Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
332 AM EDT Sat Apr 27 2024

Valid 12Z Sat Apr 27 2024 - 12Z Tue Apr 30 2024

...Central Rockies...
Days 1...

A surface low pressure consolidating in the lee of the Rockies
early Saturday will deepen briefly through the day before slowly
weakening as it ejects to the northeast and into the Central Plains
Saturday night. This low will deepen in response to impressive
synoptic ascent through LFQ upper diffluence as an upper jet
rotates through the base of an amplifying longwave trough, with
this trough deepening into a closed low before advecting into the
Plains late D1. As this low deepens and shifts to the east, it will
be accompanied by an increasingly intense deformation axis on its
NW side, which will overlap effectively with mid-level fgen to
drive ascent into the DGZ, and increasing upslope flow on easterly
winds into the Front Range and other CO Rockies. In this area,
there is additionally good consensus for CSI/CI as mid-level
theta-e lapse rates fall to below 0C/km in conjunction with pockets
of -EPV. This suggests snowfall rates will likely exceed 1"/hr, and
in the more intense convection could be 2-3"/hr as noted by the WPC
prototype snowband tool. This deformation axis will pivot in the
vicinity of the Front Range much of Saturday, and although snow
levels will likely hover around 8,000 ft, impressive cold air
dragging down to as low as 6,000 ft is probable which could bring
significant accumulations into the foothills and Palmer Divide as
well before everything shuts off Sunday morning. For the I-25 urban
corridor from Cheyenne to Colorado Springs, it is possible even the
lower elevations could mix with snow or even get some light
accumulations, but the marginal thermal structure should prevent
significant impacts east of the terrain except along the Palmer
Divide.

WPC probabilities have climbed once again this morning for the
Front Range, now exceeding 80% for the eastern slopes and into the
higher terrain, with a secondary maximum near Pike's Peek. The
bigger change this morning, however, has been a noted increase
along the Palmer Divide which now features probabilities as high as
60% for 6+ inches, especially west of I-25. Additional WPC
probabilities exceeding 50% for 6+ inches exist across other
portions of the CO Rockies above 8000 ft, and in the San Juans,
northern Sangre de Cristos, and portions of the Wasatch in UT.


...Pacific Northwest and Northern Rockies...
Days 2-3...

Persistent onshore flow will spread eastward into the Pacific
Northwest and Northern Rockies D2 and D3 as confluent mid-level
flow streams across the Pacific and into the region. Although the
available moisture will be near normal, there is forecast to be a
subtle increase in IVT within this confluent flow as reflected by
GEFS probabilities reaching 50-70% for 150+ kg/ms, highest on D2.
Despite the overall modest moisture, ascent will intensify as dual
shortwaves race eastward embedded within the pinched flow, to
enhance ascent to wring out the available PW, with PVA maximized
late Sunday and again late Monday. This ascent combined with the
favorable upslope flow component into the Cascades, Olympics, and
Northern Rockies due to the zonal flow will result in periods of
moderate to heavy snow, with snow levels falling to as low as 2500
ft Monday.

WPC probabilities for more than 6 inches ramp up during D2 across
the OR and WA Cascades, as well as the Olympic Range, where they
reach 50-80% in the higher elevations above 5000 ft. By D3 these
extend eastward into the Northern Rockies including in the vicinity
of the Salmon River and Sawtooth Ranges, with lighter probabilities
reaching as far east as the Absarokas. With snow levels falling,
several inches of snow is possible at the Cascade Passes, including
Snoqualmie and Stevens Passes, and especially by D3, resulting in
hazardous travel potential.


The probability of significant icing is less than 10 percent.


Weiss