992 FXUS63 KJKL 201605 AAA AFDJKL AREA FORECAST DISCUSSION...UPDATED National Weather Service Jackson KY 1105 AM EST Sun Dec 20 2020 .UPDATE... Issued at 1115 AM EST SUN DEC 20 2020 15z sfc analysis shows a dying cold front over eastern Kentucky with an area of high pressure trailing behind and moving east deeper into the state. The measurable pcpn has just about ended with the dissipating boundary over the JKL CWA leaving behind some sprinkles and a stray light shower into the afternoon. Currently, temperatures have rebounded into the lower 40s most places while dewpoints are up in the mid 30s. Meanwhile under a low overcast winds are light and variable across the area. Have updated the forecast for the afternoon mainly to add in the latest obs and trends for the T/Td/Sky grids. These updated grids have been sent to the NDFD and web servers with a freshened set of zones and SAFs to be issued shortly. UPDATE Issued at 721 AM EST SUN DEC 20 2020 The precipitation shield has diminished quickly early this morning. Freshened up the hourly PoPs through this morning, which will continue to feature a downward trend overall, before upticking slightly again this afternoon, with the approach of a short wave trough from the west. Updates will be out shortly. && .SHORT TERM...(Today through Monday) Issued at 427 AM EST SUN DEC 20 2020 The current upper level pattern features a broad trough across the central CONUS, with short wave energy currently swinging through the Ohio and Tennessee valleys. Another more defined vorticity center is currently spiraling across Kansas, with an even more significant short wave rotating through the Manitoba/Ontario border just north of Minnesota. At the surface, high pressure is retreating across the western Atlantic, while another more modest center is currently positioned across Texas. Weaker low pressure is filling near Lake Superior, with a diminishing cold front aligned south into the Ohio Valley and continuing through the mid-Mississippi Valley. Another area of low pressure is located across the northern Gulf of Mexico. Precipitation is ongoing across the majority of eastern Kentucky in response to the approaching trough, and especially a transient upper level jet streak. Temperatures range from the mid 30s north of the Mountain Parkway to the upper 30s and lower 40s across our south. The models are in good agreement with the progressive long wave pattern to march on across the CONUS through early next week. The central CONUS trough will shift east towards the Eastern Seaboard through the period. This trough will become more amplified with time, as the combination of short wave energy digging across the lower Mississippi Valley/Deep South, along with short wave energy digging across the Great Lakes/Ohio Valley regions, reinforces this trough into Monday. At the surface, the northern Gulf low will move northeast and gain strength as it shifts towards the western Atlantic by early Monday. Meanwhile, surface ridging will nose in across the Deep South and through the central Appalachians. On Monday, a fast- moving low pressure system will cruise across the Great Lakes. This will allow a cold front to move through the Ohio Valley. For eastern Kentucky, the widespread precipitation will diminish through dawn, as deeper moisture and lift exit. A few light rain showers/sprinkles will continue to threaten during the day, as another short wave trough approaches from our west, although the core of this energy will pass well to our south. Highs today will rebound into the mid 40s for most locations, as low level flow will remain out of the southwest, although clouds will likely be hard to shed today. Tonight will be mostly dry, although clouds may continue to plague the region. Lows will be mainly in the mid 30s, although if some clearing can take place, then lower 30s along with some patchy fog development would be more likely. A tighter pressure gradient will be in place across the Ohio Valley on Monday out ahead of the fast-moving low. This will allow for breezy southwest winds of 10 to 15 mph, with gusts in the 25 to 35 mph range at times during peak mixing. Highs will be in the upper 40s to lower 50s, with some light precipitation threatening our northeast areas late in the day, where better mid and upper level support will reside. .LONG TERM...(Monday night through Saturday) Issued at 550 AM EST SUN DEC 20 2020 0z Tuesday--0z Thursday... Model agreement is generally good for the first 48 hours of the long term. At the beginning of the period, an upper level trough and weak clipper low will be dropping through the Great Lakes Monday night while an upper level ridge builds eastward from the Rockies to the Plains. The clipper low will quickly drag a weak cold front across the Ohio Valley during the evening hours. The best chances for seeing any precipitation with this feature will be over northeast Kentucky (especially northeast of KY-15) closer to the clipper low and stronger cooling aloft. Model soundings suggest that the steepening low-level lapse rates will support a few convective showers with the front and lingering upslope showers for much of the night over the higher terrain. The soundings also show that the top of the convective layer will be marginally cold enough for ice crystals. However, temperatures ahead of this front will be in the 40s, initially keeping all precip in liquid form. As colder air filters in overnight, those temperatures will settle back into the 30s, allowing for some snow to mix in before the showers come to an end. There will also be decent low-level height gradient across the Lower Ohio Valley for the first part of the night which will support a stout 35-45kt