Area Forecast Discussion
Issued by NWS Raleigh/Durham, NC

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000 FXUS62 KRAH 150521 AFDRAH Area Forecast Discussion National Weather Service Raleigh NC 1230 AM EST Fri Dec 15 2017 .SYNOPSIS... A pair of frontal zones will merge over south-central NC tonight, and along which an area of low pressure will develop and track, to off the srn middle Atlantic coast through Fri. High pressure will follow and migrate across the southeastern US through the weekend. && .NEAR TERM /TONIGHT/... As of 920 PM Thursday... WV satellite and 00Z upr air data depicted this evening a positively- tilted trough extending from the wrn shore of Hudson Bay swwd across the upr Midwest, Four Corners, and nrn Mexico. The trough spanned both the nrn and srn branch of the upr jet, with associated split stream flow evident across the OH valley and nrn middle Atlantic states and over the Deep South, respectively. These two streams are forecast to merge and intensify to near 200 kts over the srn middle Atlantic states during the next 12-18 hrs, as the nrn stream portion of the aforementioned trough axis migrates sewd, and the srn stream bulges slightly nwd in the wake of an inconsequential srn stream perturbation now migrating across the Carolinas. At the surface, a lead, weak cold front/trough axis that settled across cntl NC earlier Thu was analyzed at 02Z from the TX Gulf coast enewd through s-cntl MS/AL, n-cntl GA/SC, and srn NC. A more marked air mass change is apparent with a trailing front analyzed, and evident as a fine line in AKQ radar data during the past couple of hours, from the nrn OBX nwwd to swrn VA, then swwd through wrn NC. This frontal zone was also evident at both 925 and 850 mb between RNK and GSO this evening. In fact, there was a 22 C temperature difference between CHS and PIT at 850 mb, indicative of the sharp temperature gradient across the aforementioned frontal zones - and related to the extreme 200-300 mb winds forecast to develop over NC/VA on Fri. These two low level frontal zones are forecast to merge in a trough/convergence axis now developing over s- cntl NC per recent surface obs, with associated light nely flow, to calm within the trough axis, likely to encompass cntl NC tonight. The models suggest the flow just above the surface, in the 925-850 mb layer, will become sly and ascend isentropically-atop the surface front, such that initial passing mid-high cloudiness will yield to a scattered to broken stratus layer over the nrn Piedmont and nrn Coastal Plain, between US 64 and the VA border, by or shortly after 12Z. While associated saturation and warm rain process light rain may develop there, a relatively dry sub-cloud layer below 1500 ft will cause most of that precipitation to evaporate before reaching the ground south of the VA border, should it even develop within that shallow moist layer. Otherwise, there may be some patchy fog within the aforementioned trough axis over srn NC, from near VUJ to FAY to CTZ, owing to calm/radiational cooling of a marginally more moist surface layer characterized by surface Td`s generally between 30-34 degrees there, relative to mid 20s to the north. No changes planned for previously forecast lows within a few degrees of 30. && .SHORT TERM /FRIDAY AND FRIDAY NIGHT/... As of 253 PM Thursday... The aforementioned low pressure wave will quickly lift to the NE Friday morning as a northern stream short wave, currently over the central Plains, moves east and sweeps across our area during the afternoon. As the wave departs and the short wave trough axis moves across, dry NW flow will return in their wake, clear the sky out by late afternoon. Other than clouds early in the day, look for fair weather with highs from the mid 40s NW to lower 50s SE. Clear and cold Friday night with high pressure building over the area, light or calm winds, and good radiational cooling conditions. That should set us up for lows dipping down in the mid 20s...perhaps even colder in the usual spots. && .LONG TERM /SATURDAY THROUGH THURSDAY/... As of 247 PM Thursday... Surface high pressure over the area will produce sunny skies with slightly cooler than normal temperatures to kick off the weekend. Highs Saturday will reach the upper 40s north to low 50s south followed by lows at or slightly below freezing Saturday night. The high pressure will edge offshore Sunday, with cloudiness increasing in return flow and associated moisture advection along with low amplitude ridging aloft on Sunday. The warm air advection and rising heights will produce modestly warmer highs in mostly the mid and upper 50s. Skies will become cloudy with scattered showers Sunday night as a series of waves begin to lift northeast into the area in deep southwest flow off the Gulf. The low amplitude ridge will linger through Tuesday, providing a conveyor for these waves and associated periods of enhanced shower activity. As we remain in the warm and moist airmass through the period, highs will be similar both Monday and Tuesday, ranging from upper 50s north to low and mid 60s south. Very difficult to forecast how much rain we might get given the duration of the event and model difficulties with timing and strength of the waves. A strong but relatively flat short wave in the northern stream moving across the Tennessee and Ohio Valleys will push a cold front east, effectively cutting off our moisture Tuesday night. The upper flow is basically zonal and the high pressure building in behind the front is only modestly colder than the air it is replacing, so highs will take only a mild hit Wednesday and Thursday...mostly in the low and mid 50s. && .AVIATION /00Z FRIDAY THROUGH TUESDAY/...
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As of 1230 AM Friday... VFR parameters should persist across central NC through 08Z. After 08Z, chances will increase for the potential for a deck of stratus to develop with MVFR ceilings 1500-2500ft probable, predominantly across the northern TAF sites. This deck of ceilings expected to persist until mid day, then gradually dissipate. Some isolated fog may also develop at FAY near daybreak, but low confidence, as mid to upper level cloud deck could dictate extent and/or duration. There is a high chance that VFR parameters will occur Friday night through Sunday morning. The next threat for sub VFR parameters is expected late Sunday into early next week as a low pressure system approaches from the west-sw.
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&& .RAH WATCHES/WARNINGS/ADVISORIES... None. && $$ SYNOPSIS...MWS NEAR TERM...MWS SHORT LONG TERM...mlm AVIATION...JJM/WSS is the U.S. government's official web portal to all federal, state and local government web resources and services.