Area Forecast Discussion
Issued by NWS Raleigh/Durham, NC

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000 FXUS62 KRAH 131501 AFDRAH Area Forecast Discussion National Weather Service Raleigh NC 1000 AM EST Sat Jan 13 2018 .SYNOPSIS...
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Chilly high pressure will build into the Carolinas through the weekend. This high will retreat to our northeast Monday, bringing more seasonable temperatures on Tuesday. A strong upper level trough will cross the region Tuesday night through Wednesday night.
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&& .NEAR TERM /THROUGH TONIGHT/...
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As of 1000 AM Saturday... Patches of very light rain and drizzle along and ahead of the 850 mb trough will be exiting the Coastal Plain this morning, leaving the area dry for the rest of the day. Drier air is pouring in behind the front, with dewpoints falling through the 30s, and behind the line of clearing now extending through the central Piedmont, values are falling into the 20s and teens. Post-clearing gustiness of the NW winds will further dry out the low levels, however the post-trough 850 mb winds veer all the way around to a more northerly direction with weak confluence evident, such that broken (if not briefly overcast) stratocu may hang around for the rest of the day across northeast sections of the forecast area. Overall, the earlier forecast handled this well, so very few adjustments are needed. Temps will level off across the NW and fall a bit before leveling off in the east, so while daytime "highs" will be 42-56, afternoon temps are likely to be in the 40-51 range. -GIH Earlier discussion from 635 AM: Temperatures, while cooling off in the Piedmont, have not cooled as quickly per earlier thinking. Thus, have adjusted hourly temperatures today to reflect reality this morning. This required a nudge upward in high temperatures for today. Should start to see an erosion in the cloud deck in proximity of the Yadkin/Pee Dee River Valley through 14Z, progressing toward the Triangle region around mid-day or early afternoon. earlier discussion... Per water vapor imagery, negative-tilt upper trough swinging through the Carolinas and central Appalachians early this morning. The attendant surface cold front pushing through the western Piedmont at 07Z and will continue to lift east-ne through the pre-dawn hours. Sfc front passage noted by sfc winds veering to west-nw direction and the dewpoints dropping a solid 10-12 degrees. Cyclonic flow through the lower half of the atmosphere should maintain a good deal of cloudiness early this morning with decreasing cloudiness anticipated southwest-to-northeast coinciding with the passage of the 850mb trough and the lifting nwd of the 700mb trough later this morning. Low level cold air advection will initially cause temperatures to fall this morning then stabilize around mid day-early afternoon, then resume to fall once again by mid afternoon as the 925-850mb flow becomes nly. Thus, have temperatures falling into the 30s to lower 40s across the Piedmont by late morning then recovering into the low-mid 40s before dropping around mid-afternoon. Across the coastal plain and Sandhills, temperatures will fall through the 40s this morning then stabilize/recover to around 50 degrees by early afternoon. Clouds expected to linger longer north and east of Raleigh with an isolated shower possible in this region through mid morning. Tonight, skies will become mostly clear, especially after midnight, though the GFS depicts a patch of stratocu overspreading the region from the east this evening, then diminishing well after midnight. Currently having a tough time justifying this solution as 925-850mb nly and land based. Will depict some cloudiness this evening over the region but not to the extent of the GFS. Min temps in the 20-25 degree range with wind chill values by early Sunday ranging from 10- 15 above NW to the upper teens south.
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&& .SHORT TERM /SUNDAY AND SUNDAY NIGHT/... As of 310 AM Saturday... 850mb thermal trough resides immediately to our NW Sunday and Sunday night while sfc high pressure builds into the region. While expecting a good deal of sun, afternoon temperatures will be 12-15 degrees below normal, generally in the 35-40 degree range. With the sfc ridge projected to be overhead Sunday night, sfc wind will decouple around sunset. Temperatures will cool quickly in the cold dry air mass, so expect minimum temperatures by early Monday morning between 15 and 20 degrees. The normally colder locations in the Piedmont will likely be in the 10-15 degree range. && .LONG TERM /MONDAY THROUGH FRIDAY/... As of 415 AM Saturday... Medium range guidance remains in good agreement regarding the development and slow ewd progression of highly amplified flow aloft across the CONUS this period. Within this pattern, an initially strongly positively-tilted, and similarly strongly meridional trough aloft, will pivot across the MS Valley Mon-Tue, then to the East Coast while assuming a neutral tilt, by Wed-Thu. While some timing and amplitude details remain, spread in 00Z guidance is much less than 24 hrs ago. At the surface, increasingly modified Arctic high pressure will be in the place from Atlantic Canada to the sern US early next week, while an occluding clipper low will have migrated to the Great Lakes through Tue. Along the trailing Arctic front, an initially flat, triple point wave will meanwhile develop from the mid MS Valley to the cntl Appalachians through Tue. A warm front preceding the triple point is expected to cross cntl NC Mon night, but not manifest as a noticeable increase in surface temperatures until diurnal heating on Tue. The Arctic boundary is then forecast to collapse east of the Appalachians and across cntl NC Tue night, with following Arctic high pressure that will build into, and modify over, the sern US through the end of the week. Regardless of whether or not a closed low forms within the aforementioned meridional trough, strong jet dynamics and deep layer frontogenesis accompanying the upper jet/front and related low level frontal zone should be sufficient to support at least a light precipitation event over cntl NC Tue night-Wed. That precipitation would likely fall in the form of light rain or rain mixed with snow early Tue night, since temperatures will have warmed considerably, into the upr 40s to lwr 50s for most Tue afternoon; and the models are typically too quick in surging cold air masses east of the cntl Appalachians of VA/NC. However, partial thickness values and boundary layer temperatures are both forecast to become supportive of a west to east changeover to snow Tue night-Wed morning. This system certainly has the potential to produce measurable snow over the middle Atlantic states, including cntl NC, but forecasting such an occurrence with any degree of confidence at this time range would be premature. Indeed, if the trough aloft is weaker and more positively-tilted as it approaches NC, then a light rain, to inconsequential flake event would result. Stay tuned. Temperatures are expected to otherwise turn colder by Wed, before moderating through the end of the week. && .AVIATION /12Z Saturday through Wednesday/... As of 635 AM Saturday... Widespread MVFR ceilings across central NC will gradually lift into VFR ceilings this morning. Sfc winds have veered to a northwest direction in the western Piedmont in the wake of a departing cold front. This front should pass KFAY and KRWI prior to 15Z. Sfc winds will remain gusty with gusts 18-24kts probable through mid afternoon. Ceilings will diminish this evening with mostly clear skies anticipated Sunday through Monday night. An upper level storm system will bring the next threat for sub VFR ceilings late Tuesday night into Wednesday along with the possibility of snow showers. && .RAH WATCHES/WARNINGS/ADVISORIES... None. && $$ SYNOPSIS...Hartfield NEAR TERM...Hartfield/WSS SHORT TERM...WSS LONG TERM...MWS AVIATION...WSS

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