685  
FOUS11 KWBC 230821  
QPFHSD  
 
PROBABILISTIC HEAVY SNOW AND ICING DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
420 AM EDT FRI APR 23 2021  
 
VALID 12Z FRI APR 23 2021 - 12Z MON APR 26 2021  
   
..PACIFIC NORTHWEST, GREAT BASIN, AND CALIFORNIA
 
 
DAYS 2-3..  
 
A CLOSED MID-LEVEL LOW DROPPING OUT OF THE GULF OF ALASKA TODAY  
WILL PROGRESS SOUTHEASTWARD TOWARDS THE PACIFIC NORTHWEST, OPENING  
AND COMING ONSHORE THE OR COAST SUNDAY EVENING. THIS LOW WILL BE  
PRECEDED BY INCREASING WARM AND MOIST ADVECTION TO ITS SOUTH IN  
RESPONSE TO INTENSIFYING CONFLUENCE AND A DIRECT PACIFIC MOISTURE  
TAP. ADDITIONALLY, A DOWNSTREAM JET STREAK IS LIKELY TO  
STRENGTHEN, FURTHER DRIVING MOISTURE ONSHORE, AND PWS ARE PROGGED  
TO REACH AS HIGH AS +1.5 STANDARD DEVIATIONS ABOVE THE CLIMO MEAN  
IN RESPONSE TO THE INCREASED 700-500MB MOIST FLOW. IVT WITHIN THIS  
SATURATING COLUMN IS ONLY FORECAST TO REACH 250 KG/M/S ACCORDING  
TO BOTH ECENS AND GEFS ENSEMBLE MEANS, BUT A LONG DURATION OF  
MODEST IVT WILL SUPPORT HEAVY PRECIPITATION EXPANDING ACROSS MUCH  
OF THE WEST THROUGH THE BROADENING DEEP LAYER ASCENT.  
ADDITIONALLY, W/SW MID-LEVEL FLOW WILL LIKELY AMPLIFY ASCENT  
THROUGH OROGRAPHICS, ESPECIALLY IN THE SIERRA.  
 
THE PROLONGED WAA WILL INITIALLY DRIVE SNOW LEVELS UP TO AROUND  
6000FT AS PRECIPITATION OVERSPREADS THE REGION SATURDAY, BUT A  
COLD FRONT MOVING EASTWARD THROUGH THE WEEKEND SHOULD ALLOW LEVELS  
TO FALL TO AROUND 4000FT ON SUNDAY. GUIDANCE OVERNIGHT HAS EASED  
SLIGHTLY ON PRECIPITATION AND SNOWFALL, WITH BOTH NBM  
PROBABILITIES AND WSE MEANS LOWER THAN WITH PREVIOUS RUNS.  
HOWEVER, THE SPREAD IN THE GUIDANCE REMAINS SIGNIFICANT, PRIMARILY  
IN THE SIERRA. THIS LEADS TO A SOMEWHAT LOWER CONFIDENCE FORECAST,  
SO ALTHOUGH IT IS LIKELY HEAVY SNOW WILL OVERSPREAD MUCH OF THE  
REGION, EXACT AMOUNTS ARE STILL IN QUESTION, ESPECIALLY BY D3. WPC  
PROBABILITIES ON D2 ARE LOW TO MODERATE FOR 6 INCHES IN THE  
NORTHERN ROCKIES, AND SOME OF THE HIGHER TERRAIN AROUND THE  
SISKIYOUS/TRINITIES OF NORTHERN CA. BY D3, SNOWFALL COVERAGE IS  
LIKELY TO EXPAND AS THE BEST FORCING SPREADS ACROSS THE REGION.  
WPC PROBABILITIES FOR 6 INCHES ARE MODERATE FROM THE OR CASCADES  
THROUGH THE ABSAROKAS AND RANGES OF NW WY, AS WELL AS THE NORTHERN  
CA RANGES AND SIERRA. HOWEVER, THE HEAVIEST SNOW IS LIKELY IN THE  
SIERRA WHERE MORE THAN 12" OF SNOW IS POSSIBLE IN THE HIGHER  
TERRAIN ON D3.  
 
FOR DAYS 1-3, THE PROBABILITY OF SIGNIFICANT ICING (0.25 INCH OR  
MORE) IS LESS THAN 10 PERCENT.  
 
WEISS  
 

 
 
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