Area Forecast Discussion
Issued by NWS Raleigh/Durham, NC

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000 FXUS62 KRAH 201052 AFDRAH Area Forecast Discussion National Weather Service Raleigh NC 652 AM EDT Thu Apr 20 2017 .SYNOPSIS... A cool wedge of high pressure extending into the Carolinas early today will give way to a surge of warmer air this afternoon. A cold front will approach from the northwest late Friday, then stall over North Carolina on Saturday. A strong storm system will move across the Carolinas on Sunday. && .NEAR TERM /THROUGH TONIGHT/... As of 430 AM Thursday... Latest surface analysis shows a 1024 MB surface high centered over GA and SC associated with an in-situ area of cold air damming. An associated region of fog with visibilities less than 1/2 mile and low clouds across the western Piedmont continues to extend eastward this morning. The pattern and some high res NWP suggests the fog will thicken and expand this morning and we have accordingly issued a Dense Fog Advisory until 10am. While the CAD is relatively weak, the lack of a well defined erosion mechanism will allow the fog and low clouds to persist in many locations through mid morning before giving way to partly to mostly sunny skies by midday. As the surface high moves offshore this afternoon, a general southwest to westerly flow aloft will develop resulting in warm advection. Low level thickness values increase into the 1394 to 1400m range by late in the day, supportive of highs in the lower to mid 80s across most of the area. Convection allowing models and other guidance suggest some convection will develop across the higher terrain of the Appalachians this afternoon in a region of warm advection. This scattered convection will shift eastward late this afternoon possibly affecting the western Piedmont and then diminish as it moves into the eastern Piedmont early this evening. Will cover the precipitation chances will low end chance PoPs in the Triad and slight chance PoPs in the Triangle for the late afternoon and early evening hours. Any lingering convection will dissipate by mid evening. Partly to mostly cloudy skies are expected tonight with a mild southwesterly wind and lows in the lower to mid 60s. -Blaes && .SHORT TERM /FRIDAY THROUGH FRIDAY NIGHT/... As of 430 am Thursday... A closed upper level low will move across the Great Lakes on Friday and then open up as it moves into eastern Canada on Friday night. A trailing cold front will approach central NC during the afternoon and then drop into northern NC toward daybreak Saturday and stall. The warm air mass ahead of the front with friday afternoon highs in the mid 80s to near 90 and dew points climbing into the upper 50s to lower 60s will contribute to 500-1000 J/Kg of MLCAPE. While the upper trough is fairly modest, there will be sufficient forcing with the front to generate scattered convection that will shift southeast across central NC during the afternoon and evening. Bulk shear values are expected to range between 20 and 25 kts suggesting some storm organization, largely in the form of multicellular clusters. Steep lapse rates with a deep boundary layer suggests main threat will be stronger wind gusts in strong to marginally severe thunderstorms. The convective intensity will wane in intensity during the evening with the front providing a focus for widely scattered showers or thunderstorms overnight. Lows will range in the lower to mid 60s. -Blaes && .LONG TERM /SATURDAY THROUGH WEDNESDAY/... As of 415 AM Thursday... While the overall forecast pattern described previously has changed very little in recent days, mesoscale/diabatic influenced from convection Fri and Fri night will modulate the location of subsequent convection and the related effective frontal zone through the weekend. As such, confidence in forecast details remains lower than average. A trough now extending along and just offshore the wrn NOAM coast, and compromised of multiple embedded circulations/ perturbations, will amplify across the Rockies and consolidate into a closed low in the mid MS Valley vicinity by early Sat. The trailing/upstream perturbations will pivot around this mean low in positive tilt fashion across the Mid and Deep South on Sun, before moving off the Southeast coast with an increasingly neutral to negative tilt through early Tue. An associated surface cyclone will track ENEwd to Ewd along the preceding frontal zone draped across the srn and central Appalachians and srn Middle Atlantic states late Sat through Sun, then to the coast of the Carolinas, where it will slow, by early Mon. That front, which will have slowed over srn VA Fri night, will likely sag Swd into at least the nrn NC Piedmont Sat, with an associated low level convergence axis provided by both the front and the aforementioned preceding outflow - likely near or just S of the VA state line. Convection will become diurnally-enhanced along and N of the front, with semi-organized storm multi-cell storm modes, and perhaps at least transient supercells in the vicinity of the low level convergence --and enhanced shear-- axis. Some slight NWd retreat of the boundary will be possible Sat night- early Sun afternoon, downstream of the aforementioned surface cyclone approaching from the W. That would suggest the better chances of convection would (briefly) pivot into the far NW portions of, or just out of, the RAH CWFA through early Sun. Another round of convection, some organized and strong to severe, will then accompany the surface cyclone and attendant cold front as they cross the Carolinas Sun afternoon and night. As the surface low slows beneath the aforementioned complex upper trough, deformation/stratiform rain and low overcast are likely to linger over the ern Carolinas, including at least the ern half of the RAH CWFA, late Sun night through at least Mon. Cooler (than previous days) temperatures will result, with highs likely a category or two below average on Mon. Drying and warming conditions will return as the low finally moves away from the coast, and return flow overspreads the Southeast, Tue- Wed. && .AVIATION /12Z THURSDAY THROUGH MONDAY/... As of 215 AM Thursday... Areas of IFR and LIFR ceilings and some fog across much of the western and northern Piedmont including the KGSO terminal will persist overnight and expand east and southeast. VFR and MVFR conditions across the remainder of the Piedmont and the Coastal Plain including the KRDU, KRWI and KFAY terminals will deteriorate as low clouds and fog expand into the area overnight. Widespread IFR and LIFR conditions are expected during the few hours around daybreak. Light mainly southwest winds will weaken further and become calm in many locations providing additional support for fog development. Aviation conditions will improve between 13Z and 16Z as increasing sunshine and a westerly component to the low level flow results rising ceilings and erosion of the fog with the most aggressive improvement arriving after 14Z with just about all locations reaching VFR conditions by 16Z with SCT clouds at 4-5kft and periods of BKN clouds at 10-15kft. Southwesterly winds will increase to 7 to 10 kts this afternoon. Isolated showers and thunderstorms will develop across the NC mountains during the afternoon and push east possibly affecting the western Piedmont between 19Z and 23Z. Coverage and confidence in the amount of convection prohibits inclusion in the TAF at this time. Looking further ahead... A cold front will approach the region on Friday and then stall across northern NC Friday night and linger into Saturday and Sunday. A storm system will sweep east across the Carolinas late Sunday. These features will result in increased chances of adverse aviation conditions with sub-VFR ceilings with scattered showers and thunderstorms through much of the weekend. Improving conditions are expected on Monday. -Blaes && .RAH WATCHES/WARNINGS/ADVISORIES...
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Dense Fog Advisory until 10 AM EDT this morning for NCZ021>024- 038>040-073>075-083>086.
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&& $$ SYNOPSIS...BLAES NEAR TERM...BLAES SHORT TERM...BLAES LONG TERM...MWS AVIATION...BLAES

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