• HVYRAIN: Excessive R 1/2

    From Weather Alert@21:1/175 to All on Wed Sep 23 17:42:00 2020
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    QPFERD

    Excessive Rainfall Discussion
    NWS Weather Prediction Center College Park MD
    1202 PM EDT Wed Sep 23 2020

    Day 1
    Valid 16Z Wed Sep 23 2020 - 12Z Thu Sep 24 2020

    ...A MODERATE RISK OF EXCESSIVE RAINFALL IS IN PLACE FOR PORTIONS
    OF THE LOWER MISSISSIPPI VALLEY...

    16Z Update: Only minor changes to the Moderate risk area over
    portions of LA into west central MS. Still on track for a swath of
    3-5" across this corridor today into tonight...with localized
    totals of 5-8" a possibility. This will result in numerous to
    widespread flooding concerns...some of which could be locally
    significant in nature. Currently seeing a pretty strong convective
    cluster over western LA northeast of the center of Beta. As we
    head into late this afternoon through tonight moisture convergence
    ahead of Beta is forecast to increase in magnitude and become more
    well defined. We should also begin to see an increasing
    interaction with the upper jet to the north. Meanwhile upstream
    instability over 1000 J/kg will be feeding into the area. Thus
    where we see this overlap of strong lower level moisture
    convergence and ~500 j/kg of MLCAPE...we should see an
    increasingly robust convective cluster move northeast across the
    Moderate risk area. This environmental setup seems to support the
    QPF coming out of the 12z CAMs and recent HRRR runs (although the
    maxes in these runs could be a tad too high). Some flash flood
    threat will also continue this afternoon from southeast AR into
    northern MS closer to the stronger mid/upper level forcing ahead
    of the trough and south of the upper jet. Rainfall rates will not
    be as robust here...nonetheless some brief heavier rates and
    rainfall totals of 2-4" supports some continued flood risk through
    the afternoon hours.

    Chenard

    ...Previous Discussion...

    ...Middle/Upper Texas Coast into southern Louisiana...
    Post-Tropical Cyclone Beta is expected to pick up forward speed
    during the day on Wednesday and track northeastward from the Upper
    Texas Coast into central/northern Louisiana, reaching western
    Mississippi Thursday morning. Precipitable water values near and
    just ahead of Beta's surface circulation were 2.2 to 2.5 inches as
    of 06Z per GPS data, and this relatively small pocket of high
    moisture is expected to more-or-less maintain into Louisiana
    Wednesday afternoon. A SW to NE low level convergence axis
    (925-850 mb) is forecast to set up to the northeast of Beta's
    circulation with roughly 20-30 kt of winds converging from central
    Louisiana into western Mississippi. Mean deep-layer flow will
    parallel the orientation of the convergence axis, supporting the
    potential for multiple rounds of heavy rain across portions of the
    Lower Mississippi Valley.

    A significant limiting factor for high rainfall rates and flash
    flooding is weaker forecast instability to the north of the
    surface low track, with a 00Z model consensus showing less than
    250 or 500 J/kg MUCAPE. South of the low level convergence
    boundary, however, instability up to 1500 J/kg is expected with
    southerly transport of moisture/instability into the Lower
    Mississippi Valley. Rainfall rates of 1-2 in/hr are expected but
    locally up to 3 in/hr cannot be ruled out. Rainfall intensity is
    expected to pick up later Wednesday afternoon and overnight,
    continuing into Thursday morning. The track of Beta showed
    relatively good agreement in the models, but some solutions took
    the storm farther south. WPC followed the tracks near the 00Z
    NAM_nest, 00Z ARW and 00Z NSSL...but feel the 24 hour rainfall
    total point maxima of 8-10 inches are too high. Areal average
    rainfall totals of 2-4 inches seems more reasonable with local
    maxima of 4-6 inches with 3-hour rainfall rates in excess of 3
    in/hr. A Moderate Risk was coordinated with SHV/LCH/LIX/JAN for
    the Day 1 period.

    ...Upper Mississippi Valley...
    Ahead of a shortwave trough moving into the Dakotas Wednesday
    evening, thunderstorms are expected to develop shortly after 00Z
    across north-central Minnesota just north of a surface front
    oriented from WSW to ENE. Precipitable water values of 1.0 to 1.3
    inches in the pre-convective environment and MUCAPE values in the
    1000 to 2000 J/kg range are forecast across south-central to
    north-central Minnesota. As southwesterly 850 mb winds of 30+ kt,
    ahead of a surface trough extending into the Central Plains,
    ascend over the front, mean storm motions are expected to parallel
    the frontal boundary supporting training. There is a relatively
    brief window between 00Z and 09Z where flash flood potential will
    be greatest, before instability weakens to the north and forward
    propagation increases toward the south, ahead of the surface low
    and in the wake of the surface trough axis. Rainfall rates of 1 to
    1.5 in/hr are expected which may cause some flash flooding. The
    00Z models were fairly tightly clustered with their axis of QPF
    and the Marginal Risk encompasses all of the latest 00Z HREF
    members and the 00Z NAM/GFS/ECMWF.

