Area Forecast Discussion
Issued by NWS Raleigh/Durham, NC

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932 FXUS62 KRAH 210248 AFDRAH Area Forecast Discussion National Weather Service Raleigh NC 950 PM EST Fri Jan 20 2017 .SYNOPSIS...
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Several waves of low pressure will impact the region through Monday.
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&& .NEAR TERM /OVERNIGHT/...
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As of 950 PM Friday... Evening upper air analysis depicts a narrow s/w ridge of high pressure over our region aloft while a wavy surface boundary stretched along our border with South Carolina. A very saturated air mass in the lowest 2500 ft has resulted in a deck of low clouds across the nw half of the forecast area. Across the southeast half, patchy high clouds were drifting overhead; though, as the night wears on, expect low clouds and/or areas of fog to develop over this region. A few sites have reported visibilities as low as a quarter of a mile. Checking in with county law enforcement officials reveal that the dense fog was spotty and not causing any travel issues at this time. Currently have a SPS for pockets of dense fog through 1 AM. Do not plan to issue an advisory at this time. If I had to, it would be over the se third of the forecast area though once the low clouds develop or move in, the sfc visibility will improve above a quarter of a mile. Will continue to monitor. Do not expect to see much on the radar until closer to daybreak over our far southern counties. Thus, aside from a patch or two of drizzle, expect no rain overnight. Min temps mid-upper 40s northeast third to the lower 50s south.
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&& .SHORT TERM /SATURDAY THROUGH SATURDAY NIGHT/... As of 300 PM Friday... Most guidance is agreement in depicting a nearly stationary front over southern and eastern sections of our region Saturday into Saturday night. There should continue to be lingering in-situ damming conditions northwest of the boundary - with areas of fog and overcast conditions. To the southeast, conditions will warm again depending on the amount of sun that can break through. The first in the series of mid/upper level waves will approach the region from the south-southwest late in the day. Showers and isolated thunderstorms are expected, with likely POP forecast along and SE of the boundary in the mid to late afternoon into the evening. It appears that any thunderstorms should remain below severe criteria in the SE, with NIL threat NW. Highs generally 55-60 NW ranging to 70 SE. This first round of showers/isolated thunderstorms should move down east and away overnight. This will leave cloudy skies with a chance of light rain or drizzle. Showers and even isolated thunderstorms are then expected to begin to affect the SW portion of the region before 12z/Sunday. We will begin to ramp up the POP again with the stronger system`s approach from the SW for Sunday. Lows should hold in the 50s to lower 60s SE. && .LONG TERM /SUNDAY THROUGH THURSDAY/... As of 310 pM Friday... ...Storms, some potentially severe, and heavy rain expected Sunday and Sunday night... A deep upper trough will evolve over the southern US this weekend, eventually becoming a closed low over the Southeast by Sunday, per latest guidance. The main energy feeding the evolutions is just now entering the southwestern US. The associated surface low will track across the Deep South and then deepen near the southern Appalachians Sunday. The NAM features a Miller B surface low pattern on Sunday with a low west of the mountains and secondary low forming over the Coastal Plain, while the GFS/ECMWF/Canadian all show essentially one low tracking somewhat atypically through western NC. It looks like there will be two main episodes of precip; one early Sunday associated with a leading, lower amplitude wave in the southwesterlies, and then a second with the closed low and associated DCVA/cold front. There are still a lot of small scale differences and uncertainties in the timing and magnitude of impacts, which may include severe storms and/or flooding. Severe threat: Forcing for ascent will not be lacking on Sunday given lead disturbances aloft and the eventual 100+ meter height falls late Sunday Deep layer shear is plenty strong in excess of 50kt, and low-level hodographs are long and increasingly cyclonically curved owing to a 40-50kt LLJ and the isallobaric response to the deepening surface low. The CAD airmass that developed this morning may still be around in the western Piedmont, and thus the early morning convection is more likely to be elevated than later in the day when backed low level flow has a better chance to advection the warm sector inland. The late day convection should evolve into a squall line as the forcing becomes more linear. Highs in the mid 60s northwest to mid 70s southeast, along with dewpoints potentially in the low/mid 60s, below mid-level lapse rates of 6.5-7C/km could definitely support CAPE values of near 1000 J/KG. Destablization is always a tough thing for models to handle in the pattern, and they typically over mix the eastern edges of the CAD airmass and overestimate SBCAPE. There is always the potential for convection along the Gulf Coast to alter the downstream mass fields and disrupt moisture transport. However, the strengthen of the surface low does warrant some concern for damaging winds and a few tornadoes inland to maybe the US-1 corridor later Sunday? QPF and Flooding: Total QPF is between 2-3 inches through the weekend, which may not be a significant flooding threat when spread out over a couple of days. However, PW around 1.25" and a cutoff low to our SW are favorable for at least some minor flooding. The upper low will pull out on Monday bu may still support some convection Monday afternoon as the cold pool aloft crosses the area. POPs trend down more appreciably Monday night. Height rises over the region Tuesday and Wednesday will result in a return to near or just above normal temps. Models then project a sheared shortwave moving through the Midwest that would result in a frontal passage and a low end chance of rain around midweek. && .AVIATION /00Z SATURDAY THROUGH WEDNESDAY/... As of 630 PM Friday... 24-Hour TAF period: Aside from a brief period of clearing at KFAY, all TAF sites remain MVFR/IFR as of 00Z. Expect conditions to continue deteriorating overnight and remain sub-vfr through the TAF period. By 06Z, expect cigs in the the 100 to 300 ft range with visibilities at or below 1 SM. Expect fog to develop for at least a few hours tonight at all terminals. Guidance suggests some slight and brief improvement Saturday afternoon, but conditions should remain sub-vfr. Winds will also be largely calm overnight and light Saturday afternoon. Looking ahead: Sub-VFR conditions are expected with showers and even some thunderstorms at times Saturday night through Monday morning. A return to VFR conditions expected Monday afternoon and Tuesday. && .RAH WATCHES/WARNINGS/ADVISORIES... None. && $$ SYNOPSIS...MWS NEAR TERM...WSS SHORT TERM...BADGETT LONG TERM...SMITH AVIATION...KC/BADGETT

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