Area Forecast Discussion
Issued by NWS Raleigh/Durham, NC

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000 FXUS62 KRAH 231746 AFDRAH Area Forecast Discussion National Weather Service Raleigh NC 145 PM EDT Sun Apr 23 2017 .SYNOPSIS... A strong mid and upper level storm system will move slowly east across the Tennessee Valley into the Carolina`s through Tuesday. A surface low pressure will track from Georgia east to the Carolina coast through Tuesday. && .NEAR TERM /This Afternoon/... As of 1155 AM Sunday... Flood Watch through 800 AM Tuesday... Driven by the 1022+ mb surface high pressure that was nosing down into our region from PA/NY, strong CAA with the low level NE wind had overspread all our region as of early this morning. The main cold front had settled into northern SC, and extended to the main low pressure located over NW Georgia. Temperatures were a good 30 degrees colder today than this time on Saturday. Readings were in the 40s NW ranging into the 50s elsewhere. Rain was widespread over the western half of NC west and south through the Tennessee Valley and northern GA. Rain continued to develop and overspread areas along and NW of a line from Albemarle to Raleigh and Roanoke Rapids. This covers all the Piedmont and portions of the northern Sandhills and northern Coastal Plain. Rainfall thus far has been generally 0.50 to 1 inch in the west with between 0.25 and 0.50 east - locally 1+ inches. This is just about as expected thus far. The maximum rain thus far has been in a SW to NE corridor from the Mountains along the northern tier of NC and southern VA where 1 to locally 3 inches has been observed. The rainfall thus far has simply wet down the dry ground as many areas had received only 50 to 75 percent of normal precipitation in the past 1 to 2 months before this rain even began. Widespread rain is expected to continue to develop and spread northward this afternoon over much of western and central NC, with the best lift forecast to be located over the western and northern part of the state much of the afternoon. This will continue to place areas along and west of a line from Albemarle to Raleigh to Roanoke Rapids in the more widespread rain, with areas of light rain and drizzle over the Sandhills and Coastal Plain. Most of the rain will be light to moderate, but will accumulate on the order of 0.05 to 0.10 per hour in the western Piedmont. These rainfall rates will not produce flooding, but simply saturate the ground, priming the region for significant runoff tonight and Monday. This is when the rain is forecast to become heavy, with rainfall rates to increase significantly. A quick peep at the new model data supports the ongoing forecast and reflects the current radar and satellite trends very well. The entire positively tilted mid/upper low near Memphis is forecast to slowly track SE through the TN valley to over northern GA later tonight and Monday, evolving into a negatively tilted system aloft with time. Strong mid level lift is forecast to arrive over much of southern and western areas later today and tonight, then in the east on Monday. The negatively tilted system will tap very deep moisture which will be pulled NW from the Gulf Stream and western Atlantic into northern SC and much of NC. Thus, the heavy rain with storm totals of 3 to 5 inches still appears likely if not probable. && .SHORT TERM /Tonight through Tuesday/...
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As of 145 PM Sunday... ...Flood Watch 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
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&& .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 /18Z Sunday through Friday/...
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As of 145 PM Sunday... IFR to LIFR CIGS with MVFR VSBYS in rain and fog are expected through the TAF period. Period of heavy rain will occur at KINT/KGSO tonight, then all areas Monday and Monday night. The rain should begin to taper to drizzle on Tuesday. However, low CIGS and VSBYS below MVFR range should continue through 12z/Tuesday. 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.
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&& .HYDROLOGY... As of noon today... ...A Flood Watch is in effect through 800 AM Tuesday... Flooding of creeks and small streams is expected to begin later today and tonight as the ground saturates and the rainfall becomes heavier. This is expected to occur over the western Piedmont, including the Triad region first, then shift east with time (later tonight and Monday) to bring the threat across all areas. 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 due to rainfall deficits over the late winter and spring, so this event will turn that around and possibly eliminate much of a drought threat for the rest of the spring season. && .RAH WATCHES/WARNINGS/ADVISORIES... Flood Watch through Tuesday morning for NCZ007>011-021>028-038>043- 073>078-083>086-088-089. && $$ SYNOPSIS...Badgett NEAR TERM...Badgett SHORT TERM...Hartfield/MLM LONG TERM...MLM AVIATION...Badgett HYDROLOGY...Badgett/MLM is the U.S. government's official web portal to all federal, state and local government web resources and services.