Area Forecast Discussion
Issued by NWS Jackson, KY

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000 FXUS63 KJKL 180030 AAA AFDJKL AREA FORECAST DISCUSSION...UPDATED National Weather Service Jackson KY 730 PM EST Fri Nov 17 2017 .UPDATE... Issued at 654 PM EST FRI NOV 17 2017 Updated sky grids to slow advancement of cloud cover. Dry air and calm winds in the valleys this evening have allowed temps to cool past the forecasted lows for tonight. Adjusted temps in the eastern valleys based on these current trends. Once clouds move in to the area, the temps should become more steady, if not warm through the rest of the night. && .SHORT TERM...(This evening through Saturday night) Issued at 415 PM EST FRI NOV 17 2017 High pressure at the sfc and aloft is beginning to shift east of the area. Meanwhile a mid level trough with a few embedded shortwaves is beginning to approach from the west. The individual shortwaves will begin to phase as the trough near the MS River on Saturday with a near neutral tilt. The trough will take on more of negative tilt on Saturday night. At the same time, the surface low is expected to track from the Central Plains to the Mid MS Valley tonight and then deepen rather quickly in response to the mid level trough deepening from late tonight through Saturday night as it tracks from the Mid MS Valley and to the mid MS and toward the Adirondacks to St Lawrence valley area pending model differences. The GFS is generally deepest and further west compared to NAM or ECMWF runs. WPC guidance is generally a compromise between the two with position and strength of surface low at the end of the period. As the high moves east and the low and associated front and upper trough begin to approach, winds aloft will increase as the pressure gradient increases across the lower OH Valley region. A low level jet will also increase overnight across the lower OH valley region. In addition, initially high clouds will begin to thicken and lower between 0Z and 6Z with mid level clouds possible late as a warm front approaches. Some showers or sprinkles will be possible across the northwest late as the front nears with possibly a stray rumble of thunder, but downslope flow should preclude any showers or sprinkles in the southeast. With light winds and high pressure initially in place, eastern valleys should decouple prior to sunset and drop 15 degrees or more through late evening before the clouds and pressure gradient increase leading to a leveling off of temperatures if not a rise for those valley locations. Winds fields aloft will remain rather strong ahead of the cold front on Saturday with some mixing down of momentum possible in the morning behind the warm front with some limited solar insolation. Then, showers and even a few embedded thunderstorms ahead of the front will be capable of mixing down the higher momentum. SPC has place the western portion of the area in a Marginal Risk for Severe Thunderstorms with this. The highest winds aloft are generally in the late afternoon and evening. Time height sections indicate that theta e and saturated equivalent potential temperature lines become nearly vertical along and or just behind the cold front during the evening. This is the period when the threat for widespread wind gusts to around 40 mph or higher appears most likely. Lingering low level moisture may be sufficient with cold air advection occurring, 850 mb temperatures falling to below 0C by the end of the period and a west to west northwest upslope low level flow for some rain or snow showers in the higher terrain before sunrise on Sunday. .LONG TERM...(Sunday through Friday) Issued at 437 PM EST FRI NOV 17 2017 Forecast starts off Sunday morning with an exiting cold front and precip tapering off from west to east. A strong shot of post frontal cold air could result in some snow mixing in with the rain as it tapers off Sunday morning, though not expecting any accumulations as ground temperatures will lag. The exception will be the highest terrain, mainly Black Mountain, but precip will be so light by Sunday morning, only a trace or so will likely occur before it mixes with rain and then tapers off soon after. High pressure is expected to move in overhead Sunday night. As was the case yesterday and last night, there is still some question as to if enough llvl moisture will become trapped under the subsidence inversion to keep clouds across the region as we head into the overnight. This could have a dramatic impact on the expected low temperatures. GFS shows the clouds scattering out across the region generally 3Z and after, while the NAM is a bit slower, showing the scattering occurring after 6Z. Went ahead and did a bit of a ridge valley split for Sunday night/Monday morning, however did not go that aggressive given the uncertainty on if these clouds will stick around or not. Return flow on the backside of the exiting high pressure will take hold by Monday night. Light southerly flow could provide a better environment for decoupling, so did drop valley temps (mainly the sheltered eastern ones) down quite a bit. An upper level trough is expected to shift east through the Upper Great Lakes during the day Tuesday, with a surface system expected to follow in its path. This will drag a weakening cold front southeast across the region Tuesday night into Wednesday, however all indications point at this frontal system being dry as it moves across the Ohio Valley, with the only impact being a slight uptick in clouds and a reinforcement of cold northerly flow. Temperatures are expected to remain below normal through the rest of the forecast period as this northerly flow continues ahead of another approaching large high pressure system. && .AVIATION...
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(For the 00Z TAFS through 00Z Saturday evening) ISSUED AT 730 PM EST FRI NOV 17 2017 Clouds will increase and lower tonight, out ahead of an approaching cold front from the west. Southeast winds of around 5 kts, will increase and become southerly at around 10 kts, with gusts up to 20 kts becoming more likely between 06 and 12z out west. South southwest winds will continue to increase to 10 to 20 kts, with gusts of 25 to 35 kts during the day on Saturday. Some scattered shower activity will be possible between 06 and 18z, with more widespread showers moving in after 18z. Ceilings/visibilities will lower down to MVFR towards the end of the period, as the cold front begins to cross eastern Kentucky.
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&& .JKL WATCHES/WARNINGS/ADVISORIES... Wind Advisory from 7 AM Saturday to 1 AM EST Sunday for KYZ044- 050>052-058>060-068-069-079-080-083-084-104-106-108-111-114. Wind Advisory from 10 AM Saturday to 4 AM EST Sunday for KYZ085>088-107-109-110-112-113-115>120. && $$ UPDATE...CGAL SHORT TERM...JP LONG TERM...JMW AVIATION...GEOGERIAN is the U.S. government's official web portal to all federal, state and local government web resources and services.