Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
420 AM EDT Tue May 07 2019

Valid 12Z Tue May 07 2019 - 12Z Fri May 10 2019


...Northern and Central Rockies onto the High Plains...

A closed low over the CA/AZ border will open as it ejects east
across NM through tonight. The system then turns northeast up the
plains Wednesday as a northern stream trough digs over the
Intermountain West and reaches the UP of MI Thursday. The digging
northern stream shortwave reaches AZ Wednesday with a reinforcing
shortwave shifting southwest from MT to the Great Basin Wednesday
night through Thursday night. Moisture advection across the
Intermountain West will persist through the period. Snow levels
will initially be high, 7000-9000 ft, but will drop to 4000-6000
ft Wednesday into Thursday under the trough. The moistening column
and synoptic ascent will be aided by upslope flow on the
developing low-level easterly/northeasterly winds. This setup is
favorable for heavy snow and the axis of heaviest snow is likely
to shift southward through the forecast period, such that the
highest probabilities for heavy snow are likely in the ranges of
WY on day 1, the CO Rockies on day 2, and the San Juans/Sangre De
Cristos on day 3. Low to moderate probabilities for 12 inches
exist each day in the highest terrain of each aforementioned area
with high probabilities for six inches. Moderate probabilities for
4 inches exist in the High Plains of southwest WY and the NE
Panhandle as well as the Black Hills, on day 2 in the
northeasterly flow Wednesday.


...Upper Midwest...

Surface low pressure from the remnants of the southern stream low
currently over the CA/AZ border will round the developing northern
stream trough over the Intermountain West and track from the OK
Panhandle along a baroclinic gradient Tuesday night through
Thursday. While this system will generally be warm and accompanied
by rainfall, low level thermal profiles well north of this surface
low will be marginally cold enough for snowfall. Low-level
warm/moist advection will accompany left exit region diffluence
and increasing 700-600mb frontogenesis to produce strong ascent,
potentially capable of dynamically cooling the column enough for
accumulating snowfall. The NAM and its related high-res models are
still likely too aggressive with the snowfall owing to low-level
thicknesses which are too cool compared to the rest of the model
suite. The rest of the global consensus paints some light snow
from far northern WI into the UP of MI, which has caused a slight
uptick in snowfall accumulation forecasts across the area. WPC
probabilities for four inches are low though the trowal crosses
the area Wednesday night which may allow for better sticking. Wrap
around flow crosses Lake Superior and enhances snow for the UP
Thursday morning/Day 3.


The probability of significant icing is less than 10 percent for
Days 1-3.

Jackson