Area Forecast Discussion
Issued by NWS Jackson, KY

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000 FXUS63 KJKL 121434 AFDJKL AREA FORECAST DISCUSSION National Weather Service Jackson KY 934 AM EST Tue Dec 12 2017 .UPDATE...
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Issued at 934 AM EST TUE DEC 12 2017 Have updated temperatures and precip type for today and tonight based on latest obs and model data. Once cloud ice production begins, the steep low level lapse rates headed in will result in rain vs. snow temps being fairly warm in the upper 30s today. UPDATE Issued at 710 AM EST TUE DEC 12 2017 The snow is starting to develop over the area. Have updated the forecast mainly to add in the latest T and Td obs/trends. Did fine tune the PoPs and will likely wipe out the freezing pcpn with an update before 8 am. Look for a new set of zones, updated SPS, and HWO with that issuance. The updated grids have been sent to the NDFD and web servers.
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&& .SHORT TERM...(Today through Wednesday) Issued at 305 AM EST TUE DEC 12 2017 07z sfc analysis shows a fairly strong area of low pressure moving east through northwest Pennsylvania. A cold front from this system is poised to enter Kentucky from the northwest. Much colder air trails the front along with brisk northwest winds. Out ahead of it, good mixing through the night thanks to a moderately strong low level jet is keeping the temperatures up even in the sheltered valleys where readings are down to the low and mid 40s. However, a hole in the cloud cover is pushing east into eastern Kentucky and would expect to see better cooling beneath this for the next few hours - but the winds mixing warmer air from aloft and locations surrounding the valleys will likely keep them well above freezing through dawn. Otherwise, temperatures are mild ahead of the front range from the low 50s in the far southeast to the lower 40s in the northwest with most places in the mid to upper 40s. Dewpoints, meanwhile are running in the upper 20s to lower 30s across the area. Sfc winds are gusting most places, from the southwest in the far east to more westerly in the west, at between 20 and 25 mph. Not much pcpn is showing up on radar ATTM even behind the front - though some drizzle is showing up in the obs upstream. Do expect it to start popping up in our northwest over the next couple of hours but falling into a relatively warm boundary layer with limited opportunity to wetbulb cooler. As such, am now looking at more in the way of liquid drizzle/sprinkles through dawn - owing to a lack of ice crystals in the column initially. Snow arrives shortly after dawn most places once the column has had a chance to cool. There also is a small potential for freezing rain/sprinkles in the couple to few hours following sunrise as the colder air moves in, however warm sfc temperatures - road pavement readings are in the upper 30s to low 40s ATTM - will limit this concern to elevated sfcs like railings, decks, and perhaps untreated bridges/overpasses will tailor the ongoing SPS to address these concerns initially. The snow expectations are elaborated on further down this discussion. The models are in excellent agreement aloft as this zero hour as the next system moves in. Twin pol troughing will swing into the Ohio Valley today with a lead impulse passing to our northeast by 12z. The core of the trough will then press into the Ohio Valley through the afternoon with plenty of energy and height falls working over eastern Kentucky. Just as quick as it arrives today this trough will lift out to the northeast tonight with heights rebounding in northwest flow. The flow will swiftly become more zonal by the end of the day Wednesday with the next impulse moving in from the west. This latter feature is similarly handled in the GFS and ECMWF, but lead the lagging CMC. Will favor the blend here sticking close to the majority consensus. Otherwise, have used the higher resolution models most heavily through today for specific forecast details with better than average confidence. Sensible weather today will see the front and its dynamics bring showers to the area going over to - from west to east snow during the morning. Some instability will be available to help drive this convection into the afternoon for our eastern zones as the snow patches/bands scatter out. A better Great Lake moisture fetch looks to set up just northeast of the area later this afternoon and evening likely sparing the JKL CWA from an extended snow threat. Have gone above the blended PoPs this morning and into the afternoon in favor of the scenario depicted by the HRRR and CAMs. Look for temperatures to fall through the day with some insolation limiting accumulations compared to the last event. Additionally, the road sfc temperatures will be much higher than they were Saturday again minimizing icing impacts. Accordingly, will continue to handle this event with an SPS focusing on variable visibility concerns with any squalls given the strength of the low level wind field. These northwest winds bring in colder air tonight and have gone a bit lower than the blend in our more open areas and the ridges for lows tonight. Similarly, 45 on Wednesday, kept those locations as touch cooler with variable cloud cover between quick hitting systems/clippers keeping the sunshine from contributing much. Winds were raised today and on Wednesday as the blend seems to be too influenced by the low biased URMA for situations such as these. Look for the SPS to continue through the early evening for the CWA with less focus on the freezing pcpn potential in favor of highlighting the more convective snow showers later this morning and this afternoon with even some squalls possible. .LONG TERM...(Wednesday night through Monday) Issued at 245 AM EST TUE DEC 12 2017 The extended will be fairly active and will feature several periods of precipitation, as multiple weather systems move across the eastern portion of the CONUS. The first episode of precipitation is slated to occur Wednesday night through late Thursday morning, as a weak area of low pressure aloft moves across the Ohio Valley. Rain and showers will be on tap for that period, with little if any snow accumulations expected. The second period of precipitation is expected Thursday night through late Friday night. Temperatures during this period should be a bit cooler, so more snow is expected across the area. Very light snow accumulations may occur early Friday night, as a clipper type system moves quickly through the region. A third period of precipitation is forecast for Sunday through Monday. This precip will be associated with a larger scale and more potent area of low pressure that is progged to move out of the southern Plains and eventually across the Tennessee and Ohio Valley regions. Temperatures should be warm enough during the passage of this system for mostly rain to occur across the area. Some snow showers may mix with rain Sunday night into early Monday morning, as some colder air filters into the region. If any snow accumulations do occur Sunday night, they will be very light and likely confined to grassy areas and elevated surfaces. Temperatures during the extended will vary quite a bit, with some days featuring highs in the 30s and 40s, and other days with highs in the 40s and 50s. Nightly lows will not be quite as varied, with min values in the 20s and 30s expected. && .AVIATION...(For the 12Z TAFS through 12Z Wednesday morning) ISSUED AT 725 AM EST TUE DEC 12 2017 A clipper system is working through the area this morning with a mix of light rain and snow moving through the area. A changeover to all snow will occur from northwest to southeast late this morning as the column cools. Temperatures will drop throughout the day with strong CAA. Some of the more robust snow showers coupled with gusty winds will lead to IFR or lower VIS at times. Winds will be gusty through the day with some locations seeing gusts of 20 to 25 knots at times. Winds will be west to northwest through the period. Conditions will improve later in the afternoon from southwest to northeast with VFR cigs returning and the snow showers scattering out - though any place hit by one of the stronger shower could see vis and cigs briefly drop into the IFR or lower range. Winds will settle down after sunset from the northwest at generally around 10 kts. && .JKL WATCHES/WARNINGS/ADVISORIES... NONE. && $$ UPDATE...HAL SHORT TERM...GREIF LONG TERM...AR AVIATION...GREIF

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