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FOUS11 KWBC 090830
QPFHSD
PROBABILISTIC HEAVY SNOW AND ICING DISCUSSION
NWS HYDROMETEOROLOGICAL PREDICTION CENTER CAMP SPRINGS MD
329 AM EST MON NOV 09 2009
VALID 12Z MON NOV 09 2009 - 12Z THU NOV 12 2009
DAYS 1-3...
PACIFIC NORTHWEST/NORTHERN ROCKIES...
ON MONDAY /DAY ONE/ THE UPPER TROUGH MOVES CLOSER TO THE PACIFIC
NORTHWEST...SO THE UPPER LEVEL JET MOVES ONSHORE AND INTO THE
NORTHERN ROCKIES BY THE END OF THE PERIOD.
UPPER LEVEL DIVERGENCE MAXIMA WILL COMBINE WITH LOW LEVEL WARM
ADVECTION AND UPSLOPE FLOW TO DRIVE SNOW SHOWERS ACROSS THE WA
CASCADES.
ON TUESDAY /DAY TWO/ POST-FRONTAL ONSHORE SOUTHWEST FLOW AND
PERSISTENT 90-100 PERCENT SFC-500 MB RH SHOULD RESULT IN AREAS OF
SNOW MAINLY OVER THE WA CASCADES WHERE MODEL QPF GUIDANCE SUPPORTS
A POTENTIAL FOR 4 INCH PLUS AMTS.
GUIDANCE IS SIMILAR IN TAKING THE LONGWAVE ERN PAC TROUGH
EWD...AND APPROACHING THE NW COAST ON WED /DAY THREE/. THIS
SHOULD LEAD TO THE UPPER JET PROGRESSING FURTHER INLAND TO THE
MOUNTAINS OF SOUTHERN ID AND WESTERN WY. THE DEEP LAYER SOUTHWEST
FLOW ADVECTS PACIFIC MOISTURE INTO THE REGION AND MOISTENS THE
AIRMASS. THE UPPER DIVERGENCE/LOWER CONVERGENCE COUPLET LEADS TO
ASCENT AND SNOW DEVELOPING IN THESE AREAS. THE GFS/NAM/ECMWF/SREF
MEAN/ MODELS ARE IN AGREEMENT IN SHOWING QPF MAXIMA IN THE RANGES
OF SOUTHERN ID/WESTERN WY.
ADDITIONAL SNOW SHOWERS ARE EXPECTED WED ACROSS THE WA/OR CASCADES
WITH A MDT THREAT FOR 4 INCH AMTS INDICATED. LAPSE RATES CONTINUE
TO STEEPEN TO NEAR 8 DEG C/KM IN THE WA CASCADES...LEADING TO
CONVECTIVE SNOW SHOWERS. MANUAL PROGS BLENDED THE GFS/SREF
MEAN/ECMWF QPF AND THERMAL PROFILES TO DETERMINE SNOW
AMOUNTS/PROBABILITIES.
THE PROBABILITY OF SIGNIFICANT ICING IS LESS THAN 10 PERCENT.
PETERSEN
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