• Area Forecast Discussion

    From digimaus@618:618/1 to All on Thu Mar 13 20:42:33 2025
    Here's what I was talking about in MIN_ADMIN:

    ===
    515
    FXUS64 KMRX 132336
    AFDMRX

    Area Forecast Discussion
    National Weather Service Morristown TN
    736 PM EDT Thu Mar 13 2025

    ...New AVIATION...

    .SHORT TERM...
    (This evening through Friday night)
    Issued at 242 PM EDT Thu Mar 13 2025

    Key Messages:

    1. Warm temperatures, deep mixing, and gusty winds on Friday will
    promote elevated fire danger levels.

    2. High winds ramp up across the mountains Friday night. Gusts of
    60-70mph likely overnight, before even stronger winds arrive in
    the long term.

    Discussion:

    Some isolated showers and thunderstorms continue to
    develop/persist across the region this afternoon. This seems
    largely tied to combination of terrain (Cumberland plateau and the Appalachians) and the northward advance of mid 50s dewpoints.
    Will leave the widespread slight chance PoPs in the forecast
    through the afternoon but I don't expect there will be much more
    coverage than what is currently on radar. That said, model
    soundings show very steep lapse rates so some decently tall storms
    capable of small hail and gusty winds if any develop. The window
    of opportunity for any convection will persist through 03z or
    thereabouts. Afterwards, dry conditions are expected through
    Friday.

    By late tonight, the upper low/shortwave responsible for the
    isolated showers across the region will have shifted well to our
    southeast. Heights will then rise ahead of the system that will
    bring us severe storms on Saturday. As such, tomorrow should be
    warmer than even today, with mid and upper 70s forecast for the TN
    valley. Winds will begin to pickup as well on Friday. Model
    soundings show that we mix out to nearly 7000 ft AGL tomorrow
    afternoon, which will tap into some 25-30kt flow aloft and help to
    promote gusty afternoon winds. This deep mixing is also going to
    lead to low afternoon RH values once again. Will need to
    coordinate with fire weather partners on whether any headlines
    will be needed. I say that because while lower elevations won't be
    near advisory levels during the afternoon tomorrow, the wind field
    aloft is going to strengthen considerably as we head into the
    evening hours. To that end, we've gone ahead and upgraded the high
    wind watch to a warning for the mountain zones. There could be
    some overlap these stronger winds with the lower RH values which
    would elevate fire danger levels for sure.

    Friday night, winds will continue to ramp up in the mountains as
    a strong H85 wind field begins to spread across the area in
    advance of the severe weather event. Put gusts to 80mph in the
    high wind warning. While those types of gusts should largely hold
    off until the long term period, gusts of 60-70mph will certainly
    be possible Friday night.

    &&

    .LONG TERM...
    (Saturday through next Thursday)
    Issued at 242 PM EDT Thu Mar 13 2025

    Key Messages:

    1. A very strong and dynamic system will impact the region Friday
    night through early Sunday morning with a high-end, long-duration
    mountain wave event likely across the mountains and foothills
    through Saturday night.

    2. A significant severe weather outbreak is likely across the region
    with the greatest threat in our area being along the Cumberland
    Plateau through southeast Tennessee and southwest North Carolina.
    The greatest threat is for significant and widespread damaging winds
    with tornadoes and large hail also possible.

    3. Flooding of urban areas and flood prone locations will be
    possible due to heavy rainfall rates Saturday night.

    4. Drier and cooler conditions arrive Sunday night into Monday,
    followed by a warming trend and another system by Wednesday.

    Discussion:

    Saturday morning, we will be under the influence of broad upper-
    level diffluence with a potent shortwave vort max preparing to eject
    from the longwave trough axis across the Arklatex region. An
    occluding surface low will be across the Great Lakes region with a
    trailing cold front across the Mississippi Valley. Due to an
    increasing upper-level jet divergence, a subtle surface low feature
    will develop along this cold front across the Lower Mississippi
    Valley and race northeastward Saturday afternoon and night. This
    will bring an increasing risk of a dangerous significant severe
    weather outbreak for portions of the Mid-South and Tennessee Valley
    across Mississippi, Alabama, and Tennessee. Supercell thunderstorm
    structures capable of large, long-track tornadoes will be possible
    as well as QLCS convection during the evening and overnight hours
    with a continued high-end damaging wind risk and a continued risk of tornadoes. Behind the front, cooler and drier weather arrive early
    to mid next week. The following sections are grouped based on order
    of occurrence and with respect to our area:

    1. Mountain Waves/Fire Weather

    A strengthening pressure gradient and increasing 850mb LLJ over 50
    kt will result in mountain wave high winds increasing into Saturday
    morning and during the day. The LLJ will increase to over 70 kt by
    Saturday evening with intense high-end mountain wave high winds
    expected. Based on extent of winds and dry conditions seen in recent
    days, fire weather is of significant concern, especially in the
    mountains and foothills. The High Wind Watch will be upgraded to a
    High Wind Warning since we are now within 24 to 36 hours of the high
    winds beginning and confidence continues to increase in these
    intense winds for the mountains and foothills.

