Scientific Forecaster Discussion

Area forecast discussion National Weather Service Greenville-Spartanburg SC 825 PM EDT Mon Sep 26 2016 Synopsis... a cold front will cross our area tonight and be near the Carolina coast by Tuesday evening. An upper level low will drop south into the Ohio Valley by Wednesday and linger over the central Appalachians through the end of the week. && Near term /through Tuesday/... as of 8 PM, line of convection producing very heavy rainfall extends from near Forest City, to Lincolnton to near Salisbury NC this evening. Although its movement is deliberate, it has been progressive, so rainfall amounts have generally been limited to around an inch per hours. However, there have been locally higher amounts of 2-3 inches. This activity will have to be monitored closely as it moves across Metro Charlotte between 9pm and midnight, owing to the poor antecedent conditions that exist there after this morning's heavy rain event. Based upon the upstream rainfall rates, it's certainly not a given that additional flash flooding will develop there, but it's likely to get close in the areas that were hit hard this morning. Otherwise, the part of the convective band pushing off the SC Blue Ridge is less cohesive/more spotty in nature, but there are signs that it may be trying to reorganize along the I-85 corridor, which is a good thing since this area is so desperate for rain. In fact, other than immediate Charlotte area, the potential for Hydro problems is quite low, with most locations along and south of the I-85 corridor expected to just see beneficial rain (although some quite a bit more so than others). Otherwise, over the next 24 hours, big-picture-wise a large mid/upper low is expected to slowly drop down from western Ontario to the upper Great Lakes, which will bring mid/upper troffing down across the western Carolinas. This should drive a surface cold front from the Tennessee Valley eastward across the srn Appalachians tonight and Tuesday morning. The front will probably get hung up across the mtns thru most of the day on Tuesday. Weak upper divergence will get strung out across the region tonight and Tuesday as the right entrance region of the upper jet moves overhead, but mid-level forcing is not that impressive. Most of what the front does will be the result of deep convection firing along and ahead of the feature this afternoon and evening, and then the weak forcing overnight. The guidance suggests holding onto a likely precip prob mainly out across the NC foothills and wrn Piedmont into the overnight hours. Min temps tonight should be mild. On Tuesday, with the front in our midst, precip chances should ramp up especially over the eastern zones where a likely was included. Temps will be at or above normal again. && Short term /Tuesday night through Thursday night/... as of 245 PM tuesday: showers and thunderstorms will be scattered along and southeast of Interstate 85 Tuesday evening as the front slowly drops south. This activity will slowly diminish but may linger into Wednesday morning. Most of Wednesday should be dry as cooler air moves into the region. A fairly potent upper level low pressure system will drop southeast across the Ohio Valley and central Appalachians impacting the area Wednesday night and Thursday. Several disturbances will swing around the parent low bringing rainfall to the area. The highest chance of rain will be in the upslope areas of western North Carolina while the lowest chance will be in northeast Georgia and upstate South Carolina where downsloping low level flow will exist. All locations will be cooler as the impacts of the low translate across the area. && Long term /Friday through Monday/... as of 315 PM EDT upper low is expected to be over Bristol Tennessee Thursday evening then lift north to eastern Ohio Friday evening. The GFS forecast has been accepted as the better performing operational model. The European model (ecmwf) is better today but still slower in its evolution of the cut off closed low lifting out and departing New England. With the lower atmospheric thickness values associated with this upper low and more cloud cover Thursday night into Friday, this will keep temperatures several degrees below normal. Very light rain amounts are possible across the NC mtns and north of I-40 Thursday evening then becoming increasingly limited to the Tennessee border areas until Friday evening when it GOES away to the north of our forecast area. The closed and cut off upper low will begin to open to the westerly flow at the north end of the Great Lakes on Saturday. The low will fill and become absorbed near Montreal Sunday night. We will be at the southern end of a large area of surface high pressure Sunday into Monday. The GFS has the high centered over north central Quebec at 12z Tuesday ridging down across our area. The orientation of the low level wind flow with this high may produce upslope wind by Monday night into Tuesday which may result in light rain in the Lake Jocassee to Tryon to Lenoir areas. Temperatures will certainly be several degrees below normal Friday then slowly rise to near normal as the low moves away from our region over the weekend. By Monday, temps may even be a little above normal since the high may not be strong enough and far enough away to have minimum influence toward wedge conditions. && Aviation /01z Tuesday through Saturday/... at kclt: a significant line of convection producing very heavy rainfall extends from about 50 miles west through 30 miles north of the airfield as of 2345z. Expect this to be on the terminal between 01z and 02z, and should produce MVFR conditions, initially gusty winds, and periodic IFR visby. A tempo has been included from 01-04 to account for the lower conditions. Stronger convection should move east of the terminal btw 03-04z, but at least vcsh will persist for a couple of hours after that. With the expectation of heavy rain developing over the terminal this evening, chances for development of fog and/or low stratus are increasing for the early morning hours, but opted to simply hint at this with scattered IFR/MVFR clouds and MVFR visby for now, and will re-evaluate at 06z. Otherwise, prevailing winds should turn toward the SW through the period. While additional shower activity cannot be ruled out Tue morning, the best chances for convection will exist during the afternoon, with vcsh/prob30 warranted during this time. Elsewhere: stronger convection has moved east of kavl/khky, although rain showers will persist for another couple of hours, possibly resulting in periods of MVFR visby. The upstate SC terminals are also expected to get in on the convective action very shortly, but convection is expected to be considerably more tame than that affecting kclt, so only expect MVFR conditions, with perhaps very brief IFR possible. Otherwise, IFR conditions are expected to develop at kavl/khky by around 06-07z, if not sooner (this potential handled with a tempo) as the low level air is saturated due to rainfall. Chances will also exist for restrictions early Tue morning at the upstate terminals. This has been handled with scattered MVFR clouds at some MVFR visby at some terminals, but this will be readdressed at 06z. While additional shower activity cannot be ruled out Tue morning, the best chances for convection will exist during the afternoon, with vcsh/prob30 warranted during this time. Outlook: cold front will slowly cross the region through Tuesday. Dry high pressure builds in slowly behind the front on Wednesday, which should bring VFR conditions in most places through the end of the week. Confidence table... 00-06z 06-12z 12-18z 18-00z kclt high 88% high 94% high 90% high 100% kgsp high 88% high 89% high 85% high 100% kavl high 82% Med 76% high 86% high 98% khky high 81% Med 62% Med 70% high 100% kgmu high 88% high 89% Med 78% high 100% kand high 100% high 86% high 85% high 100% 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...deo near term...jdl/PM short term...SW long term...deo aviation...jdl

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