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Winter Weather Forecast Discussion
 
(Caution: Version displayed is not the latest version. - Issued 0808Z Oct 21, 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
408 AM EDT TUE OCT 21 2014

VALID 12Z TUE OCT 21 2014 - 12Z FRI OCT 24 2014


...DAY 1...

THERE IS A RISK OF SNOW AT THE HIGHER ELEVATIONS OF THE
BITTERROOTS INTO THE NORTHERN ROCKIES AS COUPLETS OF UPPER
DIVERGENCE AND LOWER LEVEL CONVERGENCE COMBINE WITH THE EVENTUAL
ONSET OF LOWERING LOW/MID LEVEL HEIGHTS/THICKNESS VALUES.  SNOW
LEVELS SHOULD START PRETTY HIGH BUT SETTLE TO AROUND 5000 FT OVER
SOUTHWEST MT...WHICH WAS WHERE THE ENSEMBLES INDICATED ENOUGH
SUPPORT TO WARRANT SEVERAL SPOTTY SLIGHT RISK AREAS AT THE 4 INCH
THRESHOLD.

MODEL DIFFERENCES LINGER...WHICH PROPAGATES INTO THE FORECAST IN
THE FORM OF SUBTLE BUT IMPORTANT DIFFERENCES IN THE THERMAL FIELD.
 GIVEN MORE RUN TO RUN CONSISTENCY BY THE GFS...GAVE IT MORE
WEIGHT BUT INCLUDED SOME OF THE COLDER NAM THINKING THAT THE GFS
WAS A BIT TOO WARM.

ANOTHER ELEMENT ADDING UNCERTAINTY WILL BE HOW MUCH SNOW CAN
ACCUMULATE ON THE WARMER SURFACES.  THINK THE AMOUNTS WILL BE
LIMITED BUT THAT COULD CHANGE IF THERE IS A PERIOD OF STRONG
MESOSCALE FORCING WHICH RESULTS IN SIGNIFICANT ACCUMULATIONS IN A
SHORT TIME. GIVEN PERSISTENT AND DEEP SOUTHWEST FLOW...DO NOT
EXPECT IT TO BE A MAJOR PLAYER BUT IT WILL BE POSSIBLE.

THE PROBABILITY OF SIGNIFICANT ICING IS LESS THAN 10 PERCENT.

...DAYS 2 AND 3...

THE APPROACH OF ANOTHER SHORTWAVE WILL BRING A SECOND PERIOD OF
SNOW...THIS TIME THE BEST SUPPORT LOOKS TO BE IN THE NORTHERN
CASCADES OF WA GIVEN COLD ADVECTION AND SOME DYNAMIC COOLING
COMBINE FORCES.

THE DEEP LAYER SOUTHWEST FLOW IS FORECAST TO CONTINUE ACROSS THE
PACIFIC NORTHWEST...SO COLD ADVECTION WILL BE SLOW TO DEVELOP AS
HEIGHTS ARE SLOW TO LOWER WHICH IS WHY WE STILL EXPECT THE PRIMARY
THREAT FOR MODERATE TO HEAVY SNOW IN THE U.S. TO BE CONFINED TO
THE NORTHERN CASCADES IN WA.  IN ADDITION...THERE SHOULD BE
ANOTHER SURGE OF WARM/MOISTURE ADVECTION IN THE LOW-MID LEVELS
WITH THE 700 MB JET CROSSING THE NORTHERN WA CASCADES LEADING TO A
RESURGENCE IN PRECIPITATION.

ONCE AGAIN...WPC WAS USED FOR QPF GUIDANCE AND PRIMARILY GFS
THERMAL FIELDS WERE USED FOR THE DAY 2 FORECAST.  DAY 3 WAS A
GFS/ECMWF COMBINATION FOR QPF WITH MORE WEIGHTING GIVEN TO THE
GFS.

IN THE RANGES OF SOUTHWEST MT AND NORTHWEST WY...THE DEPARTURE OF
THE UPPER WAVE ON DAY 2 LEADS TO A TREND TOWARDS DRYING ALOFT AND
SINKING MOTION DEVELOPING...SO SNOW AMOUNTS SHOULD BE LIGHT GIVEN
THE SHORT DURATION.

THE PROBABILITY OF SIGNIFICANT ICING IS LESS THAN 10 PERCENT.

BANN