Area Forecast Discussion
Issued by NWS Jackson, KY

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303 FXUS63 KJKL 220846 AFDJKL AREA FORECAST DISCUSSION National Weather Service Jackson KY 446 AM EDT Tue Aug 22 2017 .SHORT TERM...(Today through Wednesday) Issued at 445 AM EDT TUE AUG 22 2017 A cold front will be the main culprit during the short term portion of the forecast. Aloft, troughing and NW flow across the region will allow a cold front to push SE towards the state throughout the day. It is then expected to traverse southward across eastern Kentucky starting between 0 and 3Z this evening, and exiting to our SE between 12 and 15Z Wednesday. As models come into a more fine tuned agreement of this system, so has the POP forecast. As the front moves across the state, strong surface high pressure to our north will begin to sink southward. Winds will quickly shift to a more northerly direction and allow much more stable and dry air to filter in. This will likely result in a fairly sharp cut off of precip behind the frontal passage tonight and early Wednesday morning, though upslope flow could allow it to linger a bit longer into Wednesday afternoon along the Virginia border. Elsewhere, mostly clear conditions will be in play by Wednesday, with continued NW flow aloft and Nrly flow at the surface keeping temperatures below seasonable normals, in the upper 70s with low humidity. Latest CAMs are all in agreement that convection will move through in a line structure ahead of the frontal boundary, with a secondary line possibly developing along the actual front just behind. The first round will affect eastern Kentucky through the afternoon, dissipating the farther it gets east. The second line then looks to pose the biggest punch as it moves across during the late afternoon and early evening, weakening as we head into the overnight. Also can`t rule out a few individual cells developing out ahead of these lines. Forecast soundings continue to support decent instability parameters as these lines move through the region. 0 to 6km sheer still looks to be between 30 and 40 kts through the afternoon which again is decent, but not overly impressive. In context, we are located quite a distance from the actual low center, so the overall best lift and sheer will be located farther to our NE where more sustained/larger QLCS`s are expected. SPC did not mention the concern of spin ups along the line in our region, and reasonably so. Freezing levels are quite high, and given the progressive nature of the linear system, hail, while possible, is not the main concern. Instead, still looking at more of a momentum transfer wind event as mixing occurs in the heavier showers/storms and along the leading edge of the line. Heavy rain will also be possible, with PWATS peaking around or just above 2 inches during the late afternoon/evening time frame. Thankfully we have had a decent period of drying across much of eastern Kentucky, so this should not prompt much of a flooding concern, especially given the progressive nature of the system. That being said, can`t rule out some isolated instances in low lying areas, urban areas, or locations that may experience several rounds. Overnight, the loss of heat and instability should weaken the system and cut off severe potential, along with general thunder potential. Dropped thunder down to chance and then slight chances in the prefrontal environment through the overnight. Once the front does pass through, convection will quickly be cut off all together. .LONG TERM...(Wednesday night through Monday) Issued at 359 AM EDT TUE AUG 22 2017 The models are in good agreement with a more amplified long wave pattern to hold on through the beginning of this weekend. Troughing will mainly rule in the East, with ridging in the West. Details concerning the evolution of the pattern by this weekend and early next week become more murky, especially with regards to the now remnants of Harvey. As such, did not deviate from the blended guidance. Dry weather will be on tap for eastern Kentucky through Friday, as high pressure controls to our north. A weak inverted trough will then nose in from the southwest, bringing just a slight chance of showers to our Virginia border counties on Saturday. POPs will diurnally increase into next week as the inverted trough appears to sharpen up more; however, much will depend on the outcome of Harvey. Temperatures will average below normal through the period, with highs mainly in the mid to upper 70s, with lows in the mid to upper 50s. && .AVIATION...(For the 06Z TAFS through 06Z Tuesday night) ISSUED AT 145 AM EDT TUE AUG 22 2017 VFR conditions and light winds will prevail at the TAF sites through early this afternoon. Valley fog is expected to develop through this morning with some local IFR conditions, however fog is not expected to affect the TAF sites. Clouds will be on the increase by around dawn but bases should remain VFR. An approaching cold front will then bring scattered thunderstorms this afternoon, with the storms becoming more numerous by evening. As such, kept with VCTS starting around 16 to 17Z, increasing to predominate TSRA by 21 to 22Z. We lose some instability heading into Tuesday night, but showers will continue to be numerous with maybe more scattered thunderstorms. Any shower or thunderstorm could reduce visibilities quite dramatically as they pass over a TAF site, however given the uncertainty of impacts and timing this far out, went prevailing MVFR during the numerous precip time frame. Winds will generally be from the SW through the TAF period, generally under 10 knots prevailing. && .JKL WATCHES/WARNINGS/ADVISORIES... NONE. && $$ SHORT TERM...JMW LONG TERM...GEOGERIAN AVIATION...JMW

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