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Winter Weather Forecast Discussion
 
(Latest Discussion - Issued 0802Z Apr 25, 2014)
 
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Abbreviations and acronyms used in this product
 
Geographic Boundaries -  Map 1: Color  Black/White       Map 2: Color  Black/White


PROBABILISTIC HEAVY SNOW AND ICING DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
402 AM EDT FRI APR 25 2014

VALID 12Z FRI APR 25 2014 - 12Z MON APR 28 2014



DAYS 1 THROUGH 3...  

...WEST COAST/GREAT BASIN/NORTHERN AND CENTRAL ROCKIES...

AN AMPLIFYING LONG WAVE TROUGH MOVES FROM THE EASTERN PACIFIC
ONSHORE ACROSS CA DAY 1.  300 MB DIVERGENCE/700 MB CONVERGENCE
MAXIMA TRIGGER SNOW SHOWERS ACROSS THE BLUE MNTNS OF NORTHEAST
OR...THE SAWTOOTH AND BITTERROOTS IN IDAHO AND RANGES OF WESTERN
MONTANA...AND TETONS OF NORTHWEST WYOMING.

RAIN AND SNOW SPREAD SOUTHEASTWARD ACROSS CALIFORNIA WITH A WELL
DEFINED UPPER DIVERGENCE/LOWER CONVERGENCE COUPLET COMBINING WITH
UPSLOPE FLOW TO PRODUCE A PERIOD OF HEAVY SNOW ACROSS THE SIERRA
NEVADA RANGE OF CA.  THE HEAVIEST SNOW IS MORE LIKELY TO FALL
ACROSS THE SIERRA WITH 12 INCH AMOUNTS POSSIBLE IN HIGHER
ELEVATIONS. SNOW DEVELOPS DOWNSTREAM IN THE RANGES OF CENTRAL TO
NORTHEAST NV IN THE VICINITY OF THE 700 MB FRONT AND CONVERGENCE
MAXIMA.

MANUAL PROGS USED THE 12Z-00Z ECMWF AND 18Z GFS QPF/TEMPERATURE
PROFILES TO GIVE THE MOST WEIGHTING IN THE FORECAST DAYS 1-3.


ON DAY 2...WITH THE UPPER TROUGH EXITING CA AND CROSSING UT AND
CO...THE COUPLET OF UPPER DIVERGENCE/LOWER CONVERGENCE CONTINUES
IN ADVANCE OF THE UPPER TROUGH.  THIS CAUSES
SNOW ACROSS THE UTAH WASATCH RANGE...RANGES OF WESTERN CO AND ALSO
ACROSS THE MOUNTAINS OF NW WYOMING AND RANGES OF CENTRAL AND
WESTERN MONTANA. 

THE APPROACH OF A LOW AMPLITUDE 700 MB TROUGH/NEGATIVELY TILTED
UPPER TROUGH WITH EMBEDDED DIVERGENCE MAXIMA FROM THE NORTHEAST
PACIFIC INTO WESTERN WA/OR RESULTS IN THE COLUMN MOISTENING AND
SNOW SHOWERS DEVELOPING AS THE AIRMASS BETWEEN 850-600 MB ATTAINS
A HIGH RELATIVE HUMIDITY OF 90-99 PERCENT AND ASCENT DEVELOPS
850-600 MB.

BY DAY 3...THE DEEP TROUGH IS NEGATIVELY TILTED AND CONTAINS A
CUTOFF LOW AS IT SLOWLY MOVES TO THE NORTHEAST ONTO THE CENTRAL
HIGH PLAINS.  THE UPPER DIVERGENCE/LOWER CONVERGENCE MAXIMA EXITS
NORTHERN CO AND PERSISTS IN EASTERN WYOMING AND MONTANA...WITH THE
COLD FRONT IN EASTERN WY AND MT DRAWING CLOSE ENOUGH TO THE SD
BORDER TO ALLOW A CHANGE OVER TO SNOW IN THE BLACK HILLS.  THE
UNCERTAINTY RELATES TO TEMPERATURE PROFILES AS INITIAL CONDITIONS
ON THE HIGH PLAINS ARE TOO WARM FOR SNOW.  HEIGHT FALLS AND THE
COLD FRONTAL PASSAGE LEAD TO COOLING AND EVENTUAL CHANGE TO SNOW
ON PORTIONS OF THE EASTERN MT AND WY HIGH PLAINS.  CONSEQUENTLY
SNOW PROBABILITIES ARE HIGHER FOR AREAS WITH LONGER DURATION COLD
LIKE THE BLACK HILLS AND SNOWY RANGE OF WY.

IN ADDITION...A PERSISTENT JET MAXIMA EXTENDS ACROSS OR...WITH
MORE SNOW ACROSS THE WASHINGTON AND OREGON CASCADES AS POST
FRONTAL DEEP LAYER WESTERLY FLOW PERSISTS ALONG WITH CONTINUED
ASCENT 850-700 MB AND HIGH LAYER RELATIVE HUMIDITY. AS A MID-UPPER
RIDGE STARTS TO BUILD THE LATTER HALF OF THE PERIOD...BOTH THE
GFS/ECMWF INDICATE DEVELOPING UPPER SINKING MOTION TO DECREASE
SNOW SHOWER COVERAGE/INTENSITY THE LATTER HALF OF THIS PERIOD.

THE PROBABILITY OF SIGNIFICANT ICING IS LESS THAN 10 PERCENT DAYS
1-3.

PETERSEN