Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
346 PM EDT Thu Mar 16 2023

Valid 00Z Fri Mar 17 2023 - 00Z Mon Mar 20 2023

...Great Lakes & Interior Northeast...
Days 1-2...

The Great Lakes will reside beneath two co-located regions of jet
streaks; one over the Southern Plains and another over Lake
Superior, which will aid in the deepening of a wave of low
pressure as it tracks into the eastern U.P. of Michigan by early
Friday morning. As the wave of low pressure tracks northeast this
afternoon, periods of snow will continue from central MN and up to
the MN Arrowhead, with any lingering rain/snow line over far
northern Wisconsin switching over to all snow by this evening. As
the low deepens over the U.P. late afternoon and into the evening
hours, northeasterly winds will pick up and keep banded,
lake-enhanced snowfall rates in place across the aforementioned
areas through the evening, then across the U.P. late tonight into
Friday morning. Snowfall rates of 1-2"/hr are expected while SLRs
steadily rise though the period, eventually reaching as high as
18:1 along the U.P. Friday morning. The low will remain
progressive in its movement east-northeast into southeast Canada,
but troughing on the backside of low, as well as daytime heating
helping to increase surface-3km lapse rates (sfc - 850mb delta-Ts
of 30-35F), will support more snow bands off Lake Superior during
the midday to afternoon hours with SLRs now approaching 20:1. The
sharp contrast in sfc-850mb delta Ts will linger into the
overnight, combined with the 30-35 kt 850mb northerly flow will
keep snow bands around into Saturday morning. Snow bands will
gradually diminish across the U.P. and far northern MI by Saturday
afternoon as high pressure builds in from the west. Latest
forecast calls for over a foot of snow along the U.P. of MI with
localized amounts approaching 2 feet possible through Saturday
afternoon. Areas forecast to pick up 6-12" include the MN
Arrowhead, far northern WI, and in northwest MI (localized amounts
>12" likely in these areas). The WSSI shows a stripe of Moderate
to Major Impacts along and just south of the northern U.P. coast
through Saturday.

Farther east, the same anomalously cold 850mb temps (NAEFS shows
850mb temps below the 10th climatological percentile for
mid-March), will advect east over Lakes Erie and Ontario.
Similarly to Lake Superior, delta-Ts between the surface-850mb
over the two Great Lakes are forecast to be 30F. The latest NAM
even suggests some modest MUCAPE available over the eastern
portions of the Lakes (250-500 J/kg). This combined with the
strong 30-40 knot W-WNW winds will result in intense singular lake
effect snow bands late Saturday morning and continuing into Sunday
morning. Within these bands, snowfall rates of 1-2"/hr are likely
with some rates perhaps approaching 3"/hr. The event is still a
couple days out, but residents should take note that the weather
conditions could change drastically over the course of the day on
Saturday, starting off fairly quiet Saturday morning to seeing
heavy bands of lake effect snow or even snow squalls Saturday
afternoon and evening. The snow squalls could track quickly across
northern New England on Saturday afternoon out ahead of, or along
a secondary cold front approaching from the west. WPC has Key
Messages for this storm system listed below.

...Southern Rockies & West Texas...
Days 1-2...

A shortwave trough approaching from the Southwest is allowing for
diffluent flow over the Southern Rockies at the same time a cold
front races south through the southern High Plains. The
northeasterly low level flow will prompt upslope flow into the
Sangre De Cristo of far southern CO and northern NM where heavy
snowfall rates are expected this evening and into Friday morning.
Latest WPC PWPF shows 40-60% probabilities for snowfall totals >8"
in parts of the Sangre De Cristo, as well as the San Juans of
southwest CO. The experimental PWSSI indicates there is a 60-80%
chance for Moderate impacts in these mountain ranges through
Friday evening.

As the cold front races south into central and western Texas,
temperatures will continue to drop in West TX. Meanwhile, the
aforementioned upper trough in the southwest will also makes its
way through the Four Corners region Friday evening, as will a weak
500mb disturbance through northern Mexico. A strengthening jet
streak Friday night will place a diffluent right-entrance region
over West TX and as far east as central TX. At 850mb, a ribbon of
moisture flux will be advected into the Davis Mountains and the
Big Bend around the same time 700mb moisture is directed into the
region. The easterly 850mb flow will cause upslope enhancement for
precipitation along the eastern slopes of the Davis Mountains,
while westerly 700mb flow also leads to some upslope flow on the
western slopes. This is a recipe for periods of heavy snow in the
Davis Mountains, as well as potentially in the foothills to the
south and east of there. Currently, WPC PWPF puts the odds fod >4"
of snow in the Davis Mountains at 20-40% with the highest
elevations closer to the 40% probabilities. Despite recent warmth,
snowfall is coinciding during the overnight hours Friday and rates
could be heavy at times, which does give a window for snow
accumulations on all surface into Saturday morning. Snow may
continue into the daytime hours Saturday but if rates become
lighter, most accumulations will be limited to trees and grassy
surfaces. The same can be said east towards the Big Bend and the
more hilly terrain west of San Antonio where the is a chance for a
coating of snow Friday night and into early Saturday morning.

...California...
Day 3...

After a much needed break the next couple days, the next wave of
low pressure to bring inclement weather to the Golden State
arrives on Sunday. It arrives courtesy of a 500mb disturbance
supplying a surge of 850-700mb moisture flux. Compared to recent
storm systems, this storm on Sunday will not pack nearly as much
of a punch, but will still produce additional heavy snowfall in
the Sierra Nevada. Latest WPC PWPF shows 30-40% probabilities
along northern and central Sierra Nevada, as well as the highest
elevations of the Trinity mountains in northern California. This
could lead to some hazardous travel conditions in complex terrain
during the day on Sunday.


...Key Messages for March 16-18 Winter Storm...

--A storm system will intensify as it tracks east across the U.P.
of Michigan tonight with heavy lake enhanced snow shifting east
along the south shore of Lake Superior.

--The storm drifts northeast into Canada Friday with lake effect
snow in Michigan Friday through Saturday.

--Heavy snow rates exceeding 1â€/hr are likely within both lake
enhanced and lake effect bands, producing hazardous travel and
rapidly changing road conditions. The greatest snowfall is
expected around Lake Superior where local amounts of 1-2 feet of
accumulation are forecast.

--A reinforcing cold front Saturday may allow snow squalls to
shift across interior sections of the Northeast. 


Mullinax