Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
349 AM EST Thu Nov 29 2018

Valid 12Z Thu Nov 29 2018 - 12Z Sun Dec 02 2018


...Western U.S....
Days 1-3...

On Day 1 (Thu), A potent shortwave and associated Pacific jet max
will move onshore and then across California and then into NV.  A
130 kt jet was observed at Oakland CA this evening.  The strong
jet is transporting anomalously high Precipitable Water of over +3
sigma into California.  The increasing moisture transport combined
with jet induced lift provides a high confidence forecast of heavy
snow in the CA Sierra Nevada Range that will be ongoing and then
spread inland into NV and the UT Uintas/Wasatch Thu night.

The initial wave crossing UT/AZ early Fri morning brings a period
of snow there before continuing downstream into NM/CO/WY and then
onto the Plains. Several inches of snow are likely in the
mountains of these states, but the progressive shortwave leads to
a low likelihood of 12 inches of snow on day 2.

Further west in CA/OR/WA on day 2 (Fri), the next upper trough and
jet maxima are forecast to stream onshore.  The NAM was
forecasting a 300 mb jet max of 130-150 kt by 00z Sat in northwest
CA, with related upper divergence maxima favoring lift in the
Cascades of southern OR and northwest CA, along with the Shastas. 
The jet builds southeast Fri night, extending the area of snow
into the northern CA Sierra NV.

So Fri night into Sat, the enhanced moisture and intense lift will
produce heavy snowfall in the Sierras of California, where another
foot of snow accumulation is possible, with more than 4 feet
possible during the 3 day period.

On Day 3 (Sat) the jet is forecast to continue building southeast
across the CA Sierra NV range and then southern NV and UT,
followed by AZ. As the jet builds south, upper divergence maxima
in the vicinity cross the ranges of southern UT into northern AZ,
along with 700 mb convergence maxima.  So several inches of snow
are possible in these areas, as well as near the upper low as it
drifts near the OR/NV border.

...Northern to North Central Plains Days 2/3 and Upper Ms Valley
Day 3...

A 700 mb wave moves east from WY/CO onto the central high plains
Fri afternoon, evening, with an embedded closed low developing and
moving east roughly along the KS/NE border by Sat morning. 700 mb
warm/moisture advection and convergence develops within the jet
axis with several inches of snow possible where it it is cold
enough for snow in north central NE to south central SD. Precip
type uncertainty exists further south in Nebraska as warm
advection presents the possibility of mixed precip types or a
change over to rain, limiting snow amounts.

On day 3 (Sat and Sat night), the low turns east northeast with
the consensus 700 mb low moving towards Iowa by Sun morning.
North/northwest of the low track, mid level frontogenesis and 700
mb theta-e advection and convergence support a band of heavy snow
extending from the Nebraska/SD border counties across northwest
Iowa and possibly southern MN and southern WI. Precip type
uncertainty exists south of this area in southeast NE to southern
IA and southern WI near the IL border.  QPF uncertainty exists
further north in northern SD to central MN and northern WI, with
tight gradients of liquid equivalent amounts given drier air to
the north.

Increasing 700mb deformation/frontogenesis and a developing TROWAL
will produce intense snow rates, and a stripe of heavy snow is
likely in southeast SD/northeast adjacent NE, where potential is
there for a foot of snow. The 12-00z ECMWF, 00z NAM, 00z UKMET,
and 00z GFS FV3 show a foot of snow with some north-south variance
in location. As the circulation drifts northeast, the snow extends
downstream into adjacent northern IA/southern MN and possibly
portions of WI.  WPC probabilities have increased, and show a
moderate risk for 4 inches, with low probabilities now existing
for 8 inches, of snow accumulation during Day 3.

Warm air advection near the low level circulation presents the
possibility of a mixture with
sleet/freezing rain in northeast NE across northwest IA and
adjacent MN.  WPC probabilities show a slight risk for 0.1 inches
of accretion, focused over southern MN/northwest IA/southeast SD.

Further northwest in eastern MT and WY, the lingering low-mid
level front and 700 mb trough provides convergence in confluent
flow, with light-moderate snow Sat afternoon and night. Several
inches are possible due the slow movement of the boundary.

...Upstate NY/Western New England Sat/Sat night...
Areas of icing are possible in elevated terrain from the NY
Catskills to the Adirondacks, Berkshires, and Green Mountains of
VT as a surge of mid level warm/moist advection moves across these
ranges while surface subfreezing cold air remains in place.  Once
the mid levels warm above freezing, sleet and freezing rain are
expected before the surface warms and the precip becomes all rain.
Higher freezing rain amounts in the NAM and ECMWF led to a low
risk of significant (quarter inch) icing on Day 3.

Petersen