Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
332 PM EST Thu Nov 08 2018

Valid 00Z Fri Nov 09 2018 - 00Z Mon Nov 12 2018


...Great Lakes (Days 1 and 2)...

A sharply digging shortwave into the Upper Midwest will lead to
cyclogenesis in the Great Lakes from Friday into Saturday, with a
surface low moving from near Lake Erie into Ontario and Quebec.
This should lead to a transition from synoptically driven snow on
Friday, to lake effect snow on Friday Night and Saturday before
dry air advection and a surface high (and thus weakening winds)
settles into the region. Heavy snowfall appears most likely from
far northwest Wisconsin, into the Upper Peninsula of Michigan, and
far northern Lower Michigan. Those areas will be closer to the
track of the mid-level (700-500mb) low and a pronounced surface
trough should extend into the Lake Superior region northwest from
the incipient surface low on Friday and Friday evening. Therefore,
a low-level deformation zone is expected to persist in the same
areas for about 12 hours on Friday, with the potential for heavy
snow.

The heaviest snow bands should be near the cusp of the surface
trough, where the convergence is maximized and the pressure
gradient begins to increase. This will focus stronger snow bands
and be the leading edge of the cold air advection and transition
to lake effect snow. Hi-res models are explicitly forecasting
several hundred j/kg of lake-induced CAPE near the Lake Superior
adjacent portions of the UP of Michigan, this could allow for some
convective snow bands and heavier snow rates for a time. The
probability of exceeding 12 inches of snow from near Ironwood, up
to the Keweenaw Peninsula, and over to Marquette is over 25
percent. And the probability of exceeding 6 inches of snow is over
50 percent across most of the UP of Michigan, and northwest Lower
Michigan. Therefore, there is high confidence in heavy snowfall
and these probabilities have increased with this forecast issuance.

...Interior New England (Days 2 and 3)...

To the east of the developing low in the Great Lakes, significant
warm-air advection should lead to a broad area of precipitation in
the Northeast. Residual cold air across the region may be
sufficient for some snow initially, even at lower elevations, and
this is implied by most NWP models with a surface ridge extending
down the east side of the northern Appalachians. However, the
strength of the low-level jet should lead to most snow being
confined to higher terrain areas relatively quickly. Complicating
the snow forecast is an expected dry slot that should be punching
into New England on the southeast side of the low, and this may
lead to a short (6-12 hour) period of more significant
precipitation when low-mid level WAA is maximized. Furthermore,
models indicate the potential for riming and a possible changeover
to sleet near the end of that time window. The NAM shows the rime
factor increasing and percent of frozen precip decreasing just
ahead of the dry slot, and this may further reduce the time window
for accumulating snow to fall.

Nevertheless, there should be strong enough northward moisture
flux and low-mid level flow perpendicular to the terrain to
generate a burst of heavy snow. The greatest likelihood would be
from northern New Hampshire into northwest Maine in the higher
terrain areas, where the probability of over 6 inches of snow is
over 50 percent. However, it should be noted that some lower
amounts are still a reasonable possibility in some areas, as
implied by the positive snow depth change forecasts from the GFS,
NAM and 3km NAM, which are generally under 6 inches and account
for some melting and compaction.

...Northern and Central Rockies (Days 2 and 3)...

A fast-moving clipper shortwave should dig from the Canadian
Rockies into the Northern Plains spreading some snow into portions
of the Rockies in the United States. The highest probabilities of
heavy accumulating snow will be in the mountains of Montana, and
possibly extending down into the Big Horn Mountains of northern
Wyoming. The snowfall accumulations should be tied closely to the
highest terrain.


The probability for significant ice (0.25 inches or more) is less
than 10 percent outside of several valleys in interior New England.


Lamers