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Winter Weather Forecast Discussion
 
(Caution: Version displayed is not the latest version. - Issued 0815Z Apr 18, 2014)
 
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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
415 AM EDT FRI APR 18 2014

VALID 12Z FRI APR 18 2014 - 12Z MON APR 21 2014

...PACIFIC NORTHWEST AND NORTHERN ROCKIES...

TWO SHORTWAVE TROUGHS WILL INFLUENCE THE REGION THE NEXT SEVERAL
DAYS...ONE EARLY FRIDAY FOLLOWED BY ANOTHER TROUGH SATURDAY
NIGHT/SUNDAY.  THE FIRST TROUGH QUICKLY PROGRESSES ACROSS THE
NORTHERN ROCKIES TODAY WITH 300 MB DIVERGENCE TRIGGERING A BRIEF
PERIOD OF ASCENT IN ADVANCE OF THE UPPER TROUGH.  HOWEVER WITH THE
TROUGH DEPARTING AND THE LOSS OF THE UPPER JET AND DRYING THROUGH
A DEEP LAYER LIMIT THE EVENT DURATION...KEEPING POTENTIAL AMOUNTS
MINIMAL. 
THE NEXT SHORTWV SHOULD RESULT IN SOME SNOW IN THE HIGHER
ELEVATIONS OF THE WASHINGTON CASCADES ON DAY 2.  THE NEGATIVELY
TITLED UPPER TROUGH MOVES ONSHORE FROM THE NORTHEAST PACIFIC AND
CROSSES THE CASCADES ON DAY 2. UPPER DIVERGENCE MAXIMA IN ADVANCE
OF THE TROUGH AXIS AND LOW LEVEL MOISTURE ADVECTION RESULT IN A
PERIOD OF PRECIP WITH SNOW IN THE WA CASCADES UNTIL THE TROUGH
PASSES AND ANOTHER PERIOD OF DRYING ALOFT DEVELOPS IN THE REGION. 
THE MODELS ARE IN GOOD AGREEMENT ON THE 700 BM WAVE DEAMPLIFYING
AS IT MOVES ONSHORE AND INLAND...WEAKENING CONVERGENCE SIGNATURES
ACROSS THE REGION WITH TIME.  THE 00Z NAM WAS OUT OF PHASE WITH
OTHER SOLUTIONS AND THUS GIVEN LESS WEIGHTING THAN THE 12-00Z
ECMWF/GFS/UKMET/21Z SREF MEAN QPF/TEMPERATURE PROFILES.  

...NORTHERN MN...

THE MODELS SHOW POTENTIAL FOR A ROUND OF MIXED PRECIP ACROSS
NORTHERN MN.  SURFACE WET BULB TEMPERATURES SHOULD INITIALLY BE AT
OR BELOW FREEZING.  STRONG WARM ADVECTION IN THE LOW TO MID LEVELS
ACCOMPANIED BY INCREASING MOISTURE RESULT IN SNOW DEVELOPING FRI
NIGHT OVER NORTHWEST MN AND THEN SAT MORNING OVER THE ARROWHEAD OF
MN.  RAPID WARMING TO AND ABV FREEZING IN THE 850-700 MB LAYER
RESULTS IN SNOW QUICKLY CHANGING TO SLEET AND FREEZING RAIN.  THE
STEADY FRONTAL PROGRESSION RESULTS IN THE MIXED PRECIP EVENT
WINDING DOWN AS WARMING AND DRYING OCCUR ALOFT.

GIVEN THE SPLIT IN PRECIP TYPES AND PRECIP OCCURRING HALF OR LESS
OF THE PERIOD...A SLIGHT RISK OF SIGNIFICANT FREEZING RAIN WAS
SHOWN EACH PERIOD. THE 00Z GFS AND 12Z ECMWF SHOW LESS FREEZING
RAIN THAN THE NAM AND WERE PREFERRED...GIVEN HOW QUICKLY
TEMPERATURE AND MOISTURE PROFILES CHANGE AND LEAVE A LIMITED
PERIOD WHEN BOTH ARE FAVORABLE FOR FREEZING RAIN.

THE PROBABILITY FOR SIGNIFICANT ICING IS LESS THAN 10 PERCENT DAY
3.

PETERSEN