Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
354 AM EST Tue Jan 29 2019

Valid 12Z Tue Jan 29 2019 - 12Z Fri Feb 01 2019


...Great Lakes...

Heavy lake effect snows are expected downstream from Lake Superior
in the UP of MI and Lake Michigan in the western part of lower MI,
spreading downstream to areas off Lakes Ontario in the Tug Hill
and off Lake Erie in Buffalo and the south towns over the next few
days.
Bands of a couple of feet of snow are possible where the most
persistent activity sets up in the lee shore snow belts. 
Probabilities are highest off the east end of lake Ontario due to
sustained fetches along the major axis of the lake, enhanced by
orographic lift in the Tug Hill.
 
Westerly winds will support heavy totals farther south as well
along the eastern shores of Lake Michigan downstream from where
the lake is wider.  Periods of snow are expected to continue to
impact portions of Michigan through Wednesday before waning on
Thursday. It may take until Friday for the event to wind down off
Lake Ontario.

...Southern Appalachians to the Mid Atlantic...

A strong cold front associated with the large amplifying upper
trough in the East is forecast to push southeast into the southern
Appalachians around daybreak and then across th mountains into the
Piedmont as the day progresses.  1-3 inch snow accumulations are
likely in the southern to central Appalachians, with the potential
locally heavier amounts of 4 inches across portions of the region.
The fast progression of the 700 mb wave limits the duration of
lift when it is cold enough for snow. 

As the front moves east of the mountains, expect the changeover to
occur later during in the day, resulting in some light
accumulations but with potentially significant impacts along the
central to northern Mid-Atlantic I-95 corridor during the
afternoon and early evening hours.  Snow amounts gradually
increase heading north into northeast PA in the Poconos due to a
longer duration when it is cold enough for snow. 

...Northeast...

Low pressure developing along the cold front is expected to
accentuate amounts as it lifts northeast from the northern
Mid-Atlantic into New England Tuesday evening.  Portions of   the
Catskills, Adirondacks, and Berkshires, southern Green and White
Mountains may see significant snow accumulations by Tuesday
evening, with WPC probabilities indicating a High Risk for amounts
of 4 inches or more on Day 1 and moderate or higher risks for 8
inches in most ranges.  The models have lowered snow accumulations
further north towards the NY/Canadian border, where downslope flow
off the Adirondacks should lower amounts. 

This wave will continue to lift north through northern New England
Wednesday morning, supporting significant snow amounts across
interior Maine, where several additional inches are possible.  The
snows wind down as low pressure departs north out of the state Wed
evening.

...California and the Southwest Days 2-3...

A compact upper low dropping southeast from the eastern Pacific
into Southern California is expected to produce some locally
heavy, high elevation snows along the Sierra on Thursday.
The slow arrival leads to primarily precip in the coastal ranges
Wed night where it is too warm for snow.  On Thu as the 700 mb
wave and low progress towards the southern CA coast, so the
enhanced moisture and lift is focused on the higher terrain of the
ranges of southern CA and southern Sierra Nevada mountains, where
several inches of snow are possible.  The threat ends with the 700
mb wave moving out of CA into the southwest, with light snow
possible in the higher elevations of southern NV, UT, and AZ as
the wave approaches Fri.

...Mid MS Valley to the Ohio Valley Day 3...

The models show a mid level axis of confluent flow with a
combination of 700 mb relative humidity over 90 percent and ascent
from modest frontogenesis supporting an area of light snow
expected to develop from the mid MS Valley and steadily move
downstream across the Ohio Valley, possibly reaching the central
Appalachians by Fri morning.  The ECMWF/GFS/UKMET show under a
quarter inch liquid equivalent precipitation for the 24 hour
period, so a general 1-3 inch snowfall is expected.

The probability for significant icing is less than 10 percent
nationwide days 1-3.

Petersen