Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
350 PM EDT Tue Oct 09 2018

Valid 00Z Wed Oct 10 2018 - 00Z Sat Oct 13 2018


Days 1 through 3...

...Central Plains into the Upper Mississippi Valley...

A negatively tilted long wave trough tracks from the Southern
Plains toward the Upper MS Valley during Day 1, spinning up
surface low pressure on a frontal boundary over the Central
Plains. The combination of deepening moisture and lowering heights
poses the threat for locally heavy snowfall, especially over
portions of the Northern Plains and Upper MS Valley. The thermal
portion of the forecast was based on a multi model blend, while
the QPF portion of the forecast was based on the most recent WPC
QPF.

Day 1...
An area of enhanced lift in the 700-500 mb layer ahead of a
negatively tilted trough moving from the Southern Plains toward
the Upper MS Valley should result in surface low pressure forming
on a frontal boundary over the Southern Plains. As the surface low
tracks northeast, there is a multi model signal for an area of mid
level frontogenetic forcing to form from west central NE across
central SD into southeast ND, mainly between 10/06z and 10/12z
(from southwest to northeast). Model soundings along this axis
showed moisture and lift in the dendritic growth zone, which could
result in banded precipitation on this axis.

Temperatures in the 925-850 mb layer suggest a tight snow/rain
gradient on the front side of the precipitation shield, with rain
changing to snow as colder air becomes involved in the lower level
circulation. A long axis of 4 to 8 inches was stretched from north
central NE across central ND into southeast ND. These amounts are
supported by the 12z HREF snowfall mean, as well as members of the
00z ECMWF ensemble output showing 8+ inch snowfall along this axis
as well.

Toward the end of Day 1 (11/00z), the best lift crosses northwest
MN. Model soundings here also indicated the potential for moisture
and lift in the dendritic growth zone, though the mid level
forcing is not as robust as further southwest.

Day 2...
The best lift associated with the deepening surface low and and
mid level trough crosses the northern MN during the first part of
Day 2 (mainly between 11/00z and 11/06z). Model soundings
indicated moisture and lift in the dendritic growth zone, which
could support local 4 to 6 inch snowfall amounts here. It should
be noted that the several members of the 00z ECMWF ensemble output
showed the potential for 8+ inches of snowfall over northern MN.
This much snowfall is possible should the main frontogenetic
forcing linger longer than currently expected.

The probability of significant icing (quarter inch or more) is
less than 10 percent

...Rockies/Great Basin...

Short wave energy moving from the Great Basin into the Northern
Rockies during Days 1 and 2 provides sufficient lift for locally
heavy snowfall. After the short wave lifts out, upslope flow along
and ahead of a frontal boundary banked up against the Rockies
could result in generally light snowfall amounts on Day 3. The
thermal portion of the forecast was based on a multi model blend,
while the QPF portion of the forecast was based on the most recent
WPC QPF.

Day 1...
Short wave energy tracking from ID/NV early in the period crosses
WY by the end of the period. The best mid level lift occurs early
in the period (generally before 10/12z) across northern UT into
southwest WY. The lift interacts with enough moisture in the
column for snowfall, and model soundings across portions of UT
into southwest WY and the CO Rockies moisture and lift in the
dendritic growth zone. Snow levels drop to as low as 6500 to 7500
feet with the passage of the short wave. There is enough QPF to
support an area of 6 to 12 inches of snowfall over the northern
Wasatch and Uinta Mountains, with lesser amounts across the
southern Wasatch in UT, as well as the CO Rockies. The exception
here is the San Juan range in CO, where upslope flow could produce
local 8 inch snowfall amounts. All of these values are supported
by members of the most recent ECMWF ensemble output.

Day 2...
As the short wave pulls away during Day 2, the best mid level lift
exits before 11/12z. However, upslope flow associated with a
frontal boundary banking up against the Rockies from northwest WY
into central CO makes the most of the moisture in the column to
support local 4 to 6 inch snowfall amounts over the Grand Tetons
in WY, as well as the higher peaks of the CO Rockies. These
amounts are well supported by members of the most recent ECMWF
ensemble output.

Day 3...
The column dries during Day 3, which limits the effectiveness of
the continued upslope flow, focused by the frontal boundary
extending from western MT into CO. Much of the guidance showed
local 1 to 3 inch snowfall amounts, mainly across the CO Rockies.

The probability of significant icing (quarter inch or more) is
less than 10 percent

Hayes