Scientific Forecaster Discussion

000 fxus62 kgsp 212354 afdgsp Area forecast discussion National Weather Service Greenville-Spartanburg SC 654 PM EST Tue Feb 21 2017 Synopsis... a frontal boundary will push across the area tonight and Wednesday, bringing a good chance for some modest rainfall. This will be followed by brief drying, before another frontal system brings additional chances for rain and thunderstorms late Friday or Saturday. Overall, daytime temperatures will remain well above normal through the weekend. && Near term /through Wednesday/... as of 645 PM EST tuesday: light upglide induced precip continues to spread into the region from the south as moisture advects northward ahead of the stacked system, which is now located across the lower MS River Valley. Latest cams favor continued light dz/shra through the evening, with chances increasing overnight, especially across the i85 corridor regions of northeast Georgia and the western upstate. Thus, little change was needed to pops, however did go ahead with a few tweaks to align with these cam trends. Otherwise, the remainder of the fcst is unchanged with this update. As of 200 PM upper low will drift from the arklamiss to near Tampa, Florida by the end of the day on Wednesday. This will place our area under a col in the upper flow with the northern jet stream staying well to the north over the Great Lakes. In the low levels, south/southeasterly flow will continue to gradually advect more moisture into the area. Upslope lift along the south and east-facing Escarpment should begin to produce increasing showers this evening thru the overnight. Overall forcing will remain weak, and guidance in good agreement on low quantitative precipitation forecast in this flow regime. The moisture will produce widespread stratus tonight, which will keep temps elevated well above normal with lows in the upper 40s to mid-50s. On Wednesday, the upslope flow and deep moisture should keep at least isolated to scattered light showers along the Escarpment thru most of the day, while the rest of the area should remain fairly socked in with low clouds. A 1028 mb surface high will drift off the mid-Atlantic coast and weaken. So it's tough to say how much of an in-situ wedge may linger across the Piedmont thru the afternoon. In any case, the clouds should keep a lid on temps, and highs are expected to "only" warm into the low to mid-60s across most of the area. If the low clouds manage to erode along the south and east fringe of the cwfa, temps may get into the low 70s per the met MOS. This may also unveil a little SBCAPE in the southern upstate. Lapse rates look rather weak in the soundings, so only a slight chance of thunder will carried in the southern fringe. && Short term /Wednesday night through Friday/... as of 215 PM EST tuesday: the closed upper level low pressure system near the West Coast of Florida crosses the Florida Peninsula through Thursday. North of this system, 850 mb southerly flow will continue but slowly weaken through this time. With copious low level moisture in place, the upglide and upslope flow will lead to isolated to scattered showers across the area Wednesday night and Thursday. Weak instability will lead to isolated thunderstorms and rain Thursday afternoon. With the low level inversion eroding and the less widespread and showery nature of the precipitation, highs and lows will increase to 15 to 20 degrees above normal. Mid and upper southwesterly flow develops on Friday between the weakening upper low off the southeast Atlantic coast and a deepening upper low and trough over the upper Midwest. The weak surface ridge over our area erodes as a cold front moves into the MS valley. Southerly 850 mb flow lingers through the period along with low level moisture. Expect isolated to scattered convection to develop across the mountains and foothills of the Carolinas and NE Georgia. Highs and lows will remain nearly steady from 15 to 20 degrees above normal. && Long term /Friday night through Tuesday/... of 220 PM tuesday: a strong, but weakening short wave trough will lift from the miss valley through the Ohio Valley/mid-Atlantic and off the northeast coast early in the medium range period. This will give way to the return of a progressive/quasi-zonal regime early next week, before the flow begins to amplify again late in the period as the next western storm system approaches The Rockies. Early in the period, a cold front associated with aforementioned strong short wave will sweep across the Tennessee Valley. However with much of the deep-layer forcing expected to lift west and north of the southern Appalachians, an expected weakening frontal circulation, and the likelihood that the meager pre-frontal surface-based instability will deplete further as it approaches western NC, it continues to appear as if any convective band accompanying the front will lose considerable steam as it moves into the forecast area late Fri night/early Saturday, with the bulk of significant shower activity likely passing north of the area. Pops during this time will range from likely across the western-most NC mtns, to only a low chance across much of the Piedmont. Conditions will dry out considerably behind the front on Saturday, although Max temps will once again range from 10-20 degrees above climo. A brief period of northwest flow rain and snow showers will be possible Sat night, but chances will be slight at best. The remainder of the period will be generally dry and unseasonably warm (although 5-10 degrees cooler than what has been seen for much of this week.) A couple of short wave troughs may pass near the region early next week, but their potency will be weak in the fast zonal flow, so any chances for showers will be low. More substantial chances for precipitation will likely hold off until after the end of this forecast period. && Aviation /00z Wednesday through Sunday/... at kclt and elsewhere: a mixture of VFR/MVFR and IFR expected through this fcst cycle as moisture continues to increase ahead of a stacked low pressure system centered over the MS valley this evening. Expecting low VFR cigs to prevail initially as upglide -shra/-dz spreads across the region from the south. With that, cigs will gradually lower from the southwest to MVFR then IFR, possibly to LIFR/vlifr by early morning which is featured in taf via prob30s. Expecting multiple waves of -shra/dz to spread atop the area during this time as well, therefore opted to prevail vcsh initially, leading into tempos for shra at all sites after midnight, before prevailing shra at all sites aside for kclt given its proximity to the better moisture advection and waves of upper divergence associated with the approaching upper low. Precipitation will taper into late morning, before increasing yet again into late afternoon. As such, restrictive visb/cigs will recover as well to MVFR and/or low VFR to round out the period. Winds through the night will be a bit tricky as current swly flow backs enely thanks to precip induced strengthening of the insitu wedge across the western Carolinas, therefore did carry enely flow through midday before flow veers sly yet again as the wedged high retreats for good. Outlook: moist southerly to easterly low-level flow will persist across the area until a cold front pushes thru on Saturday. This will keep high chances of morning stratus and possibly fog each day thru Saturday morning. Confidence table... 00-06z 06-12z 12-18z 18-00z kclt high 97% high 84% high 96% high 86% kgsp high 97% high 86% high 94% high 97% kavl high 86% high 93% high 91% high 95% khky high 98% high 93% high 87% high 93% kgmu high 95% high 86% high 93% high 87% kand high 93% high 90% high 81% high 86% 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... Georgia...none. NC...none. SC...none. && $$ Synopsis...Wimberley

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