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FXUS64 KMRX 060315 AFDMRX

Area Forecast Discussion National Weather Service Morristown TN 1015 PM EST Tue Dec 5 2023

New UPDATE

UPDATE
Issued at 1003 PM EST Tue Dec 5 2023

Overall, only minor changes were made with the evening update.
Currently, showers continue to move through the area, along with the surface front. Due to very low freezing levels and some convective enhancement, these showers have been able to produce sleet and gusty winds. With respect to the forecast, temperatures have been cooler in northeastern areas than previously expected, which did lead to a reduction in hourly values and earlier mention of rain/snow mix. Overall, the forecast for accumulations did not change based on a less agressive indication from high-res guidance but a possibly quicker changeover, especially further north.

SHORT TERM
(This evening through Wednesday)
Issued at 244 PM EST Tue Dec 5 2023

Key Messages:

1. Northwest flow snowfall will bring snow accumulations to the East TN mountains and parts of southwest VA tonight and into Wednesday, accumulations are expected mainly above 2500 ft.

Discussion:

Currently light radar returns across southwest VA and far northeast TN, but very little if any is making it to the surface at this time as the lower levels of the atmosphere are still fairly dry. This precipitation is associated with a low moving into the Great Lakes Region while an area of enhanced vorticity is moving around the southern edge of the trough over the Ohio/Tennessee Valleys. This is generating some weak lifting that will continue to strengthen as we move into the overnight hours. Moving into the night time hours we'll see an increase in isolated to scattered shower activity at the base of the trough as the atmosphere continues to saturate and temperatures steadily drop.

Initially precipitation should be all liquid, then as we approach midnight temperatures in the higher elevations will drop below freezing and a switchover to snow is expected. Forecast soundings continue to show that areas above 2,500 feet in elevation have the best chance to see long durations of cold enough temperatures to support snowfall throughout the much of the evening and overnight hours. Snowfall rates in the higher elevations look like they'll really ramp up in intensity for a few hours sometime between 06-15z, as the 850/700mb jet begins to increase in strength helping to enhance to orographic lift. Think that we should see our heaviest snowfall before sunrise with intensity dropping off later in the morning... However the LLJ remains fairly stout during the daytime tomorrow, so we could see prolonged periods of light snow or flurries in the very highest elevations as every drop of moisture tries to get wrung out along the mountains. This is likely going to be a very elevation based event, and cannot rule out the possibility of over 7 inches at the very tops of the southern Appalachian Mountains. However, most places near or above 2,500 feet are more likely to see closer to 1-4 inches, with amounts quickly dropping off below 2,500 feet... And no accumulations expected once you get down into the Valley. Will keep the Winter Weather Advisory as is, with the main messaging being to be cautious if traveling over the mountains tonight and into tomorrow morning.

For most people living in the valley the chilly morning will eventually make way to more sunny skies in the afternoon and temperatures back up into the 40's, but will still be about 5-10 degrees cooler than today.

LONG TERM
(Wednesday night through next Tuesday)
Issued at 244 PM EST Tue Dec 5 2023

Key Messages:

1. Dry weather is expected Thursday and Friday.

2. A strong system is expected late this weekend. Could bring mountain wave winds, widespread rainfall, and even some thunderstorms to the forecast area.

Discussion:

A small rollercoaster is in play for the latter half of the week into the weekend, we will be rebounding in temperatures after the chilly cold of Wednesday, and dry weather will persist through Friday night. After that we hit the fun portion of the coaster, with a major trough sweeping across the country into our neck of the woods this weekend. The GFS is 6 to 12 hours quicker than today's latest Euro solution, which isn't atypical for timing differences between the two. Otherwise the general forecast evolution will feature 40 to 50 knots of 850mb winds coming up and over the mountains, providing the next round of mountain wave winds. Roughly when the winds are peaking, the rain will move in. Rainfall won't last too long, likely under a 24 hour period, but will be fairly moderate to heavy. Ensembles even at this range are beginning to support the deterministic solutions, with 50 to 60% probabilities in an inch or greater in a 24 hour period, and the two global models both output swaths of an inch across the area. Rain wise this is great news for the drought situation, and keeps the region on track for further recovery.

Once temperatures decrease Sunday night, some wraparound moisture on the backside of the overall system could result in some light snow or a rain/snow mix as the overall system heads out, but this is low confidence. After the precipitation moves out, we dry out again on through Tuesday.

AVIATION
(00Z TAFS)
Issued at 628 PM EST Tue Dec 5 2023

Currently, showers and even some rumbles of thunder are moving in from the west and will affect the TAF sites through the evening.
Thunder has been very isolated and not expected to affect the terminals. Some frozen precipitation has been mixed in as well.
During and after the showers, there will be a gradual decrease in aviation conditions to MVFR with rain (possibly mixed with snow)
continuing around TRI into the morning. During the day on Wednesday, a breezy northwest wind is expected with MVFR ceilings continuing through at least half of the day. Some sprinkles/flurries are possible around TYS and TRI. Conditions should hopefully improve to VFR by the evening hours.

PRELIMINARY POINT TEMPS/POPS
Chattanooga Airport, TN 40 49 31 57 / 20 0 0 0 Knoxville McGhee Tyson Airport, TN 38 46 29 54 / 70 10 0 0 Oak Ridge, TN 37 46 28 53 / 70 0 0 0 Tri Cities Airport, TN 35 43 26 51 / 80 40 0 0

MRX WATCHES/WARNINGS/ADVISORIES
NC...NONE.
TN...Winter Weather Advisory until 4 PM EST Wednesday for Blount Smoky Mountains-Cocke Smoky Mountains-Johnson-Sevier Smoky Mountains-Southeast Carter-Southeast Greene-Southeast Monroe-Unicoi.

VA...Winter Weather Advisory until 4 PM EST Wednesday for Russell- Wise.

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