Area Forecast Discussion
Issued by NWS Jackson, KY

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000 FXUS63 KJKL 170538 AFDJKL AREA FORECAST DISCUSSION National Weather Service Jackson KY 1238 AM EST Sun Dec 17 2017 .UPDATE... Issued at 1238 AM EST SUN DEC 17 2017 The latest surface analysis shows high pressure parked to out southeast and this is the biggest driver for our sensible weather tonight. Only minor changes needed for the near term forecast with grids in good shape at this point. the biggest issues we will be looking at for the forecast package will be the onset of precipitation today and potential for lowering POPs overall. The models continue to trend slower and drier with this system, as the associated wave dampens as it moves northeast and dry air will be difficult to overcome. UPDATE Issued at 735 PM EST SAT DEC 16 2017 Hourly grids have been updated based on recent observations. The eastern valley KY Mesonet locations and KEKQ have been dropping off faster since around sunset compared to the inherited forecast. Hourly grids have been adjusted accordingly to lower hourly temperatures for the next few hours and in some cases min T was lowered. Otherwise, no changes were needed at this time. && .SHORT TERM...(This evening through Sunday night) Issued at 339 PM EST SAT DEC 16 2017 Current conditions across the area feature high pressure under control across the region with clear skies over eastern Kentucky and afternoon high temperatures around 50. Heading into tonight, models have slowed the approach of the next system moving in from the southwest significantly. Thus, will see a decent ridge to valley temperatures difference setting up tonight as skies will likely be clear through much of the night with only some thin cirrus creeping in late tonight. Heading into tomorrow, the forecast challenge will be when the profile saturates allowing precip to reach the surface which at this point with the model trends, looks to be by 18Z. Once the precip reaches the surface, the moisture present will be enough for the entire area to measure but due to the progressive pattern and duration, QPF looks to be around a tenth. In fact, most of the rainfall will exit by early Sunday night with low level moisture hanging across the area into Monday morning. So will be dealing with a lingering stratocumulus layer likely lasting into the day on Monday. Thus will leave low temps Sunday night into the upper 30s with the lingering ceilings keeping the surface somewhat insulated. .LONG TERM...(Monday through Saturday) Issued at 220 PM EST SAT DEC 16 2017 The models are in good agreement aloft at least through the first half of the extended portion of the forecast before starting to diverge toward the end of the week. They all depict stout ridging over the Southeast with fast and nearly zonal flow through the Ohio Valley carrying just weak packets of energy above us through Tuesday morning. While the northern stream remains active through the Great Lakes, Kentucky`s next weather maker will come from a trough in the southern stream moving out of New Mexico rolling east during the day, Tuesday. This wave does dampen as it approaches by Wednesday with the GFS slightly weaker and faster than the ECMWF. Its passage that night will allow heights to rebound on Thursday ahead of a much stronger and full latitude trough digging into the Four Corners region. This is where the model spread becomes rather large with the ECMWF bringing the core of its trough east into the Southern Plains quicker than the stronger GFS Friday morning. The ECMWF also releases a good portion of its energy into the northern stream that then quickly spreads through the northern Ohio Valley while the GFS has hardly any hints of this action - keeping its core energy well to the southwest. The model split magnifies heading into the weekend with strong ridging from the GFS pushing well into Kentucky from the Southeast while the ECMWF is spreading lower heights east into the lower and mid Mississippi Valley. Needless to say, confidence is low for the Christmas weekend weather given these large model discrepancies. For now will not deviate too far from the model blend at these time steps until there is a clearer break toward one solution or another. Sensible weather will feature near normal conditions to start the work week, though lingering stratus will be a concern through the day Monday (and well into Tuesday) limiting the warming potential even as high pressure slides off to the east. This will likely set up a larger than normal temperature gradient across the area from southwest to northeast into Tuesday. Warm air will move more effectively into the area for Tuesday afternoon but a mostly dry frontal boundary will then shift south enough to renew the temperature gradient across the area into Wednesday as a southern wave passes just south of the area - though potentially strafing our southern tier with some pcpn in the form of light rain. Quiet wx takes hold through the rest of the work week with another gradual warm up occurring for all of eastern Kentucky into Friday. Low pressure running northeast to our west and then passing Kentucky by the northeast will then set up a baroclinic zone through the Ohio and Tennessee Valley later that night and into Saturday with significant uncertainty as to the placement of the colder and warmer air throughout the area. The GFS shows strong overrunning across this boundary from an open Gulf leading to ample pcpn for the JKL CWA Friday night and through the weekend split by the cold air intrusion making for significant concerns of winter wx across our area. Meanwhile, the ECMWF slips its cold front right on through the state and well to our southeast taking with it any real potential for significant winter weather lacking anywhere near the overrunning signals of the GFS with its baroclinic zone well off to the southeast of the state. Accordingly, will downplay any of the more extreme aspects of this potential situation in favor of a more benign interpretation of the blends. Given the high level of uncertainty and dynamic potential, though, this will have to be watched closely in upcoming model runs. Only made some minor adjustments to temperatures through the period - even at night given different advection patterns throughout the extended and mostly plentiful cloud cover. As for PoPs, have downplayed the potential for midweek in the far south and again Friday into Saturday given the high degree of uncertainty at this point. && .AVIATION...(For the 06Z TAFS through 06Z Sunday night) ISSUED AT 1223 AM EST SUN DEC 17 2017 We are starting the period off VFR and this will remain the trend for much of this TAF cycle. The feature of interest for the TAF period is an upper level shortwave which is currently located across the Southern Plains tonight. This feature will move northeast and weaken as it does so. This will bring a shot of light rain to the area through the day on Sunday. The impacts will be minimal till we get closer to the end of the cycle when MVFR CIGS will become the norm across most TAF sites. The winds are expected to remain light out of the south and southwest through the TAF period. && .JKL WATCHES/WARNINGS/ADVISORIES... NONE. && $$ UPDATE...DJ SHORT TERM...SHALLENBERGER LONG TERM...GREIF AVIATION...DJ

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