Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
347 PM EDT Thu Mar 17 2022

Valid 00Z Fri Mar 18 2022 - 00Z Mon Mar 21 2022

...Central Plains...
Day 1...

A surface low pressure moving out of Texas will track northeast
along a cold front towards the Great Lakes tonight and Friday.
Strong CAA behind this front will help sharpen the elevated
baroclinic gradient, with this fgen being aided by impressive
upper diffluence in the LFQ of a subtropical jet streak. At the
same time, a closed low will eject across OK, with robust
deformation occurring to the NW through interaction with a
northern stream shortwave. Where these features overlap,
impressive ascent is likely, and the guidance is converging on a
band of heavy snow developing across KS.

While the guidance is in good agreement in both the placement and
timing of this feature, some uncertainty persists as to how much
snow will occur. The precipitation will initially be rain, but as
forcing becomes intense, dynamic cooling of the column will occur
causing a p-type transition from rain to snow. Despite surface
temperatures that will likely remain right around 32F, intense
omega through CSI and possibly upright convection will produce a
pivoting band of snowfall with rates likely eclipsing 1"/hr as
shown by the HREF and WPC snowband prototype tool. With convective
snowfall likely, snowfall amounts above the 90th% of the WSE and
NBM are plausible, although a modestly shallow DGZ (SREF
probabilities for 100mb of depth 10% or less) and antecedent
hostile conditions leading to some warming/compaction will at
least somewhat limit snowfall. Still, heavy wet snow falling at
1-2"/hr will likely contribute to significant impacts, possibly
into the morning commute of Friday. WPC probabilities have
increased and are now 20-40% for 4" in a N-S stripe centered near
Hays, KS, and locally more than 8" is possible in isolated
locations.


...Pacific Northwest...
Days 2-3...

A shortwave dropping onshore the WA coast Saturday will elongate
into a full latitude trough with energy splitting into southern
stream and northern stream shortwaves across the Great Basin on
Sunday. PVA and height falls associated with this trough will
combine with increasing LFQ diffluence downstream of a powerful
Pacific jet streak which will arc onshore Saturday night to
produce deep layer ascent spreading eastward through the weekend.
Moisture during this time will steadily increase as well in WAA
ahead of a surface cold front, and within the upper jet structure.
Together this will produce expanding coverage of heavy snow D2 and
D3. For D2 /Friday night and Saturday/, WPC probabilities for more
than 6 inches of snow are above 20% in the Olympics and northern
WA Cascades. By D3 the trough pushes inland and moisture surges to
+1 standard deviation above the climo mean with respect to PW. WPC
probabilities for 6+ inches reach above 20% again in the Olympics
and WA Cascades, but also spread into the Northern Rockies and
Tetons.


For Days 1-3, the probability for receiving at least 0.10" of
freezing rain is less than 10 percent across the lower 48.

Weiss