603  
FOUS30 KWBC 260844  
QPFERD  
 
EXCESSIVE RAINFALL DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
343 AM EST TUE JAN 26 2021  
 
DAY 1  
VALID 12Z TUE JAN 26 2021 - 12Z WED JAN 27 2021  
 
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF  
THE CENTRAL CALIFORNIA COAST...  
 
A DEEP LAYERED CLOSED LOW OFF THE PACIFIC NORTHWEST COAST WILL  
HELP DRIVE A FRONT AND ATMOSPHERIC RIVER (AR) SOUTH ACROSS CA BY  
LATER TODAY INTO TONIGHT. OVERALL LOOKS LIKE A MODERATE INTENSITY  
AR, WITH IVT VALUES APPROACHING THE CLIMATOLOGICAL 97-99TH  
PERCENTILE, BUT REMAINING A TAD BELOW MORE EXTREME VALUES. THE AR  
IS ALSO PRETTY PROGRESSIVE THROUGH THE DAY 1 PERIOD, WHICH WILL  
CAP THE UPPER BOUND OF RAINFALL TOTALS THROUGH 12Z WED. WITH THAT  
SAID, THE LARGER SCALE FORCING IS QUITE STRONG WITH THIS SYSTEM.  
MODEL FORECASTS SHOW PRETTY GOOD COUPLING OF UPPER LEVEL  
DIVERGENCE AND STRONG LOW LEVEL CONVERGENCE AS THE FRONT AND AR  
SHIFTS SOUTHWARD. THE IMPRESSIVE LOW LEVEL ONSHORE FLOW AND  
CONVERGENCE WILL LIKELY HELP DRIVE A BAND OF HIGHER INTENSITY  
RAINFALL AS THE SYSTEM SHIFTS SOUTHWARD, WITH THE UPPER DIVERGENCE  
HELPING SUPPORT THE MAINTENANCE OF ANY HEAVIER RATES.  
 
ANTECEDENT CONDITIONS HAVE BEEN QUITE DRY OVER CA, WITH STREAM  
FLOWS GENERALLY RUNNING WELL BELOW NORMAL, AND SOIL MOISTURE ALSO  
RUNNING BELOW AVERAGE FOR LATE JANUARY. THUS, IN THE GENERAL, THE  
1-3" OF RAIN EXPECTED TODAY SHOULD BE PRIMARILY BENEFICIAL RAINS.  
HOWEVER THE EXPECTATION OF A SOUTHWARD PROGRESSING BAND OF HIGHER  
RAINFALL RATES DOES SUGGEST THE POTENTIAL FOR SOME FLASH FLOOD  
IMPACTS...PRIMARILY NEAR BURN SCARS AND ANY OTHER MORE SUSCEPTIBLE  
AREAS. THE 00Z HREF DEPICTS HIGH PROBABILITIES OF HOURLY RAINS  
EXCEEDING 0.5", WITH INDICATIONS THAT HOURLY MAGNITUDES WILL PEAK  
AROUND 0.75" WITHIN NARROW LOCALIZED SWATHS. AMOUNTS OF THIS  
MAGNITUDE WILL LIKELY EXCEED THRESHOLDS AT SEVERAL AREA BURN  
SCARS. THIS IS CONFIRMED WHEN LOOKING AT THE HREF PROBABILITIES OF  
EXCEEDING 1 HR FFG, WHICH SHOW ANYWHERE FROM A 20-70% CHANCE OF  
EXCEEDANCE OVER SEVERAL BURN SCARS. THUS WHILE IMPACTS OUTSIDE OF  
BURN SCARS WILL LIKELY REMAIN LOCALIZED AND MOSTLY LIMITED IN  
SCALE GIVEN THE PROGRESSIVE AR AND DRY ANTECEDENT  
CONDITIONS...IMPACTS COULD BE MORE SIGNIFICANT WITHIN/NEAR THESE  
BURN SCAR AREAS, MAINLY AFTER ~05Z TONIGHT. FOR THIS REASON WE  
WENT AHEAD AND UPGRADED MUCH OF THE CENTRAL CA COAST TO A SLIGHT  
RISK.  
 
THE AR IS FORECAST TO STALL OUT AS WE HEAD INTO DAY 2...WHICH MAY  
RESULT IN EVEN MORE SIGNIFICANT IMPACTS ACROSS PORTIONS OF  
MONTEREY AND SAN LUIS OBISPO COUNTIES...SEE THE DAY 2 ERO AND  
DISCUSSION FOR MORE DETAILS ON THIS THREAT.  
 
CHENARD  
 
DAY 2  
VALID 12Z WED JAN 27 2021 - 12Z THU JAN 28 2021  
 
...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR THE CENTRAL  
CALIFORNIA COAST...  
 
