MESOSCALE PRECIPITATION DISCUSSION 0090
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
345 PM EDT FRI APR 06 2018

AREAS AFFECTED...NORTHERN/CENTRAL CA...INCLUDING THE NORTHERN
SIERRA-NEVADA

CONCERNING...HEAVY RAINFALL...FLASH FLOODING POSSIBLE

VALID 061945Z - 070745Z

SUMMARY...ATMOSPHERIC RIVER CONDITIONS CONTINUE ACROSS CENTRAL AND
NORTHERN CA. HEAVY RAINFALL WILL CONTINUE THROUGH THE DAY WITH
INCREASING CONCERNS FOR RUNOFF PROBLEMS AND LOCALIZED FLASH
FLOODING AS RAINFALL RATES INCREASE FURTHER.

DISCUSSION...THE LATEST GOES-15 WV IMAGERY SHOWS A DEEP LAYER
SOUTHWEST PLUME OF MOISTURE AND EMBEDDED SHORTWAVE ENERGY TAKING
AIM ON CENTRAL AND NORTHERN CA IN ASSOCIATION WITH A WELL-DEFINED
ATMOSPHERIC RIVER. ALL OF THIS IS FOCUSING OUT AHEAD OF AN
AMPLIFYING SHORTWAVE TROUGH ALONG AND WEST OF 140W. THE MOISTURE
ANOMALIES ARE EXTREMELY IMPRESSIVE...WITH ESSENTIALLY A WIDESPREAD
AXIS OF NEAR-RECORD OR RECORD PWATS FOR THIS TIME OF THE YEAR
EMBEDDED WITHIN THE DEEP LAYER WARM CONVEYOR BELT. IN SOME CASES
THE PWATS ARE OVER 1.75 INCHES OFFSHORE...AND EVEN AREAS OF
CENTRAL CA JUST NORTH OF THE SAN FRANCISCO BAY AREA ARE SEEING
GPS-DERIVED PWATS OVER THE LAST COUPLE OF HOURS OF 1.6 TO 1.7
INCHES WHICH WOULD DECIDEDLY BE AN ALL-TIME RECORD FOR THE MONTH
OF APRIL.

BROAD AND RELATIVELY STRONG ISENTROPIC LIFT DOWNWIND OF THE UPPER
TROUGH COUPLED WITH THE ANOMALOUS MOISTURE TRANSPORT WILL CONTINUE
TO FOCUS MODERATE TO HEAVY RAINFALL ACROSS CENTRAL AND ESPECIALLY
NORTHERN CA GOING THROUGH THE AFTERNOON AND EVENING
HOURS...HOWEVER EXPECT THERE TO BE SOME INCREASE IN THE RAINFALL
RATES AS THE LOW LEVEL WIND FIELD INCREASES AND BECOMES ORIENTED
ORTHOGONAL TO THE COASTAL RANGES WHICH WILL ENHANCE OROGRAPHIC
ASCENT. THIS WILL BE AIDED ALSO BY A DIFFLUENT UPPER JET STRUCTURE
ALOFT AND EMBEDDED VORT ENERGY FOR PERIODICALLY ENHANCED LIFT.

RAINFALL RATES THUS FAR HAVE GENERALLY REMAINED CLOSE TO A 0.25
INCH/HR AND OCCASIONALLY REACHING TO NEAR A 0.50 INCH/HR...BUT
OVER THE NEXT SEVERAL HOURS...EXPECT RAINFALL RATES TO BEGIN
LOCALLY REACHING UP INTO THE 0.50-0.75 INCH/HR RANGE NORTH OF THE
SAN FRANCISCO BAY AREA. ALREADY AS MUCH AS 2 TO 4 INCHES OF RAIN
HAS OCCURRED ACROSS PARTS OF MENDOCINO...LAKE AND SONOMA COUNTIES
AND THESE AREAS WILL LIKELY SEE SOME OF THE HEAVIEST ADDITIONAL
RAINFALL THROUGH THIS EVENING WITH ADDITIONAL TOTALS THROUGH
MIDNIGHT OF 3 TO 5 INCHES POSSIBLE. SOMEWHAT LESSER AMOUNTS ARE
EXPECTED FARTHER NORTH UP OVER HUMBOLDT AND TRINITY COUNTIES. ALL
OF THIS IS CONSISTENT WITH THE LATEST HIRES CAM CONSENSUS
INCLUDING THE HRRR.

FARTHER INLAND...EXPECT HEAVY RAINFALL TO FOCUS ACROSS THE
FOOTHILLS AND WEST-FACING SLOPES OF THE SIERRA-NEVADA. THE DEPTH
OF WARM AIR IS QUITE HIGH AND EVEN THE HIGHER PEAKS OF THE SIERRA
WILL GENERALLY BE SEEING RAIN WITH THIS EVENT. ADDITIONAL RAINFALL
AMOUNTS HERE OF AS MUCH AS 3 TO 4 INCHES WILL BE POSSIBLE TOO
THROUGH MIDNIGHT.

GIVEN THE ADDITIONAL RAINFALL TOTALS WITH THIS EVENT...THERE WILL
BE GROWING AND LOCALLY SIGNIFICANT CONCERNS FOR RUNOFF
PROBLEMS...INCLUDING MUDSLIDES AND POSSIBLE DEBRIS FLOWS. SOME
ISOLATED FLASH FLOODING WILL BE POSSIBLE IN AND AROUND ANY BURN
SCAR AREAS. WILL CONTINUE TO MONITOR.

ORRISON

ATTN...WFO...EKA...HNX...MFR...MTR...REV...STO...

ATTN...RFC...CNRFC...NWRFC...

LAT...LON   42102394 42022328 41562236 40702171 39672060
            38541969 37911935 37762002 38192083 39072167
            39802197 40152248 39962292 39512288 38912261
            38232210 37732190 37542231 37782299 38472376
            38472376 39882444 41862464