Area Forecast Discussion
Issued by NWS Greer, SC

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000 FXUS62 KGSP 312332 AFDGSP Area Forecast Discussion National Weather Service Greenville-Spartanburg SC 732 PM EDT Sat Oct 31 2020 .SYNOPSIS... Cool high pressure will weaken tonight as a cold front approaches from the northwest. The cold front will cross the region Sunday, ushering in the coldest air of the season for the early part of next week. Dry high pressure will then linger across the area through the middle part of the week. && .NEAR TERM /THROUGH SUNDAY/...
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700 PM Update...Issued a High Wind Watch for the nrn NC mtns effective Sun afternoon through daybreak Mon. A strong cold front will mix down stg winds tomorrow evening and thru the overnight. Winds will approach high wind criteria and due to the recent rainfall and very moist soils...trees and powerlines will likely be blow down. A high wind warning or wind advisory will be issued on subsequent shifts. The fcst is in good shape. Still seeing good Atl moist flux interacting with sfc wedge making for widespread cloudiness with Ci shield moving in from the west. May not get a good look at blue moon, but there could be some breaks occasionally. 500 PM Update...Increased sky cover across the Upstate and NE GA as se/ly moisture adv has increased atop the llvl wedge. This has held localized temps down a few degrees and have adj/d the grids down in these areas. As of 230 PM: Sfc high pressure will migrate east off the New England coast, while a strong cold front pushes into the Ohio Valley tonight. The anticyclonic flow around the departing high will advect some Atlantic moisture into the area this evening, deepening overnight as decent isentropic lift develops atop the FA. Guidance seems to be trending wetter with more robust QPF response. I`m still reluctant to go likely or higher on PoP, given model track record of overdoing QPF response in these shallow return-flow situations. For now, have bumped up PoP to around 50% overnight, highest across the Piedmont. At the very least, should see expanding stratus deck across the entire area. Temps will be held up due to strong WAA and the clouds, bottoming out in the upper 30s highest peaks to mainly mid to upper 40s elsewhere. QPF should be light. On Sunday, the strong cold front will cross the area during the day as a deep upper trough digs across the region. The arrival of the front will actually lower PoPs, except along the TN border, given it will be dry and flow turns out of the WSW just ahead of it. The biggest concern with the fropa will be the onset of gusty winds in the mountains, with strong CAA ramping up by early aftn. Will likely need a wind advisory for portions of the NC mountains starting around midday tomorrow, but will let the midnight assess one more cycle of guidance. Temps will be tricky, as skies clear and some downslope flow develops east of the mountains. But the CAA may be enough to overwhelm a lot of that. So sticking with max temps a few degrees below model consensus. Still will be near normal east of the mountains.
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&& .SHORT TERM /SUNDAY NIGHT THROUGH MONDAY NIGHT/... As of 225 PM EDT Saturday: The first part the new week looks busy with non-precipitating weather concerns, none of which will yet be addressed beyond the HWO. After the axis of a relatively deep upper trof crosses the region Sunday evening, we will be set up for a classic advective freeze across the mtns, courtesy of cold advection on a strong NW flow in the wake of the trof. However, the date on Monday, that being 2 November, is problematic because by policy the NWS does not issue Freeze Warnings across the mountains after 1 November. That makes our expected advective freeze a day late as far as the mountains are concerned. Meanwhile, the NC Foothills and much of the NW Piedmont are also looking favorable for a freeze early Monday morning, so we WOULD have to issue a Freeze Warning there if the fcst does not change. It might ultimately be less confusing to ignore the usual turn-off date for the mtns and issue a Warning everywhere that freezing temps are expected (with the exception of the nrn mountains which have been officially turned off as of this past morning). Winds at 850 increase markedly late Sunday evening with the guidance showing a 50kt wind at 850 with cold advection, which is usually good for a Wind Advisory. We rarely issue third-period advisories, so we will punt this to the next shift to let them issue at a more appropriate time. Moisture looks meager and limited in this NW flow on Sunday evening, such that any precip is expected to remain to our north. Will not be surprised, however, if some locations over the nrn mountains get their first snow flurries of the season (almost on schedule it seems). We will address that later as well. Needless to say, temps will be below normal. On Monday, with high pressure building in throughout the day and cold advection continuing, high temps will remain about ten degrees below normal and will struggle to get out of the 50s east of the mtns. There is some indication that orographic cirrus will develop downwind of the higher terrain, which will help keep temps cooler, maybe even a few degrees cooler than expected. The sfc high will nose in from the southwest Monday night allowing winds to diminish. Low temps early Tuesday will once again be down in the frost range east of the mtns, but the air mass will be so dry that frost may not be able to form in most areas across the Piedmont and foothills of NC and the Upstate. && .LONG TERM /TUESDAY THROUGH SATURDAY/... As of 126 PM EDT Saturday: The medium range looks rather quiet, even as the upper pattern changes slowly throughout the week. Deamplification is expected through the middle part of the week as the upper trof moves farther off the East Coast and a flat upper ridge moves eastward. This should allow for surface high pressure to set up shop over the srn/central Appalachians on Tuesday, where a chunk of this sfc high will get stranded for the rest of the week. The culprit in this development will be a rex blocking pattern that forms to our west late in the week. The result would be a building upper anticyclone that moves down over the OH/TN Valley region to the Carolinas by the weekend. Confidence isn`t all that great in such a blocking pattern, but for now, the fcst will remain dry in spite of some weak return moisture shown by the GFS next Saturday. In this pattern, temps may start out a few degrees below normal during the middle of the week, but end up about five degrees above normal by the end of the week. && .AVIATION /00Z SUNDAY THROUGH THURSDAY/...
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At KCLT and elsewhere: A low level wedge will remain in place overnight while good moisture flux will lift overhead and produce -shra across the FA, beginning arnd 09z and not pushing east until noon or the early afternoon. These showers will bring flight restrictions, mainly low CIGs in the MVFR to IFR range during this time. Winds will remain sw/ly across the non/mtns during the afternoon and become gusty ahead of a cold front. At KAVL...winds will remain se/ly then shift nw/ly arnd 20z with moderate gusts developing. Outlook: Clearing skies and gusty NW winds expected in wake of the cold front Sunday aftn. Winds gradually weaken on Monday, followed by quiet VFR conditions likely thru the end of next week. Confidence Table... 23-05Z 05-11Z 11-17Z 17-18Z KCLT High 100% High 95% High 83% High 90% KGSP High 95% High 86% Med 65% Med 65% KAVL High 100% High 86% Med 63% High 100% KHKY High 100% High 82% Low 48% High 80% KGMU High 95% High 86% Med 61% Med 70% KAND High 95% High 95% Med 65% Med 65% 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
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&& .GSP WATCHES/WARNINGS/ADVISORIES...
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GA...None. NC...High Wind Watch from Sunday afternoon through Monday morning for NCZ033-049-050-501-503-505. SC...None.
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&& $$ SYNOPSIS...PM NEAR TERM...ARK/SBK SHORT TERM...PM LONG TERM...PM AVIATION...SBK

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