Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
340 PM EDT Sun Mar 21 2021

Valid 00Z Mon Mar 22 2021 - 00Z Thu Mar 25 2021

...Pacific Northwest into the Great Basin...
Days 1-3...


...Central and Southern Rockies...
Days 1-3...

Two distinct storm systems will bring heavy snow to the region
through the middle of the upcoming week.

Tonight through Monday night, a shortwave moving across the Four
Corners will deepen and close off as it pushes over the TX
Panhandle before lifting away to the northeast and across the
Midwest by Tuesday morning. At the same time, a subtropical jet
streak extending from the Pacific will arc eastward placing the
favorable diffluent LFQ for ascent more robustly atop the Central
Rockies. This enhanced diffluence combined with the height falls
and divergence associated with the closed mid-level feature will
drive surface low development in the lee of the Rockies early
tonight. This low is then likely to strengthen gradually as it
lifts northeast from NM into KS by Monday night, and then towards
the Midwest on Tuesday. As this low develops, ascent will be
enhanced by increasing E/NE low-level flow which will provide an
upslope component into the Front Range, Sangre De Cristos, and
Palmer Divide. Robust available moisture being acted upon by this
intense deep layer ascent will produce heavy snowfall much of
Monday and Monday night before the system pulls off to the
northeast and bringing an end to the forcing. WPC probabilities
are high on D1 for 6 inches in the Front Range, Palmer Divide, and
northern Sangre De Cristos with locally up to 12 inches possible.

After a brief respite on D2, yet another shortwave diving out of
the Pacific Northwest Tuesday will close off near the Four Corners
region late Tuesday night leading to height falls and intense
mid-level divergence across the Southern Rockies Wednesday while
yet another upper divergence maxima pivots overhead in response to
a strengthening jet streak. The setup for D3 appears very similar
to D1, but further south, with more abundant moisture, and colder
as the back door front should be well into New Mexico by this
time. A colder column with strong ascent aided by renewed upslope
will produce heavy snow across much of the High Plains of NE NM
and SE CO, with heavy snow extending back into the Sangre De
Cristos and San Juans. WPC probabilities are already high for 12
inches in these areas D3, with locally much higher amounts
possible. In the major cities of Santa Fe and Albuquerque,
confidence is lower due to shadowing, but several inches of
snowfall is possible there as well.


...Pacific Northwest through the Northern Rockies...
Days 1-3...

A shortwave dropping out of British Columbia will move into
Washington state Monday morning and then continue to progress
southeastward towards the Great Basin on Tuesday. This will be
accompanied by the LFQ of a jet streak dropping southward,
transporting abundant moisture into the region. Ascent within the
850-700mb layer becomes intense, within a region of high
RH/moisture. These features together will drive a wave of surface
low pressure and associated cold front southward as well,
providing some enhanced low-level convergence, along with some
upslope on the windward side of the terrain. Moderate to heavy
precipitation will follow these features southeastward, and WPC
probabilities are high for 6" on D1 in the Cascades, Olympics,
Northern Rockies, and Blue Mountains of OR. Locally more than 12"
is possible in some of these regions where upslope becomes
maximized. As this system continues southeast D2, moderate to
heavy snow is likely in the Great Basin ranges, Wasatch of UT,
Absarokas, Wind Rivers, and Big Horns where WPC probabilities are
moderate for 6 inches.

While less of an accumulation impact, guidance is coming into
better agreement that as the shortwave digs southeast and steepens
the mid-level lapse rates atop the cold front, scattered to
widespread convective snow showers or snow squalls may occur.
0-2km theta-e lapse rates fall as low as -7C/km coincident with
the cold front and at least modest instability. These snow squalls
could impacts parts of WA/OR/ID/NV/MT Monday evening and Monday
night. The HREF probabilities suggest a high potential for
snowfall rates briefly exceeding 1"/hr, so while this should not
accumulate much in any squall, severely restricted visibility and
briefly dangerous travel is possible.

After a brief dry period, another shortwave and associated jet
energy may approach the Pacific Northwest coast late on D3. This
could reintroduce heavy snow to the Cascades and Olympics with
more than 6" of snow possible.


For Days 1-3, the probability of significant icing (0.25 inch or
more) is less than 10 percent.


Weiss