Area Forecast Discussion
Issued by NWS Raleigh/Durham, NC

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000 FXUS62 KRAH 210257 AFDRAH Area Forecast Discussion National Weather Service Raleigh NC 957 PM EST Sat Jan 20 2018 .SYNOPSIS... High pressure will drift across the southeastern United States through tonight, then offshore on Sunday. A cold front will approach from the west Monday, and cross central NC on Tuesday. && .NEAR TERM /THROUGH Sunday/... As of 1000 PM Saturday... As high pressure settles in over central NC, the forecast problem of the night will be what influence does the remaining snow pack have on the formation of fog overnight. While earlier simulations of most notably the SREF, and WRF ARW showed a strong signal for fog during the early morning hours on Sunday, the 21z run of the SREF has backed off considerably. The 00z sounding at GSO shows a lot of dry air in the low levels and METAR observations in many places showing large dewpoint depressions. If there was to be fog overnight, the most favored location would be an area south of the Triad into the southwest Piedmont and east to the Sandhills. The biggest problem with this location getting fog is that temperatures could be below freezing in some locations when this happens, leading to a brief freezing fog episode. Looking at persistence, fog did not occur last night with a larger snow pack in place, but the more ideal location of the surface high for radiational cooling could counteract the smaller snow pack. Winds, while calm in some locations are stirring at times to upwards of 5 knots which could also keep temperatures up a bit. The upshot is a low confidence nuisance event at best with very little to no impact. Overnight lows will bottom out in the low to mid 30s. Any potential fog will dissipate after sunrise. Some mid level clouds are possible Sunday afternoon as temperatures rise into the low to middle 60s and obliterate any snow left on the ground. && .SHORT TERM /SUNDAY NIGHT THROUGH TUESDAY/... As of 330 PM Saturday... May see a better potential for areas of fog late Sunday night into early Monday morning, primarily across the Piedmont, though increasing high cloudiness may inhibit fog development in vicinity of the Yadkin/Pee Dee river valley. Min temps mostly in the mid 30s. Monday, a deep cyclone over central Plains will slowly lift into and across the Great Lakes Monday night and Tuesday. Swly low level flow will continue to pull warm air into the Carolinas. This warm air coupled with ample sunshine will push temperatures well into the 60s along and east of highway 1. Thickening cloud coverage over the west will inhibit sunshine to result in cooler temps, holding the nw Piedmont in the upper 50s to around 60. At this time, expect bulk of the showers to hold off until after sunset, though a stray shower or two cannot be ruled out in vicinity of Winston-Salem by late afternoon. The trailing sfc cold front associated with the deep cyclone will approach from the west Monday evening, and cross our region early Tuesday. Thickening clouds and steady sly flow will likely maintain mild conditions Monday night with some places likely not dropping much below 55 degrees. Better support aloft over the NW Piedmont suggest better shower coverage in this region, with support waning farther to the south and east. Currently not expecting abundant rainfall with this system as forcing not overly impressive. Rainfall totals will likely vary from less than a half inch NW to under a quarter of an inch SE. Morning clouds and scattered showers Tuesday morning will giveway to increasingly sunny skies. 850mb front does not cross region until the afternoon, so we should have time to warm before low level cold air advection commences late in the day. This required an adjustment up in high temperatures Tuesday. Currently expect max temps Tuesday to be in the 60-65 degree range. && .LONG TERM /TUESDAY NIGHT THROUGH SATURDAY/... As of 212 PM Saturday... Tuesday night and beyond: Quiet weather with near to above seasonal temperatures are expected into early Saturday. The upper air pattern Tuesday night features a departing short wave trough aloft and cold front. A period of short- lived strong cold advection Tuesday night results in clearing skies and temperatures that fall to near normal in the upper 20s to lower 30s. Weak short-wave ridging develops on Wednesday and temperatures reach he mid to possibly upper 50s. A short-wave trough and an associated cold front with very little moisture swings across the region late Wednesday/early Thursday. Skies will remain mainly clear with limited sensible weather impacts. Ridging aloft with increasing heights build into the area for Thursday into early Saturday. Skies will remain generally clear with moderating temperatures. Highs on Thursday will range in the lower to mid 50s, in the mid to upper 50s on Friday and in the mid 50s to lower 60s on Saturday. A higher amplitude trough that moves onto the Pacific coast on Thursday reaches the central Plains by Saturday night. Deep layer southerly flow transports moisture northward from the Gulf and Pacific as a surface wave develops in the mid-Mississippi Valley. Both the 12Z operational GFS and EC have trended slower with the precipitation arrival in our region, especially with the precipitation spreading northward across GA and the Carolinas as the surface high is slower to depart. Will keep PoPs confined to the Saturday night period for now. -Blaes && .AVIATION /00Z SUNDAY THROUGH THURSDAY/... As of 700 PM Saturday... 24 Hour TAF Period: A lot of discrepancy in the short term models over the extend of fog during the early morning hours on Sunday morning. The fog in the models appears to be related to the left over snow pack over the Triad and as far east as RDU. Models are split on the effects of the leftover snow on visibility and the SREF points to a fairly extensive fog event where as other short term models such as the HRRR are keeping visibilities at VFR levels. Have decided to go with IFR visibilities after 6z in the Triad and after 9z at KRDU. Any fog will rebound to VFR after sunrise. Some mid level clouds can be expected on Sunday especially later in the day as RH cross sections point to a lot of moisture in the 4-6 KFT range. No precipitation is expected and winds should remain light and variable as surface high pressure remains in control of the pattern. Long Term: After another bout of possible fog early Monday morning, VFR conditions are expected through Monday night before a weak frontal system approaches the area by Tuesday morning. Sub-VFR conditions can be expected through Tuesday evening before a return to VFR on Wednesday. && .RAH WATCHES/WARNINGS/ADVISORIES... None. && $$ SYNOPSIS...WSS NEAR TERM...Ellis SHORT TERM...WSS LONG TERM...BLAES AVIATION...Ellis is the U.S. government's official web portal to all federal, state and local government web resources and services.