PROBABILISTIC HEAVY SNOW AND ICING DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
319 PM EDT SAT MAY 11 2013

VALID 00Z SUN MAY 12 2013 - 00Z WED MAY 15 2013


DAY ONE...

...UPPER GREAT LAKES...

CURRENT GUIDANCE...NAM/SREF MEMBERS AND MEAN AND HI-RES
NMM/ARW...SHOW FAVORABLE CONDITIONS FOR LAKE ENHANCED SNOW SHOWERS
CONTINUING THROUGH TONIGHT INTO EARLY SUNDAY ACROSS UPPER
MICHIGAN...SUPPORTING LOW PROBABILITIES OF 4 INCHES OR MORE OF
SNOW.  THE COLD AIR DEEPENS AND BOUNDARY LAYER TEMPERATURES FALL
TO NEAR FREEZING.  THE LONG FETCH ACROSS LAKE SUPERIOR INTO THE
EASTERN UP OF MI IS FAVORABLE BUT THE MODELS SHOW TEPID LOW LEVEL
ASCENT OVERNIGHT SOUTH OF THE LAKE PLUS DRYING OCCURRING FROM WEST
TO EAST ON SUNDAY...LIMITING EVENT POTENTIAL.  THE POTENTIAL IS
ALSO LIMITED BY INITIAL TEMPS ABV FREEZING EARLY THIS EVENING
LEADING TO A POSSIBLE MIX OF PRECIP TYPES BEFORE A CHANGEOVER TO
SLUSHY SNOW. MANUAL PROBABILITIES OF FOUR INCHES WERE LOW DUE TO
THE COMBINED UNCERTAINTIES.

THE PROBABILITY OF SIGNIFICANT ICING IS LESS THAN 10 PERCENT.


DAY 2...

THE PROBABILITY OF HEAVY SNOW AND/OR SIGNIFICANT ICING IS LESS
THAN 10 PERCENT.


DAY 3...

...PACIFIC NORTHWEST...

THE MODELS/ENSEMBLE MEANS SHOW AN UPPER LEVEL TROUGH PROGRESSES
ACROSS THE PACIFIC NORTHWEST INTO THE NORTHERN ROCKIES MON-MON
NIGHT.  THE MODELS SHOW A 300 MB JET STREAK CROSSES FROM THE
NORTHEAST PACIFIC ACROSS WA STATE AND THEN INLAND INTO ID/WESTERN
MT IN CONJUNCTION WITH THE TIMING OF THE UPPER TROUGH. A 300 MB
DIVERGENCE/700 MB CONVERGENCE COUPLET CROSS THE CASCADES INTO THE
NORTHERN ROCKIES TO PRODUCE PRECIP...INCLUDING SNOW AT HIGHER
ELEVATIONS.  THE STEADY MOVEMENT OF THE UPPER TROUGH KEEPS THE
PRIMARY PERIOD OF ASCENT WITHIN ABOUT A 6 HOUR WINDOW...SO
POTENTIAL IS LOW FOR A HEAVY SNOW EVENT AS A MID-UPPER RIDGE
BUILDS IN AND DRYING ALOFT ENSUES IN BETWEEN SYSTEMS. 

THE FINAL RESULT IS FOR LOW AND MODERATE PROBABILITIES OF 4 INCHES
OR MORE OF SNOW FOR THE NORTHERN CASCADES OF WASHINGTON...AS THERE
IS A SPLIT IN THE MODEL/ENSEMBLE SOLUTIONS AS TO WHETHER THERE IS
ENOUGH QPF TO SUPPORT FOUR INCHES OF SNOW. 

THE PROBABILITY OF SIGNIFICANT ICING IS LESS THAN 10 PERCENT.

PETERSEN