NWS Snow Discussion

Current Discussion
000 
fous11 kwbc 302058
qpfhsd

Probabilistic heavy snow and icing discussion
NWS weather prediction center College Park MD
457 PM EDT Mon Mar 30 2015

Valid 00z Tue Mar 31 2015 - 00z Fri Apr 03 2015


Days 1 to 2...

...Great Lakes/northeast...

A relatively compact and fast moving shortwave system will amplify
somewhat as it moves east southeastward through Tuesday evening.
As a surface low tracks across the Midwest and Middle Atlantic
States...a band of rain and snow will fall to its north and east.
Since this system is occurring on the last day of March...there
will be many factors influencing where and how much snow will
accumulate.  While temperatures throughout much of the atmosphere
appear to support the snowfall...as evidenced by relatively low
thickness values...boundary layer temperatures and the insolation
from late March sunlight could work against significant
accumulations.  Conversely...relatively high snowfall rates and
intensity even over short periods could also play a significant
role promoting accumulations. It is not necessarily clear which
factors will be most important from Michigan across Pennsylvania
and northern New Jersey.  If snow is light...much of it will not
accumulate. If snow falls heavily...temperatures will cool and
snow could add up quickly.  While neither of these scenarios
necessarily favor one over the the other...the forecast here is
based on the potential for some significant accumulations since it
is possible that snowfall may fall relatively quickly...promoting
accumulations at least on non Road surfaces.

While there is some good agreement with the morning model
runs...the NAM/Canadian are most bullish with qpf with greater
than .75 inches across a more narrow strip of central lower
Michigan and northern Pennsylvania while maximum amounts in the
12z ECMWF/GFS are around .5 inches.  To arrive at a compromise
solution using a variety of input from several models...we arrive
at a low to moderate probability for greater than 4 inches across
central lower Michigan and across northern Pennsylvania.  The lack
of a high probability likely relates to the uncertainties
mentioned above.

Days 1 to 3...

...Pacific northwest/northern rockies/central rockies...

A broad upper level trough with multiple embedded shortwaves will
move eastward from the Pacific coast over the next 3 days with
dropping snow levels and Pacific moisture streaming inland.  On
day 1/Monday night through Tuesday...snowfall accumulations
exceeding 4 inches will be confined to the higher elevations of
the Washington and Oregon Cascades. As colder air spreads eastward
across the northwest on day 2/Tuesday night into
Wednesday...heavier snow spreads across the mountains of Idaho
into southwestern Montana into northwestern Wyoming...as well as
the Washington Cascades. 

A separate shortwave crosses the central Pacific coast on
Wednesday and begins to amplify southeastward toward The Four
Corners area setting the stage for some significant snow
developing along the foothills of the Colorado rockies where there
is the potential for at least 4 to 8 inches of snow.

The probability of significant icing is less than 10 percent.

Kocin

$$



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