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

VALID 12Z SAT MAY 11 2013 - 12Z TUE MAY 14 2013

...UPPER GREAT LAKES...

CURRENT GUIDANCE...PARTICULARLY THE HIGHER-RESOLVING GUIDANCE SUCH
AS THE NAM AND HI-RES WINDOW RUNS OF THE NMM/ARW...SHOW FAVORABLE
CONDITIONS FOR LAKE EFFECT SNOW DEVELOPING BY LATE TODAY AND
CONTINUING THROUGH EARLY SUNDAY ACROSS UPPER MICHIGAN...SUPPORTING
LOW PROBABILITIES OF 4 INCHES OR MORE OF SNOW.  THE AIR IS
INITIALLY TOO WARM FOR SNOW BUT SHOULD BECOME INCREASINGLY COLD
ENOUGH BY EARLY TONIGHT AS THE COLD AIR DEEPENS AND BOUNDARY LAYER
TEMPERATURES FALL TO NEAR OR JUST ABOVE FREEZING.  THE STRONG
FETCH ACROSS LAKE SUPERIOR WILL COMBINE WITH MARGINAL SURFACE TO
850 MB LAPSE RATES TO FAVOR THE DEVELOPMENT OF SOME LAKE EFFECT
SNOW-BANDS.  USED AN INTERMEDIATE SOLUTION INVOLVING THE NAM/NAM
CONUS-NEST AND HI-RES WINDOW RUNS OF THE NMM/ARM TO HELP OFFSET
THE POTENTIALLY HIGH LIQUID EQUIVALENT TOTALS OF THE NAM CONUS
NEST.  MOST IF NOT ALL SNOW SHOULD DIMINISH OR CONVERT TO RAIN BY
EARLY OR MIDDAY SUNDAY.

THE PROBABILITY OF SIGNIFICANT ICING IS LESS THAN 10 PERCENT.

...PACIFIC NORTHWEST...

SNOW LEVELS ARE EXPECTED TO LOWER TO AROUND 4000 TO 5000 FEET
ACROSS THE NORTHERN CASCADES BY MONDAY NIGHT AS THE LONGWAVE RIDGE
CURRENTLY OVER THE REGION STEADILY ERODES OVER THE NEXT SEVERAL
DAYS.  THE MOST SIGNIFICANT SHORTWAVE TO INFLUENCE THE REGION
SHOULD ARRIVE BY LATE MONDAY...WITH A PREFERENCE FOR THE
INTERMEDIATE TIMING/STRENGTH OF THE NAM AND 12Z ECMWF WHICH
ALIGNED BEST WITH THE GLOBAL ENSEMBLE MEANS.  THE FASTER GFS AND
00Z ECMWF WERE ALSO CONSIDERED BUT RECEIVED LESS WEIGHTING.  THE
FINAL RESULT IS FOR LOW AND MODERATE PROBABILITIES OF 4 INCHES OR
MORE OF SNOW FOR THE NORTHERN CASCADES OF WASHINGTON MONDAY NIGHT.

THE PROBABILITY OF SIGNIFICANT ICING IS LESS THAN 10 PERCENT.

JAMES