Area Forecast Discussion
Issued by NWS Raleigh/Durham, NC

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000 FXUS62 KRAH 150736 AFDRAH Area Forecast Discussion National Weather Service Raleigh NC 236 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/...
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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. Saturday coming shortly...
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As of 235 AM Friday... The surface high will shift offshore on Sunday as a warm front lifts northward through the area Sunday eve/night, resulting in south- southeasterly return flow and warm advection into Central NC. Meanwhile aloft, the Southeast US will be under an upper level ridge, while a deep trough dominates over the Rockies and Southwest. A shortwave will break off from the low, migrating east-northeast through the Midwest toward the Mid-Atlantic. As of the latest model runs, the shortwave should ride the northern periphery of the ridge and stay north of the area Sunday/Sunday night. Temperatures will be on the rise, with highs in the mid to upper 50s and lows in the low to mid 40s. Although the models are in much better agreement with the overall upper level pattern, there are still some significant differences in the temperature and precipitation forecasts. The closed low over the Southwest US will become more of an open trough on Monday, gradually deamplifying as it lifts ENEward into the Midwest Tuesday/Tuesday night. The southwest flow into Central NC will increase during this time, increasing the moisture advection and thus chances of precipitation. Meanwhile, a cold front will drop southward toward and into Central NC Tuesday/Tuesday night. The highest chances for and amount of rain will be across the south, ahead of the front. The weakening trough turned shortwave may enhance the precipitation along the front Tuesday night as it passes near/over the area. Temperatures will be unseasonably warm Monday and Tuesday, with highs in the upper 50s to mid 60s and lows in the mid to upper 40s. Expect a slight decrease in temperatures for mid week, albeit still a bit above normal (highs in the mid to upper 50s and lows in the 30s Wed night and 40s Thu night).
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&& .AVIATION /00Z FRIDAY THROUGH TUESDAY/... 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. && .RAH WATCHES/WARNINGS/ADVISORIES... None. && $$ SYNOPSIS...MWS NEAR TERM...MWS SHORT LONG TERM...KC AVIATION...JJM/WSS is the U.S. government's official web portal to all federal, state and local government web resources and services.