AN UPPER-LEVEL TROUGH/LOW CENTERED OVER THE EASTERN PACIFIC IS  
EXPECTED TO RELOAD WITH ADDITIONAL ENERGY AND AMPLIFY SOUTHWARD ON  
WEDNESDAY/WEDNESDAY NIGHT. AS IT DOES SO, AMPLE MOISTURE WILL BE  
DIRECTED INTO CALIFORNIA IN THE FORM OF A POTENT ATMOSPHERIC  
RIVER. THE AR IS FORECAST TO REORIENT ITS AXIS FROM APPROXIMATELY  
WSW-ENE TO MORE SSW-NNE AS THE PERIOD PROGRESSES, BUT AFFECT  
BASICALLY THE SAME LAND AREA OF COASTAL CENTRAL/SOUTHERN  
CALIFORNIA WITH DEEP MOISTURE INFLOW AND FORCING. THIS WILL CAUSE  
SEVERAL INCHES OF RAIN FOR THAT REGION--WIDESPREAD 5" AMOUNTS ARE  
EXPECTED IN THE 24 HOUR PERIOD, WITH SOME AREAS RECEIVING 10" OF  
RAIN FOR AN EVENT TOTAL. WHILE ONLY CONSIDERED A MODERATE AR IN  
TERMS OF INTENSITY (BY MEASURES SUCH AS THE SCRIPPS INSTITUTION OF  
OCEANOGRAPHY CENTER FOR WESTERN WEATHER AND WATER EXTREMES), AND  
WHILE RAIN RATES MAY NOT BE AS HIGH AS THEY CAN GET DUE TO LACK OF  
INSTABILITY, THE PERSISTENCE OF THE AR/ITS DURATION AFFECTING THE  
SAME AREA WILL BE THE MAIN ISSUE CAUSING HEAVY RAIN AND FLASH  
FLOODING. OTHER MEASURES OF ANOMALOUSNESS SUCH AS THE ENSEMBLE  
SITUATIONAL AWARENESS TABLE INTEGRATED WATER VAPOR TRANSPORT FIELD  
DO SHOW VALUES OVER THE 99TH PERCENTILE FOR THIS AR. THE MODERATE  
RISK IS LOCATED OVER PARTS OF MONTEREY COUNTY (ALONG AND WEST OF  
U.S. 101) AND THE WESTERN PORTIONS OF SAN LUIS OBISPO COUNTY, AND  
ENCOMPASSES THE DOLAN AND RIVER BURN SCARS AS THOSE WILL BE HIGHLY  
SENSITIVE TO FLASH FLOODING.  
 
TATE  
 
DAY 3  
VALID 12Z THU JAN 28 2021 - 12Z FRI JAN 29 2021  
 
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR THE CENTRAL  
CALIFORNIA COAST...  
 
ON THURSDAY INTO THURSDAY NIGHT, THE DEEP UPPER TROUGH MENTIONED  
IN THE DAY 2 ERO DISCUSSION IS FORECAST TO BEGIN MOVING MORE  
PROGRESSIVELY SOUTHEASTWARD, AND WILL SHIFT THE AFOREMENTIONED  
ATMOSPHERIC RIVER SLOWLY SOUTHWARD TOWARD SOUTHERN CALIFORNIA WITH  
IT. THE AR SHOULD REMAIN STRONG (ABOVE THE 99TH PERCENTILE IN  
TERMS OF INTEGRATED WATER VAPOR TRANSPORT) THROUGH THE DAY ON  
THURSDAY BEFORE WEAKENING. THERE IS SOME QUESTION HOW QUICKLY THE  
AR AND HEAVY PRECIPITATION WILL MAKE ITS WAY SOUTH, BUT 00Z MODEL  
GUIDANCE CAME INTO BETTER AGREEMENT WITH THE LOCATION OF THE  
HIGHEST QPF, AFTER 12Z/18Z GUIDANCE WAS MORE SPLIT WITH A FARTHER  
NORTH ECMWF AND FARTHER SOUTH GFS/CMC. A SLOWER MOVEMENT WOULD  
LEAD TO ADDITIONAL ISSUES ACROSS THE AREAS THAT WILL SEE MULTIPLE  
INCHES OF RAIN ON DAY 2. AS IT STANDS, THE CURRENT QPF SHOWS 3-5"  
OF RAIN FOR THE 24 HOUR PERIOD ACROSS PORTIONS OF SANTA BARBARA  
AND VENTURA COUNTIES. ANOTHER 2" OR SO OF RAIN COULD ALSO FALL  
FARTHER NORTH OVER PARTS OF MONTEREY AND SAN LUIS OBISPO COUNTIES  
ESPECIALLY EARLY THURSDAY. THUS, A SLIGHT RISK OF EXCESSIVE  
RAINFALL IS IN PLACE FOR THOSE AREAS, GIVEN THE VERY WET  
ANTECEDENT CONDITIONS MONTEREY AND SAN LUIS OBISPO COUNTIES WILL  
FACE FROM WEDNESDAY'S RAIN AND CONSIDERING THE RIVER AND DOLAN  
BURN SCARS ON THE NORTHERN END, AND WITH SEVERAL INCHES OF RAIN  
POSSIBLE FARTHER SOUTH.  
 
TATE  
 
DAY 1 THREAT AREA: HTTPS://WWW.WPC.NCEP.NOAA.GOV/QPF/94EPOINTS.TXT  
DAY 2 THREAT AREA: HTTPS://WWW.WPC.NCEP.NOAA.GOV/QPF/98EPOINTS.TXT  
DAY 3 THREAT AREA: HTTPS://WWW.WPC.NCEP.NOAA.GOV/QPF/99EPOINTS.TXT  
 

 
 
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