123  
FXUS66 KSTO 131130  
AFDSTO  
 
AREA FORECAST DISCUSSION  
NATIONAL WEATHER SERVICE SACRAMENTO CA  
330 AM PST FRI DEC 13 2019  
 
   
SYNOPSIS
 
 
LIGHT TO MODERATE VALLEY RAIN AND MODERATE TO HEAVY MOUNTAIN SNOW  
FRIDAY THROUGH SATURDAY NIGHT. MOUNTAIN TRAVEL DELAYS ARE LIKELY.  
DRIER WEATHER SUNDAY AND EARLY NEXT WEEK.  
 

 
   
DISCUSSION
 
 
UPPER LEVEL FLOW REMAINS MAINLY ZONAL FLOW OVER NORCAL TODAY.  
MULTIPLE IMPULSES ALONG THE FLOW WILL BRING ANOTHER WEAK PACIFIC  
MOISTURE PLUME INTO NORCAL TODAY. PERIODS OF LIGHT TO MODERATE  
PRECIPITATION IS EXPECTED TO RETURN TO NORCAL TODAY THROUGH  
SATURDAY CREATING TRAVEL CONCERNS OVER THE SIERRA. THE FIRST WAVE  
OF PRECIPITATION TODAY IS EXPECTED TO BEGIN IN THE NORTHERN  
MOUNTAINS & VALLEY EARLY THIS MORNING SLOWLY SPREADING SOUTH  
THROUGH THE DAY. HIRES MODELS SHOW HEAVIER BAND OF PRECIPITATION  
REACHING THE CENTRAL VALLEY BY LATE AFTERNOON, IMPACTING THE  
EVENING COMMUTE. SNOW AMOUNTS OF AROUND 10 TO 15 INCHES ARE  
FORECAST AT PASS LEVEL WITH THE HEAVIEST FALLING AFTER 4PM THIS  
AFTERNOON THROUGH TONIGHT. SNOW LEVELS WILL BE HIGH TODAY  
GENERALLY ABOVE 6500-7000 FEET, BEGINNING TO FALL OVERNIGHT TO  
AROUND 4500 TO 5500 FEET BY EARLY SATURDAY. LIQUID PRECIPITATION  
TOTALS OF A FEW HUNDREDTHS UP TO HALF AN INCH ARE EXPECTED IN THE  
VALLEY WITH A HALF AN INCH TO 2 INCHES FORECAST IN THE MOUNTAINS.  
WINDS WILL BECOME BREEZY FRIDAY OVER THE MOUNTAINS WITH GUSTS UP  
TO 30 TO 40 MPH POSSIBLE AT HIGHER ELEVATIONS. THIS COMBINED WITH  
FALLING SNOW COULD CREATE LOW VISIBILITIES OVER MOUNTAIN PASSES.  
 
MODELS CONTINUE TO INDICATE A BRIEF BREAK IN PRECIPITATION  
SATURDAY LATE MORNING AND AFTERNOON BEFORE A TROUGH ORIGINATING  
FROM THE GULF OF ALASKA BRINGS ANOTHER ROUND OF PRECIPITATION  
SATURDAY EVENING THROUGH THE NIGHT, MAINLY LIMITED TO THE  
MOUNTAINS AND FOOTHILLS. AS THIS IS A COLDER SYSTEM, SNOW LEVELS  
WILL BE AROUND 4000 TO 4500 FEET. HOWEVER, THIS WILL ALSO BE NOT  
AS WET WITH ADDITIONAL LIQUID PRECIPITATION AMOUNTS OF A FEW  
HUNDREDTHS IN THE VALLEY TO UP TO HALF AN INCH IN THE MOUNTAINS.  
ADDITIONAL SNOW AMOUNTS OF 4 TO 8 INCHES ARE POSSIBLE ABOVE 5000  
FEET WITH A FEW INCHES POSSIBLE DOWN TO 4000 FEET. CONFIDENCE IS  
STILL FAIRLY LOW AS HIGHER RESOLUTION MODELS ARE NOT BRINGING IN  
AS MUCH PRECIPITATION WITH THIS WAVE AS GFS & EURO.  
 
