THE WEATHER PREDICTION CENTER

COLLEGE PARK, MD


PROBABILISTIC HEAVY SNOW AND ICE DISCUSSION




PROBABILISTIC HEAVY SNOW AND ICING DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
336 PM EST SUN NOV 23 2014

VALID 00Z MON NOV 24 2014 - 00Z THU NOV 27 2014


...PACIFIC NORTHWEST/GREAT BASIN/NORTHERN AND CENTRAL ROCKIES...

THE MODELS FCST MOSTLY LIGHT SNOW AMOUNTS ON DAY 1 AS THE MODELS
CONTINUE TO SHOW A POWERFUL AND ELONGATED UPPER JET BUILDING
DOWNSTREAM OF A HIGH AMPLITUDE RIDGE OVER THE NORTHEAST PACIFIC.
THE JET STREAMS FROM WA INTO ID/WESTERN MT/UT WITH SPORADIC UPPER
DIVERGENCE MAXIMA COMBINING WITH UPSLOPE CONDITIONS TO PRODUCE
SNOW SHOWERS IN THE RANGES FROM WA/ID/WESTERN MY/WESTERN
WY/NORTHERN UT/WESTERN CO.

ON WED THE DEVELOPMENT OF A LOW LEVEL FRONT OCCURS OVER MT
SANDWICHED BETWEEN STRONG HIGH PRESSURE OVER NORTHERN CA/NORTHERN
NV AND ANOTHER HIGH OVER WESTERN CANADA. LOW LEVEL FRONTOGENESIS
MAXIMA EXTEND ACROSS NORTHERN WA TO NORTHERN ID TO WESTERN MT AND
NORTHERN WY.  WARM ADVECTION AND 700 MB CONVERGENCE MAXIMA OCCUR
AS THE 700 MB JET STREAM ACROSS NORTHERN WA/ID/WESTERN MT...AND
THESE FACTORS COMBINE WITH THE FRONTAL LIFT TO PRODUCE SNOWS IN
THE FAVORED MOUNTAINS.

ON TUE NIGHT-WED...THE STRONG UPPER JET STRADDLES THE US/CANADIAN
BORDER NEAR MT/ID/NORTHEAST WA.  UPPER DIVERGENCE MAXIMA IN THE
RIGHT ENTRANCE REGION ARE LIKELY TO PRODUCE MORE SNOWS IN NORTHERN
ID AND WESTERN MT AS THE 700 MB JET NEAR 60 KT FCST BY THE GFS
CROSSES THESE AREAS...CONTINUING THE CLUSTERS OF 700 MB
CONVERGENCE.  THIS COMBINES WITH THE PERSISTENT LOW LEVEL FRONT
ACROSS NORTHERN ID AND WESTERN MT IN PRODUCING ASCENT AND
RESULTANT SNOWS IN THE REGION. THE UNCERTAINTY RELATES TO THE
FRONTAL TIMING AS THE 12Z ECMWF HAS MOVED THE FRONT FASTER ACROSS
NORTHERN MT...CUTTING BACK ON POTENTIAL QPF/SNOW AMOUNTS.

MANUAL PROGS GAVE MORE WEIGHTING TO THE 12Z ECMWF AND 12Z
UKMET...AS THE GFS AMOUNTS ARE ALMOST TWICE AS MUCH AS THE GEFS
MEAN AMOUNTS.

...MID-UPPER MS VALLEY AND UPPER MIDWEST/GREAT LAKES...

MODELS HAVE MADE ADJUSTMENTS AND NOW CLUSTER PRETTY WELL ON THE
PATH AND INTENSITY
OF THE LOW DEVELOPING IN THE MID MS VALLEY AND THEN CROSSING THE
GREAT LAKES MONDAY MORNING.
GIVEN THE NAM MOVED EAST FROM PRIOR RUNS...THIS CUT BACK ON HEAVY
SNOW POTENTIAL FOR WESTERN WI.   THE MODELS AGREE BETTER ON HIGHER
QPF TOTALS AFTER IT BECOMES COLD ENOUGH FOR SNOW IN EASTERN IOWA
TO NORTHERN IL/ADJACENT SOUTHERN WI. IN SPITE OF BETTER CYCLONE
FORECASTS...THERE ARE STILL
DIFFERENCES ON POST-FRONTAL QPF AND DURATION/INTENSITY OF SNOW
ONCE THE RAIN TO SNOW TRANSITION OCCURS.  LAKE ENHANCEMENT AND
LAKE EFFECT ARE ALSO EXPECTED TO BOOST SNOWFALL TOTALS DOWNWIND OF
LAKES SUPERIOR AND MICHIGAN. 

...NORTHERN PLAINS TO UPPER MS VALLEY TUE NIGHT-WED...
AN AREA OF LIGHT SNOW IS FCST ACROSS NORTHEASTERN NORTH DAKOTA AND
MN AS THE NEXT 700 MB WAVE MOVES SOUTHEAST FROM THE NORTHERN
PLAINS TOWARDS THE MID-UPPER MS VALLEY. MOSTLY LIGHT SNOWS ARE
EXPECTED WITH LOCAL ENHANCEMENT POSSIBLE NEAR THE LK SUPERIOR
SHORE IN MN..AS UPPER DIVERGENCE MAXIMA OCCUR NEAR THE SLOWLY
MOVING UPPER TROUGH AXIS.  MANUAL PROGS USED A MULTI-MODEL AND
SHORT RANGE ENSEMBLE MEAN BLEND.

...SOUTHERN TO CENTRAL APPALACHIANS/MID ATLANTIC/NORTHEAST....

THE MODELS/ENSEMBLE SHOW THE LIKELIHOOD OF A CYCLONE TO DEVELOP IN
THE COASTAL WATERS OFF THE SOUTHEAST COAST AND MOVE ROUGHLY
PARALLEL TO THE COAST OF THE MID ATLANTIC AND THEN NEW ENGLAND.
TRACK TIMING AND INTENSITY DIFFERENCES RESULT IN DIFFERENCES IN
THE SNOW AXIS/ORIENTATION.  THOSE SOLUTIONS LIKE THE 12Z UKMET/12Z
ECMWF THAT ARE CLOSER TO THE COAST ALLOW WARM AIR TO PENETRATE
INLAND WHEREAS THE MORE OFFSHORE CYCLONE TRACKS OF THE 09Z SREF
MEAN/12Z GEFS MEAN ALLOW SNOWS IN THE WASHINGTON DC TO
PHILADELPHIA AND NEW YORK CITY METRO AREAS.  SNOW OCCURS WITHIN
THE MID LEVEL FRONTAL CIRCULATION/DEFORMATION ZONE NORTHWEST OF
THE CYCLONE.  THE HIGH MAGNITUDE OF THE FRONTOGENESIS COULD LEAD
TO A NARROW BAND OF HEAVY SNOW WITHIN A COUPLED JET REGION INLAND
AWAY FROM THE COAST. 

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

PETERSEN








Last Updated: 336 PM EST SUN NOV 23 2014