Area Forecast Discussion
Issued by NWS Jackson, KY

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000 FXUS63 KJKL 111806 AAA AFDJKL AREA FORECAST DISCUSSION...UPDATED National Weather Service Jackson KY 106 PM EST Mon Dec 11 2017 .UPDATE...
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Issued at 107 PM EST MON DEC 11 2017 Hourly grids have been freshened up based on recent observations and satellite trends. This led to warming up temperatures a few degrees from the previous forecast across the southern two thirds of the area. Otherwise, the main forecast concern remains the light wintry precipitation potential late tonight and on Tuesday. QPF for the event should 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 are underdone and that good chance (50 percent) or even low end likely pops (60 percent) will probably be needed for the event. Temperatures on precipitation arrival in most locations will likely be near freezing if not above with several hours above freezing for most locations and for the entire night in many areas falling not falling below freezing tonight after above normal temperatures today. The exceptions to this will be elevations above 3000 feet near the VA border and some of the deeper valleys that may fall above freezing around or prior to midnight before temperatures rise there to near if not above freezing before dawn as clouds arrive and pressure gradient 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 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. Marginal to possibly warm road and ground surfaces 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 6 AM and 10 AM or during the morning commute with snow showers and flurries lingering in the southern and easternmost locations into the evening. Additional model guidance will be monitored as the event approaches including the convective allowing models. UPDATE Issued at 630 AM EST MON DEC 11 2017 The forecast is looking good so far this morning, so no major update is planned at this time. Used the latest obs to freshen things up and establish new trends in the hourly forecast grids.
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&& .SHORT TERM...(Today through Tuesday) Issued at 427 AM EST MON DEC 11 2017 A complicated forecast is in store for the short term across eastern Kentucky. Light precipitation is expected to begin moving into the area late tonight ahead of an approaching and fast moving area of low pressure aloft. Precipitation should begin as drizzle and then transition to a mix of freezing drizzle, drizzle, and light snow showers during the early morning hours just before sunrise on Tuesday. Once we get more ice aloft early Tuesday morning, the precipitation should then transition over to a mix of rain and snow showers, and then all snow showers as temperatures fall to around freezing by late morning and early afternoon. There could be enough ice accumulation to create some slick spots on area roadways early Tuesday morning, so caution should be used if out driving during that time. Any snow accumulations on Tuesday should be light and confined mainly to ridgetops, grassy areas, and elevated surfaces. Temperatures will be fairly mild today, with highs forecast to reach the mid to upper 40s for most locations. A few spots along the Tennessee border may see the lower 50s, and a few north locales may only reach the lower 40s. We will see dry conditions today through early tonight along with generally partly cloudy skies. Overnight lows should fall to around freezing. Tuesdays highs are expected to max out in the low to mid 30s, most likely early in the morning before the bulk of the expected snow begins to fall. Temperatures will then likely begin falling during the day, as our upper level weather system begins to pull east of the area. .LONG TERM...(Tuesday night through Sunday) Issued at 445 AM EST MON DEC 11 2017 The models are in general agreement aloft through the extended portion of the forecast, though they still differ significantly on some of the key smaller scale features that will affect the area during this time frame. For the bulk of the long term, troughing will dominate the northeast quarter of the nation. The core of this trough will move from the central Great Lakes east to northern New England from Tuesday night into Wednesday. As it does so the heights will rebound over eastern Kentucky from very low values Tuesday evening. The flattening northwest flow in the wake of the larger trough`s departure will support a minor wave rolling east into Kentucky Wednesday night - strongest in the ECMWF as it has been for the past few runs. This wave now has some support from the CMC but the GFS is weaker and further southwest with its version of the impulse. Accordingly, confidence is lower than normal for the mid portion of the extended. The models do line up better by the end of the work week as a full latitude trough takes shape with plenty of energy spreading into the Deep South. Again the ECMWF is the strongest with this portion of the trough than the other models but that has been the case for the past several runs. Leaning towards the Euro solution will limit the impacts for eastern Kentucky, though. The models all carry this trough through the area by Friday evening with heights on the rebound for the weekend in northwest, to increasingly zonal, 5H flow. Upstream another smaller trough will take shape and head toward the area for Sunday, but the pattern buckles enough that eastern parts of the nation will see southwest flow ahead of this trough along with some decent height rises. The meat of this trough reached the JKL CWA Sunday afternoon in the ECMWF but not until that night in the GFS. This uncertainty and the amount earlier in the period makes a general model blend the best starting point option but some adjustments will be needed at times to lean more strongly toward the ECMWF - in general. Sensible weather will feature a very cold start to Wednesday in the wake of a fast moving clipper system with readings in the teens most places and brisk winds driving wind chills down into the single digits around dawn. After a chilly day Wednesday the night will be milder than Tuesday`s as a WAA pattern takes hold at least briefly ahead of the next clipper system inbound for Thursday. This one looks to bring another quick shot of colder air into eastern Kentucky along with the threat for additional light pcpn. This will take the form of rain showers during the day possibly mixing with snow showers by evening before continuing as scattered light snow showers through the night into Friday morning for most of the area - departing late in the far east. Warmer conditions truly move in on Saturday and continue into Sunday with low 50s anticipated for highs to close out the weekend though a southern sfc wave looks to bring rain chances to the area from the southwest, as well. Made only minor adjustments to temperatures through the period mainly for terrain effects Wednesday and Friday/Saturday nights. Again more substantial adjustments were made to PoPs and snow chances through the week beefing up the potential lingering in our higher terrain Tuesday night and again for Thursday evening into the day Friday. && .AVIATION...
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(For the 18Z TAFS through 18Z Tuesday afternoon) ISSUED AT 1255 PM EST MON DEC 11 2017 VFR conditions are currently observed and should prevail in all locations until the 6Z to 10Z period when MVFR ceilings arrive although low and mid level clouds will begin arriving around if not prior to 6Z as a clipper system moves into the area. Mixed freezing drizzle and snow will be possible along and in ahead of the clipper system cold front with snow showers possible after the first hour or two of precipitation. Any glaze from icing should be very minimal. The snow showers or mixed wintry precipitation could bring brief periods of IFR in some locations. winds will also be gust at times, especially during the last 6 to 12 hours of the period when gusts as high as 20 to 25 kt will be possible. Winds will average out of the south to southwest through the 3Z to 6Z period, before shifting to the west and the northwest 6Z to 12Z and then remaining northwest through the end of the period.
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&& .JKL WATCHES/WARNINGS/ADVISORIES... NONE. && $$ UPDATE...JP SHORT TERM...AR LONG TERM...GREIF AVIATION...JP

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