066
FOUS11 KWBC 201903
QPFHSD
PROBABILISTIC HEAVY SNOW AND ICING DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
303 PM EDT MON APR 20 2026
VALID 00Z TUE APR 21 2026 - 00Z FRI APR 24 2026
..CALIFORNIA
DAYS 1-2...
GOES-WEST SATELLITE IMAGERY CURRENTLY SHOWS A LARGE, CLOSED, UPPER
LEVEL LOW SLOWLY SPINNING OVER THE EASTERN PACIFIC OCEAN OFF THE
COAST OF CA. THIS ANOMALOUSLY DEEP UPPER LOW (500MB HEIGHTS FALLING
TO THE 2.5 PERCENTILE LEVEL OF THE CFSR CLIMATOLOGY) WILL
GRADUALLY INCH CLOSER TO THE CA COAST TONIGHT BEFORE SWINGING
INLAND TUESDAY AFTERNOON AND EVENING AS FORWARD SPEED INCREASES.
WHILE THE UPPER LOW IS FORECAST TO INTERACT WITH INCOMING SHORTWAVE
ENERGY DROPPING SOUTHWARD OUT OF WESTERN CANADA AND THEN BREAKDOWN
INTO SEVERAL SMALLER SCALE WAVES ROTATING AROUND AND WITHIN THE
PARENT TROUGH SOON AFTER MAKING LANDFALL, PRONOUNCED ASCENT THROUGH
HEIGHT FALLS, PVA, AND THE LFQ OF A JET STREAK PIVOTING ONSHORE
DOWNSTREAM WILL HELP EXPAND PRECIPITATION ACROSS CA TONIGHT AND
THROUGH THE DAY ON TUESDAY. THE HEAVIEST PRECIPITATION IS LIKELY TO
BE FOCUSED DURING THE DAYLIGHT HOURS ON TUESDAY AS A TRANSIENT,
MODESTLY STRONG ATMOSPHERIC RIVER (IVT NEARING 500 KG/M/S AND PW
ANOMALIES OVER 200% OF NORMAL) PUSHES INTO CA WITHIN THE CONFLUENT
FLOW SOUTHEAST OF THE TROUGH. GIVEN MARGINAL THERMALS AND ELEVATED
SNOW LEVELS, MOST OF THE PRECIPITATION ACROSS CA IS LIKELY TO FALL
AS RAIN. THAT BEING SAID, SNOW IS LIKELY IN THE HIGHEST TERRAIN OF
THE SIERRA AND SHASTA/TRINITY REGION AS SNOW LEVELS FALL FROM
AROUND 6000-7000 FT DOWN TO AS LOW AS 4500 FT, POTENTIALLY LOCALLY
LOWER AS STEEP LAPSE RATES AND IMPRESSIVE ASCENT DYNAMICALLY COOL
THE COLUMN (THE NBM 10TH% SNOW LEVEL DROPS TO 3500 FT IN THE
SIERRA).
TAKING INTO ACCOUNT THE COMBINATION OF DYNAMIC FORCING, PLUME OF
ANOMALOUS MOISTURE, AND LOWERING SNOW LEVELS, HEAVY SNOW
ACCUMULATIONS ARE EXPECTED, ESPECIALLY ACROSS THE SIERRA, BUT ALSO
INTO THE HIGHER ELEVATIONS OF THE NORTHERN CA TERRAIN AS WELL. WPC
PROBABILITIES INDICATE A HIGH RISK (>90%) FOR AT LEAST 12 INCHES OF
SNOW IN THE HIGHER SIERRA (ABOVE 6000 FT), WITH LIGHTER
ACCUMULATIONS DOWN TO 4500 FT. HEAVY SNOW IS ALSO LIKELY IN THE
SHASTA/TRINITY REGION AS REFLECTED BY WPC PROBABILITIES THAT ARE
ABOVE 70% FOR AT LEAST 8 INCHES. WITH WSSI-P SHOWING >90%
PROBABILITIES FOR MODERATE WINTER STORM IMPACTS AND A 10-30% CHANCE
OF MAJOR IMPACTS, TRAVEL ACROSS THE SIERRA PASSES WILL LIKELY BE
DIFFICULT AT TIMES THROUGH WEDNESDAY. SNOW INTENSITY WANES DURING
THE DAY ON WEDNESDAY BEFORE TAPERING OFF BY WEDNESDAY NIGHT.
..NORTHERN ROCKIES
DAYS 2-3...
THE SAME SYSTEM THAT WILL BRING HEAVY SNOW TO CALIFORNIA WILL
CONTINUE ITS TREK INTO THE NORTHERN ROCKIES WEDNESDAY INTO THURSDAY
AS SEVERAL PIECES OF SHORTWAVE ENERGY PINWHEEL AROUND AND WITHIN A
NEGATIVELY-TITLED UPPER TROUGH. ALTHOUGH MOISTURE WILL GRADUALLY
DECAY (IVT LESS THAN 150 KG/M/S SPILLING INTO THE INTERMOUNTAIN
WEST), A STRENGTHENING SURFACE LOW ACROSS THE NORTHERN HIGH PLAINS
WILL WORK TOGETHER WITH CONTINUED MID-LEVEL SOUTHWEST FLOW
(FUNNELING THE PACIFIC MOISTURE) TO PRODUCE WIDESPREAD
PRECIPITATION ACROSS THE REGION BEGINNING WEDNESDAY MORNING.
SUFFICIENT SYNOPTIC ASCENT INTO THIS MOISTENING COLUMN WILL
MANIFEST AS AREAS OF HEAVY SNOW, INITIALLY ONLY ABOVE 8000 FT, BUT
THEN FALLING TO AROUND 5000 FT BY THE END OF THE FORECAST PERIOD AS
THE UPPER TROUGH SWINGS OVERHEAD. WPC PROBABILITIES SHOW THERE IS
A HIGH RISK (>70%) FOR WIDESPREAD LIGHT TO MODERATE SNOWFALL
ACCUMULATIONS EXCEEDING 4 INCHES FROM THE BLUE MOUNTAINS OF OR
THROUGH THE SAWTOOTH/SALMON RIVER RANGES OF ID, INTO THE ABSAROKAS,
LITTLE BELTS, TETONS AND OTHER AREAS AROUND YELLOWSTONE NP, AND AS
FAR NORTH AS THE LEWIS RANGE AROUND GLACIER NP. LOCALLY, MORE THAN
8 INCHES IS POSSIBLE (50% CHANCE) NEAR THE ABSAROKAS, LITTLE
BELTS, AND TETONS.
ALTHOUGH CURRENT MODEL SNOWFALL ACCUMULATIONS OUTSIDE OF THE
HIGHER TERRAIN ARE MODEST, THERE IS AT LEAST SOME CONCERN THAT LOW-
LEVEL SOUTHERLY FLOW DOWNSTREAM OF THE SURFACE CYCLONE WILL PIVOT
CYCLONICALLY AND LIFT THE ACCOMPANYING THETA-E RIDGE INTO A TROWAL
ACROSS MONTANA AND THE WESTERN DAKOTAS D3 INTO D4. AS OF THIS
TIME, THE PROBABILITIES FOR HEAVY SNOW REMAIN LOW, BUT THIS WILL
NEED TO BE MONITORED FOR POTENTIALLY IMPACTFUL ACCUMULATIONS
DROPPING INTO LOWER ELEVATIONS WEDNESDAY INTO THURSDAY.
THE PROBABILITY FOR SIGNIFICANT FREEZING RAIN EXCEEDING 0.1 INCHES
ACROSS THE CONUS IS LESS THAN 10 PERCENT.
MILLER/WEISS
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