Area Forecast Discussion
Issued by NWS Raleigh/Durham, NC

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000 FXUS62 KRAH 170228 AFDRAH Area Forecast Discussion National Weather Service Raleigh NC 1030 PM EDT Fri Mar 16 2018 .SYNOPSIS... A cold front will stall and remain stationary just south of central NC through Saturday. An area of low pressure will cross the area Saturday night, pushing the front farther away from our region. This will allow an area of high pressure and associated cooler air mass to build into the Carolinas on Sunday. && .NEAR TERM /OVERNIGHT/...
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As of 1030 PM Friday... Little change required to the near term forecast. 00Z GSO sounding tells the story this evening...the atmosphere is bone dry. The precipitable water value is only 0.10 of an inch. Even at this time in the evening, relative humidity values are still in the 20s to the lower 30s (very unusual for this time of year and time of night). Based on this and the lack of moisture advection upstream, will eliminate the pre dawn slight chance PoPs in the west, and lower the PoPs for Saturday morning across the Piedmont. Otherwise, the dry air mass and subsiding sfc wind has permitted temperatures to cool into the 40s north half, and the lower 50s south. Increasing mid-high level clouds after 06Z will moderate nocturnal cooling, primarily across the piedmont. Thus expect min temps to vary from the lower 30s across the northern coastal plain where the skies will remain clear until just prior to daybreak, to the lower 40s across the south-southwest.
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&& .SHORT TERM /SATURDAY AND SATURDAY NIGHT/... As of 245 PM Friday... The front will begin to lift northward as a warm front ahead of the approaching system toward daybreak Saturday. As a result, a more southerly warm, moist advection will kick in, slowly thickening and lowering the cloud coverage. Meanwhile, a weak upper disturbance will move east across the area, which will enhance layer lift and increase PoPs progressively from the west, with better chances of rain increasing into late day. Highs will be mild and expect a strong gradient across the area ranging from mid 50s across the northeast to near 70 in the south. Our best rain chances will be early Saturday night as a much stronger mid level wave moves southeast out of the Ohio Valley and across the area. The frontal zone, which is expected to lie across the northern tier, will separate higher chance (50%) PoPs north from more scattered coverage (20-30%) across the south, with diminishing chances by Sunday morning as the wave departs. Mid level lapse rates will be steep, with modest favorable shear ahead of the short wave. Diurnal timing will be unfavorable, of course, and as such, perhaps we could have an elevated rumble or three, but will hold off the introduction of thunder at this point. Low temperatures will range from upper 30s across the north to the mid and even upper 40s across the south. Rainfall amounts will be light, a couple of tenths of an inch over most areas. && .LONG TERM /SUNDAY THROUGH FRIDAY/... As of 400 PM Friday... The forecast large scale pattern and overall forecast rationale remain unchanged. That pattern will consist of a high latitude blocking ridge over the nwrn N. Atlantic to near Greenland and a negative height anomaly tucked beneath it over sern Canada, with an associated slowing/blocking of the flow characteristic of a negative NAO phase. Resultant confluent flow aloft over the Northeast and mid Atlantic, at the base of the sern Canada vortex, will consequently cause an upstream shortwave perturbation to get sheared on a sly track across the srn middle Atlantic states Mon night and Tue, with associated Miller "Type B" cyclogenesis along the srn fringe of a cold air damming high anchored by the confluence aloft. The result will be an associated high to likely probability of rain/showers late Mon-Tue, with nrn (climatologically-favored) counties likely to remain in a cooler and more stable wedge air mass, while srn ones experience milder and slightly unstable conditions, with an associated slight chance of thunder Tue aft. Thereafter, medium range guidance has trended toward a deeper and more srn solution regarding shortwave perturbations rotating through the base of the l/w ern U.S. trough, such that a mid-upr level low may briefly close off as it crosses the cntl Appalachians and srn middle Atlantic states Tue night-Wed. Such a scenario would favor the development and ewd migration of a second frontal wave across the Carolinas, with deepening as it tracks up the middle Atlantic coast, and only slowly so owing to the blocking pattern downstream. As such, probability of precipitation has been increased into the solid to high chance range for Tue night-Wed, with some of that in the form of deformation band wintry precipitation over portions of the middle Atlantic, including as far south as s-cntl VA, or even w- cntl NC. With each passing day deeper into the month of March, however, the likelihood of meaningful wintry precipitation in our area grows smaller. Will trend the forecast toward climo at this time and favor all liquid. A drying and clearing trend will commence as the ern U.S. trough gradually relaxes and lifts away through the end of the week. && .AVIATION /00Z SATURDAY THROUGH WEDNESDAY/... As of 730 PM Friday... 24-Hour TAF Period: VFR conditions are expected to continue across all TAF sites through 00Z Sunday. A cold front has stalled across south central North Carolina Friday evening, with little in the way of migration expected overnight. Winds are expected to continue to diminish after dark, likely settling into the 5 to 10kt sustained range. A gradual direction shift is forecast to occur overnight, from a northerly wind to south/southwesterly wind expected just after daybreak. Upper-level clouds will also increase from west to east, and gradually lower from 25kft around daybreak to roughly 5 to 7kft by sunset Saturday evening. Looking ahead: Upper level disturbances tracking ESE across the area (along the frontal zone) will bring increasing chances for sub-VFR conditions Sat night into Sunday. VFR conditions may return briefly Sun afternoon, then another threat of sub-VFR conditions is possible Monday into the middle of next week as a series of disturbances passes overhead. Confidence is not high during the Sat-Mon time frame, with wide variation among models regarding timing of these disturbances, so aviation interests should check back for the latest. && .RAH WATCHES/WARNINGS/ADVISORIES... None. && $$ SYNOPSIS...WSS NEAR TERM...WSS SHORT TERM...mlm LONG TERM...MWS AVIATION...JJM is the U.S. government's official web portal to all federal, state and local government web resources and services.