• HVYSNOW: Winter Storm RM

    From Dumas Walker@21:1/175 to All on Tuesday, March 12, 2024 08:02:00
    FOUS11 KWBC 120740

    Probabilistic Heavy Snow and Icing Discussion
    NWS Weather Prediction Center College Park MD
    340 AM EDT Tue Mar 12 2024

    Valid 12Z Tue Mar 12 2024 - 12Z Fri Mar 15 2024

    Day 1...

    The final in a series of shortwaves will push onshore this morning
    over WA/OR and then dive southeast towards the Great Basin while
    amplifying. This feature will rapidly intensify into a strong
    closed low over the Great Basin on Thursday as anomalous ridging
    bulges along the Pacific Coast. This evolution will bring an end,
    finally, to the repeated rounds of precipitation across the
    Northwest, especially to terrain above 2500-3500 ft. Ascent across
    the region will be driven primarily via height falls and PVA
    associated with this shortwave, but a modest 90kt Pacific jet
    streak is also likely to orient WNW to ESE, driving favorable LFQ
    diffluence into the area as well. This synoptic lift combined with
    upslope flow in the W/NW oriented ranges will produce heavy
    snowfall as moisture surges ahead of the best ascent on the
    downstream confluent flow, although PW and IVT anomalies according
    to NAEFS are generally near normal. Still, there will be
    plentiful moisture to be wrung out by the impinging ascent, and
    WPC probabilities for more than 6 inches of snow are high (>70%)
    in the Olympics, along the WA/OR Cascades, and eastward including
    portions of the Northern Rockies, in the NW WY ranges, and south
    to the Ruby Mountains and along the Wasatch of UT.

    ...Central Rockies, Southern Rockies, & Central High Plains...
    Days 1-3...

    ***Significant and long lasting winter storm likely to produce
    heavy snow across much of the terrain of the Central and Southern
    Rockies, with impactful snowfall becoming more likely into the
    lower elevations of the High Plains as well.***

    A northern stream shortwave will dig out of British Columbia late
    Tuesday, and then deepen rapidly as the mid-level flow becomes
    increasingly amplified. A potent upstream ridge blossoming across
    the eastern Pacific will become intense, characterized by
    700-500mb height anomalies reaching as much as +4 sigma near
    British Columbia by the end of the forecast period, driving
    equally impressive downstream height falls as a 500mb closed low
    sags S/SW into the Desert Southwest by Friday morning. The primary deterministic global members have continued to trend deeper and SW
    with this closed low as it cuts off beneath the omega block across
    the Pacific. While this low then continues to spin slowly and
    retrograde, it will produce an extended temporal duration of
    impressive synoptic lift through divergence downstream across the
    Four Corners and Central Rockies, with additional ascent occurring
    in the post-frontal upslope regime as a cold front sags southward
    through the Plains. The prolonged period of synoptic lift and
    upslope NE flow will result in this long duration precipitation

    While it is almost certain that ascent will be prolonged and
    impressive, there are still some questions marks about moisture
    and position of greatest ascent. The guidance does appear to be
    converging on a deeper more SW aligned system, creating confluent
    flow emerging out of the Pacific and streaming northeast,
    especially within the 700-500mb layer. This will increase column
    moisture, but NAEFS ensemble tables suggest overall PW will be
    near normal within this axis. However, at the same time, a leading
    850-700mb wave will spin out of the primary gyre, causing a local
    backing the lower level flow leading to the emergence of enhanced
    moisture being drawn northward from the Gulf of Mexico. There are
    still considerable model differences in this lead evolution, but
    the trends have been for a slightly deeper but farther south low.
    This is important because the downstream moist isentropic upglide characteristic of the flow around this feature will likely lift
    the theta-e ridge into an impressive TROWAL, pivoting SW around
    the 850mb low and transporting additional moisture and instability
    back into the High Plains. This is reflected by higher PW
    anomalies in a NE to SW arc across the High Plains, and with
    sfc-700mb flow likely strengthening out of the NE during this
    time, it will transport significant moisture to prolong and
    enhance snowfall, especially into the Front Range. During this
    time as well, the setup appears to match the conceptual model for
    a pivoting band of snow somewhere across eastern CO or into the
    Central High Plains where a potent deformation axis and
    overlapping fgen surge omega into the deepening DGZ. Confidence is
    low in this evolution, and the column is marginally supportive for
    heavy snow, but significant dynamic cooling could occur in this
    band to allow for rapid snowfall accumulation. However, the higher
    confidence part of this forecast is more about how the
    intensifying NE flow around the 850mb low will help transport
    additional moisture into the Rockies, which within the slow moving
    synoptic ascent will produce heavy snowfall across much of the
    terrain. Snow levels during the event will gradually cool,
    starting around 5000-7000 ft, and dropping to 3500-5000 ft by
    Friday. WPC probabilities across WY and CO D2-3 exceed 70% from
    the Big Horns south into the Laramies, along the Front Range, into
    the Palmer Divide and down through the Raton Mesa and Sangre de
    Cristos. 2 or more feet of snow is likely in the higher terrain of
    the Front Range, with locally up to 1 foot possible elsewhere.
    Along the I-25 urban corridor and into the High Plains, amounts
    will be more modest, but several inches of snow should create
    impacts along this corridor as well.

    The second phase of this event will begin during D3 as the closed
    upper low over the Desert SW continues to amplify, resulting in
    the prolonged mid-level divergence across the Four Corners. This
    will lead to increasing 300-310K isentropic ascent surging NE from
    AZ/NM into UT/CO, producing waves of heavy precipitation starting
    late D3, primarily into the terrain north of the Mogollon Rim and
    points northeast back into the San Juans and CO Rockies. WPC
    probabilities D3 surge across the Four Corners, reaching above 70%
    for 6+ inches along the Mogollon Rim, the White Mountains of AZ,
    southern Wasatch and into the San Juans.

    The probability of receiving at least 0.1" ice is less than 10%
    across the CONUS through Day 3.

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