Area Forecast Discussion
Issued by NWS Jackson, KY

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000 FXUS63 KJKL 120327 AFDJKL AREA FORECAST DISCUSSION National Weather Service Jackson KY 1027 PM EST Mon Dec 11 2017 .UPDATE...
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Issued at 1027 PM EST MON DEC 11 2017 The latest surface analysis shows a low pressure now across northern Ohio and a cold front extending southward from that. This dry cold front is moving across central Kentucky and leading to gusty winds along and behind the front. Given this have updated winds and gusts to better deal with the trends. This has also further complicated hourly temperature forecast with mixing leading to a 15 degree jump at Quicksand Kentucky Mesonet site. There remains some question on the degree of mixing that will remain in the sheltered valleys spots, as an area of clearing pushes toward the area. Given the uncertainty related to how the temperatures will react plan on keeping the isolated spots of patchy freezing rain going towards dawn. Despite this thinking the overall trend would be for little impacts in relation to the freezing rain based on road temperatures. Otherwise this is more of a minor update to deal with the latest obs and trends. UPDATE Issued at 750 PM EST MON DEC 11 2017 The evening surface analysis shows a double barrel low pressure across the Great Lakes and Ohio Valley regions, with one across the UP of Michigan and the other closer to home across central Illinois. These lows are associated with two different upper level shortwave that will help drive the weather over the next 24 hours or so. The initial struggle in the short term will be temperatures, with some valleys seeing quick drop offs this evening. However, satellite and obs have indicated an area of mid to high level clouds are streaming across central and eastern Kentucky and subsequently the temperatures have leveled off. This leads to the next challenge this evening, there is a some clearing on the backside of this initial cloud deck before lower clouds move eastward behind the clearing. This will be critical for potential wintry precipitation toward dawn, but will wait see how temperatures trend through the evening for any required changes. At this point will only need to make minor changes with SKY grids for potential clearing. Also updated with latest obs and trends for temps/dews/winds.
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&& .SHORT TERM...(This evening through Tuesday night) Issued at 415 PM EST MON DEC 11 2017 The main forecast concern remains the light wintry precipitation potential late tonight and on Tuesday along and behind a cold front associated with a clipper system to track to the north of the OH River tonight across the southern Great Lakes reaching western NY by sunrise on Tuesday. Temperatures on precipitation arrival in most locations will likely be near freezing if not above, with many locations above freezing for the entire night. This is following above normal highs mainly in the 50s today across the entire region. The exceptions to this will be elevations above 3000 feet elevation near the VA border which should fall below freezing late tonight and some of the deeper valleys that may fall below freezing around or prior to midnight before temperatures possible rise to near if not above freezing before dawn as clouds arrive and pressure gradient and winds increase and the boundary layer becomes more mixed. Also at precipitation arrival/development, the clouds may contain supercooled water and little if any ice for the first hour or two late tonight particularly with southwest extent toward the TN border. This makes the initial precipitation type uncertain between needle or small flakes, drizzle or even a touch of freezing drizzle on some elevated surfaces in the highest elevations and perhaps a few normally colder valley locations at onset. Marginal to warm road and ground surfaces anticipated on onset also makes the extent of impacts uncertain. As it stands right now, most of the scattered precipitation that does fall for the bulk of the area should fall between 5 AM and 11 AM or during parts of the morning commute with snow showers and flurries lingering in the southern and easternmost locations into Tuesday evening. The model consensus is that any precipitation from about 8 AM on should be all snow. QPF for the event will be light and some locations may not receive measurable precipitation at all. However, given the convective nature of the event with falling temperatures aloft steepening up lapse rates, feel that MOS guidance and model blend pops were underdone for much of the central and western parts of the area and that good chance (50 percent) to likely pops (60 percent) were in order for this event. At this point, it appears that most locations should receive a dusting to a half of an inch of snow accumulation due to much warmer ground going into this event compared to the last one. A light glaze of ice could occur at onset one the highest elevations and few of the coldest valleys. In addition to the falling snow, winds should gust as high as 20 to 30 mph at times late tonight and on Tuesday. The temperature curve for tonight will be complex as already noted with most locations dropping into the low to mid 30s by 9 AM EST. Otherwise, temperatures should gradually fall on Tuesday with cold advection continuing. The coldest of the airmass should be over the area on Tuesday night with widespread lows in the teens. The top of Black Mtn should drop into the upper single digits above zero. .LONG TERM...(Wednesday through Monday) Issued at 406 PM EST MON DEC 11 2017 There are multiple systems to contend with, including rain and snow, and model discrepancies ballooning late in the period. Forecast confidence takes a hit as time goes by. Wednesday should be a quiet day, before a clipper type system is expected to pass by to our north on Wednesday night. Any precip of substance is expected to be to our north on Wednesday night into Thursday, with just a potential for flurries over the northeast part of the JKL forecast area. After a break in the weather Thursday afternoon into Thursday night, an upper level impulse rotates through the persistent eastern CONUS trough late Thursday night and Friday. The GFS is more aggressive with precip than the ECMWF, but still light. A compromise yields chance POPs. Any precip would probably be snow. After another quiet day on Saturday, the final system affects the area on Sunday. The GFS has a low track through Kentucky and taps into southern stream moisture, resulting in generous precip in our area. Temperatures support primarily rain. Meanwhile, the ECMWF low track is north of the Ohio River and the system does not tap into as much moisture, and it`s precip for our area is much lighter. For now, have used high chance POPs Sunday into Sunday night. Will expect changes to be necessary as models eventually converge on a solution. && .AVIATION...(For the 00Z TAFS through 00Z Tuesday evening) ISSUED AT 622 PM EST MON DEC 11 2017 An area of mid to high clouds continues move east across western and central Kentucky and starting to move into the Bluegrass this evening. Overall VFR mid to high clouds will remain the story till we move toward 8Z to 10Z timeframe when we will see CIGS begin to lower toward MVFR conditions. A clipper system will work into the area toward dawn and the onset of precipitation will be tricky, with the potential of early drizzle and/or freezing drizzle. The best shot of freezing drizzle will be in the shelter valley locations that manage to go below freezing before thinker cloud cover move in tonight. This clipper precipitation will transition to all snow through the morning into the afternoon as temperatures drop throughout the day. Some of the more robust snow showers coupled with gusty winds will lead to IFR or lower VIS at times. The winds tonight will remain light to begin the TAF period before becoming gusty toward the 8Z to 10Z timeframe. Some locations will see gusts of 20 to 25 knots at times Tuesday morning into Tuesday afternoon. Winds will begin out of the south and southwest and veer to west and northwest through the period. Also added a period of LLWS early tonight, as the low level jet increases before surface winds. && .JKL WATCHES/WARNINGS/ADVISORIES... NONE. && $$ UPDATE...DJ SHORT TERM...JP LONG TERM...HAL AVIATION...DJ

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