stabilizes and begins to dry further. Temps are tricky with a wide spread in model statistical guidance, not surprising considering that any amount of insolation will cause temps to jump up. Have held onto a scenario that reflects a longer-lived stable pool, suggesting highs from the mid 60s NW to the mid 70s in the far east-central CWA. Will trend pops down and out slowly Tue night, with skies trending toward partly cloudy everywhere except the NE, where a lingering low chance of showers will be retained overnight. Lows 53-58. -GIH && .LONG TERM /WEDNESDAY THROUGH MONDAY/... As of 345 pm Monday... The biggest story in the extended will be a transition to a amplified western trough and eastern ridge, especially into the weekend. Global models are in good agreement that a shortwave and associated surface cyclone will lift out for the Central Plains and across the Great Lakes Wednesday and Thursday, and a trailing cold front associated will encroach upon the CWA from the west Thursday. The removed upper forcing and lack of moisture return suggests this waning precip with the front, which itself may never actually move into NC as the subtropical ridge begins to expand across the Southeast by Friday. Main impacts look to be an isolated shower or storm in the west late Thursday or isolated storm late Thursday. The synoptic pattern will favor near record heat by Saturday, owing to a 590+dm H5 ridge, Bermuda highs and H7 anticyclone over SC. Thickness are progged at 1410m Saturday, 50m above normal and indicative of upper 80s to lower 90s. The main question is how quickly another shortwave ejecting out of the Central/Southern Plains, this one potentially stronger, will cause the ridge to retreat. Heights are forecast to lower, more so across western NC, favor low/80s west to upper 80s east Sunday and Monday, with a chance of storms associated with the upper wave and approaching cold front by Monday. && .AVIATION /18Z Monday through Saturday/... As of 155 PM Monday... High confidence in poor aviation conditions through at least 06z tonight. Thick IFR and occasionally LIFR based clouds will persist over central NC as a deep and strong low pressure system slowly crosses the area through Tue, exiting to our northeast Tue night. Periods of rain through 06z will affect all sites with occasional MVFR to IFR vsbys, along with the mainly IFR cigs. Cigs will slowly lift Tue morning to MVFR with decreasing vertical extent as drier air just off the ground arrives from the S. The rain will decrease in coverage and intensity later tonight, although scattered showers will persist after sunrise Tue as the core of the upper low passes overhead. Surface winds will be mainly from the NE at 7-12 kts through tonight, occasionally gusting to 15-20 kts during the evening and overnight hours, especially at GSO/RDU/RWI as an easterly low level jet at 1000-2000 ft AGL extends into the area. This low level jet may lead to a risk of low level wind shear at INT/GSO/RDU/RWI from late evening into the overnight hours. Surface winds will then decrease late tonight (after 09z) and become more from the N or NW Tue. Looking beyond 18z Tue, cigs should remain MVFR through Tue night, with infrequent MVFR vsbys in scattered showers, although cigs may drop back to IFR through Tue night especially at INT/GSO as a cool stable air mass persists there. After the low shift to our NE Tue night, high pressure ridging will take over, and we`ll see cigs rise to MVFR then VFR through Wed morning. VFR conditions will then dominate through Sat. -GIH && .HYDROLOGY...
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As of 420 PM Monday... ...Flood Watch remains in effect through 12Z Tuesday... Much of the rainfall so far has been moderate, with most places so far spared by the heavier rain rates. This is expected to change through the evening hours as increasing lift and moisture lead to an uptick in rain rates. Several creeks around central NC are experiencing rises, with a few approaching minor flooding, and this should persist through tonight, with retreating water levels Tuesday. Urban and street flooding, particularly in the typical flood-prone areas, have been common all across central NC. A few main stem rivers are projected to approach or exceed flood stage over the next couple of days in response to this rainfall, including parts of the Yadkin, Haw, Little, Cape Fear, Rocky, and Tar Rivers. Rainfall since last night has been around 1 to 3 inches, and an additional 1-3 inches of rainfall are expected through tonight, with locally higher totals possible. -GIH
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&& .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 LONG TERM...CBL/Smith AVIATION...Hartfield HYDROLOGY...Hartfield

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