THE WEATHER PREDICTION CENTER

COLLEGE PARK, MD


PROBABILISTIC HEAVY SNOW AND ICE DISCUSSION




PROBABILISTIC HEAVY SNOW AND ICING DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
409 AM EST FRI NOV 28 2014

VALID 12Z FRI NOV 28 2014 - 12Z MON DEC 01 2014

DAYS 1 TO 3...

...PACIFIC NORTHWEST/NORTHERN ROCKIES/NORTHERN PLAINS/UPPER
MIDWEST...

OVER THE NEXT 3 DAYS...THERE WILL BE A SHIFT IN THE UPPER LEVEL
PATTERN AS HEIGHT FALLS OUT WEST HELP TO REPLACE A DEEP LAYER
RIDGE THERE WITH A TROF.  IN ADDITION...ONE STRONG SHORTWAVE
TROUGH DROPPING SOUTH AND EAST FROM BRITISH COLUMBIA WILL MOVE
ACROSS THE NORTHWEST PORTION OF THE COUNTRY...CREATING THE
POTENTIAL FOR HEAVY SNOW IN THE NORTHWEST MOUNTAINS.

THE HEAVY SNOW ON DAY 1/FRI WITH AMOUNTS OF 1-2 FEET OF SNOW WILL
BE LIKELY ACROSS THE ROCKIES IN NW MONTANA. THE HEAVY AMOUNTS
RESULT FROM THE LOW LEVEL JET MAX AT 700 MB CROSSING THE REGION
WITH A COUPLET OF 300 MB DIVERGENCE/STRONG 700 MB CONVERGENCE AND
ALSO UPSLOPE FLOW. THE PRIMARY UNCERTAINTY IS IN RANGES FURTHER
WEST AS WARM AIR FROM THE PACIFIC MOVES ONSHORE AND
INLAND...KEEPING SNOW ELEVATIONS HIGH AND LIMITING SNOW AMOUNTS IN
THE LOWER ELEVATIONS OF THE OR CASCADES.

A WAVY COLD FRONT DROPS SOUTH AS THE MID-UPPER TROUGH AMPLIFIES
ACROSS THE NORTHWEST.  ON DAY 2...THE UPPER JET MAX CROSSES ID
INTO NORTHWEST WY AND ADJACENT MT.  AS IT SINKS SOUTH ACROSS
ID...DRIER AIR WILL ADVECT INTO THE RANGES OF IDAHO...LEADING TO A
DECREASING TREND IN SNOW COVERAGE/INTENSITY.
STRONG 700 MB CONVERGENCE LEADS TO A SECONDARY MAXIMA IN SNOW OVER
NORTHWEST WY.

ON DAY 3...THE LOW LEVEL FRONT SAGS SOUTH TOWARDS THE CO ROCKIES
AND UT WASATCH.  UPPER DIVERGENCE/LOWER CONVERGENCE MAXIMA EARLY
IN THE PERIOD LEADS TO SNOW MAXIMA IN THE FAVORED UPSLOPE AREAS OF
THE MOUNTAINS OF NORTHERN CO AND NORTHERN UT.  A WEAK UPPER TROUGH
CROSSES THE NORTHERN AND CENTRAL SIERRA...BUT IS FOLLOWED BY AN
AMPLIFYING UPPER RIDGE AND DRYING IN THE -12 TO -18C LAYER.  THIS
SHOULD MAKE IT DIFFICULT TO ACHIEVE SNOW GROWTH THE LAST SEVERAL
HOURS OF DAY 3/MON...SO POTENTIAL FOR HEAVY SNOW IS LIMITED.  

MANUAL PROGS USED GREATER WEIGHTING TOWARDS THE 12-0Z
ECMWF...GFS...AND 21Z SREF MEAN FOR QPF AND TEMPERATURE FORECASTS
USED TO DERIVE THE SNOW FORECASTS.

THE PROBABILITY FOR SIGNIFICANT ICING IS LESS THAN 10 PERCENT.

PETERSEN





Last Updated: 409 AM EST FRI NOV 28 2014