000
FOUS11 KWBC 092012
QPFHSD
PROBABILISTIC HEAVY SNOW AND ICING DISCUSSION
NWS HYDROMETEOROLOGICAL PREDICTION CENTER CAMP SPRINGS MD
412 PM EDT FRI MAY 09 2008
VALID 00Z SAT MAY 10 2008 - 00Z TUE MAY 13 2008
DAY 1...
THE MODELS INDICATE A CLOSED UPPER LOW DRIFTS EAST OUT OF THE
NORTHERN ROCKIES...WITH DIFLUENT AREAS ALOFT LEADING TO
PRECIPITATION CONTINUING ACROSS EASTERN MT AND ND. A COUPLED JET
REGION AND ASSOCIATED DIVERGENCE MAXIMA ALOFT CROSSES ND.
LOW LEVEL COLD ADVECTION OCCURS...CHANGING RAIN TO SNOW.
THE MODEL QPF FORECASTS STILL SHOW SIGNIFICANT DIFFERENCES IN
AMOUNTS AND LOCATIONS.
AN ECMWF/SREFMEAN/GFS BLEND WAS USED. THE GLOBAL ENSEMBLE MEAN
WAS WARMER THAN THE OPERATIONAL ECMWF/UKMET/GFS/SREFMEAN/CANADIAN
GLOBAL MODELS...SO THE LATTER MAJORITY WERE PREFERRED.
CONSEQUENTLY...THE PROBABILISTIC MAPS SHOW A SLIGHT RISK OF 4
INCHES TO ACCOUNT FOR THE QPF UNCERTAINTY/DURATION OF SNOW.
WET/ABV FREEZING GROUND TEMPS WHICH MAY ACT TO LIMIT ACCUMULATION
AT THE ONSET OF PCPN WHERE INTENSITY IS LIGHT.
GFS MODEL SOUNDINGS SHOW DEEP LAYER NORTHWEST FLOW IMPINGING ON
THE BIGHORN RANGES ROUGHLY 06-18Z...SO AREAS WHERE THIS FLOW TURNS
UPSLOPE FAVOR A PERIOD OF SNOW...WITH THE MODEL CONSENSUS
SUPPORTING SEVERAL INCHES. SIMILAR AMOUNTS ARE EXPECTED IN THE
NORTHERN CO ROCKIES...WHERE A PERIOD OF SNOW OVERNIGHT IS EXPECTED
AS DEEP WEST FLOW MOISTENS AS AN UPPER LEVEL JET MAXIMA CROSSES
THE REGION. AN UPPER DIVERGENCE MAXIMA COMBINES WITH UPSLOPE FLOW
IN WEST FACING TERRAIN TO PRODUCE SNOW.
THE PROBABILITY OF SIGNIFICANT ICING IS LESS THAN 10 PERCENT.
DAY 2...
ANOTHER AMPLIFYING TROF WILL BE MOVING INTO THE PACIFIC NORTHWEST
AND MAKING ITS WAY EAST ACROSS THE WA CASCADES. HIGHLY DIFLUENT
FLOW AHEAD OF THE SYSTEM AND EMBEDDED DIVERGENCE MAXIMA ALOFT OVER
A LOW LEVEL FRONT SUPPORT A REGION OF BROAD SCALE LIFT.
CONSEQUENTLY...SNOW WILL BE DEVELOPING IN THE WA CASCADES. THE
GFS SHOW PRECIPITABLE WATERS OF HALF AN INCH...SUPPORTING
SIGNIFICANT ACCUMULATIONS. THE MODEL CONSENSUS IS ABOUT HALF AN
INCH LIQUID EQUIVALENT IN THE NORTHERN CASCADES...SO A MODERATE
RISK OF FOUR INCHES IS INDICATED.
THE PROBABILITY OF SIGNIFICANT ICING IS LESS THAN 10 PERCENT.
DAY THREE...
SNOW IS MOST LIKELY AT THE HIGHEST LEVELS OF THE NORTHERN ROCKIES
AS THE UPPER LEVEL TROUGH AND ASSOCIATED JET MAXIMA CROSS THE
REGION. THE UPPER DIVERGENCE MAXIMA COMBINE WITH DEEP WEST FLOW
TO PRODUCE SNOW AND THEN THE MID LEVEL TROUGH PASSAGE IS FOLLOWED
BY WEAKENING WINDS AND DECAYING ASCENT. SNOW ELEVATIONS DROP LATE
IN THE PERIOD AS COLD ADVECTION FOLLOWS THE FRONTAL PASSAGE. THE
ECMWF/GFS/GFS PARALLEL/CANADIAN MODELS AND SREFMEAN CONSENSUS WAS
USED TO INDICATE THE RISK OF 4 TO 8 INCHES OF SNOW IN NW WY.
THE PROBABILITY OF SIGNIFICANT ICING IS LESS THAN 10 PERCENT.
PETERSEN
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