Area Forecast Discussion
Issued by NWS Jackson, KY

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000 FXUS63 KJKL 201837 AAC AFDJKL AREA FORECAST DISCUSSION...UPDATED National Weather Service Jackson KY 237 PM EDT Thu Jul 20 2017 .UPDATE...
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Issued at 225 PM EDT THU JUL 20 2017 Hourly grids were freshened up based on recent observations. This led to no substantial changes. Trends in very isolated convection in Central KY will be monitored for any possible need for isolated pops for a few locations. Overall, the ridge and associated warm temperatures aloft are keeping the cumulus from substantial vertical development. UPDATE Issued at 1138 AM EDT THU JUL 20 2017 High pressure at the surface remains in place across the southeastern Conus while mid and upper level ridging is centered to the west of KY. Cirrus is currently passing overhead with some cumulus also having developed in the moist airmass with dewpoints near 70. The previous forecast was largely on target with only some slight adjustments made to high temperatures in a few locations based on Coop highs from yesterday with hourly temperatures freshened up based on these trends. Most locations at 1500 feet elevation msl and below should again reach 90 or the lower 90s today while higher elevations climb well into the 80s. Heat indices in most locations should peak in the upper 90s to 100 range or just above 100. Deeper valley locations as well as locations in the far north and west should have the highest chance for heat indices peaking at 100 or just above 100. The SPS has been freshened up for this wording with the heat also mentioned in the HWO. Overall, though, no substantial changes were needed at this time. UPDATE Issued at 653 AM EDT THU JUL 20 2017 Ended up tweaking lows down a few degrees based on temperature trends. As sun continues to rise, temperatures will quickly be on the increase from here on out. Pretty good fog development going on across eastern KY, especially in the river valleys. This too should continue to impact the region for the next hour or two before quickly dissipating with the rising temperatures. Also tweaked forecast for dew points and winds in the near term to make sure they were on track with current conditions. All updates have been updated and sent to NDFD/web.
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&& .SHORT TERM...(Today through Friday) Issued at 417 AM EDT THU JUL 20 2017 Upper level ridging will continue to build into the region today. It will dampen somewhat tomorrow in the northern portion of the state as a strong upper level low over Hudson bay allows for more WNW to ESE flow across the northern half of the U.S. to shift a bit southward into the northern Ohio Valley. That being said, even with this small shift, the upper level ridge will still have dominant control. At the surface, high pressure will also be large and in charge across the state today. The continued building heights will attribute to yet another day of well above normal temperatures, reaching 90 or maybe even topping it in most locations. A few fair weather cu will be possible during the afternoon, but otherwise conditions should remain dry under this regime. By Friday, however, a cold front will push towards the region from the north. Flow will become more S to SW, allowing more moisture to advect in from the south and boosting humidity levels a bit (though not a strong surge). This will cause two concerns. First, the presence of the frontal boundary just to our our north and a warm/more humid airmass in place, could spark isolated to scattered showers and thunderstorms during the day, especially in the afternoon. Best chances will be in the northern CWA and along the high terrain in the SE. Second, the S/SW flow will boost temps another degree or two, into the low 90s for highs. This will combine with the additional humidity in the atmosphere to produce heat indices just over 100 degrees in most locations. While we are still below heat advisory criteria based on the forecast, will go ahead and issue an SPS for heat concerns this day. .LONG TERM...(Friday night through Wednesday) Issued at 422 AM EDT THU JUL 20 2017 An upper level ridge will be over the Tennessee Valley, and surface high pressure over the lower Great Lakes and Appalachians at the start of the period. The high pressure at the surface and aloft will be weakening. In the upper levels, this will allow for slight cooling and a modest increase in wind as a northeast CONUS trough develops and the westerlies drop further south. At the surface, increased flow will help to bring Gulf of Mexico moisture into the area. The combined result will be an overall increase in shower/thunderstorm coverage as we move through the weekend. A cold front is expected to approach from the northwest Sunday and Monday, and this is when the highest POP will be carried. The GFS and ECMWF agree on the front passing through from northwest to southeast late Monday and Monday night, allowing for somewhat cooler and less humid air to move in to finish the period. Until then, conditions will be uncomfortably hot and humid. The hottest weather is expected on Friday, before an increase in clouds and precip starts to hold temperatures down. && .AVIATION...
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(For the 18Z TAFS through 18Z Friday afternoon) ISSUED AT 235 PM EDT THU JUL 20 2017 Surface high pressure and an upper level ridge building into the area will continue the rather stagnant weather pattern, with VFR conditions and light winds prevailing through much of the period. Cu has developed in the 4 to 6kft range but should dissipate toward 23Z. High clouds will increase somewhat overnight, especially over the northern half of Kentucky, and stray convection cannot be completely ruled out during the 6Z to 18Z period ahead of an approaching cold front for SYM, SJS and possibly JKL. Patchy valley fog should again affect mainly non TAF locations in the southern part of the area. Some MVFR vis will be possible at LOZ and SME between 5Z and 13Z. Winds will remain light and variable through the period.
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&& .JKL WATCHES/WARNINGS/ADVISORIES... NONE. && $$ UPDATE...JP SHORT TERM...JMW LONG TERM...HAL AVIATION...JP

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