Area Forecast Discussion
Issued by NWS Greer, SC

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168 FXUS62 KGSP 230840 AFDGSP Area Forecast Discussion National Weather Service Greenville-Spartanburg SC 340 AM EST Sat Feb 23 2019 .SYNOPSIS... The wet pattern will continue today into tonight before a robust cold front finally moves through the region late tonight through Sunday morning. The early part of next week should be dominated by dry high pressure. A cold front may move through the region from the northwest on Wednesday, but moisture with this front remains highly uncertain. Better moisture could arrive toward next weekend. && .NEAR TERM /THROUGH TONIGHT/... As of 300 AM: High pressure centered over New England will continue to ridge down the East Coast today, maintaining cold-air damming. A robust upper ridge is centered over the eastern Caribbean. In between these features, a broad warm frontal zone extends eastward out of developing low pressure in the Plains. Widespread rainfall is occurring across Tennessee as a result, with lighter and more spotty rainfall extending across the Carolinas where it sits atop the CAD wedge. Given WSW to W flow around the upper ridge, some of the Tennessee rain will advect into NW NC and our northern zones over the course of the morning, and categorical PoPs are fcst there. Further south there will be frequent opportunities for brief showers and drizzle, warranting high chance to likely PoPs. The continued northward shift of the warm front suggests chances lessen for a time later today, though we will retain at least weak upglide over the wedge so we can`t let the precip mention drop entirely. Temps will continue to reflect the CAD, remaining more or less steady in the 40s over most of the CWFA; the un-wedged valleys in and around the Smokies look to experience mid 50s by aftn. Expect little change in temps and sky cover through most of tonight, for that matter. The Southeast will be in the warm sector of the aforementioned low as it moves toward the Great Lakes, but the ensuing warm advection will only maintain the CAD in our neck of the woods. 850mb flow will strengthen thru the period, suggesting not only increasing upslope enhancement to PoPs, but strong sfc winds on the higher ridgetops. Though gusts do not look especially strong given WAA and limited mixing, we feel a Wind Advisory is warranted for elevations above 3500 feet. Precip rates will increase overnight; as the sfc low continues moving east, the LLJ and moisture plume will cross the area in the early morning hours. Locally heavy rain can be expected, especially in the usual spots along south-facing terrain. With a number of river gages already above bankfull stage or in minor flood across the area, and a large swath of our territory north of I-85 having received 3" or more of rain in the past week--more like 6" along the Escarpment--we have issued a new Flood Watch effective now, and valid until noon Sunday. This will give time for the cold front to scour out all the precipitation, and some time for rivers to crest after it ends. A small amount of instability will accompany the front as it moves into the area early Sunday, and the strength of the gradient allows 0-3km bulk shear to peak at 40-60 kt. Nearly saturated prog profiles reveal a very "skinny" CAPE profile, and the environment is characterized by SHERB values less than 0.7 prior to the end of the period at 12z. Nonetheless we plan to carefully monitor any convective elements that develop. && .SHORT TERM /SUNDAY THROUGH MONDAY NIGHT/... As of 230 AM EST Saturday: Weak convection associated with a cold frontal passage should move quickly east of the mountains Sunday morning, departing from the Blue Ridge mountains around 12Z and passing east of I-77 by 15Z. Very little surface-based instability is expected with the fropa east of the mountains, especially with the wedge boundary slow to retreat northward, but any sbCAPE uncovered with the morning fropa would affect the southern piedmont during that narrow morning window. Otherwise, anticipate rapid post fropa drying through the day on Sunday, with temperatures in the upper 60s in many places east of the mountains, touching 70 in a few spots. Hydro problems are likely to linger along the slower response main stem rivers and reservoirs through Sunday. Winds