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231
FOUS11 KWBC 230834
QPFHSD

PROBABILISTIC HEAVY SNOW AND ICING DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
433 AM EDT WED APR 23 2014

VALID 12Z WED APR 23 2014 - 12Z SAT APR 26 2014


DAYS 1 THROUGH 3...


...WEST COAST/NORTHERN ROCKIES/GREAT BASIN...


SHORT WAVES ASSOCIATED WITH AN APPROACHING LONG WAVE TROUGH WILL
AFFECT THE WEST COAST AND ROCKIES EACH OF THE NEXT THREE DAYS.
THERE WAS VERY GOOD 00Z MODEL SUITE AGREEMENT WITH RESPECT TO THE
EVOLUTION OF THE LONG WAVE TROUGH...THOUGH THE 00Z NAM WAS A BIT
FASTER THAN THE CONSENSUS. WITH THIS IN MIND...THE QPF PORTION OF
THE FORECAST WAS BASED ON A MULTI MODEL BLEND (MINUS THE 00Z NAM).
THE THERMAL PORTION OF THE FORECAST WAS DERIVED FROM A BLEND OF
THE 00Z NAM/GFS/21Z SREF MEAN.

THE FIRST SHORT WAVE IN THE TRAIN AFFECTS THE PACIFIC NORTHWEST
AND NORTHERN ROCKIES DURING THE SECOND HALF OF DAY 1. THE LIFT
ASSOCIATED WITH THE SHORT WAVE IS AUGMENTED BY DIVERGENCE IN THE
LEFT FRONT QUADRANT OF A 90 KNOT JET STREAK APPROACHING SOUTHERN
OR. AHEAD OF THE SHORT WAVE... A LOW TO MID LEVEL SOUTHWEST FLOW
INJECTS MOISTURE INTO THE SYSTEM. FREEZING LEVEL RISE TO BETWEEN
5000 AND 6000 FEET ACROSS THE WA AND OR CASCADES (AND AS HIGH AS
8000 FEET ACROSS THE OLYMPIC RANGE). THE RISING FREEZING LEVELS IN
THE PACIFIC NORTHWEST SHOULD CAP SNOWFALL TO BETWEEN 2 AND 6
INCHES ACROSS THE WA CASCADES. A FEW MEMBERS OF THE LATEST ECMWF
ENSEMBLE SUPPORT 8+ INCHES OF SNOW ACROSS THE WA CASCADES.
HOWEVER...GIVEN THE RISE IN FREEZING LEVELS AND THE PROGRESSIVE
NATURE OF THE SHORT WAVE...THIS POTENTIAL LOOKS LOW AT BEST.

FURTHER EAST...THE FREEZING LEVELS ACROSS THE NORTHERN ROCKIES DO
NOT CHANGE MUCH ON DAY 1... AND THE UPSLOPE FLOW HELPS MITIGATE
THE LOSS OF MOISTURE IN THE FLOW. THE QPF SUPPORTS AREAS OF 4 TO 6
INCHES OF SNOW ACROSS THE HIGHER TERRAIN (GENERALLY ABOVE 4000
FEET) ACROSS THE BITTERROOTS OF ID.

A WEAKENING SHORT WAVE CROSSES THE PACIFIC NORTHWEST COAST EARLY
ON DAY 2. FREEZING LEVELS REMAIN HIGH AHEAD OF THE SHORT
WAVE...LIMITING THE IMPACT OF WINTER WEATHER OVER THE PACIFIC
NORTHWEST. SNOWFALL ACROSS THE WA CASCADES SHOULD AVERAGE BETWEEN
3 AND 6 INCHES ABOVE 6000 FEET...AND 4 INCHES OR LESS ACROSS THE
OR CASCADES. THE LATEST ECMWF ENSEMBLE SOLUTION STILL HAS A FEW
MEMBERS WITH 8+ INCHES OF SNOW IN THE WA CASCADES...BUT AGAIN THE
HIGH FREEZING LEVELS SHOULD RELEGATE THIS POSSIBILITY TO LOW AT
BEST.

THE LONG WAVE TROUGH APPROACHES THE WEST COAST LATE ON DAY
2....AND CROSSES THE CA COAST BY THE END OF DAY 3. CRASHING MID
LEVEL HEIGHTS SUPPORT LOWERING FREEZING LEVELS TO NEAR VALLEY
LEVELS BY THE END OF THE PERIOD...ESPECIALLY ACROSS CENTRAL CA.
THE CORE OF THE COLDEST AIR WITH THE LONG WAVE TROUGH TRACKS
ACROSS CENTRAL CA DURING THE SECOND HALF OF THE DAY...
PLACING THE BEST LIFT (AND MARGINAL INSTABILITY DUE TO STEEPENING
MID LEVEL LAPSE RATES) HERE. THE COMBINATION OF LIFT AND UPSLOPE
FLOW PLACES THE SIERRA NEVADA RANGE IN THE CROSS HAIRS FOR HEAVY
SNOW. THE QPF SUPPORTS A LARGE AREA OF 6 TO 12 INCHES OF SNOW
ACROSS THE CHAIN. INTERESTINGLY...ONLY A FEW OF THE LATEST ECMWF
ENSEMBLE MEMBERS SUPPORT 12+ INCHES OF SNOW ACROSS THE REGION.

