Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
402 PM EDT Mon May 06 2019

Valid 00Z Tue May 07 2019 - 00Z Fri May 10 2019


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

Closed mid-level low across Southern California will open and
eject northeast over the Four Corners Tuesday before pushing into
the Great Lakes Wednesday/Thursday. Following quickly, two
northern stream shortwaves will drop southward from British
Columbia late Tuesday and again early Thursday, maintaining broad
cyclonic flow within a positively tilted tough across the West.
This will persist waves of moist advection and synoptic ascent
embedded within periodic jet maxes lifting to the northeast. Given
the orientation of this trough, persistent moist advection will
drive PWATs to +1 to +2.5 standard deviations above climo at times
through the week. Snow levels will initially be quite high,
7000-9000 ft, but will drop to 4000-6000 ft D2/D3 as the front
banks against the east side of the mountains in advance of cold
high pressure sinking southward from Canada. 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
NW Wyoming on day 1, the Front Range of CO on day 2, and the San
Juans/Uintas/Sangre De Cristos on day 3. The highest probabilities
for 12 inches exist across the aforementioned day 3 areas, where a
moderate risk exists. Lower probabilities exist on days 1 and 2.
Additionally, modest probabilities for 4 inches exist in the High
Plains of WY, NE, and SD, especially the Black Hills, on day 2/3.


...Western Great Lakes...

An area of low pressure will move northeast 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
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 U.P. of MI, which has caused a
slight uptick in snowfall accumulation forecasts across the area.
WPC probabilities have increased to 30 percent for the U.P., but
predominantly accumulations should be less than 4 inches, and most
efficient on grassy and elevated surfaces.


The probability of significant icing is less than 10 percent.

Weiss