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FOUS11 KWBC 151914
QPFHSD

PROBABILISTIC HEAVY SNOW AND ICING DISCUSSION
NWS HYDROMETEOROLOGICAL PREDICTION CENTER CAMP SPRINGS MD
311 PM EDT SUN MAR 15 2009

VALID 00Z MON MAR 16 2009 - 00Z THU MAR 19 2009


DAY 1 THROUGH DAY 3...
PACIFIC NORTHWEST/NORTHERN ROCKIES...
A PERSISTENCE FORECAST ACROSS WASHINGTON...NORTHERN
OREGON...IDAHO...WESTERN MONTANA AND NORTHWEST WYOMING AS
DEEP/COLD AND UNSTABLE CYCLONIC FLOW CONTINUES TO STREAM EASTWARD
ALONG AND NORTH OF THE STRONG ZONAL 300 MB JET CORE. THERMAL
PROFILES SUGGEST LITTLE OR NO WARMING ABOVE 4000 FEET FOR MONDAY
AND SLIGHT WARMING IN THE H85-H5 LAYER BEGINNING LATE
TUESDAY...WITH A STABLE LAYER/DRIER AIR AT H7 BEGINNING TO DEVELOP
ALONG THE COASTLINE TUESDAY NIGHT. THIS IMPLIES THAT PRECIPITATION
WILL BE IN THE FORM OF SNOW ACROSS THE CASCADES AND 4500-5500 FEET
MSL IN THE SAWTOOTH/BITTEROOTS DURING THE NEXT 48 HOURS.
ACCUMULATING SNOW LEVELS THEN RISE TO/NEAR 5000-6000FT MSL FOR
WEDNESDAY...WITH ONLY THE HIGHEST TERRAIN SEEING A GENERAL 4-8
INCH SNOWFALL ACCUMULATION.

ALONG THE WASHINGTON COAST AND NORTHWEST OREGON COAST...THE VERY
COLD TEMPS ALOFT...IE H5 TEMPS OF M30-M35C...WILL OCCASIONALLY
PRODUCE DEEP CONVECTION OFFSHORE...WITH HEAVY PRECIPITATION MOVING
INLAND...AND BOUNCE LOCAL SNOW LEVELS IN THE 06Z-18Z TIME
FRAME...TO SEA LEVEL/VALLEY FLOORS. ANTICIPATE 1-2 INCH PER HOUR
SNOWFALL RATES...GIVEN THE CONVECTIVE INSTABILITY
PARAMETERS/1000-850 MB THICKNESS PROGS. THE JET CARRIES THESE VERY
COLD TEMPERATURES ALOFT ACROSS NORTHEAST OREGON...IDAHO...MONTANA
AND NORTHERN WYOMING...ALLOWING DIURNAL CONVECTIVE SNOW SHOWERS TO
DEVELOP MONDAY AND TUESDAY AFTERNOON/EVENING...AT/ABOVE 5000 FEET
MSL. A DEEP-LAYERED WESTERLY FLOW ALOFT WILL FAVOR THE HIGH
TERRAIN OF NORTHERN IDAHO...WESTERN MONTANA AND THE TETONS...WHERE
TWO-DAY TOTALS (MON-TUE) OF 1-2 FEET ARE LIKELY ABOVE 5500 FT MSL.
A NOTE...A MODEL CONSENSUS QPF FORECAST ACROSS THE
SAWTOOTH/SALMON/CLEARWATER RANGES ARE SUGGESTING THE TWO-DAY
SNOWFALL TOTALS WILL RANGE FROM 30-48 INCHES. PLEASE SEE QPFPFD
FOR ADDITIONAL DETAILS.

THE PROBABILITY OF SIGNIFICANT ICING IS LESS THAN 10 PERCENT.

DAYS 1 AND 2...
NORTHERN SIERRA-SISKIYOUS...
WITH THE STEADY STATE SYNOPTIC-SCALE FLOW PATTERN AND EXPANSIVE
300MB JET CORE IN PLACE...EXCESSIVE MOISTURE ORIGINATING IN THE
EAST PAC/GULF OF ALASKA...COUPLED WITH GOOD AGREEMENT FROM MODEL
DATA WITH RESPECT TO THERMAL PROFILES/QPF FORECASTS WARRANT A
MENTION OF HEAVY WET SNOWFALL ACCUMULATION ABOVE 5000 FEET
MSL...ACROSS THE TRINITY ALPS AND MT SHASTA AREAS. FROM MT LASSEN
SOUTHWARD INTO THE NORTHERN SIERRA...ACCUMULATING SNOWS ARE
EXPECTED TO BE IN THE 5500-6500 FT MSL RANGE FROM NORTH TO SOUTH
RESPECTIVELY. TIMING OF MODERATE TO HEAVY PRECIPITATION IN THE
CRITICAL 06Z-18Z TIME FRAMES...IN ASSOCIATION WITH STRONG VERTICAL
MOTION...WOULD SUGGEST DOWN DRAFTS IN STRONGER SHOWERS SHOULD
BRIEFLY KNOCK DOWN LOCAL SNOW LEVELS INTO 3500-4000 FT MSL
RANGE...LATE TONIGHT/MONDAY MORNING AND LATE MONDAY NIGHT/EARLY
TUESDAY.

THE PROBABILITY OF SIGNIFICANT ICING IS LESS THAN 10 PERCENT.
VOJTESAK
$$