Area Forecast Discussion
Issued by NWS Raleigh/Durham, NC

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369 FXUS62 KRAH 221532 AFDRAH Area Forecast Discussion National Weather Service Raleigh NC 1135 AM EDT Tue Aug 22 2017 .SYNOPSIS... A pre-frontal surface trough will develop across western NC today, in advance of a strong cold front that will approach from the northwest tonight and cross central NC late Wednesday and Wednesday night, reaching the coast on Thursday. && .NEAR TERM /THROUGH TONIGHT/...
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As of 1135 AM Tuesday... Current forecast is in good shape with only minor adjustments required. Late morning surface analysis shows the remains of a stationary front across SC and GA with a developing lee trough across western VA and the western Carolinas. Recent GOES-16 visible satellite imagery shows a developing cumulus field across central NC with the most widespread and thickest cumulus to the southeast across the Coastal Plain and coastal region in a region of deeper moisture and localized convergence just inland. Scattered showers and thunderstorms are developing in this region which will impact much of Sampson, and southern portions of Wayne and Cumberland Counties. Elsewhere, dry weather is expected today with partly cloudy skies. Current forecast highs in the lower 90s look on target. -Blaes Previous discussion...The strong 594 dm mid/upper level anticyclone centered over the SE US this morning will begin to break down today, with H5 heights falling slightly by the afternoon/evening with the approach of the northern stream trough and attendant cold front moving into the Ohio and Tn Valleys. At the surface, weak surface high centered off the NC coast will shift further offshore this afternoon, allowing the development of pre-frontal sfc trough in the lee of the mountains. Central NC remains in a void of appreciable synoptic scale forcing/lift through tonight. And as such, convective rain chances are expected to remain rather low and diurnally driven, confined across the far SE zones in the vicinity of the sea breeze and across the western part of the state along the developing pre-frontal trough. Nocturnal cooling/stabilization this evening should lead to dry conditions by late evening, and it should remain dry until the approach of the cold front Wednesday afternoon. Highs in the lower 90s with lows in the lower to mid 70s.
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&& .SHORT TERM /WEDNESDAY AND WEDNESDAY NIGHT/... As of 330 AM Tuesday... On the southern periphery of the main belt of westerlies accompanying the northern stream trough and low pressure system lifting northeast through SE Canada, a cold front will advance SE into the area Wednesday afternoon and will cross Central NC Wednesday evening/night, exiting the coastal plain counties AOA daybreak Thursday. Strong insolation within the moist airmass featuring dew points in the lower to mid 70s, in advance of the cold front, will likely support moderate instability on the order of 1500-2500 J/kg across central and eastern portion of the forecast area Wednesday afternoon. A pre-frontal convection band/line is expected to develop across the western Piedmont by the early afternoon, with the potential for the line to strengthen/become better organize as it pushes eastward into the more juicier more unstable air maximized across the central Piedmont, Sandhills and Coastal Plain counties. Unidirectional SWLY shear of 20 to 25kts would support a primary threat of straight-line damaging winds with NCAPE values ~0.25 suggesting rather fat CAPE profiles along and east of US 1 which will make for prolific lightning producers. Latest Day 2 outlook from SPC has all of central NC in a marginal risk threat and will convey this threat in the HWO with the great severe threat likely to happen between 2pm to midnight. This pre-frontal band of convection will likely serve as the effective cold front, with only isolated/slight chance pops along the actual cold frontal passage late Wednesday night/early Thursday morning. Highs in the upper 80s NW to lower 90s. Lows Wednesday night will depend on the timing of the cold front, with some cooler mid to upper 60s possible across the NW Piedmont, and lower/mid 70s SE. && .LONG TERM /THURSDAY THROUGH MONDAY/... As of 330 AM Tuesday... A series of s/ws rotating through a l/w trough over the eastern U.S. will cause generally dry and not as hit conditions to central NC this period. The lead s/w will aid to drive a sfc cold front through central NC early Thu morning. This feature expected to lie just south and east of the Triangle at 12Z Thu, and should exit our southeast counties by early afternoon. Forcing along the boundary interacting with abundant moisture and modest instability will support scattered showers and possibly a thunderstorm. Over the nw piedmont, partial clearing should occur though cloudy periods will be probable at least in the morning, and chances for rain appear to be less than 20 percent as subsidence behind the front should suppress shower development. The next s/w expected to cross to our north Friday. This system will enhance the cloud cover over our region and a stray shower or two cannot be ruled out, primarily north and northeast of Raleigh. The big story, will be the arrival of a notably drier and cooler air regime as sfc dewpoints will drop into the upper 50s and lower 60s while afternoon temperatures will be in the 80-85 degree range. A modifying Canadian high pressure system will extend into central NC by Saturday, enhancing the drier cooler air mass in place. Afternoon temperatures once again will average several degrees below normal, near 80-lower 80s with lower/comfortable humidity values. Forecast confidence wanes Sunday into Monday as latest medium range guidance suggesting the potential for a tropical system to develop and cross the Atlantic waters offshore of the SE U.S. While chances for this system to move closer to the coast appear small at this time, this system will need to be monitored later in the week. Circulation around this system and the sfc high to our north may create breezy conditions over the coastal plain Sunday with a notable uptick in cloud cover, though forcing for showers should remain well to our east. If the parent sfc high moves east Monday, our low level flow will become more easterly over our region. This fetch will lead to a moistening over the lower half of the atmosphere, resulting in more clouds than sun. If this occurs, temperatures early next week may struggle to reach 80 over most of central NC. && .AVIATION /06Z TUESDAY THROUGH SATURDAY/... As of 732 AM Tuesday... 24 Hour TAF period: LIFR to MVFR fog/stratus will lift and disperse, returning to VFR AOA 15z. An isolated shower/thunderstorm can`t be ruled out on this afternoon, but much too low to include any mention in TAFS. Low-level flow will become more west- southwesterly this evening and tonight, with a slight 3 to 5kt breeze overnight, which should limit fog/stratus development. Looking beyond the 24 hour TAF period: A cold front Wednesday will bring adverse conditions to the area through Thursday when the front settles along the coast. High pressure will bring a return to VFR conditions Friday through the weekend. && .RAH WATCHES/WARNINGS/ADVISORIES... None. && $$ SYNOPSIS...CBL/Badgett NEAR TERM...Blaes/CBL SHORT TERM...CBL LONG TERM...WSS AVIATION...CBL/Ellis is the U.S. government's official web portal to all federal, state and local government web resources and services.