Area Forecast Discussion
Issued by NWS Jackson, KY

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000 FXUS63 KJKL 211737 AFDJKL AREA FORECAST DISCUSSION National Weather Service Jackson KY 137 PM EDT Mon Aug 21 2017 .UPDATE... Issued at 137 PM EDT MON AUG 21 2017 Will be updating NDFD shortly to account for latest observational trends. Three areas of thicker cirrus are moving across the forecast area, one around Lake Cumberland, another along the TN-KY border and a third moving ESE from the central Bluegrass towards the central part of our forecast area. Scattered cumulus is also developing. The movement of the cirrus patches is high enough that the SW part of the area where the percentage of the eclipse is the highest will only be dealing with scattered cumulus during the height of the eclipse about an hour from now. It will be hit and miss whether an individual cumulus cloud will be over the sun during the height of the eclipse though. UPDATE Issued at 1238 PM EDT MON AUG 21 2017 Updated NDFD for latest observational trends. With the partial eclipse to begin within the next hour the main focus was to update sky cover based on satellite trends. Satellite shows a thicker patch of cirrus moving into the Cumberland Valley and another area of thicker cirrus moving across the Bluegrass region into the northern part of our forecast area. At least for the southwest part of the forecast area where the partial eclipse will have the highest percentage the thicker patch of cirrus will have moved east of the area by the height of the partial solar eclipse. UPDATE Issued at 1039 AM EDT MON AUG 21 2017 Updated NDFD for latest observational trends. There was very little change in the ongoing forecast. Sky cover still looks to be minimal during the eastern KY partial eclipse this afternoon. Some thin cirrus will move across the area from the thunderstorms over IL, but it appears that the thicker cirrus will remain north of our area. UPDATE Issued at 745 AM EDT MON AUG 21 2017 Freshened up the hourly temperatures, dew points, and sky cover through this morning. Patchy fog will burn off by around 9 am, with some thin cirrus moving in from the northwest out ahead of a weakening thunderstorm complex located across the Midwest. && .SHORT TERM...(Today through Tuesday) Issued at 358 AM EDT MON AUG 21 2017 Surface high pressure is centered near the mid-Atlantic, with a quasi-stationary boundary aligned from the Great Lakes to the central Plains, with a secondary warm front extending southeast into the Ohio Valley. Aloft, an upper level ridge rules across the southeast CONUS, with flatter flow across the majority of the Mississippi Valley. A more vigorous short wave trough is moving southeast out of the southwest Canada, currently moving through the British Columbia/Alberta border. Eastern Kentucky is enjoying another most clear night, with areas of valley fog. Some debris high clouds have been making it into the area from the west, but these remain pretty thin. The next 24 hours will remain dry, as the upper level ridge to our southeast remains in control. Today still looks like an overall good viewing day for the full eclipse. Highs will peak in the upper 80s to near 90, with some cumulus and thinner high clouds to contend with during the day, but nothing extensive. Tonight, skies will remain mostly clear initially, as afternoon cumulus dries up. Meanwhile, the Canadian short wave trough will deepen and continue to move southeast, reaching the Great Lakes region by dawn Tuesday. Some high clouds will move in by early Tuesday morning, keeping fog a bit less extensive. Lows will generally range from 65 to 70 degrees. As the short wave trough continues to amplify across the U.S./Canadian border on Tuesday, low pressure will organize and deepen rapidly along the surface boundary to the east. The cold front will move southeast across the Ohio Valley Tuesday afternoon, with showers and thunderstorms breaking out along and ahead of the boundary. The blended POPs yielded likely values from the northwest during the afternoon, and given the forcing and instability, this seems reasonable. Additionally, stronger 0-6km bulk shear values than we have seen in a while will be on tap, supporting a severe thunderstorm threat, especially across the northwest half of the area. Will highlight this threat in the HWO. .LONG TERM...(Tuesday night through Sunday) Issued at 358 AM EDT MON AUG 21 2017 Forecast will start out with a cold front approaching the region Tuesday evening, and traversing from NW to SE Tuesday night and during the first half of the day Wednesday. Showers and thunderstorms will be possible throughout the night, but will peak during the day Wednesday just ahead of the frontal passage. That being said, once the front passes, winds will quickly shift to a more northerly direction and instability will drop off. Lingering precip on the back side of the frontal passage through the day Wednesday will therefore likely be in the form of rain, and should come to an end by Wednesday evening as the frontal boundary shifts southeast of the state. A large area of high pressure will take hold Wednesday night and through the rest of the work week. A strong troughing pattern will still be in place aloft, promoting NW flow across the state. This, in conjunction with northerly winds at the surface, will help to keep temperatures well below normal. Highs will generally be in the upper 70s with low humidity through Friday. By Saturday and Sunday, models are still in fair agreement showing the upper level troughing moving east of the region but some shortwaves developing and moving into the Ohio Valley under continued NW flow. That being said, they are not in good agreement as far as the exact timing and magnitude of these waves across the state. Surface winds also suggest a weak surface low passing across the region during the day Sunday. However, not confident on the exact location and timing given the uncertainty in models aloft. Regardless, there is the possibility for some unsettled weather during the weekend. Right now model consensus is for mainly diurnally driven isolated to scattered showers and thunderstorms, with best coverage during the day Sunday. Surprisingly both the GFS and ECMWF are both putting similar areas of coverage across far SE KY during the day Sunday as of now, so confidence was a bit higher here. But expect this forecast to continue evolving over the next few days. && .AVIATION...(For the 18Z TAFS through 18Z Tuesday afternoon) ISSUED AT 137 PM EDT MON AUG 21 2017 VFR conditions and light winds will prevail at the TAF sites through early Tuesday afternoon. Valley fog is expected to develop tonight with some local IFR conditions, however fog is not expected to affect the TAF sites. An approaching cold front will bring scattered thunderstorms Tuesday afternoon, with the storms becoming more numerous by Tuesday evening. && .JKL WATCHES/WARNINGS/ADVISORIES... NONE. && $$ UPDATE...SBH SHORT TERM...GEOGERIAN LONG TERM...JMW AVIATION...SBH

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