409 FXUS62 KGSP 060227 AFDGSP Area Forecast Discussion National Weather Service Greenville-Spartanburg SC 927 PM EST Fri Feb 5 2021 .SYNOPSIS... Dry and cool high pressure will briefly be in control of our weather tonight and Saturday morning. Low pressure will spread moisture north across our region late Saturday and Saturday night then move off the East Coast on Sunday. Dry high pressure builds back into the area on Monday. Unsettled weather possible through the rest of the week. && .NEAR TERM /THROUGH SATURDAY/... As of 930 pm EST Friday: No changes will be made to the existing Winter Storm Watch late this evening given the trend of the 18Z model runs toward a slightly stronger warm nose in profiles. Any Warning or Advisory decisions will be made overnight after a full review of the incoming 00Z model runs to assess the latest trends. For the rest of tonight, the rate of cooling of hourly temperatures in the forecast has been slowed a bit for the early overnight hours given the abundant high clouds with the upper jetlet late this evening. Still anticipate that the high clouds may be sufficiently thin at times, with some clearing from the west, to permit temps to reach forecast min values. Conditions will remain quiet into Saturday morning as dry but weak high pressure persists near the southern Appalachians. On Saturday, another low begins to organize over the Northern Gulf of Mexico and lift northeast toward our area late day. Associated PoPs ramp up tomorrow aftn and early evening. Based on the latest model guidance/soundings, our region should see precipitation generally begin as rain across the foothills, Piedmont, and lower mountain valleys, with primarily snow or a snow along the higher ridges, and a rain/snow mix along mid slopes of the mountains. Light mountains accumulations will be possible by early evening before the heart of the moisture rolls in later. The increasing clouds will keep temps a touch below climo for daytime Saturday, especially with precipitation breaking out from the south during the afternoon. && .SHORT TERM /SATURDAY NIGHT THROUGH MONDAY/... As of 200 pm EST Friday: Coastal low will continue to develop as shortwave crosses the TN Valley Saturday night and approaches the southern Appalachians early Sunday morning. In the event the warm front has not shifted north over the CWA by the start of the period 00z Sun, it should by 06z. Evaporative cooling will play a big role in determining the sfc temp and thus p-type at onset, with in-situ CAD developing. Based on current prog sounding trends, once the low levels moisten enough for precip to reach the surface, snow would predominate wherever sfc temps are cool enough. The warm front will be fairly broad; stratiform QPF appears to follow its leading edge north of the CWA during the evening, while the warmer air necessary for a transition to sleet and/or freezing rain looks to lag until overnight. There's the usual spread among the guidance as far as how far northwest into the CWA that warm air will affect p-type. The NAM has made its customary trend toward colder sfc temps and a stronger warm nose; to boot, it is colder at the onset of precip. These trends have not yet been backed up by another available model aside from SREF members (which are based on same core as NAM). That notwithstanding, the other members' profiles would support mainly snow initially where cold enough, with sleet/mix overnight at "peak warm nose" in many areas between the Escarpment and I-85. It appears most likely that the majority of the Piedmont will remain just above freezing, which looks to limit accumulation, at least early on during the snowy period. A slight downward trend in the temps would lead to a jump in the southeastern extent of the accumulations. Right now it looks like an advisory-level event for the counties near I-85, so won't do anything with them just yet as far as headlines. Will add more of the NC Foothills and NW Piedmont to the Winter Storm Watch we issued earlier today, thinking they are most certain to need an upgrade, and/or most likely to see some jump in accums due to a cooler sfc fcst. Winds will turn westerly as the sfc low progresses up the NC and then VA coastline, and as the shortwave axis passes east of the area. This will shut off the upglide and Piedmont precip Sunday morning, and result in mainly NW-Flow type impacts for the mountains. That said, moisture is not particularly deep nor forcing particularly good due to mediocre 850mb winds. Thus, the remaining snowfall may not prove too exciting. Temps are currently expected to rise into the upper 40s and lower 50s across much of the Piedmont on Sunday, but that could prove a little too warm if snow/sleet accum does occur. Quasi-zonal flow will continue aloft through Monday, with dry high pressure returning. Max temps will trend upward slightly. && .LONG TERM /MONDAY NIGHT THROUGH FRIDAY/... As of 225 PM EST Friday: Guidance still shows at the start of this period a general zonal upper pattern over our area with a series of weak short waves passing through the flow. On Monday night the surface high moves off the East Coast as active weather resumes from the west. A strong cold front moves across the mid and lower Mississippi Valley and TN and Ohio Valleys Tuesday. This front stalls out over Florida and sets up a potential wave train from Wed through the end of the week. Cold Canadian air filters down from the Dakotas and it is a matter of timing for when waves move up from the Gulf. The ECMWF has 2 main lows...one late Tuesday crossing SC with the second crossing Thursday night and early Friday. The GFS gradually gets organized in mid week with the one main low crossing GSP early Friday. These features change in location and strength each model run which affects the forecast. At this time it appears Thursday night and Friday morning are the period of greatest concern for wintry weather mainly over the mountains and NC foothills. Temperatures near normal mid week and getting colder at the end of the week. && .AVIATION /03Z SATURDAY THROUGH WEDNESDAY/... At KCLT and elsewhere: Mainly high clouds will persist the first half of the TAF period, with steadily lowering VFR bases through the day on Saturday. The deeper moisture ahead of an approaching surface low will move back into the terminal forecast area quickly during the late afternoon and early evening hours to produce MVFR cigs at most locations, with IFR cigs encroaching from the south toward the end of the period. Snow appears more likely just after the current TAF period ends from KAVL to KHKY, but the 30-hour forecast at KCLT will include a -RASN mention with the changeover line so close to the I-85 corridor. Anticipate winds turning around the dial at the foothill and Piedmont sites, with some flavor of westerly tonight and more easterly in direction on Saturday and Saturday evening. Winds will toggle from NW at KAVL early to SE late. Outlook: More widespread precipitation and restrictions are expected the rest of Saturday night into early Sunday as a low pressure system lifts over the area from the Gulf of Mexico. Drying is then expected for the later part of Sunday through Monday. Moisture and restrictions may return on Tuesday. Confidence Table... 02-08Z 08-14Z 14-20Z 20-00Z KCLT High 100% High 100% High 100% High 94% KGSP High 100% High 100% High 100% High 91% KAVL High 100% High 100% High 100% High 100% KHKY High 100% High 100% High 100% High 88% KGMU High 100% High 100% High 100% High 91% KAND High 100% High 100% High 100% Med 73% The percentage reflects the number of guidance members agreeing with the scheduled TAF issuance flight rule category. Complete hourly experimental aviation forecast consistency tables are available at the following link: www.weather.gov/gsp/aviation && .GSP WATCHES/WARNINGS/ADVISORIES... GA...Winter Storm Watch from Saturday evening through Sunday morning for GAZ010-017. NC...Winter Storm Watch from Saturday evening through Sunday morning for NCZ033-035-048>053-056-058-059-062>065- 501>507-509. SC...Winter Storm Watch from Saturday evening through Sunday morning for SCZ001>003. && $$ SYNOPSIS...DEO NEAR TERM...HG/JPT SHORT TERM...Wimberley LONG TERM...DEO AVIATION...HG