FURTHER EAST...THE COMBINATION OF INSTABILITY...SYNOPTIC SCALE
LIFT AND LOCAL UPSLOPE FLOW SHOULD PRODUCE A LARGE AREA OF 3 TO 6
INCHES OF SNOW ACROSS THE HIGHER TERRAIN OF NORTHEAST NV. A SHORT
WAVE OVER THE NORTHERN PORTION OF THE LONG WAVE TROUGH CLIPS THE
NORTHERN ROCKIES LATE ON DAY 3. THE COMBINATION OF INSTABILITY AND
UPSLOPE FLOW SHOULD SUPPORT 4 TO 6 INCHES OF SNOW IN THE HIGHER
ELEVATIONS OF THE BITTERROOTS OF ID...THE ROCKY MOUNTAINS FRONT
RANGE IN MT...AND THE GRAND TETON RANGE IN WY.


...UPPER MS VALLEY/GREAT LAKES....

A CLOSING MID LEVEL SYSTEM OVER THE NORTHERN PLAINS AND UPPER MS
VALLEY ON DAYS 2 AND 3 WILL PROVIDE THE LIFT AND MOISTURE
NECESSARY FOR A SIGNIFICANT LATE SEASON WINTER STORM ACROSS THE
UPPER MS VALLEY AND GREAT LAKES. THE ECMWF APPEARS TO HAVE HAD THE
BEST HANDLE ON THE SYSTEM FOR THE PAST FEW MODEL CYCLES...WITH THE
GFS NOT TOO FAR BEHIND. WITH THIS IN MIND...THE QPF PORTION OF THE
FORECAST WAS BASED ON A BLEND OF THE 00Z ECMWF/GFS. THE THERMAL
PORTION OF THE FORECAST WAS BASED ON A BLEND OF THE 00Z NAM (WHICH
WAS THE COLDER SOLUTION)...THE 00Z GFS (WHICH WAS THE WARMER
SOLUTION)...AND THE 12Z ECMWF (WHICH WAS SOMEWHERE IN
BETWEEN...LEANING TOWARD THE COLDER 00Z NAM SOLUTION).

MOISTURE IN THE INCREASING UPGLIDE (ON THE NOSE OF A 35 TO 45 KNOT
SOUTHEAST FLOW) SATURATES THE COLUMN FROM NORTHEAST MN INTO
NORTHERN WI BY THE END OF DAY 1. MODEL SOUNDINGS SUGGEST THAT
BOUNDARY LAYER TEMPERATURES MAY BE TOO WARM FOR SNOW
INITIALLY...BUT EVAPORATIVE COOLING COULD ALLOW SNOW TO DEVELOP ON
THE NORTHERN EDGE OF THE BEST LIFT APPROACHING 12Z THU.

THE CLOSED MID LEVEL LOW TRACKS FROM THE NORTHERN PLAINS INTO THE
GREAT LAKES DURING DAY 2... AND SURFACE LOW PRESSURE DEVELOPS ON A
LOW LEVEL BAROCLINIC ZONE ACROSS UPPER MS VALLEY. THE STRONG
SOUTHEAST INFLOW MAINTAINS STRONG UPGLIDE NORTH OF THE 850 MB LOW.
THE BEST LIFT BECOMES ORIENTED NORTHWEST THROUGH SOUTHEAST ACROSS
MN INTO WI AND THE UP OF MI...AND IS EXPECTED TO OCCUR BETWEEN 18Z
THU AND 06Z FRI. THERE IS A MULTI MODEL SIGNAL THAT A TROWAL
DEVELOPS NORTH OF THE 850 MB ACROSS MN...WI INTO NORTHERN MI...AS
THE BEST LIFT ORGANIZES INTO A ZONE OF MID LEVEL FRONTOGENETIC
FORCING ACROSS THE ABOVEMENTIONED AREA.

