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
344 PM EDT SAT OCT 25 2014

VALID 00Z SUN OCT 26 2014 - 00Z WED OCT 29 2014


DAYS 1 THROUGH 3...

...PACIFIC NORTHWEST/NORTHERN AND CENTRAL ROCKIES...

A DEEP LAYER LONGWAVE TROUGH IS FORECAST TO CROSS THE PACIFIC
NORTHWEST AND NORTHERN ROCKIES ON DAY 1...AND THEN ACROSS THE
NORTHERN PLAINS AND EXITING THE CENTRAL ROCKIES TO THE CENTRAL
PLAINS ON DAY 2.
THE FORECAST WAS DERIVED FROM A BLEND OF THE 00Z ECMWF/12Z NAM/12Z
GFS QPF AND TEMPERATURE PROFILES.

ON DAY 1...THE STRONGEST MOISTURE FLUXES AND ANOMALIES CROSS WA
STATE AND THEN NORTHERN ID AND NORTHWEST MT.
PRE-FRONTAL 700 MB CONVERGENCE/300 MB DIVERGENCE OCCURS AHEAD OF
THE WAVE...RESULTING IN ASCENT ACROSS THE NORTHWEST AND NORTHERN
ROCKIES.  
SEVERAL INCHES ARE POSSIBLE ACROSS THE NORTHERN CASCADES OF
WA......SOUTHERN CASCADES OF OREGON...THE SEVEN DEVILS AND
SAWTOOTH RANGES OF ID...AND THEN INTO THE RANGES OF NORTHWEST
WY/ADJACENT SOUTHWEST MT ON SUNDAY.
SNOW CONTINUES EARLY DAY 2 IN WY AS THE UPPER TROUGH CONTINUES TO
MOVE EAST...WITH A SECONDARY AREA IN THE RANGES OF CENTRAL TO
NORTHERN ID IN DIFLUENT FLOW ALOFT AS THE 300 MB JET MAXIMA
CROSSES WA AND OR AND THEN ID ON MON.
LOWERING SNOW LEVELS AND PERSISTENT UPSLOPE FLOW WILL PRODUCE
AREAS OF LIGHT SNOW ACROSS THE HIGHER TERRAIN OF NORTHWEST WY. 

ON DAY 3...THE UPPER RIDGE PROGRESSES INLAND ACROSS THE NORTHWEST
AND NORTHERN ROCKIES...FOLLOWED BY ZONAL WEST TO EAST ALOFT. 
THE 300 MB JET MAX FCST BY THE GFS/ECMWF TO 110 KT PRODUCES
DIVERGENCE IN THE LEFT JET EXIT REGION ACROSS THE OLYMPICS AND
CASCADES.
STRONG 700 MB THETA-E ADVECTION AND CONVERGENCE ON THE NOSE OF A
50 KT JET MAXIMA NEAR THE WA/OR COAST HELPS TO PRODUCE WIDESPREAD
ASCENT.
HEIGHT FALLS LATER IN THE PERIOD START TO RESULT IN DECREASING
SNOW LEVELS ONCE THE WARM ADVECTION CEASES.
THE PRIMARY UNCERTAINTY IN THE SNOW FORECAST IS THE INITIAL BURST
OF WARM ADVECTION RAISES SNOW ELEVATIONS TO NEAR THE PEAKS OF THE
CASCADES...SO CHANCES ARE A TRANSITION TO RAIN OCCURS AT LOWER
ELEVATIONS. 


THE PROBABILITY OF SIGNIFICANT ICING IS LESS THAN 10 PERCENT.

PETERSEN





Last Updated: 344 PM EDT SAT OCT 25 2014