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FXUS64 KMRX 081339 AFDMRX

Area Forecast Discussion National Weather Service Morristown TN 939 AM EDT Wed May 8 2024

New UPDATE

UPDATE
Issued at 930 AM EDT Wed May 8 2024

Wanted to send out an early update this morning to account for the convection moving into the area. The morning round of convection will be stronger than previously anticipated based on the current radar trends, projected trends from the HRRR, and current parameters shown on SPC mesoanalysis. All hazards will be possible with this morning round, large hail, high winds, flooding, as well as a tornado risk. HRRR shows this activity moving across the central and northern TN Valley over the next few hours and then eventually down across the east TN mountains, such as Sevier and Cocke counties. Last few HRRR runs continue to show helicity tracks across this corridor as the activity moves through. Have updated the forecast to account for these changes.
Please pay close attention to the forecast today as it will be an evolving situation throughout the day with numerous rounds of showers and storms with all hazards possible.



SHORT TERM
(Today and tonight)
Issued at 341 AM EDT Wed May 8 2024

Key Messages:

1. A regional severe weather outbreak is increasingly likely from late this afternoon through tonight across the Mississippi River Valley and Tennessee Valley.The threats include widespread damaging winds, very large hail, and tornadoes (including EF-2+ strength).
The tornado threat is highest along and west of Interstate 75.

2. Scattered to possibly numerous instances of flooding are likely, especially west of Interstate 75. This threat will be most amplified from this evening to Thursday morning.

Today and Tonight

Currently early this morning, a 120+ kt 250mb jet extends into the western Great Lakes with a southern jet of near 100 kts extending from the southern Mississippi Valley. A very broad warm sector is also in place with a cold front near the Mississippi River and a warm front near the Great Lakes. Also, strong to severe convection is ongoing across the Ohio River Valley and will continue to progress southward through the morning hours. By daybreak, the aforementioned northern jet will progress towards the eastern Great Lakes with the left-exit region of the southern jet being placed directly over Tennessee and westward. This will create a strongly divergent pattern aloft. The timing of when this initial convection arrives is still somewhat uncertain, but a later timing (early afternoon) would allow for more destabilization beforehand and could slightly limit instability for the evening/overnight period. In any case, deep-layer shear by the early afternoon will reach near or above 40 kts with MLCAPE in excess of 2,000 J/kg. During this time, the flow pattern will still be fairly light and still somewhat uni- directional, but the thermodynamics and overall profiles are still sufficient for storm organization and a damaging wind and hail threat.

By the late afternoon through tonight, the wind profile will strengthen and become increasingly more veered with opportunity for additional destabilization beyond 2,500 J/kg of MLCAPE, especially if there is some break in earlier activity. During this timeframe, effective shear is expected to reach or exceed 50 kts with very right-turning hodographs. The CAMs still differ some on the timing and evolution of convection, but most solutions suggest widespread organized supercells by the late afternoon along and north of Interstate 40 with one or more line segments arriving through the evening hours. While the low-level shear isn't too strong, impressive low-level instability and veering wind direction yield pretty high confidence in STP values to exceed 2 or 3, especially along and west of Interstate 75. In the mid-levels, lapse rates of around 7 Celsius/km or greater can be expected with very impressive CAPE in the -10 to -30 Celsius region of 800 to 1,000 J/kg. While the CAMs differ on exact timing and storm mode, widespread storms are indicated anytime from the afternoon through the overnight hours, initially focused north and then shifting south. The latest HRRR data suggest a broad swath of updraft helicity tracks across much of the area and focused especially along and west of Interstate 75. In any case, it should be noted that this system has already produced numerous tornadoes, including some strong to violent, across the Great Plains to places along and north of the Ohio River Valley. This event will also almost certainly continue to the overnight hours, presenting a nocturnal severe weather threat.

Another important aspect of this event is duration, intensity, and coverage of convection. Places further south may not see as much coverage during the day, but this will likely occur more from the evening to early Thursday morning. The CAMs are indicating high probabilities for over 3" of rainfall with notable probabilities (20 to 30 percent) for 5" or more of rainfall. Just as with the tornado threat, this is elevated even more further west. Based on the strong signals and likelihood of significant rainfall, a Flood Watch was issued for the entire area. The threat is certainly not uniform area- wide, but the signals suggest rainfall and antecedent conditions sufficient for scattered to maybe even numerous instances of flooding.

LONG TERM
(Thursday through Tuesday)
Issued at 341 AM EDT Wed May 8 2024

Key Message:

1. Cooler weather late week and into the weekend behind the cold front.

2. Lingering showers possible Friday with shortwaves within the northwest upper flow. Quick shortwave may bring some showers and thunderstorms Saturday (highest chance northern areas).

3. Drier weather expected Sunday and Monday with a gradual warming trend. Chance of showers increases again mid-week.

Discussion.

Behind the cold front on Thursday, cooler and drier air will arrive across the region within the northwest upper flow. Troughing and cooler air aloft will result in some diurnal convection on Friday afternoon with limited potential for thunder.

A stronger shortwave is expected to move through the upper level northwest flow on Saturday with scattered showers and thunderstorms possible on Saturday afternoon, with the highest chance for precipitation across our northern counties through southwest Virginia.

Ridging begins to build on Sunday and Monday with drier weather and a gradual warming trend into early next week. Chance of rain begin to return on Tuesday and Wednesday as a system is expected to develop across the Lower Mississippi Valley, but certainty in timing

AVIATION
(12Z TAFS)
Issued at 653 AM EDT Wed May 8 2024

Very poor aviation conditions are expected from the afternoon through tonight. This is due to several different rounds of storms with severe storms likely across the entire area. Before the storms arrive, MVFR conditions are possible, but fairly quiet VFR conditions should prevail. Otherwise, reductions to below MVFR can be expected. More importantly, the storms will be particularly intense with the threat of damaging winds, hail, flooding, and even tornadoes. These threats gradually increase into the late afternoon and through the evening with little to no improvement expected overnight. In fact, additional lines of storms can be expected. Confidence is low on the timing of impact on the terminals, but rapid changes in conditions can be expected.

PRELIMINARY POINT TEMPS/POPS
Chattanooga Airport, TN 88 68 85 61 / 40 80 80 30 Knoxville McGhee Tyson Airport, TN 81 67 80 58 / 90 80 80 30 Oak Ridge, TN 80 66 82 57 / 90 80 80 20 Tri Cities Airport, TN 77 63 77 56 / 70 80 90 30

MRX WATCHES/WARNINGS/ADVISORIES
NC...Flood Watch from 2 PM EDT this afternoon through Thursday morning for Cherokee-Clay.

TN...Flood Watch from 2 PM EDT /1 PM CDT/ this afternoon through Thursday morning for Anderson-Bledsoe-Blount Smoky Mountains-Bradley-Campbell-Claiborne-Cocke Smoky Mountains- East Polk-Grainger-Hamblen-Hamilton-Hancock-Hawkins- Jefferson-Johnson-Knox-Loudon-Marion-McMinn-Meigs-Morgan- North Sevier-Northwest Blount-Northwest Carter-Northwest Cocke-Northwest Greene-Northwest Monroe-Rhea-Roane-Scott TN- Sequatchie-Sevier Smoky Mountains-Southeast Carter- Southeast Greene-Southeast Monroe-Sullivan-Unicoi-Union- Washington TN-West Polk.

VA...Flood Watch from 2 PM EDT this afternoon through Thursday morning for Lee-Russell-Scott VA-Washington VA-Wise.

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