    ...Pacific Northwest...
    A rather strong atmospheric river is forecast to impact the
    Pacific Northwest today with IVT values in excess of 750 kg/m/s,
    ahead of a cold front which was analyzed roughly 200 miles west of
    Washington at 06Z. Southwesterly 850 mb winds of 40-60 kt will
    reach the Coastal Ranges late Wednesday morning and be embedded
    within precipitable water values of 1.25 and 1.5 inches near the
    Olympic Peninsula via the 00Z GFS. Orographic lift will be
    augmented by a powerful 150 kt upper level jet streak and
    diffluent flow which will approach the Pacific Northwest Coast
    among weak instability, forecast by the 00Z hi-res model
    consensus. Rainfall rates of 0.5 to just over 0.8 in/hr are
    expected to impact the Olympics, Coastal Ranges and Washington
    Cascades, with slow eastward translation of the moisture plume
    until roughly 00Z Thursday. Beyond 00Z Thursday, the cold front
    and associated moisture axis will push inland and south at a
    steady pace, limiting the potential for higher rainfall
    rates/duration into the Oregon Cascades. The 00Z HREF supports 0.5
    in/hr rainfall rates in excess of 90 percent for the Washington
    Olympics, Coastal Ranges and Cascades, with probabilities closer
    to 50-60 percent farther south into Oregon and of a shorter
    duration compared to farther north. 24 hour totals of 3-5 inches
    are most likely for the Olympics and northern Washington Cascades,
    but locally higher values cannot be ruled out.

    Otto


    Day 2
    Valid 12Z Thu Sep 24 2020 - 12Z Fri Sep 25 2020

    ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS THE
    TENNESSEE VALLEY INTO THE SOUTHEAST...

    ...Lower Mississippi Valley/Tennessee Valley/Southeast...
    As remnants of Beta get picked up by a positively tilted long wave
    trough extending from the OH Valley into the Lower MS Valley
    during Day 2, deep moisture and instability are transported across
    the TN Valley and Southeast states. There is a fair amount of
    spread concerning just how far north the remnants are drawn
    northward, and how much instability follows, particularly into the
    TN Valley. The 00z NAM/ECMWF look too far north with the weakening
    surface system, apparently allowing too much instability to reach
    northern MS/AL. The 00z GFS looks too fast with the system, as it
    lifts out the surface low into the eastern TN Valley. The 00z
    UKMET looks like a fair proxy for the expected rainfall
    distribution, so it was used as a template for the WPC QPF and
    Excessive Rainfall Outlook.

    With the mid level trough lifting to the northeast from the OH
    Valley into the Lower MS Valley, the remnants of Beta gets lifted
    northeast. However, as mentioned earlier, there is quite a bit of
    spread with respect to how quickly this occurs, and how far north
    the surface system gets. Since the model guidance has been too
    quick to lift Beta with the trough over the past couple of days,
    it seems as though the track of the system will be further south
    than much of the guidance. With a track further south with the
    surface system, the northern edge of the instability gradient is
    expected to lie from central MS into central and possibly northern
    AL during the afternoon and evening hours (when instability
    peaks). During this time, a 50 knot jet streak passing just to the
    north across the OH Valley provides synoptic scale ascent for
    banded rainfall to extend from central and northern MS across
    northern AL into far south central TN.

    Much of the 00z high resolution guidance (including the 00z NAM
    CONUS Nest and the 00z WRF ARW) showed a band of 4.00/6.00 inches
    of rainfall along the northern edge of the instability gradient.
    While placement looks reasonable, the amounts seem too high, as
    these models appears to be bringing too much instability into
    northern MS/northern AL. The moisture plume with Beta is
    transported along the isentrope toward an elevated boundary over
    this area, so heavy rainfall is expected, but lacking deeper
    instability, the high resolution values look too high. Hourly
    rainfall rates could top out near 1.50 inches, with total rainfall
    amounts of 3.00/4.00 inches possible in spots. This area was
    spared the highest rainfall associated with Sally, and three hour
    flash flood guidance values are generally above 2.50 inches.

    (cont'd)
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