Area Forecast Discussion
Issued by NWS Raleigh/Durham, NC

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000 FXUS62 KRAH 181938 AFDRAH Area Forecast Discussion National Weather Service Raleigh NC 335 PM EDT Fri May 18 2018 .SYNOPSIS... 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. && .NEAR TERM /THROUGH TONIGHT/...
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As of 335 PM Friday... Flash Flood Watch in effect for most of central NC through Saturday evening. Near term model guidance in good agreement, maintaining a deep sly flow across central NC tonight. Disturbances embedded in this flow will trigger/sustain clusters of showers and thunderstorms. With precipitable water values well above normal, hovering between 1.8 and 2 inches, torrential downpours will accompany the heavier showers and storms. Rainfall rates on the order of 1-3 inches per hour will be common. Due to the deep sly flow, training of these heavier showers may lead to localized excessive rainfall1 and potentially flash flooding. This flooding threat appears highest over portions of the Triad through late this evening, then mostly along and east of highway 1 late tonight through Saturday evening. 12Z GFS depict a modest 850mb moisture transport axis over our western Piedmont this afternoon. This feature moves east and aligns over our eastern periphery late tonight though Saturday. This low level feature coupled with an area of upper divergence will provide decent lift through the column. thus, expect the repeating clusters of showers and isolated storms to drift from the western Piedmont early this evening into the Sandhills and Coastal Plain late tonight.
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&& .SHORT TERM /SATURDAY THROUGH SATURDAY NIGHT/...
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As of 335 PM Friday... Saturday and Saturday night, the deep sly flow will shift ewd and be positioned primarily over the eastern counties. This shift is due to the mid-upper level low currently over the TN Valley lifting into the lower Great Lakes. With the low pressure system to our NW and the Bermuda high anchored offshore, upper divergence will remain over eastern NC. Model guidance continues to depict a series of perturbations lifting nwd in the deep sly flow. With the abundantly moist and unstable atmosphere in places, rounds of showers and storms will continue through Saturday evening. Model guidance currently suggest the axis of deeper moisture and best lift drifting east, and out of our region late Saturday night. Rainfall through early Saturday evening should total one to two inches over most of central NC with locally higher amounts between 3 and 4 inches possible. Some portions of central NC have already received amounts of 3 inches. This added rainfall will likely lead to rapid run off and flooding of creeks and streams.
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&& .LONG TERM /SUNDAY THROUGH FRIDAY/... As of 215 PM Friday... ...Unsettled weather is likely to continue through much of the long term period... The upper-air pattern at the beginning of the long term period will feature a well established sub-tropical ridge centered off the Southeast coast with a shearing trough across the southern Appalachians and mid-Atlantic while an upper-level low drifts north across the eastern Gulf of Mexico. The ridge off the Southeast coast shifts east during the work week as the upper low drifts onshore across the northeast Gulf Coast and lingers in the Deep South through mid-week. Broad troughing develops across the Northeast late in the work week producing a weak northwest flow across the Carolinas by Friday as the main westerlies remain well to our north. Elevated amounts of deep layer moisture are forecast to persist across the Carolinas through much of the long term period. A persistent frontal zone across the Virginias potentially drops into the Carolinas toward end of the work week as the upper flow becomes more northwesterly. The result should be a period of scattered, mainly afternoon and evening showers and a few thunderstorms from Sunday through most of the work week. Precipitation chances may trend downward by the end of the work week depending on the position of the front. A large fraction of each day will be dry but with a threat of scattered convection. With a little change in air masses, a good deal of cloudiness and scattered convection, highs will consistently range in the lower to mid 80s with low mainly in the mid to upper 60s. -Blaes && .AVIATION /18Z FRIDAY THROUGH WEDNESDAY/... As of 205 PM Friday... A moisture rich air mass coupled with a slow moving area of low pressure will contribute to the development/maintenance of scattered to numerous showers and thunderstorms across central NC through Saturday evening. In vicinity of the heavier showers and storms, expect IFR/MVFR visibilities and ceilings. Away from the showers/storms, MVFR/low end VFR ceilings will dominate until 02Z Saturday. After 02Z, ceilings will become widespread MVFR/IFR, persisting into Saturday morning. Numerous showers and scattered storms will slowly diminish this evening in the western Piedmont, but scattered convection with torrential downpours will persist across the Sandhills, Coastal Plain and eastern Piedmont into the overnight. On Saturday, the highest coverage of showers and storms will be roughly along and east of highway 1. Convective coverage expected to be gradually decrease through the period Sunday through Wednesday. There will still be periods of MVFR/IFR ceilings in proximity of the scattered convection, and during the late night/early morning hours. && .RAH WATCHES/WARNINGS/ADVISORIES...
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Flash Flood Watch through Saturday evening for NCZ007>011-021>028- 040>043-075>078-085-086-088-089.
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&& $$ SYNOPSIS...WSS NEAR TERM...WSS SHORT TERM...WSS LONG TERM...BLAES AVIATION...WSS

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