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FOUS11 KWBC 180830
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
430 AM EDT Fri Oct 18 2024

Valid 12Z Fri Oct 18 2024 - 12Z Mon Oct 21 2024


...Central and Southern Rockies...
Days 1-2...

Potent low closes off over northern AZ this morning where it
stalls/occludes/fills through Saturday. This will result in a
prolonged period of intense synoptic ascent focused across the Four
Corners and southern/central Rockies, where height falls,
downstream divergence, and upper level diffluence in response to
modestly coupled jet streaks overlap. At the same time, a surface
cold front will shift east over the Four Corners today and over the
southern Rockies tonight. This will have a two-pronged effect of
enhancing ascent, as a wave of low pressure develops along it,
while impressive baroclinicity results in sharpening fgen to drive
locally more intense lift. Where this most intense ascent occurs,
especially in areas of upslope flow, precipitation rates will be
heavy.

Downstream of this upper low, strong moisture advection leads to
PWs +2 to +3 sigma, additionally supporting widespread heavy
precipitation. Snow levels ahead of the low are quite high,
varying between 9000 to 10000ft, but will fall to around 6500ft
underneath the upper low over AZ tonight. Day 1 PWPF for >8" is
over 50% in the higher southern UT ranges and the Kaibab Plateau
and around Humphreys Peak in northern AZ along with over 80% for
>12" additional in the San Juans of CO.

The filling low causes precip rate to drop to light to moderate
with snow levels lingering around 9000ft over the San Juans. Day 2
PWPF for additional >8" is 30-50%.


The probability of significant ice across the CONUS is less than
10 percent.

Jackson


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