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

Valid 12Z Sun Mar 21 2021 - 12Z Wed Mar 24 2021

...Central to Southern Rockies/Southwest into Central High
Plains...
Days 1-3...

On Day 1, snow remains likely across the CO foothills and front
range as a mid-upper level trough slowly approaches, with the
forward flank upper jet maxima crossing eastern CO into KS. A
combination of a well defined and persistent 700 mb convergence
maxima helps lift the available moisture, in conjunction with
upper divergence maxima and orographics in windward areas to
produce moderate to heavy snow.
Lee cyclogenesis occurs Sunday night, and this low is likely to
lift northeast into the Central Plains by Tuesday morning.

As the surface low develops over far NE NM and lifts northeast,
low-level flow will turn E/NE, producing upslope enhancement into
the Front Range and Palmer Divide, especially Monday morning. The
heaviest snow D1-2 is likely in the Front Range and Palmer Divide,
extending south into the Sangre DeCristo Mountains of southeast
CO. Locally a foot of snow is possible.

The thermal profiles east of the terrain are marginal for
snowfall, but the likelihood for a deformation axis to enhance
ascent to dynamically cool the column should produce a few inches
of snow along the I-25 urban corridor and into the high plains of
NE CO.

On Day 3, Tuesday, the next upper trough crosses NV/AZ on the way
to CO/NM.  The amplifying trough leads to increasing upper level
divergence maxima developing in southern CO and NM. A wave and
then circulation develops in southern CO, drifting south into
northern NM.
Well defined boundary layer convergence occurs along the path of
the circulation, peaking Tue night-early Wed morning in southeast
CO to northeast NM.

The GFS and ECMWF have increased QPF this cycle and potential snow
amounts in response to mid level frontogenesis, favorable
difluence aloft with embedded 3030 mb divergence maxima, plus east
northeast flow coming into the Sangre deCristo Mountains that
provide an orographic component to lift.  The prolonged ascent
leads to locally heavy snow in southeast CO/northeast NM focused
along the Sangre DeCristo mountains.The
ECMWF/GFS/NAM/UKMET/Canadian global show potential or a foot of
snow, and a moderate risk for 8 inches of snow in these areas.

Secondary maxima are expected in the southern UT mountains south
into the Mogollon Rim of AZ and White Mountains of NM as the
trailing cold front and 700 mb trough move south across these
areas, with frontal convergence providing lift in the terrain.
Forecast amounts are not as high since the sfc-700 mb convergence
strength is not as strong as over northeast NM.


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

Well defined and persistent warm/moisture advection occurs across
WA into the northern Rockies today into Monday.  Peak ascent in
the 850-700 mb layer and high layer relative humidity over 90
percent lead to conditions favorable for heavy snow in the WA
Olympics and Cascades.  The lift is accented by a 300 mb jet max
max streaming onshore and across WA state today and tonight. 
Difluent flow aloft crosses the northern Rockies as well,
combining with 700 mb ascent to lead to persistent snow in the
ranges of northern ID to northwest MT Sun night to early Mon . 
Heavy snow remains likely in the terrain of the Olympics and WA/OR
Cascades, with 2 day totals locally as high as 2 feet in the
northern WA Cascades.  WPC probabilities are high for 8 inches.

Secondary snowfall maxima are expected in the OR Blue Mountains,
and into the Northern Rockies with Day 1 plus Day 2 amounts
locally a foot. The jet streak and associated forcing weaken and
drop southeast, leading to renewed light snow accumulations in the
Absarokas, Tetons, and Big Horns.

On Day 3 the remaining snow is expected to be focused on the Wind
River Range of WY and Gillette Arch, continuing down to the
Laramie Mountains. The approach of a 700 mb ridge towards the
Pacific northwest leads to a dry period there.

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

Petersen