Area Forecast Discussion
Issued by NWS Raleigh/Durham, NC

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000 FXUS62 KRAH 181445 AFDRAH Area Forecast Discussion National Weather Service Raleigh NC 1045 AM EDT Fri May 18 2018 .SYNOPSIS...
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A Bermuda high offshore and an area of low pressure over the TN Valley will maintain a moist southerly flow over central NC through Saturday night. A series of disturbances aloft will trigger periods of showers and thunderstorms through Saturday night, producing locally excessive rainfall.
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&& .NEAR TERM /THROUGH TONIGHT/...
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As of 1045 AM Friday... 12Z upper air analysis depicts a few features that may help to initiate or support numerous showers/scattered thunderstorms with heavy rain across central NC this afternoon. At 850mb, a very moist air mass is in place with dewpoints 11-13 degrees C. A subtle/weak trough was noted across central SC into southern GA. This feature should aid to focus convective development into the NC Piedmont/Sandhills. In the mid-upper levels, some speed convergence noted over eastern NC with an area of divergence aloft over the same area. The modest forcing at the sfc and aloft coupled with the abundantly moist air mass will support numerous showers and a few storms with locally heavy rain probable. The heavy/locally excessive rain threat appears pretty much everywhere across our forecast area this afternoon-evening, then the focus will shift to areas east of highway 1. A Flash Flood Watch may become necessary for sections of central NC for later today through Saturday. Similar to Thursday, peaks of sun in between the showers/storms may be enough to warm temperatures to around 80-lower 80s this afternoon. If showers/storms become extensive, temperatures in the 70s-near 80 will be common. Tonight, the bulk of the convection across the western-southern Piedmont will gradually diminish with loss of heating. Along and east of highway 1, however, the moisture rich air mass and favorable features to initiate/sustain scattered convection will persist. This will necessitate the need for likely PoPs into the overnight hours across the Sandhills/eastern Piedmont and Coastal Plain. If heavy rains occur this afternoon, the additional heavy rain overnight will lead to rapid run-off, leading to flooding of creeks/streams and low lying areas.
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&& .SHORT TERM /SATURDAY AND SATURDAY NIGHT/... As of 315 AM Friday... Deep southerly moisture tap up the south Atlantic coast will be directed into central and eastern NC on Saturday, with a plume of PW`s > 2 inches initially over the Coastal Plain/I95 corridor. Low level forcing and instability will be relatively weak with shallow mid level lapse rates, but mid and upper level dynamics will be favorable for enhancing existing convection with mid level perturbations and a very favorable entrance region of a 70KT upper jetlet squeezed between the weakening upper short migrating across the Ohio Valley and the Bermuda ridge offshore. Will raise PoPs to 80-90% in the Coastal Plain and not try and get cute with timing given this tropical airmass` lessened reliance on the diurnal cycle. As such, expect that Saturday will be our greatest threat for flooding over the east, especially if we see significant antecedent rainfall today & tonight, given the high potential for training heavy showers. A flash flood watch may need to be considered today to cover a column or two of our eastern counties on Saturday. Highs will be within a degree north or south of 80. The moisture plume will be nudged east Saturday evening by the upper short wave moving across the area, with significant reduction in coverage and rainfall intensity overnight with persistence lows in the mid to upper 60s. && .LONG TERM /SUNDAY THROUGH THURSDAY/... As of 315 AM Friday... Scattered convection will be more prevalent in the east on Sunday as the upper short wave moves east and offshore. A weak frontal zone associated with the short wave will slow and stall across the area Monday to provide a weak focusing mechanism for convection, mainly over the south and east, on Monday and Tuesday. A low pressure area will be lifting north out of the eastern Gulf in the midweek, and will increase the chance of rain Wednesday into Friday, although details concerning exactly when and where are murky at this point. Temperatures will show little change through the period given the persistent tropical flavored airmass, with highs mostly in the 80 to 85 range and lows from 65 to 70. && .AVIATION /12Z THURSDAY THROUGH TUESDAY/... As of 735 AM Thursday... 24-Hour TAF period: Although confidence is high that showers and thunderstorms will develop again today across Central NC, the exact timing and location are still a bit uncertain. Do expect convective activity to increase mid-morning with numerous to widespread convection by the aft/eve hours. Ceilings in the 600-1500 ft range this morning should lift/scatter to predominantly MVFR/VFR by mid morning. Heavy rain will accompany any showers and thunderstorms that develop today, quickly reducing visbys where they occur. Some wind gusts in the 15-20 kt range are also possible with the storms. Looking ahead: The pattern of daytime showers and thunderstorms resulting in heavy rain and reduced visbys, with a relative lull in precipitation accompanied by sub-vfr cigs and visbys overnight, will continue into early next week. && .RAH WATCHES/WARNINGS/ADVISORIES... None. && $$ SYNOPSIS...WSS NEAR TERM...WSS SHORT TERM...mlm LONG TERM...mlm AVIATION...KC

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