Area Forecast Discussion
Issued by NWS Jackson, KY

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000 FXUS63 KJKL 021733 AFDJKL AREA FORECAST DISCUSSION National Weather Service Jackson KY 1233 PM EST Fri Dec 2 2016 .UPDATE... Issued at 1039 AM EST FRI DEC 2 2016 Stratus deck in far northern KY has drifted south as low level wind flow has shifted from west to west northwest. Cloud deck just now into Fleming County at the northern end of forecast area. Current trends and latest short range model guidance pointing towards a trend of low level winds continuing to veer to more northwesterly. This complicates the near term portion of the forecast with respect to forecasting advance of low cloud deck. Have modified sky cover to bring more clouds into the northern part of the forecast area, with a resulting decrease in afternoon maximum temperatures. UPDATE Issued at 652 AM EST FRI DEC 2 2016 Stratus deck hovering along the Ohio River this morning with isolated patches of lower clouds streaming across portions of eastern Kentucky. May see this shield shift south a bit this morning as temperatures climb toward the mid 40s near the greater cloud cover to low 50s farther south. && .SHORT TERM...(Today through Saturday) Issued at 352 AM EST FRI DEC 2 2016 A stacked low pressure system across Quebec will continue to result in cyclonic flow into the Ohio Valley, producing rounds of clouds moving across eastern Kentucky. Despite the greatest concentration of stratocumulus remaining north of the Interstate 64 corridor, cirrus will stream in downstream of an upper ridge attempting to build north across the Great Plains as an upper low digs into the Gulf of California. However, aforementioned Canadian upper low will limit height rises, subsequently keeping high temperatures in the mid 40s to low 50s as a low level cold dome remains locked in place into the Tennessee Valley. Surface ridging stretching from the northern plains to the northern gulf coast will make greater headway toward the Commonwealth today. Enough of a pressure gradient will remain in place to allow for west/southwest winds to still increase to 5-10 mph later this morning through this afternoon. This surface ridge looks to nose into southern Kentucky this evening and tonight, producing calm surface winds. Portions of the Big Sandy region through northeast Kentucky will likely see enough low level moisture to keep radiational cooling from taking full effect. The remainder of eastern Kentucky will likely also be somewhat limited in this regard as the cirrus shield thickens and becomes more widespread. Nonetheless, valley locations should cool into the mid 20s while ridges remain near 30 degrees by daybreak Saturday. Upper ridging looks to finally move overhead by later in the day on Saturday with accompanying height rises. However, top-down moistening and high pressure overhead, making for slow retreat of the lingering cold dome, will in all likelihood result in below normal temperatures once again to start the weekend, as highs top out in the mid 40s to low 50s. .LONG TERM...(Saturday night through Thursday) Issued at 356 AM EST FRI DEC 2 2016 While deamplified ridging is expected across the area through the first part the weekend, a small upper level shortwave is expected to begin pushing eastward toward the state from the upper Mississippi River Valley through the day Sunday. This shortwave should pass, mainly to our north, through 12Z Monday morning before exiting to our northeast. Zonal flow will take hold once more during the day Monday. However, attention will then turn to a closed upper level low over southern TX as of 12Z Monday, as it begins to propagate NE, reaching western TN and western KY by 12Z Tuesday. The low will dissolve slightly into a wave as it passes over eastern KY Tuesday afternoon, before getting absorbed into the overall longwave troughing pattern establishing itself across the western 2/3 of the conus. Through this point, but the GFS and ECMWF are in very good agreement with one another. Things start to break down a bit as the longwave trough begins to shift eastward toward the Mississippi River Valley and eventually the Ohio River Valley mid week. The ECMWF is a bit slower with the axis of the trough reaching the state, while the GFS has it directly across KY by 18Z Thursday. Given the uncertainty, will stick to model blend for precip and other variables during this time. The first shortwave to impact the region will bring with it a surge of upper level support, including a strong jet max. However, at the surface, high pressure should generally still be in control, though moving east of the region, with very light winds and little llvl moisture support. Most of this system will be mid and upper level dynamically driven, as winds should start feeding off of the stronger system to our SW over TX, pulling moisture off the Gulf of Mexico and northward. As such, this heightened pull of moisture should eventually overcome the stable air at the surface, and allow precip to fall through the dry layer. All models are in agreement that precip should start entering into the CWA from the south between 6 and 12Z, quickly increasing as the shortwave nears closer to the state throughout the day Sunday. Temperatures will be at or just above freezing Saturday night and into early Sunday morning, before quickly warming. Despite this, temperatures aloft during this time, in the moist layer, will be quite a bit colder. As such, as precip begins to overcome the dry air, it is possible that snow may mix with the rain, before changing over to all rain for the remainder of the day. This should not cause any actual snow accumulations. The precip and upper level dynamics should finally shift east of the region Monday, allowing for a brief respite in the precipitation. However, it won`t be long before the next upper level low begins to push northeast from TX, and brings with it another wide swath of precipitation. Both GFS and ECWMF are once again in good agreement with this system, bringing the front end of precip into the region between 0 and 3Z Tuesday, then quickly increasing to categorical coverage by 6Z and continuing throughout the morning and early afternoon as the surface low and upper level wave pass nearly overhead. As the wave moves northeast of the state and gets absorbed into the upper level flow during the day Wednesday, forcing will lesson and precip will taper off. The GFS does keep some residual moisture and pops across the CWA for the entire day Wednesday so did leave some slight chance pops in, though QPF should be minimal. Model differences start to appear in timing from here on out. The longwave pattern across the western 2/3 of the US will begin to shift eastward, bringing impulses/shortwaves across KY. The GFS shows this as early as Wednesday evening/night, while the ECWMF is almost 12 hours behind in timing. Regardless of the timing, they both agree that this could be a rather potent system, with very tight pressure gradients pulling deep southerly flow from the Gulf of Mexico and producing decent QPF values. Went with a blend of the two models for POPs and QPF, which resulted in an average of chances for both Wednesday night through Thursday, but no doubt that we will see precip at some point during this period. If the GFS pans out, the cooler overnight temps could lead to an onset of snow, transitioning to rain during the day. The ECMWF should be late enough, however, that most precip should fall as all rain. Either way, based on the combination of the two producing chance pops Wednesday night, this will also result in some snow chances. While some light accumulations are possible, generally less than an inch, as snow changes over to rain during the morning hours, all snow should quickly melt. && .AVIATION...
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(For the 18Z TAFS through 18Z Saturday afternoon) ISSUED AT 1233 PM EST FRI DEC 2 2016 Deck of stratocumulus has continued to sink southeastward across the forecast area, with MVFR ceilings now expected to prevail across the northeastern one half of the forecast area into the day Saturday. The MVFR cloud deck is expected to persist at SYM, SJS and JKL through the forecast period. It is still uncertain how far southwest the cloud deck will eventually extend, but there are some indications it could reach LOZ and SME by daybreak Saturday.
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&& .JKL WATCHES/WARNINGS/ADVISORIES... NONE. && $$ UPDATE...SBH SHORT TERM...GUSEMAN LONG TERM...JMW AVIATION...SBH is the U.S. government's official web portal to all federal, state and local government web resources and services.