will be the other major concern as cold advection develops behind the departing front and the surface pressure gradient tightens ahead of approaching high pressure. Even the more conservative GFS features a 50+ kt 850 mb jet by Sunday afternoon/evening across the northern Blue Ridge mountains. Given the mixing potential in the cold advection, the prudent course appears to be a northern mountain High Wind Watch for Sunday and Sunday night, with a Wind Advisory for the central and southern mountain through late Sunday. Any northwest flow moisture in the cold air should be shallow and rather short-lived across the western mountains, so no snow showers will be advertised. Upper flow should become rather zonal over the east on Monday, with dry surface high pressure building over to the north of our area. Maximum temperatures will be a couple of degrees below climo in the NC mountains, near climo across the rest of western NC, and a degree or two above climo in SC and northeast GA on Monday afternoon. && .LONG TERM /TUESDAY THROUGH FRIDAY/... As of 245 AM EST Saturday: There is decent agreement on dry conditions lasting through at least Tuesday, with some semblance of a surface high to the north of the area. Forecast confidence takes a nose dive from mid to late week, however, as the GFS/ECM camps differ markedly on the strength and permanence of surface high pressure to the north, and on the redevelopment of shortwave energy and troughiness in the flow aloft over the Mid and Deep South. The ECMWF features a relatively dry period of WNW flow over the region Tuesday through Thursday night, but with modest heights falls over the plains and mid MS River valley on Friday. Meanwhile, the GFS has zonal flow early, but with more southern stream shortwave activity over the deep south by Wednesday through Thursday, and a more vigorous system on Friday. All told, will stay very close to WPC and an ensemble blend for the Wednesday through Friday period. This will mean very low PoPs, possibly peaking on Friday, with mainly liquid ptypes throughout. The diurnal range on temperatures should get a bit smaller each day as clouds and PoPs slowly increase. && .AVIATION /09Z SATURDAY THROUGH WEDNESDAY/... At KCLT and elsewhere: Cold-air damming will continue to drive the pattern thru tonight, meaning IFR to LIFR cigs will persist. The only possibility of higher cigs appears to be if SHRA cause brief erosion of the lowest cloud layer via the downward motion they will induce. An area of SHRA is expected to blossom near the NC/SC border and gradually advect northward as warm front atop the CAD airmass effectively shifts in that direction. Thus, some TEMPOs feature higher cigs, though as has happened at times last night, MVFR or IFR vsby can be expected with the precip as well. Off-and-on SHRA and/or DZ will continue into tonight, with cigs lowering a bit after sunset as wedge becomes more shallow. A scouring cold front will enter the area early Sunday morning but the only effect mentionable in this TAF set is a shift to SW winds at KCLT just before dawn, and developing LLWS concerns; cigs will linger there until after 12z Sun. Outlook: Conditions will become much drier yet gusty on Sunday as a strong cold front crosses the area. VFR conds will return Sun and remain through the middle of next week. Confidence Table... 08-14Z 14-20Z 20-02Z 02-06Z KCLT Med 75% Med 72% Med 67% High 94% KGSP Low 58% Med 78% Med 72% High 100% KAVL High 89% High 92% Med 61% High 85% KHKY Med 70% Med 76% Med 61% High 100% KGMU Med 71% Med 76% Med 72% High 100% KAND Med 77% Med 78% Med 64% 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... GA...Flood Watch through Sunday morning for GAZ010-017-018-026-028. NC...Flood Watch through Sunday morning for NCZ033-035>037-048>053- 056>059-062>065-068>072-501>510. High Wind Watch from Sunday morning through Monday morning for NCZ033-049-050-501-503-505. Wind Advisory from 7 PM this evening to 7 PM EST Sunday for NCZ048-051-052-058-059-062>064. Wind Advisory from 7 AM to 7 PM EST Sunday for NCZ053-065. Wind Advisory from 7 PM this evening to 7 AM EST Sunday for NCZ033-049-050. SC...Flood Watch through Sunday morning for SCZ001>010. && $$ SYNOPSIS...HG NEAR TERM...Wimberley SHORT TERM...HG LONG TERM...HG AVIATION...Wimberley

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