• HVYRAIN: Excessive Rainfall Discussion...corrected.

    From COD Weather Processor@1:2320/105 to wx-storm@lists.illinois.edu on Fri Oct 1 22:47:37 2021
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    FOUS30 KWBC 012247
    QPFERD

    Excessive Rainfall Discussion...Corrected.
    NWS Weather Prediction Center College Park MD
    646 PM EDT Fri Oct 01 2021

    Day 1
    Valid 2244Z Fri Oct 01 2021 - 12Z Sat Oct 02 2021

    ...THERE IS SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS THE RIO GRAND
    VALLEY AND THE LOWER TX COAST...

    ...Southwest/Southern Plains...
    2200 UTC Update -- Based on the latest observational/convective
    trends, along with recent HRRR runs and 18Z HREF
    deterministic/probabilistic data (including exceedance
    probabilities), we have removed the Slight Risk across portions of
    TX into southwest LA...with the exception of the RGV and Lower TX
    Coast. Degree of upper level difluence continues to wane over TX
    as the upper low drifts from northeastern NM and into western
    OK/KS overnight. Deep-layer instability remains sufficient,
    however with weakening moisture transport and frontogenesis in the
    lower layers, despite the still-favorable thermodynamic profiles
    (MUCAPEs 1000-2000 J/kg and PWs ~2.00+ inches near the coast),
    expect convection to become less organized with time and as such a
    more limited (localized) flash flood threat going through the
    evening.

    ...Northern Plains/Upper Midwest...
    2200 UTC Update -- Minor changes were made to the Marginal Risk
    area, based on the latest observational and short-term HRRR/HREF
    guidance trends.

    Previous discussion...

    A weak surface feature moving from eastern SD into MN will bring
    periods of showers and thunderstorms to the region. Anticipate
    the better coverage and intensity of rain to be during the
    afternoon with daytime heating and thus better instability. Given
    precipitable water values are 2-2.5 standard deviations above the
    mean, rain rates could climb above 1 inch/hour, especially if
    there are repeated rounds of heavy rain. While antecedent
    conditions within this region have been fairly dry of late, heavy
    rain was observed over the past 12-24 hours helping to prime the
    soils. Therefore, a small Marginal Risk area was introduced for
    localized flash flooding impacts.=20

    Hurley/Pagano


    Day 2
    Valid 12Z Sat Oct 02 2021 - 12Z Sun Oct 03 2021

    ...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FROM THE CENTRAL
    AND SOUTHERN PLAINS INTO THE MISSISSIPPI VALLEY AND MIDWEST....

    ...2030z Update...
    No significant changes in the Marginal Risk area with this update.
    Models are coming into a better consensus with a slightly more
    progressive cadence, which (if anything) should help to limit the
    flash flood threat. The edge of the Marginal Risk was adjusted
    slightly to account for the small difference in timing, but the
    general region remains unchanged with the best probabilities for
    excessive rainfall in the center portion of the outlined area
    (though probabilities are still too low and dispersed for Slight
    Risk consideration, especially given the antecedent conditions).

    Churchill


    ...Previous Discussion...

    The closed upper level low will become absorbed by the northern
    stream trough axis dropping south from the Northern Plains into
    the Upper Midwest Saturday into early Sunday. At the surface, a
    weak surface low with a trailing cold front will focus widespread
    showers and thunderstorms from the Central and Southern Plains
    into the Mississippi Valley and Midwest region.=20=20

    Precipitable water values will surge north into the
    Lower/Mid-Mississippi values in response to the strong return flow
    ahead of the closed low and trough axis moving through the
    midsection of the country. Precipitable water values will range
    from 1.75 to over 2 inches closer to the Central Gulf Coast with
    1.5 inch values extending up into the Mid-MS Valley and Midwest.=20
    The latter values are around 2 standard deviations above the mean.
    And while this might seem conducive for heavy rain, the weak
    instability across this region will likely limit the rain rates
    observed with any convection that does develop. Regardless, as
    the low level flow becomes aligned with the 300-850mb mean wind
    parallel to the front, anticipate rain rates of around 1-1.5
    inches/hour are possible with the potential for training activity.
    Areal average precipitation will range from 1-2 inches with
    locally higher amounts possible.=20=20

    Given the fairly progressive nature of this activity and the drier
    antecedent conditions across much of the region, do not anticipate
    widespread impacts as it relates to flash flooding Saturday into
    early Sunday. However, there is a subtle signal for some training
    across eastern OK into AR as a mid-level impulses moves atop the
    surface boundary. As a result, the heaviest rain and highest
    impacts may occur within this region. Though, given model spread
    and thus a higher degree of uncertainty, decided to retain the
    Marginal Risk with no upgrade made at this update.=20

    Pagano

    Day 3
    Valid 12Z Sun Oct 03 2021 - 12Z Mon Oct 04 2021

    ...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FROM THE LOWER
    MISSISSIPPI VALLEY INTO THE TENNESSEE VALLEY...

    ...2030z Update...
    The Marginal Risk was slightly expanded and shifted eastward with
    this update, as the 12z model consensus depicts a slightly more
    progressive system. Meteorological reasoning otherwise remains the
    same with no new or upgraded risk areas necessary.

    Churchill


    ...Previous Discussion...

    As a storm system moves through the Great Lakes region, a trailing
    cold front will help focus showers and thunderstorms from the
    Lower MS Valley into the TN/OH Valleys, Sunday into early Monday.=20
    While the dynamic associated with this system are not impressive,
    they will still be most robust farther north. However, the better
    moisture and instability will be farther south. So assuming some
    subtle mid-level impulses will be moving through the mean
    southwesterly flow, widespread showers and thunderstorms could
    produce locally heavy downpours.=20

    With precipitable water values around 1.5 inches and MUCAPE values
    exceeding 1000 J/kg by the afternoon, convection could drop over 1
    inch/hour rates and potentially higher with multiple rounds of
    activity expected. The limiting factor to flash flooding could be
    the dry antecedent conditions. However, there is some signal for
    scattered showers and thunderstorms well ahead of the cold front
    (Day 2-3) that may prime the soils. In addition, there could be a
    brief period of training that would result in an increase
    potential for localized flash flooding. Therefore, a Marginal
    Risk was hoisted at this update.

    Pagano


    Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!oWUFvvIgKu6tarcYy4gi45mpkIqddqkA2SOuBiyrvAzN= yQgSk9Uydrpt4SODdSQABX97euzI$=20
    Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!oWUFvvIgKu6tarcYy4gi45mpkIqddqkA2SOuBiyrvAzN= yQgSk9Uydrpt4SODdSQABcK-ynns$=20
    Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!oWUFvvIgKu6tarcYy4gi45mpkIqddqkA2SOuBiyrvAzN= yQgSk9Uydrpt4SODdSQABfdF8brT$=20


    $$




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