NWS Snow Discussion

Current Discussion
000 
fous11 kwbc 260908
qpfhsd

Probabilistic heavy snow and icing discussion
NWS weather prediction center College Park MD
407 am EST Mon Jan 26 2015

Valid 12z Mon Jan 26 2015 - 12z Thu Jan 29 2015

Days 1 to 3...

...Central Appalachians/mid-Atlantic/New York/New England...

A shortwave trough and associated area of low pressure moving east
from the Ohio/Tennessee valleys crosses the central Appalachians
and weakens with a new storm forming off the mid Atlantic coast
this afternoon. This new low intensifies tonight into Tuesday as
it heads towards the 40n 70w benchmark. The ECMWF and NAM show a
little slowing motion with even the possibility of looping in this
area before weakening Wed and moving northeast towards the
Canadian Maritimes.

The models show an inverted low level trough persisting today in
western PA
into the central Appalachians.  Low level convergence near this
trough results in a swath of 4-6 inches near the mountains near
the WV/MD border into western PA to NY border.  

The latest models though are insisting that this storm will evolve
into a high impact winter storm/blizzard Monday night and Tuesday
for portions of the northern mid-Atlantic...eastern New York and
southern and eastern New England. This includes major metropolitan
areas along the I-95 corridor from Philadelphia through New York
City and up to Boston. The models depict a powerful deformation
zone developing and spreading inland in response to rapidly
deepening low pressure off the mid-Atlantic coast.

Southern and eastern New England are expected to see very intense
snowfall as very strong frontogenetic forcing...Atlantic moisture
transport and upper jet dynamics combine for intense snowfall
rates.  Aside from the favorable synoptic scale set-up...the moist
onshore east northeast flow favors orographic enhancement in the
terrain of western CT to the hills west of Boston. 

Mesoscale banding features are expected within the larger area of
heavy snow to result in local enhancements. There is a likelihood
for blizzard conditions to impact many areas from northern New
Jersey north across southeast New York and southern and eastern
New England.

Storm total amounts of 1 to 3 feet can be expected with the
heaviest amounts generally from New York City across southern and
eastern New England.  Several models and ensemble runs indicate
eastern MA and RI as having the highest totals. The
models/ensemble mean have trended a few inches lower on the
northern fringe of the snow area across northern NY/northern New
England.

The GFS and 00z gefs mean had weaker and more progressive cyclone
tracks vs the slower NAM and ECMWF.  Manual progs favored the 00z
ECMWF/00z NAM/21z sref mean/12z and 00z ECMWF ensemble mean qpf
and temperature profiles.


...Sierra-Nevada/Great Basin/rockies...

An upper low that is currently situated just west of the Baja
Peninsula is forecast to lift north today in response to an
upstream trough digging across the eastern Pacific. As the upper
trough moves north across Southern California tonight a divergence
maxima aloft is expected to cross Southern California into the
southern Sierra range. Low level southeast winds transport
moisture into the mountains.  Once moistening occurs...snow
showers should develop MN night and continue early Tue. 
There should be several inches of snow for the higher elevations
of the southern to central Sierra-Nevada.

The upper trough will shear downstream later Tuesday and Wednesday
across the Great Basin and into the north-central rockies. The
energy aloft coupled with favorable mid level moisture transport
and upslope flow will favor snowfall for the northern portions of
the Wasatch and into The Tetons. The western Colorado high country
will also see accumulating snow. Snow amounts locally are expected
to exceed 6 inches for these areas.  Manual progs used continuity
and a blend of the 00z models and 21z short range ensemble mean.

The probability of 0.25 inch or greater icing is less than 10
percent.

Petersen

$$



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