Area Forecast Discussion
Issued by NWS Raleigh/Durham, NC

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000 FXUS62 KRAH 140540 AFDRAH Area Forecast Discussion National Weather Service Raleigh NC 935 PM EST Wed Dec 13 2017 .SYNOPSIS... A fast moving upper level disturbance will move just north of the region tonight, and will be followed by a weak surface cold front that is expected to pass across central North Carolina Thursday morning. High pressure will then build into the area by late Thursday. && .NEAR TERM /THROUGH TONIGHT/... As of 935 PM Wednesday... WV satellite and 00Z upper air data depict a lead shortwave trough beginning to lift enewd to newd into swrn PA, while a trailing shear vorticity-dominated one centered invof srn Lake MI was digging across the lwr Great Lakes and toward the OH Valley. Related to the lead trough, a narrow band of DCVA/forcing for ascent --and an associated, approximately 100 mile wide axis of mid level moisture now over the NC Piedmont, and centered around 10 k ft per surface observations and 00Z GSO and RNK RAOB data-- will pivot east across e-cntl NC through 05-06Z. In addition, strongly veering flow through the lowest couple of kilometers (WAA) --and implied forcing for ascent-- was evident throughout the lwr OH Valley, srn middle Atlantic, and Southeast. A band of precipitation, mostly in the form of snow, not coincidentally was pivoting newd within a zone of strong QG-forcing for ascent accompanying the lead shortwave trough and preceding strong lower tropospheric WAA - a truly textbook case of QG-theory at work. At the surface, a parent, 993 mb cyclone was analyzed over far nwrn OH at 02Z, with a preceding warm front analyzed sewd from the low - across cntl OH and the VAs and on the srn flank of the aforementioned precipitation band. To the south of the warm front, sly to swly low level flow exists in the warm sector over the Carolinas. Light radar returns have steadily lowered, to about 2000 ft in a narrow, approximately 10 nm wide arc between IGX and RDU in the past hour. Although extremely narrow, and originating from the aforementioned narrow band of ceilings centered well above the surface - around 10 k ft (between 8-13 k ft per KRAX data), a 5-10 minute period of a few flurries may reach the surface as the parent moist axis shifts ewd. Otherwise, a strong swly to wswly llj evident on the RTP profiler, between 30 and 45 kts at 2 and 5 k ft, respectively, will occasionally mix to the surface and manifest as gusts up to 25 mph, as has already been the case across several Piedmont sites in the past couple of hours. Given that WAA is occurring in that layer, which will promote stability, it is likely that at least temporary surface decoupling and lessening of the surface wind to 5-6 kts or less will allow temperatures to dip into the middle 20s to around freezing by daybreak - about 10 degrees from 9 PM readings. && .SHORT TERM /THURSDAY THROUGH THURSDAY NIGHT/... As of 248 PM Wednesday... Fair weather is in store for Thursday in the wake of tonight`s departing short wave trough and the sfc cold front continuing to push south of our area. With the upper flow becoming westerly, look for the high clouds currently located over the central Plains to gradually move eastward and across our area. Otherwise, with low level thicknesses remaining close to their pre-frontal values (some 30-40m higher than today), we should see highs at least 8-10 degrees warmer than today. Overall, Thursday will be partly cloudy with highs in the lower 50s. Thursday evening the low level wind field will shift to the N/NE as high pressure over the Ohio Valley moves east. Although low level thicknesses may drop a few meters on this N/NE flow...increasing high clouds from the southwest should offset the cooler airmass. Lows in the upper 20s to around 30 under partly cloudy skies. && .LONG TERM /FRIDAY THROUGH WEDNESDAY/... As of 255 PM Wednesday... A strong upper short wave moving across the Ohio Valley will nudge a reinforcing surge of cold air into the area. Surface high pressure will then build across the area, resulting in plenty of sun, but suppressing high temperatures to the 40s Friday and Saturday. Morning lows Saturday will be mostly upper 20s, with some areas across the south only falling to near freezing. Short wave ridging will be amplifying north into the Eastern Conus by Saturday night, with increasing heights and initiation of warm air advection in return flow as the surface high moves offshore. Cloudiness will be on the increase by later Sunday with highs reaching mostly low and mid 50s after a morning low near freezing. By Monday, a low pressure area will be lifting northeast out of the Gulf of Mexico. There is a good deal of uncertainty as to how the details will play out, with model timing issues as the ECMWF has a strong fast moving short wave race across the Ohio Valley to kick the system offshore quickly, resulting in precip pretty much limited to the southeast, while the GFS`s slower short wave allows for a couple of waves to ride up the upper ridge into the area to produce more widespread precip and prolonging the duration of precip chances into Tuesday. Will maintain our ongoing forecast of chance PoPs from Sunday night through Tuesday and allow details to emerge from later model consensus. That said, precipitation type will be no problem with southern stream flow into the area allowing highs Monday and Tuesday to reach the 50s after morning lows in the low to mid 40s. Wednesday looks to be a dry and a bit cooler, but still near seasonable in the 50 to 55 range. && .AVIATION /00Z THURSDAY THROUGH MONDAY/... As of 1230 AM Thursday... There is a high probability of VFR conditions across central NC through 00Z Friday. A fast moving upper disturbance continues to the east at this hour, leaving the northern piedmont under clearing skies and a brisk Northwest wind. An attendant sfc front will cross central NC later tonight through early Thu. This system will be accompanied by a deck of mid level clouds with ceilings at or above 10000ft. These clouds will depart prior to sunrise Thu. Low level wind shear is also a concern in the proximity of KFAY and KRWI as winds just a couple thousand feet above the surface will be swly 35- 40kts for a short period of time. VFR conditions are expected to continue through Saturday night. Another area of low pressure will affect our region late Thu night through Friday. Ceilings associated with this system are expected to remain VFR. A low pressure system approaching from the west may bring a threat for sub VFR ceilings to central NC late in the weekend. && .RAH WATCHES/WARNINGS/ADVISORIES... None. && $$ NEAR TERM...MWS SHORT LONG TERM...mlm AVIATION...WSS/JJM is the U.S. government's official web portal to all federal, state and local government web resources and services.