Area Forecast Discussion
Issued by NWS Greer, SC

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701 FXUS62 KGSP 151949 AFDGSP Area Forecast Discussion National Weather Service Greenville-Spartanburg SC 249 PM EST Fri Dec 15 2017 .SYNOPSIS... Dry and cool high pressure will dominate our weather through Sunday morning. Moisture and a gradual warm-up will return to the area starting Sunday. A series of disturbances will pass across the area next week, with a prolonged period of unsettled weather. && .NEAR TERM /THROUGH SATURDAY/... As of 215 PM Friday: With high clouds, mainly opaque in nature, streaming across the Southeast today, temperatures have peaked just below normal across the Upstate and Piedmont this afternoon in the upper 40`s/around 50 degrees, while temperatures remained cold across the mountains in the 30`s. As a weak sfc boundary across the southeastern Carolinas continues to progress eastward offshore, flow will become more zonal and sfc high pressure will gradually build northeastward into the Southeast beginning tonight, where it`ll become the dominating weather pattern on Saturday. As a result, expect high clouds to decrease in both coverage and opacity tonight, allowing for ideal radiational cooling. This along with CAA will allow for temperatures to plummet overnight, with temperatures below freezing expected across the entire forecast area. However, if high clouds linger for longer than anticipated, going temperature forecast could be a bit too low. Will also note, as high pressure builds in tonight, a short-lived tightened pressure gradient across the NC mountains could bring gusty winds to the higher elevations, subsiding before midnight. As surface high pressure continues to build into the Southeast on Saturday, conditions will remain dry and cool. With nearly clear skies throughout the day, anticipate a gradual increase in high clouds late in the afternoon hours as available moisture increases ahead of the next approaching system. High temperatures on Saturday will climb into the low to mid 50`s across the Upstate and Piedmont, while remaining a bit cooler into the 40`s across the mountains. && .SHORT TERM /SATURDAY NIGHT THROUGH MONDAY/... As of 215 PM EST Friday: A shallow ridge will build over the southeast as a phasing shortwave lifts through the southern plains. Meanwhile, surface high pressure over the region will gradually migrate offshore the latter half of the weekend. The deamplifying upstream wave will generally lift north of the OH valley Sunday and then off the mid-Atlantic coast Sunday night - leaving the southeast upper ridge intact. The deeper moisture plume ahead of the phasing shortwave should impact mainly TN to the west Saturday night through Sunday morning before the moisture lays over across our region later Sunday into Sunday night, with brief mid/upper drying likely early Monday Morning. This first round should yield mostly light QPF across our region and thermal profiles should remain warm enough through the period for any precipitation to fall as liquid. The upper flow will then become more southwesterly through Monday as heights fall again over west Texas. Deeper moisture will return to the area from the southwest and some better rates and marginal instability could return to the lower piedmont. Anticipate the heaviest rainfall with any passing surface waves on Monday to slip by mainly just southeast of our forecast area. Monday mins/maxes will run several categories above climo. && .LONG TERM /MONDAY NIGHT THROUGH FRIDAY/... As of 245 PM EST Friday: Forecast confidence has taken a tumble as the 12Z ECMWF and GFS have come in with significantly different scenarios, the ECMWF has the larger change from its earlier run, for the middle of next week. For what it`s worth, the Canadian is in between the two while the GEFS mean is closer to the operational GFS but does have a hint of a slower version of the Canadian. The guidance agrees that the period starts with a split flow across the CONUS. The GFS keeps the systems more progressive moving the northern and southern stream troughs into the area by Tuesday night and off shore Wednesday. The ECMWF keeps the streams separate moving the northern stream trough east of the area by Tuesday night. The southern stream wave is much stronger and slower as a closed low which moves into the lower MS valley Tuesday night, into the TN valley Wednesday, then across the Appalachians Thursday. Of course, this creates quite a difference in the surface pattern as well. A wave forms along the stalled frontal system to the south and spreads moisture and precip into the area as it moves east. As expected, the ECMWF is quite a bit slower than the GFS. The GFS then moves a northern stream cold front through the area Tuesday night with dry high pressure for Wednesday and Thursday. The ECMWF is slower with a stronger southern stream low pressure system. This northern stream cold front moves into the area Wednesday as moisture increases ahead of the low. The low then slowly crosses the area Wednesday night with decreasing precip chances on Thursday. QPF is quite a bit higher on the ECMWF than the GFS. Temps are warm, so all of the precip would be liquid in both scenarios. Given the wide difference in solutions, have gone with a guidance blend which has chance PoP for Monday night and Tuesday with diminishing PoP for Tuesday night and a dry Wednesday. Temps ramp up from above normal Monday to well above normal for Tuesday then slowly down to around 5 degrees above normal for Thursday. Guidance does agree on a frontal system moving into the area by late Friday with some timing differences. Have PoP increasing back to the chance range during the day. Temps will be above normal. && .AVIATION /20Z FRIDAY THROUGH WEDNESDAY/... At KCLT and Elsewhere: Expect VFR through the valid TAF period. A weak, dry surface boundary just south of the forecast area will continue to progress eastward as high pressure builds in from the southwest. Expect FEW/SCT/BKN cirrus to persist through this afternoon, decreasing in opacity tonight. With the exception of the potential for gusty northwesterly winds at KAVL through early this evening, expect northwest winds 5 to 10 kts at the TAF sites through tonight, becoming light and variable/nearly calm overnight as the pressure gradient slackens. On Saturday, VFR will continue with nearly clear skies and west-northwest winds becoming west-southwest, around 5 kts. Outlook: Potential for short-lived restrictions on Sunday with rain showers. Otherwise, expect VFR as a series of dry cold fronts move through the area. Confidence Table... 15-21Z 21-03Z 03-09Z 09-12Z KCLT High 81% High 82% Med 68% Low 56% KGSP Med 70% Med 75% High 83% Med 64% KAVL High 94% High 85% Med 70% High 87% KHKY High 100% High 83% Med 66% High 100% KGMU Med 76% High 86% High 85% Med 63% KAND Med 76% Med 65% High 90% Med 61% The percentage reflects the number of guidance members agreeing with the scheduled TAF issuance flight rule category. Complete hourly experimental aviation forecast consistency tables and ensemble forecasts are available at the following link: www.weather.gov/gsp/aviation && .GSP WATCHES/WARNINGS/ADVISORIES... GA...None. NC...None. SC...None. && $$ SYNOPSIS...SW NEAR TERM...SGL SHORT TERM...HG LONG TERM...RWH AVIATION...SGL

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