    2. Severe Weather

    As mentioned above, a new subtle surface low will be developing
    beneath a region of upper-level divergence across the Lower
    Mississippi Valley on Saturday morning. This low will move rapidly
    northeast Saturday afternoon and evening with the cold front moving
    eastward. With a strong SSW 850mb LLJ around 70+ kt and upper-level
    500mb winds from the WSW at 80+ kt by Saturday evening, effective
    bulk shear will be very strong across the Mid-South and Tennessee
    Valley. 15Z RAP forecast soundings at CHA for 18z Saturday are
    showing low instability and very high shear values. MLCAPE around
    500 J/Kg with EBShear around 55 kt. ESRH between 300 to 500 m2/s2 is
    very high, and the strong low-level shear is resulting in long,
    curved hodographs favorable to tornadoes, including strong and long-
    track tornadoes. The big limitation for Saturday's severe convection
    across our region may be morning showers and thunderstorms that
    stabilize portions of the forecast area. This could stabilize the
    boundary layer and limit surface based convection. However, it still
    appears likely that strong southerly flow will lift this northward,
    and at the very least, destabilize the southern plateau and southern
    valley enough to support high-end severe weather. For this reason,
    the highest risk of severe weather across our forecast area will be
    counties along the southern Cumberland Plateau and southeast
    Tennessee.

    On Saturday afternoon, it is likely that Supercells develop along
    the cold front or prefrontal trough across northeast and east-
    central Mississippi and central and northern Alabama; these would
    begin to track northeast through the evening. These storms would
    most likely arrive in southeast Tennessee sometime between 23z and
    04z Saturday evening with a gradual upscale growth into a QLCS with
    embedded circulations as the night progresses. Storm motion will be
    very fast, around 220/65kt. The fast movement and kinematic wind
    profile will support severe thunderstorm damaging winds area-wide.
    The main tornado risk, including the most probable significant, long-
    track tornado risk, is likely to remain south and west of the
    Knoxville area where better boundary layer moisture is present, but
    there will at least be a low risk of tornadoes area-wide. There will
    be a risk of large hail with Supercell thunderstorms, but the
    overall hail risk is low to medium.

    3. Flooding

    Ahead of the approaching cold front, very strong 850mb moisture
    transport moves into our area with PWATs reaching to near record
    high values of 1.4 to 1.5 inches. This is around 3.5 sigma above
    normal values which will support heavy rain rates. Due to heavy rain
    rates, flooding of urban areas and flood prone locations is expected.

    Sunday through Wednesday

    Upslope flow keeps clouds and showers across the mountains on Sunday
    with a chance of some light snow or rime ice on Sunday night before
    dry air and clear sky conditions return on Monday. With troughing
    early week, temperatures will be cooler early in the week before
    ridging brings a return to above normal temperatures by mid-week.

    &&

    .AVIATION...
    (00Z TAFS)
    Issued at 730 PM EDT Thu Mar 13 2025

    A possible thunderstorm near TYS and possibly CHA next hour before
    dissipating so VCTS remarks. VFR flight categories however will
    continue through the 00z TAF period. Winds will be light through
    about 18z Friday, after which point the potential for gusts up to
    about 20 knots begins to increase.

    &&

    .PRELIMINARY POINT TEMPS/POPS...
    Chattanooga Airport, TN 52 76 61 73 / 10 10 50 90
    Knoxville McGhee Tyson Airport, TN 48 78 60 73 / 20 0 30 80
    Oak Ridge, TN 48 77 59 72 / 20 0 30 80
    Tri Cities Airport, TN 45 75 54 74 / 10 0 10 50

    &&

    .MRX WATCHES/WARNINGS/ADVISORIES...
    NC...NONE.
    TN...High Wind Warning from 8 PM Friday to 2 AM EDT Sunday for Blount
    Smoky Mountains-Cocke Smoky Mountains-East Polk-Johnson-
    Sevier Smoky Mountains-Southeast Carter-Southeast Greene-
    Southeast Monroe-Unicoi.

    VA...NONE.
    &&

    $$

    SHORT TERM...CD
    LONG TERM....JB
    AVIATION...TD
    ===

    -- Sean

    ... I ordered a chicken and an egg from Amazon. I'll let you know.
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    * Origin: Outpost BBS * Johnson City, TN (618:618/1)