Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
242 PM EDT Sat Mar 15 2025

Valid 00Z Sun Mar 16 2025 - 00Z Wed Mar 19 2025


...Midwest to Great Lakes...
Day 1...

Recent trends have increased the potential for a swath of moderate
snowfall to occur on the backside of a developing secondary low
pressure system racing northward from the Mid- Mississippi Valley
this evening. The mechanism behind this trend is related to a
deepening shortwave crossing the southern Plains this afternoon and
becoming more separated from the exiting upper low over Minnesota.
This leads to increased ridging ahead of this secondary shortwave
(partly amplified by latent heat release from convection across
the South this afternoon) and a repositioning of the 250 mb jet
streak over the Upper Great Lakes to produce more favorable upper
divergence and a broader precipitation shield reaching westward
enough into the cold airmass settling in behind a potent cold front
crossing the region today. While this system will be very
progressive, there is potential for snowfall rates to approach
1"/hr from northern IL and eastern WI through the western L.P. and
eastern U.P. of MI between 06Z to 18Z Sunday. However, even though
this is a D1 forecast and snow begins to develop after 06z tonight
there remains more uncertainty than to be desired. Additionally,
most areas have surface temperatures well above freezing today and
will see precip type start out as rain, leading to possible heavy
snowfall rates driving where most accumulation occurs. WPC
probabilities for at least 4" have increased across the northern
L.P. of MI to around 30-50%, where the 12z HREF depicts the
greatest chances for 1"/hr snowfall rates. Lighter amounts under
3" are anticipated to stretch along a southwest- northeast corridor
from northern IL/southeast WI to the eastern MI U.P.


...The West...
Days 1-3...

The active pattern continues across the West with an atmospheric
river impacting the Oregon Cascades and California ranges through
D2 before the associated upper trough swings across the
Intermountain West and Rockies into D3.

As a shortwave impulse at the base of the upper trough approaches
the Northwest on Sunday the resultant IVT with this setup becomes
moderate to strong, potentially reaching 500-750 kg/m/s at the
start of D1 as moisture funnels onshore along a cold front which
will elongate into the region. Although the accompanying WAA with
this atmospheric river (AR) will drive snow levels to as high as
6000 ft south of the cold front, considerable moisture and a sharp
gradient in snow levels will likely result in impactful pass-level
snow from the Cascades through the Northern Rockies and eventually
Sierra Nevada once again on D2. In fact, NAEFS SAT depicts IVT
values above the 99th climatological percentile on Sunday extending
from the OR/CA coast through the northern Great Basin, further
highlighting the impressive moisture associated with this system.

For the entire forecast period (ending 00Z Weds), high WPC
probabilities (>70%) for 12+ inches exist across the Sierra Nevada
and northern CA ranges, OR Cascades, east to the Salmon
River/Sawtooth area and northern Rockies. Event total snowfall is
likely to be extreme in the higher terrain of the OR Cascades where
4-6 feet of snow is possible, with widespread 1-3 feet in the
other higher elevations regions of the Northwest, Sierra Nevada,
and northern Great Basin.


...Central Rockies and High Plains...
Day 3...

The system impacting the West through early D3 will begin to eject
into the central Plains by the end of the forecast period. This
shortwave will be accompanied by an intensifying sub- tropical jet
streak which will begin to arc poleward downstream of this
amplifying trough. This will have the dual-pronged effect of
transporting moisture into the Central Rockies while also placing
favorable LFQ diffluence atop the region of greatest mid- level
height falls. The resultant deep layer ascent will likely interact
with a cold front dropping southeast into the area, and the setup
appears favorable for developing cyclogenesis in the central High
Plains by Tuesday morning. Where the strongest ascent overlaps the
greatest moisture, especially in areas of expanding frontogenesis,
a swath of heavy snow is likely and at this time is centered over
central WY on Tuesday. WPC probabilities for at least 4" have
increased to 40-80% throughout much of central WY, but remain
higher for at least 6" (70%+) in the Big Horns, Black Hills,
central UT ranges and CO Rockies.


This winter storm may become the next powerful cyclone across the
Plains and Upper Midwest through the middle of next week. See WPC's
Extended Forecast Discussion for more information.


Snell