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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