Area Forecast Discussion
Issued by NWS Raleigh/Durham, NC

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000 FXUS62 KRAH 250605 AFDRAH Area Forecast Discussion National Weather Service Raleigh NC 204 AM EDT Tue Apr 25 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 1000 PM Monday... Most of North Carolina resides in the left exit region of a jet streak rounding the base of a compact negatively-tilted trough across the southeastern CONUS. The center of a compact surface low pressure can be found across far eastern South Carolina and is moving northeastward with time. A secondary (and stronger) subtropical low is approaching the coast of North Carolina, and is starting to merge with the aforementioned low pressure system to its west. Closer to home, the region remains in a strong CAD regime with the wedge front delineating slightly more warm and moist air and the CAD to its west located across the eastern CWA. As the lows merge, continued DCVA (as well as frontogenetical forcing, and isentropic upglide along a wedge front draped across the southeastern CWA) will continue to enable widespread ascent. Coupled with an anomalously moist atmosphere across the CWA that increases toward the east (the 00z KMHX sounding reflects a PWAT of 1.83", breaking the previous daily record by nearly nearly 0.3"), widespread rainfall continues areawide. Currently, the heaviest rainfall is confined to two main bands--one across Edgecombe to Franklin counties and another across Wayne and Johnston counties. Both bands seem to combine across Wake, Chatham counties. While rainfall rates aren`t particularly intense within the bands (generally a few tenths per hour approaching 0.5"/hr in the heaviest segments), the long duration of the rainfall is leading to 6 hourly rainfall rates of 1-2". Unsurprisingly, recent storm reports indicate a few closed roads across the CWA with rivers rising in the heaviest bands toward flood stage. However, some slight drying is being noted across the far NE CWA into the NE coast of North Carolina (likely in association with vertical mixing of a pocket of drier air centered around 925 mb) leading to an erosion of the northern precipitation band. As the combined low pressure system translates northeastward across the coast of North Carolina, the bands of heaviest precipitation will likely pivot slightly northeastward with time. A quick peak at the latest suite of model guidance indicates a narrow axis of MUCAPE may build into the eastern CWA enabling slightly heavier precipitation rates as the band moves northeastward (especially between 03-06z). Indeed, a few flashes of lightning are occurring across the NC coast within the aforementioned precipitation band. As the band shifts northwestward, an additional 1-2" with isolated pockets of 3-4" seems likely across the NE CWA (e.g. Edgecombe, Nash, Franklin counties) as well as parts of Wake, Durham, Chatham, and Orange counties. The latest mesoscale discussion from the WPC lines up well with this reasoning. && .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 /06Z TUESDAY THROUGH SATURDAY/...
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As of 155 AM Tuesday... 24 Hour TAF Period: Adverse aviation conditions are expected to continue through the 24 hour TAF period (high confidence) as a deep mid/upper level low moves across the region. A persistent rain continues to fall across portions of central NC early this morning, resulting in generally IFR/low end MVFR conditions. However, periods of LIFR cigs will be possible in areas of light rain and/or brief lulls in the precip ahead of the mid/upper low (such as KFAY currently). Sporadic wind gusts of up to 22 kts or so will continue to be possible across the northern TAF sites early this morning, before diminishing from east to west through 12Z. Will continue to keep LLWS in the TAFs at KGSO/KINT/KRDU/RWI as easterly winds at around 2 kft are expected to be around 45-50 kts or so through 09Z. However, given this will mostly be speed shear (with generally unidirectional flow) expect this will only create marginal LLWS conditions (especially given some of the gustiness expected). Surface winds will then decrease after daybreak. While the main area of rain will shift northward of the area by 12-15Z today we can still expect some lingering showers/rain, especially moisture wrapping around the departing low pressure system across the northeastern portions of our area (KRWI/KRDU). In fact we could see a few storm develop if we are able to destabilize enough this afternoon. While cigs should improve today, possibly becoming VFR temporarily, think we should generally maintain MVFR cigs. Outlook: Cigs should remain MVFR through tonight, although cigs may drop back to IFR through this evening into the overnight. After the low shift to our NE tonight, high pressure ridging will take over, and we`ll see cigs rise to VFR Wed morning. VFR conditions will then dominate through Sat.
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&& .HYDROLOGY... 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 && .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...BB/MWS SHORT TERM...Hartfield LONG TERM...CBL/Smith AVIATION...BSD/Hartfield HYDROLOGY...Hartfield

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