089 FXUS64 KMRX 230136 AAA AFDMRX AREA FORECAST DISCUSSION...UPDATED National Weather Service Morristown TN 936 PM EDT Thu Apr 22 2021 .UPDATE... EVENING UPDATE. && .DISCUSSION... No changes are needed to the forecast this evening. Observed temps are following pretty close to the forecast in most spots, so our forecast lows in the lower 30s north to lower 40s south appear on track. Clouds have cleared out but satellite shows some high clouds in West TN that are expected to start spreading into our area around sunrise. The southern counties of the Frost Advisory look very marginal for frost due to the expected increase in clouds, but there is enough uncertainty to keep the Advisory in place for those areas. DGS && .AVIATION... 00Z TAF DISCUSSION. A quiet period for aviation as high pressure settles over the area. Winds will be calm for most of the night, with increasing midlevel clouds. Gradually, the cig will lower through the morning but remain VFR. Winds will shift to a S-SW direction tomorrow afternoon at 5-10 kts. DGS && .PREV DISCUSSION... /ISSUED 319 PM EDT Thu Apr 22 2021/ SHORT TERM...(Tonight through Friday)... Much of the forecast area lies beneath a broken low level CU field this afternoon in response to a well mixed low level atmosphere spurred on by an approaching shortwave aloft. This trough axis should shift east of the by 00z this evening, with weak height rises expected in its wake as shortwave ridging builds in. With surface high pressure still in place, and an H85 ridge extending from roughly Georgia up through Indiana overnight, one would expect prime radiational conditions to be observed. At at least for the first half of the night that will be the case as diurnally driven CU field dissipates and skies remain clear. Think we will see temperatures fall quickly between sunset this evening and 1-2 am tonight. Afterwards, there is some uncertainty about whether high clouds will be moving in. This has implications on how widespread frost and/or sub-freezing temperatures will become. A shortwave passing through northern plains, and a weaker disturbance to it's south in the confluence of northern and southern stream jet energy over southern Illinois, will help shove the surface high east of the Appalachians tonight, and the associated H85 ridge to along the Appalachian spine by daybreak tomorrow. Weak isentropic ascent will move into middle/eastern TN as a result, bringing at least some scattered mid and high clouds. High resolution temperature guidance like the ARW, shows the effects of this, effectively bumping temperatures back up above freezing by 09z or so. The question is, how certain is that cloud cover to be. To lesser extents, several pieces of guidance point to this so I did go ahead and adjust temperatures upward a bit. Resulting frost/freezing temperatures is tapered back slightly for central and southern areas. Northern areas still seem like a lock for freezing conditions. Despite the tapering back of temps and frost coverage, there's still enough left in all zones that were under an advisory or warning. As such, will be leaving that as-is for the time being. For Friday, dry conditions are expected with increasing high clouds and warm advection. Should see temperatures rise into the 60s most everywhere in the lower elevations, and into the upper 60s in the south. A western trough will traverse Arizona and New Mexico tomorrow, ejecting into Texas and the southern plains during the evening. Rain associated with this system holds off until the long term period however. CD LONG TERM...(Friday night through Thursday)... Key Messages: 1. Rain is expected from Saturday morning into Saturday night with chances for a few thunderstorms, mainly in southeast Tennessee and southwest North Carolina. 2. Strong winds are possible in the Tennessee mountains and adjacent foothills Saturday morning into the early afternoon. 3. Dry weather with a warming trend is expected into next week before chances for rain return by Thursday. Friday Night through Monday To start the period, a mid/upper shortwave and associated surface low will be moving into the Southern Plains with a strong 120+ kt jet streak across much of the eastern U.S. The aforementioned system will be the feature of focus through the weekend as it progresses into the Mississippi River Valley heading into Saturday. During this time, the jet streak moves to the north, putting southern Appalachia in its right-entrance region. Accordingly, a direct ageostrophic circulation will result in increasing upper divergence with notable MSLP falls leading to surface convergence. This is also accompanied by increasing Gulf moisture. The result is for PoPs to increase from southwest to northeast with showers resulting from this system. Based on reasonably high confidence for rain, high-end PoPs were utilized in the earlier portion of the day. Also, while there are still differences in the track of the surface low and 850mb low, the setup does suggest potential for high winds in the mountains and the adjacent foothills. This is based on the consensus for the 850mb jet to be 50+ kts and from a southeasterly direction and strong MSLP gradient. With the overall proximity of this system, the window for this to occur is relatively short, likely from about 8 a.m. until the early afternoon. Based on the orientation and strength of the jet, a high wind watch and/or wind advisory may be needed in the future. Otherwise, the main story will be showers off and on throughout the day. With some indications for afternoon instability in southeast Tennessee and southwest North Carolina, thunder was utilized with lesser confidence for strong/severe convection. If the associated warm frontal boundary moves into the area, strong/severe convection will be more likely with an overall marginal elevated setup, otherwise. The setup is not ideal with limited directional shear once low-level winds veer later in the day. As for rainfall totals, the best moisture will stay to our south with initial downsloping likely inhibiting rainfall totals across much of the area. Nevertheless, near an inch or more still seems likely in southern portions of the area. As the system moves off to the east, rain chances gradually decrease west to east ahead of the associated cold frontal boundary. Northwesterly flow will usher in seasonally cooler air into the area for Sunday with high pressure setting up to our north. CAA is also noted by falling thermal (1,000-500mb thickness) values. For the day on Monday, shortwave ridging moves into the area with height rises leading to the start of a warming trend. Tuesday through Thursday Heading into mid-week, the pattern will become increasingly amplified with significant ridging in the east and a trough over the Rockies. With 500mb height anomalies of +10 to +15 dam and surface high pressure along the Atlantic Coast, southerly flow and thermal ridging will continue a significant warming trend. Based on these considerations and model outputs, much of the area could reach into the 80s by Tuesday and Wednesday. Heading towards Thursday, focus will be on the evolution of the trough/system to our west with high uncertainty in the details at this time. With the overall consensus, 40% to 50% PoPs are justified to increase west to east by Thursday, following the associated cold frontal boundary. While the chance of rain is likely higher than these values, the timing and progression still remains largely uncertain. BW && .PRELIMINARY POINT TEMPS/POPS... Chattanooga Airport, TN 41 67 51 70 52 / 10 10 90 100 30 Knoxville McGhee Tyson Airport, TN 38 63 48 66 50 / 0 10 70 100 50 Oak Ridge, TN 37 63 47 65 49 / 0 10 60 100 50 Tri Cities Airport, TN 31 61 43 61 48 / 0 10 20 100 70 && .MRX WATCHES/WARNINGS/ADVISORIES... NC...Frost Advisory from 2 AM to 9 AM EDT Friday for Cherokee-Clay. TN...Freeze Warning from 2 AM to 9 AM EDT Friday for Blount Smoky Mountains-Claiborne-Cocke Smoky Mountains-Hancock-Hawkins- Johnson-Northwest Carter-Northwest Greene-Sevier Smoky Mountains-Southeast Carter-Southeast Greene-Sullivan-Unicoi- Washington. Frost Advisory from 2 AM EDT /1 AM CDT/ to 9 AM EDT /8 AM CDT/ Friday for Anderson-Bledsoe-Campbell-Grainger-Hamblen- Jefferson-Knox-Loudon-McMinn-Meigs-Morgan-North Sevier- Northwest Blount-Northwest Cocke-Northwest Monroe-Rhea- Roane-Scott-Sequatchie-Southeast Monroe-Union. VA...Freeze Warning from 2 AM to 9 AM EDT Friday for Lee-Russell- Scott-Washington-Wise. && $$