• Rockies Winter Storm Disc

    From Dumas Walker@21:1/175 to All on Wednesday, March 13, 2024 07:55:00
    FOUS11 KWBC 130647

    Probabilistic Heavy Snow and Icing Discussion
    NWS Weather Prediction Center College Park MD
    247 AM EDT Wed Mar 13 2024

    Valid 12Z Wed Mar 13 2024 - 12Z Sat Mar 16 2024

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

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

    Two-phased major winter storm begins this afternoon as the mid-
    level pattern begins to evolve into one supporting a long-duration
    system across the Central and Southern Rockies. The event begins
    as a wave of low pressure developing in the lee of the southern CO
    Rockies in response to a lobe of vorticity swinging east out of
    the Desert Southwest overlapped with a strengthening jet streak
    arcing northeast into the Central Rockies. The overlap of these
    two will cause rapid pressure falls in the lee of the terrain, and
    this low is likely to deepen as it moves east into the Central
    Plains by tonight before finally ejecting into the Upper Midwest
    and Great Lakes on Thursday. The guidance has continued its trend
    southward with this surface low, which will allow for more rapid
    cold advection to sink across WY/CO, especially behind the
    southward advancing surface cold front behind this leading low

    Moisture will become impressive through Thursday as both low and
    mid level moisture advects into the Rockies. In the surface-850mb
    level, southerly flow out of the Gulf of Mexico will draw PW
    northward as the attendant theta-e ridge surges cyclonically into
    a TROWAL around the deepening low. This causes an axis of elevated
    moisture anomalies reaching +1 to +2 sigma according to NAEFS
    surging through the MS River Valley and then arcing westward into
    the Central Rockies and High Plains. At the same time, an
    impressive deformation axis is likely to develop on the
    intensifying NE winds behind the front and around the back side of
    the low, which will overlap with robust fgen to enhance ascent
    and cause dynamic cooling to help cause p-type transition from
    rain to snow. While the column across the High Plains is marginal
    for snow, this dynamic cooling combined with rapid CAA behind the
    front should be sufficient for at least some snow in the lower
    elevations D1. Still, however, the most significant snowfall and
    snow rates, which will likely reach 2-3"/hr, are expected in the
    Front Range where upslope flow maximizes into the moist column.

    The guidance has continued to shift the lead impulse a little
    faster resulting in slightly lighter moisture advecting into the
    region with phase 1, and the regional soundings indicate a near
    isothermal layer at temperatures slightly above the DGZ,
    suggesting the potential for riming. This could cause SLRs to be
    slightly lower than forecast in the NBM/deterministic models,
    which has caused at least a small reduction in amounts with this
    forecast update. However, impacts are still likely to be
    significant as reflected by the WSSI-P showing higher than 50%
    chance for major impacts in the Front Range/Sangre de Cristos, and
    above a 50% chance for moderate impacts in the other Four-Corners

    WPC probabilities for more than 6 inches on D1 are above 80% from
    the Big Horns south through the Wind Rivers and Laramies, and
    then across much of the Front Range including the Palmer Divide,
    south into the Sangre de Cristos. The highest snowfall is likely
    in the Front Range with more than 2 feet possible.

    As this surface low pulls away, the best low-level moisture will
    be shunted to the east. However, it is during this time when the
    500mb low deepens towards -3 sigma at 700-500mb according to NAEFS
    over the Desert Southwest. This low will become extremely slow
    moving as it gets trapped beneath the omega ridge to its north,
    which will result in a long duration of ascent through downstream
    divergence and waves of PVA. At the same time, this persistent SW
    flow between 700-500mb will advect copious Pacific moisture
    northeast through the Four Corners, leading to part 2 of this
    system with snow quickly overspreading much of the terrain from
    the Mogollon Rim northeast through the Wasatch and CO Rockies. At
    the same time, the NE flow behind the southward sinking cold front
    will persist robust upslope ascent into the Front Range and
    eventually the Sangre de Cristos, resulting in additional heavy
    snow in these areas as moisture increases aloft and some terrain-
    induced fgen occurs. This suggests that D2 will have the most
    widespread coverage of heavy snow, with rates of 1-2"/hr or more
    likely in many areas, before the focus transitions primarily to
    the Four Corners terrain with drier air and weaker forcing
    shifting into CO.

    During D2, the greatest WPC probabilities for more than 6 inches
    of snow pivot south and west, but still peak along the Front
    Range, Palmer Divide, Sangre de Cristos/Raton Mesa, and back into
    the San Juans where they exceed 80%. Additional high probabilities
    above 60% extend into the Wasatch, Uintas, and Mogollon Rim. The
    heaviest accumulations D2 are again expected in the Front Range
    where an additional 1-2 feet is possible, and impacts along the
    I-25 urban corridor, especially across the Palmer Divides, will be
    most substantial. By D3 the event winds down across the Front
    Range, but high probabilities for more than 6 inches persist near
    the Four Corners including again in the San Juans, Wasatch, and
    portions of the Mogollon Rim. 2-day snowfall in these areas will
    approach or exceed 2 feet.

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


    **Key Messages for March 13-16 Winter Storm have been started and
    can be found on our website at https://www.wpc.ncep.noaa.gov **

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