Area Forecast Discussion
Issued by NWS Raleigh/Durham, NC

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000 FXUS62 KRAH 190135 RRA AFDRAH Area Forecast Discussion National Weather Service Raleigh NC 935 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/... As of 935 PM Friday... Flash Flood Watch in effect for most of central NC through Saturday evening. Weak shortwave impulses, emanating north ahead of the shortwave trough that will move across the TN Valley, will continue to support the re-development of scattered to numerous showers, with some embedded thunderstorms, through the remainder of the evening and overnight hours. Within the anomalously high 1.7 to 2.0" PWAT in place, shower will be very efficient rain producers, with any training convection apt to result in some flooding overnight, mainly across urban areas. Muggy overnight lows in the upper 60s to lower 70s. Previous forecast: 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. && .SHORT TERM /SATURDAY THROUGH SATURDAY NIGHT/... 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. && .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 /00Z SATURDAY THROUGH WEDNESDAY/... As of 800 PM Friday... As has been the case the last several days, the showers have started to wind down as the sun sets. A heavier band of convection is currently running north to south between KRDU and KRWI and may stay in between them. Otherwise just light rain possible at all TAF sites. Another round of rain is expected overnight and then a heavy batch of convection is expected to set up over the eastern half of the forecast area. With increased convective coverage, ceilings will most likely stay in the MVFR range much of the day on Saturday. Winds will continue to be out of the south and around 10 kts on Saturday. A continued wet pattern will bring chances for precipitation and sub- VFR conditions each day through the long term forecast. && .RAH WATCHES/WARNINGS/ADVISORIES... Flash Flood Watch through Saturday evening for NCZ007>011-021>028- 040>043-075>078-085-086-088-089. && $$ SYNOPSIS...WSS NEAR TERM...CBL/WSS SHORT TERM...WSS LONG TERM...BLAES AVIATION...Ellis is the U.S. government's official web portal to all federal, state and local government web resources and services.