MODEL SOUNDINGS SHOW LIFT AND MOISTURE IN THE DENDRITIC GROWTH
ZONE...AS WELL AS ISOTHERMAL PROFILES THROUGHOUT THE REGION OF
BEST LIFT. THESE ELEMENTS SHOULD SUPPORT A BAND OF HEAVY SNOW
ACROSS THE ARROWHEAD OF MN...THE NORTHERN REACHES OF WI...AND THE
UP OF MI. THERE WAS MULTI MODEL SUPPORT FOR 6 TO 10 INCHES OF SNOW
ACROSS THE ABOVEMENTIONED AREA...AS WELL AS A MAXIMUM OF 12 INCHES
ACROSS THE NORTHEAST TIP OF MN.  THIS IS SUPPORTED STRONGLY BY
SEVERAL MEMBERS OF THE LATEST ECMWF ENSEMBLE OUTPUT...AS WELL AS
THE MODEL CONSENSUS QPF. IN FACT THE COLDER 00Z NAM...WITH A
COLDER COLUMN AND DEEPER LIFT IN THE DENDRITIC GROWTH
ZONE...SUGGESTS AS MUCH AS 18 INCHES OVER THE NORTHEAST TIP OF MN.

THE SYSTEM WEAKENS AS THE BEST LIFT TRACKS TO THE NORTHEAST ON DAY
3. THE COLUMN REMAINS COLD ENOUGH FOR AN ADDITIONAL 1 OR 2 INCHES
OF SNOW ACROSS THE UP OF MI BEFORE THE BEST LIFT EXITS.


...NORTHERN NEW ENGLAND...

JUST HOW FAST COLDER AIR ON THE BACK SIDE OF THE CLOSED LOW OVER
THE GULF OF ME INTERSECTS MOISTURE AND LIFT IN THE FORM OF A NORTH
TO SOUTH ORIENTED DEFORMATION ZONE WILL DETERMINE HOW MUCH SNOW
FALLS ACROSS NORTHERN ME DURING THE SECOND HALF OF DAY 1. THERE
WAS GENERALLY GOOD MODEL AGREEMENT WITH RESPECT TO THE OVERALL
SYNOPTIC SETUP WITH THIS SYSTEM...SO THE QPF PORTION OF THE
FORECAST WAS BASED ON A MULTI MODEL BLEND.

THERE WERE SOME IMPORTANT THERMAL DIFFERENCES HOWEVER...AS THE
BOTH THE 00Z NAM AND 12Z ECMWF WERE COLDER IN THE COLUMN THAT THE
00Z GFS/18Z GEFS MEAN AND 21Z SREF MEAN. THE COLDER SOLUTIONS
WOULD PRODUCE SIGNIFICANT AMOUNTS OF SNOW IN THE DEFORMATION ZONE
ACROSS NORTHERN ME...WHILE THE WARMER SOLUTIONS WOULD PRODUCE MUCH
LESS. GIVEN THAT THE COLD AIR APPEARS TO LAG THE LIFT IN JUST
ABOUT ALL SOLUTIONS...THE WARMER SOLUTIONS APPEAR TO BE MORE
CORRECT. WITH THIS IN MIND...THE THERMAL PORTION OF THE FORECAST
WAS BASED ON A BLEND OF THE 00Z GFS/18Z GEFS MEAN AND 21Z SREF
MEAN.

THE BEST LIFT IN THE DEFORMATION ZONE OCCURS BETWEEN 00Z AND 12Z
THU FROM WEST TO EAST ACROSS THE ROOFTOP OF ME...WITH THE MAXIMUM
LIFT BETWEEN 06Z AND 12Z. MODEL SOUNDINGS SHOW MOISTURE/LIFT IN
THE DENDRITIC GROWTH ZONE IN THE DEFORMATION ZONE. SINCE THE ZONE
IS MOVING...THE HIGHEST SNOWFALL RATES OVER ANY PARTICULAR
LOCATION SHOULD BE FAIRLY LIMITED. AS THE COLD AIR LAGS THE
LIFT...SNOWFALL AMOUNTS SHOULD GENERALLY AVERAGE BETWEEN 2 AND 4
INCHES ACROSS NORTHERNMOST ME. FURTHER SOUTH...THE BOUNDARY LAYER
PROBABLY REMAINS TOO WARM FOR MUCH OF THE PRECIPITATION TO FALL IN
THE FORM OF SNOW.

HAVING SAID THAT...IT SHOULD BE NOTED THAT THE 00Z NAM AND 12Z
ECMWF SHOW A SWATH OF 4 TO 6 INCHES OF SNOW ACROSS PORTIONS OF
NORTHERNMOST ME...NEAR THE CENTER OF THE DEFORMATION ZONE. THESE
AMOUNTS ARE SUPPORTED BY SEVERAL MEMBERS OF THE LATEST ECMWF
ENSEMBLE OUTPUT...SO THERE IS A LOW PROBABILITY OF 4+ INCHES OF
SNOW ALONG THE PATH OF THE DEFORMATION ZONE.

THE PROBABILITY OF SIGNIFICANT ICING IS LESS THAN 10 PERCENT.

HAYES



$$






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