430 FXUS64 KMRX 180117 AFDMRX Area Forecast Discussion National Weather Service Morristown TN 917 PM EDT Tue Oct 17 2023 ...New UPDATE... .UPDATE... Issued at 912 PM EDT Tue Oct 17 2023 Not a lot of change from the previous forecast. The main complication overnight is still dealing with how quickly clouds erode. There is still a significant amount of cloud cover along I-40 and northward. However, if you loop the past few hours worth of satellite data clearing is evident, just slow. Latest runs of the HRRR support further clearing through the night which is still in line with the forecast. So still a chance for some patchy fog and/or frost for some areas if clearing can happen early enough. Only changes to forecast were adjusting hourly temps and dewpoints, as our northern areas are running a little warmer due to current cloud cover. && .SHORT TERM... (This evening through Wednesday) Issued at 327 PM EDT Tue Oct 17 2023 Key Messages: 1. Dry weather through the period, with a warming trend for Wednesday. 2. Very patchy frost possible in sheltered valleys of far NE TN and SW VA tonight, as well as in higher elevations of the East Tennessee mountains. Discussion: On the synoptic scale, deep northwesterly flow remains in place across the region as upper level troughing continues to slowly meander eastward and surface high pressure builds in from the west. Visible satellite trends depict this afternoons pesky clouds are finally showing signs of slowly clearing out in southern portions of the East Tennessee valley, however, they continue to linger in central and northern portions, as well as across higher elevations. Unfortunately model guidance has been handling this cloud cover poorly as they generally clear clouds out much quicker than what has been observed. For example, by ECMWF thoughts - the skies should be perfectly clear at the time of this discussion. As such, there remains uncertainty as to how these clouds will play out but have opted to trend sky cover towards more aggressive models such as the NAMdmg, NBM90th, and ADJMET. Have also blended slightly cooler temperatures over the next few hours given the lack of solar heating due to these clouds. How quick and to what extent the clouds clear out tonight will impact the potential for patchy fog and patchy frost in sheltered valleys and higher elevations overnight. Current forecast depicts clouds eventually diminishing around 2-3z timeframe allowing for decent radiational cooling to take place, favoring the scenario of very patchy frost in sheltered valleys of far NE TN, SW VA, and higher elevations. If the trend of models struggling with clouds persists over the next 12-16 hours, overnight lows may be a couple degrees low, in which case patchy frost would be unlikely. Either way, rather unimpactful weather tonight. Much more sunshine tomorrow afternoon as troughing lifts further eastward and H5 heights increase from 568dam to 576dam locally. Temperatures will be around 10 degrees warmer. && .LONG TERM... (Wednesday night through next Tuesday) Issued at 327 PM EDT Tue Oct 17 2023 Key Messages: 1. Widespread showers will move across the area Thursday night through Friday. 2. Orographic northwest flow rain showers expected across the mountains for Friday night and parts of Saturday. 3. Dry and mild weather for Sunday into early next week. Discussion: Wednesday night into early Thursday looks to remain dry as high pressure continues to exert influence. However, a strong upper trough and cutoff low will be digging southeast across the Midwestern states during this time. Further deepening is forecast on Thursday into Friday as a strong Pacific jet extends southeast out of Canada into the upstream side of this trough over the northern plains. For most locations, Thursday will be dry as the trough and associated cold front will be too far west to produce rainfall over our CWA I think. Will see the low level wind field strengthen however, with breezy conditions expected for most areas during the day Thu, especially over the northern plateau. Lack of strong CAD east of the Appalachians, along with unfavorable southwesterly H85 wind direction doesn't indicate there will be much of any mountain wave action but I'm sure some of the ridge and mountain tops in the Smokies will be windy. Rainfall begins to overspread the forecast area from the west Thursday night and continues into the daytime hours on Friday. Soundings indicate that some meager instability will be present ahead of the front, so could see a few rumbles of thunder being possible. Doubt there will be a ton however, as the CAPE values are less than 500 J/kg, and there's still uncertainty with respect to how much rain coverage we're going to have. Trough shifts east of us by Friday evening, transitioning any remaining precipitation to northwest flow showers confined mainly to the higher terrain of the Smokies northward into Virginia. Models show this northwesterly flow pattern and possible high terrain showers continuing well into the weekend, with another potent shortwave moving through West Virginia late Sat night to finally wrap it up. For Sunday into early next week, upper trough lifts northeast with drier air and eventually upper ridging developing over the Tennessee valley. Milder and drier conditions are anticipated especially by early next week. && .AVIATION... (00Z TAFS) Issued at 721 PM EDT Tue Oct 17 2023 VFR conditions at CHA through the period. Widespread cloud cover still affecting TYS and TRI. Hi-res models not handling current clouds very well. Therefore, low confidence forecast in terms of fog potential at TYS and TRI overnight. If clouds clear early enough, TEMPO MVFR fog at TRI. If clouds persist through most of the night, then likely no fog. Otherwise, VFR through the period. && .PRELIMINARY POINT TEMPS/POPS... Chattanooga Airport, TN 44 71 49 71 / 0 0 0 10 Knoxville McGhee Tyson Airport, TN 41 69 47 70 / 0 0 0 10 Oak Ridge, TN 42 68 47 69 / 0 0 0 20 Tri Cities Airport, TN 39 66 43 68 / 0 0 0 0 && .MRX WATCHES/WARNINGS/ADVISORIES... NC...NONE. TN...NONE. VA...NONE. && $$ SHORT TERM... LONG TERM....CD AVIATION...