Area Forecast Discussion
Issued by NWS Jackson, KY

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000 FXUS63 KJKL 170056 AAA AFDJKL AREA FORECAST DISCUSSION...UPDATED National Weather Service Jackson KY 756 PM EST Sat Dec 16 2017 .UPDATE...
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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.
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&& .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 18Z TAFS through 18Z Sunday afternoon) ISSUED AT 107 PM EST SAT DEC 16 2017 Expect VFR conditions through the entire TAF period. High pressure passing through will keep clear skies in place with southwest winds gusting up to 20 knots this afternoon before decreasing by 00z and becoming light and variable. A system will move into the area from the southwest as mid and upper level cloud cover moves in late tonight. LLWS is not expected to be an issue but southerly winds at the surface increasing to 25 and shift to the southwest will provide a notable wind shift. Low level cloud cover will likely begin to enter the area towards the end of the TAF period. && .AVIATION...
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(For the 00Z TAFS through 00Z Sunday evening) ISSUED AT 725PM EST SAT DEC 16 2017 VFR conditions should prevail in all locations through the first 18 to 21 hours of the period. Cirrus will gradually thicken and lower through the 10Z to 16Z period, before mid level and eventually low level clouds and a period of light rain begin to affect the region. As the atmosphere gradually saturates ahead of the next low pressure system, ceilings may drop into the MVFR range by the end of the TAF period, mainly in the south. Winds through the period are expected to generally be southwest to southeast at 10KT or less on average.
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&& .JKL WATCHES/WARNINGS/ADVISORIES... NONE. && $$ UPDATE...JP SHORT TERM...SHALLENBERGER LONG TERM...GREIF AVIATION...JP

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