602 FXUS63 KLMK 160015 AFDLMK Area Forecast Discussion National Weather Service Louisville KY 715 PM EST Fri Jan 15 2021 .Forecast Update... Issued at 715 PM EST Fri Jan 15 2021 A 60 mile wide band of light snow stretches roughly from Indianapolis to Bowling Green to southwest of Nashville, and is pressing eastward. It appears that this band of snow is the main show for this evening, that is, it represents the best chance of impactful snow and wind over the next few hours. Over the past hour or so visibilities in this band have generally been in the 3 to 5 mile range with wind gusts of 20 to 25 mph...although Simpson County did gust to 33 mph and there have likely been locally lower visibilities in heavier showers. This band of snow will continue to push east across central Kentucky this evening and into the Blue Grass and Lake Cumberland areas around midnight EST/11pm CST. A dusting of accumulation will be possible, as seen already in grassy areas in Dubois County. Road surface temperatures have begun to fall but are still in the 40s in the I-65 corridor...and the mid 30s to the west where the aforementioned band of snow has moved through. As air and road temperatures continue to cool tonight we could still see slick spots on area roadways...especially elevated and untreated roads. Looking deeper into the overnight...more general area of light snow currently over southern sections of Illinois and Missouri will move through and provide additional opportunities for lightly accumulating snow into early Saturday morning, along with some banding potential. Will continue with the Winter Weather Advisory already in place. && .Short Term...(This evening through Saturday) Issued at 230 PM EST Fri Jan 15 2021 ...Convective Snow Showers May Impact Travel Tonight... Ridiculously steep low-level lapse rates exceeding 9C/km up to 700mb as a deep closed low digs into the mid-Mississippi Valley. The resulting sky in some parts of the Ohio Valley looks like something you might see near Lake Erie as convection continues to blossom. Bases are still fairly high, around 5000 feet, and clouds are not very deep yet, so there is quite a bit of virga, but we have also seen some rain as well as snow or sleet pellets making it to the surface, even in the face of temps in the mid 40s. Can't rule out a rumble or two of thunder with any of these showers, regardless of the precip type, but chances are too low to mention. As we head toward sunset, we will get better saturation overhead and start to see more precip making it to the ground, mostly as snow given wetbulb effects below cloud base. Hi-res models are suggesting one or two organized, progressive bands of snow pushing east across the area during the evening. Given the reflectivity progs, this could be a legitimate snow squall setup. Air and roadway temperatures will be mitigating factors, as neither of them are likely to get below freezing. However, visibilities will be a significant impact, and the main impact early on. The likelihood of impacts from these snow showers will increase as the evening goes on and temperatures drop. Given that, the Winter Weather Advisory appears in good shape as-is, and impacts before the 7 PM start time and/or outside the advisory area, will be short- lived as temps will be warm enough that any accumulated snow would melt quickly as soon as it lets up. Will handle squall-like impacts, in or out of the advisory, with short-fuse warnings or statements. Main upper vort lobe swings through overnight, allowing more consistent but less intense snow to persist, especially in south- central and east-central Kentucky. Temperatures close to freezing will allow that snow to accumulate more easily and minor impacts to linger into Saturday morning. We will get a break for a decent portion of Saturday, but another impulse will curl southward off Lake Michigan and then SE into eastern Kentucky late in the day. Look for scattered snow showers to return to southern Indiana by mid-afternoon and swing into the Bluegrass region toward sunset. At this time, any impacts from this batch of snow would likely not happen until close to sunset on Saturday, so for now will ride with the current advisory to cover tonight's impacts, and revisit what areas, if any, would see enough impact Sat night to warrant another Winter Weather Advisory. Daytime temps on Sat will hover in the mid/upper 30s, just warm enough to limit impacts, and we could even see more of a mix west of I-65. .Long Term...(Saturday night through Friday) Issued at 234 PM EST Fri Jan 15 2021 Very large scale and broad upper level troughing will be present over the eastern two-thirds of the country by Saturday evening. Shortwaves embedded within the large scale flow will impact the region for the latter half of the weekend into early Monday. Temperature profiles during this period will largely support snow, though one timeframe we are keeping an eye on is Saturday evening into early Sunday morning. Model soundings reveal limited moisture above the -8C to -10C layer where ice crystal growth would be favored. With surface temperatures close to or just below freezing, this could result in a DZ/FZDZ scenario. Think one factor in favor of ice crystals/snow development is that low level lapse rates may be steep enough to promote some vertical growth in any precipitation that develops and push moisture into a more favored ice crystal growth zone. In any case, something to continue watching. Overall snow amounts could exceed an inch by Monday morning for several locations, but a lot of the snow will occur in spurts and likely melt down some before the next wave of snow showers moves in. The eastern Bluegrass (I-75 corridor area) has the best potential to see accumulating snow and potential impacts, and can't rule out future headlines being hoisted to account for this. We begin to transition to more of a zonal flow aloft for the early week period. Drier weather will return initially, though precipitation chances begin to increase going into Tuesday ahead of a frontal boundary. Additional chances for precipitation could continue into the mid and late week period depending on how waves evolve in the flow aloft. && .Aviation...(00Z TAF Issuance) Issued at 637 PM EST Fri Jan 15 2021 An impressive upper low spinning across southern Indiana tonight will bring snow showers to the region this evening and a more general light snow to the area overnight. The main threats will come this evening with any heavier snow showers producing severely limited visibility and gusty winds out of the WSW...though given the speedy movement of the showers the impacts at any one spot would be brief. Overnight ceilings will lower significantly, likely to IFR. Tomorrow the upper low will head to the Finger Lakes of New York and dry air will sweep into the Ohio Valley at the mid and upper levels, bringing a break to the snow threat...though low ceilings were persist. Late Saturday a small disturbance sliding down the backside of the parent low will bring another shot of scattered snow showers. Winds coming in from the west behind the surface low over the Northeast will be brisk Saturday with gusts to around 20kt. && .LMK WATCHES/WARNINGS/ADVISORIES... IN...None. KY...Winter Weather Advisory until 1 PM EST /noon CST/ Saturday for KYZ039>041-045>049-053>057-063>067-073>078-081-082. && $$ Update...13 Short Term...RAS Long Term...DM Aviation...13