Area Forecast Discussion
Issued by NWS Raleigh/Durham, NC

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926 FXUS62 KRAH 261029 AFDRAH Area Forecast Discussion National Weather Service Raleigh NC 630 AM EDT Sat May 26 2018 .SYNOPSIS... High pressure centered over the western Atlantic will extend westward across the Carolinas through Sunday. Deep southerly flow between Subtropical Storm Alberto and high pressure over the western Atlantic will transport tropical moisture north into the area late Sunday night through mid to late week, resulting in periods of moderate to to heavy rain. && .NEAR TERM /THROUGH TONIGHT/... As of 330 AM Saturday... An area of high pressure situated over Bermuda continues to ridge westward into central North Carolina, with broad troughing beginning to dig southward across the Gulf Coast States. This continues the southerly/southwesterly flow regime across the Carolina`s, characterized by a warm and humid maritime tropical airmass, with Pwat values in the 1.6 to 1.8 inch range. Early morning stratus will lift gradually after sunrise, aided by a weakening shortwave trough rotating north through the western Piedmont by 15z or so. This could initiate an early round of showers with intermittent thunder prior to the noon hour. These showers will likely decay, setting up a nice differential heating/outflow boundary near the foothills by early afternoon, which should serve as epicenter to more widespread thunderstorm development between 2pm - 8pm Saturday. This activity should be limited to areas along and west of US-1 initially, with a few more organized clusters pushing east into the northern Piedmont by nightfall. The favored seabreeze areas in the far south and eastern portions of NC could also see an enhanced period of precipitation in the early evening hours. Either way, with little sustainable forcing and limited shear, thunderstorms are expected to remain sub- severe and multicellular in nature, with heavy rainfall being the primary threat. Temperatures should range near normal, with highs settling in the low to mid 80s Saturday afternoon, and lows remaining near 70 degrees overnight into early Sunday morning. && .SHORT TERM /SUNDAY/... As of 330 AM Saturday... Sunday will provide a brief reprieve from the unsettled weather pattern, as central North Carolina finds itself between a frontal zone to the north and the developing tropical system Alberto to the South. Temperatures will rise into the mid to upper-80s during the afternoon, with a few stray showers/storms possibly sneaking south of the VA/NC border, or east of the Blue Ridge Mountains, as the frontal zone becomes more active by late afternoon. Most areas will remain dry and under broken skycover Sunday. && .LONG TERM /SUNDAY NIGHT THROUGH FRIDAY/... As of 330 AM Saturday... Following a weakness between a pair of prominent ridges over the Central US and off the SE coast, Subtropical Storm Alberto is forecast to track NNW, making landfall along the Mobile/Florida Panhandle late Monday, and will proceed slowly north through the Deep South and Tn Valley through midweek, before eventually becoming absorbed by the westerlies as it lifts into the Ohio Valley and Northeast US late Wednesday and into Thursday. Deep southerly flow channeled between Alberto and ridging off the SE coast, will draw high-daily maximum PWATS of over 2.0" north into the area starting late Sunday night and will reside over the area through mid to late week. This anomalous moisture coupled with s/w disturbances embedded in the southerly flow aloft, along with afternoon heating and resultant destabilization will set the stages for periods of moderate to heavy rains across central NC Monday through at least Wednesday. WPC`s new Day 3(Monday/Labor Day) Excessive Rainfall Outlook, which only captures the beginning of the expected wet-unsettled period, has the southeast two-thirds of the forecast area in a slight risk for excessive rainfall(risk of rainfall exceeding flash flood guidance), with the northern- northwestern third in a marginal risk. By Thursday-Friday, as Alberto`s remnants move off to the north, it`s possible that the Carolinas could get dry slotted, which could lower/limit convective coverage. With soils already wet and streams and creeks elevated from recent rainfall, additional heavy rain Monday through Wednesday will lead to a threat of flash flooding across central North Carolina, with river flooding possibly becoming a problem later in the week. This flooding threat will likely necessitate the issuance of a Flash Flood Watch in the next day or two. Residents that live in flood prone or low-lying areas, should monitor the forecast closely. Widespread clouds and rain will temper the heating with highs generally 80 to 85 Monday through Wednesday. Potential for some partial sun and stronger insolation could boost afternoon highs in the 85 to 90 range. Balmy overnight lows in the upper 60s to lower 70s. && .AVIATION /12Z SATURDAY THROUGH WEDNESDAY/... As of 630 AM Saturday... Through 12Z Sunday: MVFR/IFR conditions due to low stratus will slowly improve early Saturday morning, allowing CATs to slowly rise back to VFR by mid day. This improvement will be quickly followed by scattered shower and tstms development from late-morning through the evening hours. Best coverage will likely be at KINT/KGSO during the day, with TSRA struggling to push east into KRDU by the late afternoon/evening hours. Flight conditions will locally lower to MVFR/IFR in and around shower/tstms activity, but have settled on tempo groups with timing uncertainties remaining. SW winds should persist around 10kts mid morning Saturday with local gusts possible, especially in the direct vicinity of thunderstorms. Any remaining showers and storms should dissipate by nightfall, likely prior to reaching KFAY/KRWI. Continued WAA regime and a moist boundary layer should lead to additional stratus development overnight/early Sunday morning, with intermittent VSBY drops also possible. Looking beyond 12Z Sunday: After a period of morning stratus, a shower or storm will be possible across portions of the triad, which could exhibit brief CAT drops. Widespread adverse aviation conditions are expected Monday through Wednesday as widespread showers and thunderstorms are expected. && .RAH WATCHES/WARNINGS/ADVISORIES... None. && $$ SYNOPSIS...CBL NEAR TERM...JM SHORT TERM...JM LONG TERM...CBL AVIATION...JM

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