Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
353 PM EDT Tue Oct 16 2018

Valid 00Z Wed Oct 17 2018 - 00Z Sat Oct 20 2018


Days 1 through 3...

...Southwest/Great Basin/Central and Southern Rockies...

Lift associated with a closed mid level low over central AZ
opening up and exits the Rockies, combined with upslope flow,
produces locally heavy snowfall over the Central Rockies during
Day 1. Meanwhile, shallow cold air lingering over NM poses a
threat for icing during the same period. After the mid level
system exits, upslope flow leads to generally light snowfall
during Days 2 and 3. There was very good model agreement
concerning the mid level system, so the thermal portion of the
forecast was based on a multi model blend. The QPF portion of the
forecast was based on the most recent WPC QPF.

Day 1...
The closed mid level low tracks slowly from central AZ to
northwest AZ/southwest UT during Day 1. Lift associated with the
mid level system (as well as steepening lapse rates, which could
result in convective elements in the precipitation shield)
interacts with moisture in the southeast upslope flow over the
Central Rockies and Great Basin. The best lift is expected over AZ
into southern Wasatch Mountains between 17/06z and 17/12z, where
snow levels are expected to hover between 7000 and 8000 feet.
There should be sufficient moisture in the upslope flow to support
local 3 to 6 inch snowfall amounts across the higher terrain of
the southern Wasatch range in UT.

Further east, the best lift with the closed mid level low occurs
closer to 18/00z, when the mid level system begins the process of
opening up into a positively tilted long wave trough. The upslope
flow over southwest CO coincides with the best mid level lift,
supporting the potential for 4 to 8 inches of snowfall over the
higher terrain of the San Juan Mountains in southwest CO. It
should be noted that there has been a gradual shift in the
placement of the highest snowfall amounts with exiting mid level
system, so snowfall amounts over southwest CO have been reduced
due primarily to the shift in the model placement of QPF.

Finally, shallow cold air remains in place across much of central
and southern NM during Day 1. Model soundings showed that the cold
air was less than 1000 feet deep in some places, as increasing
warm advection occurs above the inversion. There is a multi model
signal (which includes the 12z NBM) which should local 0.10 inch
ice accumulations over south central NM, before the column warms
enough to diminish the icing threat after 17/15z.

Day 2...
The best lift with the exiting mid level system occurs across the
Central Rockies during the first part of Day 2 (generally before
19/00z) across eastern UT and southwest CO. Snow levels, generally
between 7000 and 8000 feet during the first part of Day 2, begin
to rise as mid level heights rise. The rising snow levels should
keep the 3 to 6 inch snowfall amounts relegated to elevations
above 8000 feet, with a phase change expected below this level.

Residual shallow cold air across portions of south central NM
could result in spotty light freezing rain or freezing drizzle
during the first part of Day 2. Most of the 12z guidance showed
light icing amounts here, but accumulations should remain below
0.10 inches.

Day 3...
The mid level system lifts into the Northern Plains during Day 3.
Behind the mid level system, a broad low to mid level northwest
flow results in upslope across the Central Rockies. Since moisture
becomes scant in the northwest flow, snowfall amounts should
remain below 4 inches across the higher terrain of southwest CO.


...Great Lakes/New England...

A long wave trough crossing the Great Lakes and New England during
Days 1 and 2 provides sufficient lift for generally light snowfall
over the higher terrain. There was very good model agreement with
the overall synoptic setup, so the thermal portion of the forecast
was based on a multi model blend. The QPF portion of the forecast
was based on the most recent WPC QPF.

Day 1...
A strong positively tilted long wave trough tracking from the
Great Lakes into northern New England provides an environment
conducive to the development of lake effect circulations during
Day 1. The northwest flow over Lake Superior is expected to
produce banded precipitation over the UP of MI. Since boundary
layer temperatures should be near critical (especially during day
daytime hours), accumulations of 2 to 4 inches are expected mainly
above 1000 feet. Further east across the lower Great Lakes,
conditions become better able to support lake effect circulations
during the second half of the period. However, boundary layer
temperatures are expected to remain mostly above freezing, so
light snowfall amounts are expected only in the higher terrain
(once again, generally above 1000 feet).

Day 2...
The mid level trough takes on a negative tilt as it crosses New
England during the first half of Day 2. The low to mid level flow
supports lake effect circulations over the lower Great Lakes, but
once again boundary layer temperatures should limit the areal
extent of accumulating snow. Snowfall amounts of less than 3
inches are expected in the higher terrain east of Lake Ontario.

Further east, the northwest flow becomes more conducive for
upslope snow over portions of northern New England during the
second half of Day 2. The column is fairly dry, with the best
moisture in the upslope flow expected over the higher terrain of
western ME. Local 1 to 3 inch are possible here.

The probability of significant icing is less than 10 percent.

Hayes