UPPER LEVEL RIDGING RETURNS SUNDAY, BRINGING DRY CONDITIONS BY  
SUNDAY MORNING THROUGH MONDAY. OVERNIGHT TEMPERATURES WILL DROP  
SATURDAY NIGHT AROUND 5 TO 10 DEGREES AFTER PASSAGE OF SURFACE  
FRONT, THEN WILL DROP ANOTHER FEW DEGREES SUNDAY NIGHT AS LACK OF  
CLOUD COVER WILL ALLOW RADIATIONAL COOLING EFFECTS. VALLEY  
TEMPERATURES EXPECTED TO FALL INTO THE 30S WITH WIDESPREAD TEENS  
AND 20S IN THE MOUNTAINS THROUGH MONDAY NIGHT. DAYTIME  
TEMPERATURES WILL REMAIN FAIRLY STEADY WITH A SLIGHT COOLING  
SUNDAY. HEC  
 

 
   
EXTENDED DISCUSSION (TUESDAY THROUGH FRIDAY)
 
 
THE BRIEF DRY TREND FROM THE WEEKEND WILL CONTINUE INTO EARLY  
NEXT WEEK. ON TUESDAY, THE TRANSITORY RIDGE TO LIKELY SHIFT OVER  
TOWARD THE CENTRAL GREAT BASIN AHEAD OF A COMPACT UPPER LOW  
NEARING THE CALIFORNIA COAST. GIVEN THE COMPLEX OF NATURE OF THIS  
SYSTEM SEPARATING FROM THE WESTERLIES AND EVENTUALLY CLOSING OFF,  
UNCERTAINTY STILL PLAGUES THE FORECAST. WHILE THE GFS/ECMWF  
ENSEMBLES AGREE ON THE EXISTENCE OF THIS SYSTEM, THEY CONTINUE TO  
EXHIBIT SPATIO-TEMPORAL DIFFERENCES. THE FORMER MODEL SUITE  
REMAINS THE WETTER SOLUTION WITH WIDESPREAD PRECIPITATION ENTERING  
THE PICTURE BY MID-WEEK. ON THE CONTRARY, THE ECMWF ENSEMBLE  
SYSTEM TAKES A MORE SOUTHERN TRACK FAVORING IMPACTS DOWN IN  
SOUTHERN CALIFORNIA. THERE IS ENOUGH ENSEMBLE SPREAD TO KEEP  
EITHER SCENARIO IN PLAY SO WILL CONTINUE MAINTAINING PRECIPITATION  
IN THE FORECAST. EXPECT INCREASING CLOUD COVER GIVEN THE INFLUX  
OF PACIFIC MOISTURE WITH NEAR AVERAGE TEMPERATURES FOR MID-  
DECEMBER.  
 
LOOKING TOWARD THE FOLLOWING WEEKEND, THERE ARE STILL FAIRLY  
STRONG SIGNALS OF WET WEATHER IN RESPONSE TO A FULL-LATITUDE  
TROUGH APPROACHING THE PACIFIC COAST. WHILE SOME OF THE 24-HOUR  
PROBABILITIES FOR 0.50 AND 1.00 INCHES OF PRECIPITATION HAVE COME  
DOWN A BIT, THIS WILL CONTINUE TO BE WATCHED. THE CLIMATE  
PREDICTION CENTER EXPERIMENTAL DAY 8-14 HAZARDS SUPPORTS THIS  
NOTION WITH DEC 20-22 BEING HIGHLIGHTED FOR HEAVY PRECIPITATION  
WHICH INCLUDES SNOW IN THE MOUNTAINS. ~BRO  
 

 
   
AVIATION
 
 
AREAS OF MVFR/IFR CONDITIONS, LOCAL LIFR, NEXT 24 HOURS, BECOMING  
MORE WIDESPREAD BY EARLY MORNING. PACIFIC SYSTEM WILL BRING  
WIDESPREAD PRECIPITATION AFTER 14Z-16Z FRIDAY. SNOW LEVELS  
LOWERING TO 6500 FEET FRIDAY. LIGHT WINDS GENERALLY LESS THAN 12  
KTS.  
 

 
   
STO WATCHES/WARNINGS/ADVISORIES
 
 
WINTER WEATHER ADVISORY FROM NOON TODAY TO 4 AM PST SUNDAY FOR  
WEST SLOPE NORTHERN SIERRA NEVADA-WESTERN PLUMAS COUNTY/LASSEN  
PARK.  
 

 
 

 
 
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