Area Forecast Discussion
Issued by NWS Jackson, KY

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000 FXUS63 KJKL 212005 AFDJKL AREA FORECAST DISCUSSION National Weather Service Jackson KY 405 PM EDT Thu Sep 21 2017 .SHORT TERM...(This evening through Friday night) Issued at 405 PM EDT THU SEP 21 2017 19z sfc analysis shows weak high pressure through the area. This has not been strong enough to keep the convection at bay as mostly pinprick showers and a stray thunderstorm have developed this afternoon and are drifting south. In general, the more substantial showers are favoring the Cumberland Valley while mainly sprinkles are falling from any other cells. The small increase in cloud cover and limited pcpn coverage this afternoon did not slow the temperature rise much as they are topping out in the mid 80s most places while dewpoints are generally in the mid 60s. Winds through east Kentucky are mainly light from the northeast. The models are in good agreement aloft through the short term portion of the forecast. They all agree that ridging will hold over the area into the weekend. Early on, though, some stuck energy will be around the area before fading out tonight. This will be more of smaller scale issue so that the higher resolution models will be better at capturing the sensible weather. For that reason, while a general model blend was favored, the near term CAMS guidance was leaned on most heavily for the first part of the forecast, along with some persistence for the late night with respect to fog. Sensible weather will feature a few showers and a possible storm into the first part of the evening. Again expect the convection to die out and the clouds to clear this evening setting the stage for radiational cooling and the development of fog - dense in the river valleys - toward dawn. The fog will burn off by mid morning Friday and with less support from aloft expect a convection-free day and more warm conditions as we cross into autumn late that afternoon . Similar conditions as to tonight for Friday night will lead to areas of fog developing in the valleys - locally dense near the rivers and lakes. Used the CONSShort and ShortBlend as the starting point for the grids with some adjustments to the spot temperatures at night given the anticipation of ridge to valley splits. As for PoPs, kept the elevated through 00z this evening before dropping them below mention for wx later tonight and then kept them dry into the weekend. .LONG TERM...(Saturday through Thursday) Issued at 310 PM EDT THU SEP 21 2017 Models are in good agreement through most of the extended. Mid level ridge centered over the Great Lakes will eventually weaken and dissipate as it shifts eastward into New England by the end of the period. Further aloft, a rather large center of "relative" low pressure over the Atlantic Coast will retrograde westward into the deep south before lifting back out of the region into the Atlantic. A more significant trough will be moving into the Upper Midwest and Great Lakes region at the end of the extended period. At the surface, high pressure centered over the upper Ohio Valley will remain anchored until Dy6 Wednesday when it erodes under the influence of Maria, currently forecasted to ride northward just offshore of the east coast, and an approaching storm system from the west. The storm system moving in from the west will bring a cold front into the region towards the end of the forecast window, or possibly just beyond. Timing of this surface boundary into our area is in question since the 12Z GFS has trended slower with it`s parent trough approaching from the west. The 0Z ECMWF suggests a couple of shortwave impulses rounding the base of the main trough, which may bring multiple surface boundaries (front/troughs) and reinforcing shots of cooler air through the area. The 0Z ECMWF is also about 18-24 hours faster than the 12Z GFS solution with the main trough. A brief glance at the 12Z ECMWF shows a similar trend in the timing of the system (slower) into the Ohio Valley, just not as pronounced as the GFS. The ECMWF is now more consolidated with the smaller scale features, or shortwave energy rounding the base of the main trough as well. Sensible weather remains quiet, dry, and warm through the bulk of the extended. Temperatures will generally continue to run above normal with daily highs in the 80s and overnight lows generally in the 60s. Conditions remain favorable for patchy/areas of fog most nights. As already mentioned the only potential of rain will come at the end of the forecast window as a cold frontal system approaches from the west. With uncertainty in timing and details of this storm system will only advertise slight PoPs on the last day of the extended. Suspect future runs will tend to keep Thursday dry and hold off on higher PoPs until beyond the range of the current forecast package. Eventually however, once this storm system manages to clear the area, single digit H850 temperatures will be ushered into the region providing what may be a true shot of much cooler autumn air. && .AVIATION...(For the 18Z TAFS through 18Z Friday afternoon) ISSUED AT 200 PM EDT THU SEP 21 2017 Scattered cumulus ranging from 3 to 5k feet agl are found through the area this afternoon and a stray shower or storm may cross over a TAF site. However, these should be isolated enough to keep a mention out the TAFs. Skies will clear out once again tonight, with LIFR or worse fog forming mainly in the valleys, but will allow for some visibility restrictions from this late tonight at the LOZ, SME, and SYM sites. Winds will be light and variable through the period. && .JKL WATCHES/WARNINGS/ADVISORIES... NONE. && $$ SHORT TERM...GREIF LONG TERM...RAY AVIATION...GEOGERIAN/GREIF is the U.S. government's official web portal to all federal, state and local government web resources and services.