Quantitative Precipitation Forecast
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FOUS11 KWBC 291915
QPFHSD

PROBABILISTIC HEAVY SNOW AND ICING DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
314 PM EDT MON SEP 29 2014

VALID 00Z TUE SEP 30 2014 - 00Z FRI OCT 03 2014


DAYS 1 THROUGH 3...


...NORTHERN AND CENTRAL ROCKIES...

A CLOSED MID LEVEL LOW OVER THE GREAT BASIN EARLY ON DAY 1 WILL
OPEN UP INTO A LONG WAVE TROUGH ACROSS THE ROCKIES DURING DAY
2...BEFORE TAKING ON A NEGATIVE TILT ON DAY 3 AS IT CROSSES THE
NORTHERN PLAINS. LOWERING HEIGHTS WILL RESULT IN DYNAMICAL COOLING
AND LOWERING SNOW LEVELS. A SHORT WAVE DROPPING OUT OF BRITISH
COLUMBIA ON DAY 3 WILL AID IN LOWERING SNOW LEVELS FURTHER ACROSS
THE NORTHERN ROCKIES.

AS THE CLOSED MID LEVEL SYSTEM EXITS THE GREAT BASIN ON DAY
1...LOWERING HEIGHTS WILL RESULT IN LOWERING SNOW
LEVELS...DROPPING TO AS LOW AS 9000 FEET BY THE END OF THE PERIOD.
THE BEST COMBINATION OF LIFT AND MOISTURE OCCURS ACROSS THE
NORTHERN AND CENTRAL ROCKIES DURING THE SECOND HALF OF THE PERIOD.
THE WPC QPF SUPPORTS AN INCH OR TWO OF SNOWFALL ACROSS THE ROCKIES
IN CENTRAL CO...AS WELL AS THE GRAND TETONS IN WESTERN WY.

AS THE MID LEVEL SYSTEM OPENS INTO A LONG WAVE TROUGH...SHORT
WAVES RIDING DOWN THE BACK OF THE LONG WAVE TROUGH WILL PROVIDE
ADDITIONAL SYNOPTIC SCALE LIFT ACROSS WY/CO ON DAY 2.
ADDITIONALLY...COLD AIR ADVECTION BEHIND THE SHORT WAVES WILL
RESULT IN UPSLOPE FLOW. THE COMBINATION OF LIFT AND UPSLOPE FLOW
SHOULD MAKE THE MOST OF THE AVAILABLE MOISTURE TO PRODUCE SNOWFALL
ACROSS THE UINTA RANGE IN UT...THE ROCKIES IN CENTRAL CO...AND THE
GRAND TETONS AND BIGHORN MOUNTAINS IN WY.

THE BEST COMBINATION OF MOISTURE AND LIFT SHOULD BE FOCUSED BY A
SHORT WAVE ACROSS WESTERN WY. MODEL SOUNDINGS AT KP60 IN WY SHOWED
MOISTURE AND LIFT IN THE DENDRITIC GROWTH ZONE (CENTERED AROUND
MINUS 15 CELSIUS) BETWEEN THE 01/18Z AND 02/00Z...AS WELL AS SNOW
LEVELS NEAR 8500 FEET. THESE CONDITIONS SHOULD RESULT HIGHER SNOW
TO LIQUID RATIOS ACROSS THE HIGHER TERRAIN...AND GIVEN THE WPC QPF
AMOUNTS...SNOWFALL AMOUNTS BETWEEN 4 AND 8 INCHES OF SNOW ARE
POSSIBLE ABOVE 8500 FEET.  THE SPEED OF MOVEMENT OF THE MID LEVEL
SYSTEMS...AS WELL AS LACK OF DEEPER MOISTURE... SHOULD CAP
SNOWFALL ACCUMULATIONS. THE LATEST ECMWF ENSEMBLE OUTPUT DID NOT
HAVE ANY MEMBERS SHOWING 8+ INCHES OF SNOWFALL IN THIS AREA. THE
SNOWFALL FORECAST RESULTED IN A MODERATE RISK OF 4 INCHES SNOWFALL
FOR NORTHWEST WY.

THE LONG WAVE TROUGH TAKES ON A NEGATIVE TILT AS IT CROSSES THE
NORTHERN PAINS ON DAY 3. A SHORT WAVE DIVING DOWN THE BACK OF THE
TROUGH WILL CROSS THE NORTHERN AND CENTRAL ROCKIES BY THE END OF
THE PERIOD. MOISTURE AND LIFT TIED TO THIS SHORT WAVE WILL FOCUS
ON THE ROCKIES OF CENTRAL CO...AND MODEL SOUNDINGS AT KPUM SHOWED
SNOW LEVELS NEAR 9000 FEET. MOISTURE WILL BE THE LIMITING
FACTOR...SO SNOWFALL AMOUNTS WERE LIMITED TO 2 TO 4 INCHES ABOVE
9000 FEET. THE LATEST ECMWF ENSEMBLE OUTPUT SHOWED SEVERAL MEMBERS
WITH 4+ INCH SNOWFALL AMOUNTS.

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

HAYES

$$




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