WNW 850 mb jet. Decent low-level mixing should lead to a blustery night with a few gusts of 20-25 mph possible. The forecast challenge on Tuesday will be how quickly we are able to rid ourselves of the low clouds. Rising heights will cause surface high pressure to build across the Southeast US while a lowering subsidence inversion will result in an increasingly shallow layer of low-level moisture. However, the WNW upslope flow at the lowest levels could maintain a low stratus deck for much of the day. Both the NAM and GFS time heights show quite a bit of moisture trapped below 900mb through much of the day, especially over northeast Kentucky. The current forecast calls for highs in the low to mid 40s with gradual clearing, but those values could be too warm if the clearing holds off until later in the day. High pressure will crest over the Central Appalachians Tuesday night with clear skies, light winds, and drying air in the low-levels. This will be supportive of a stout ridge-valley temperature split with lows expected to range from the lower 20s in the sheltered eastern valleys to the lower and mid 30s over the ridges. In the meantime, an amplifying trough will pass from the the West Coast states 0z Tuesday to the High Plains by 12z Wednesday and will support a deep surface low passing from the Washington state to the upper Midwest. By that time, the system's cold front will be trailing back to the SW to over Texas. Most of the models suggest that the surface low will become weaker and increasingly elongated or perhaps even morph into to multiple waves along the front. Either way, a strong southwesterly return jet (40-50 knots @ 850 mb) will develop between the approaching front and departing high during the day Wednesday, but mixing to that level will be foiled by the stout subsidence inversion. Even so, there should be enough low-level mixing for a few 15-20 mph southwesterly gusts and possibly a bit higher west of I-75. The WAA and mostly sunny skies will also send Wednesday's high temperatures surging well into the 50s and perhaps near 60 in the warm spots. 0z Thursday--Sunday... Heading into Thursday and Friday, the amplifying trough and its leading surface cold front will pass from the Plains to the East Coast, but many of the details of how this unfolds remain uncertain due to significant models spread. In particular, the models seem to be struggling with how to handle additional energy dropping in from the north and the potential formation of a transient cutoff circulation. The GFS still features the coldest, deepest trough and most progressive frontal system, but it has been among the coldest solutions in recent runs compared to the GEFs. The GEFs have also been rather variable from one run to the next. The ECWMF and its ensemble mean have been more consistent in recent runs and feature a slower, relatively warmer, and weaker trough with more notable cutoff low developing and passing through the TN valley. The Canadian is taking a middle ground solution between the ECWMF and GFS. The ECWMF was preferred for this forecast package due to its better consistency and better support from the Canadian and other models. In this scenario, the front would pass through eastern Kentucky late Thursday and would be preceded by a period of rain. By late Thursday or Thursday night, a new low pressure would likely develop along or near the Central or Southern Appalachians and then quickly lift toward New England on Christmas Day. The big question is whether or not we hang onto the deeper moisture long enough for the colder air to work in, change the precip to snow, and then cool the ground quickly enough for area- wide 1+ inch snowfall (the traditional definition of a White Christmas). Almost all of eastern Kentucky will likely, at the minimum, see a dusting of snow accumulation. However, at this point, the best chance for a White Christmas (as traditionally defined) will likely be over the higher elevations of southeast Kentucky where the brisk upslope northwest flow will keep snow showers going into Christmas Day. Areas near and north of the Mountain Parkway might also be favored for some Great Lakes- enhanced snow showers and light accumulations into Christmas morning. Regardless of how much snow we see, temperatures will be cold, struggling to reach freezing in the southern valleys and likely remaining in the 20s over the ridges and across northern Kentucky. Over the weekend, upper level ridging and surface high pressure will return, bringing clearing skies and moderating temps. Saturday morning could very well be the coldest night of the period with lows ranging in the teens, especially if we are to clear out Friday night. Highs are expected to reach the 30s on Saturday afternoon under mostly sunny skies. Sunday morning's lows could still be quite cold, especially in sheltered eastern valleys with lingering snow cover; otherwise, the moderating trend will continue with Sunday's highs likely surpassing 40 for many locales. && .AVIATION...(For the 12Z TAFS through 12Z Monday morning) ISSUED AT 721 AM EST SUN DEC 20 2020 Light precipitation will diminish across the area this morning. MVFR ceilings will lower to IFR through the day. Some improvement may occur late this afternoon into this evening; however, as we cool down tonight, ceilings may lower once again to IFR, although confidence is not high, as there is some potential for partial clearing as well. South to southwest winds of around 5 kts will increase to 5 to 10 kts during the day, before diminishing once again towards dusk. && .JKL WATCHES/WARNINGS/ADVISORIES... NONE. && $$ UPDATE...GREIF SHORT TERM...GEOGERIAN LONG TERM...GEERTSON AVIATION...GEOGERIAN