Graphic for MPD #0054

MESOSCALE PRECIPITATION DISCUSSION 0054
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
336 AM EST FRI FEB 17 2017

AREAS AFFECTED...COASTAL RANGES OF CENTRAL AND SOUTHERN CALIFORNIA

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

VALID 170835Z - 171800Z

SUMMARY...SIGNIFICANT OFFSHORE TROUGH AMPLIFICATION WILL ALLOW
HEAVY RAINS TO APPROACH AND BEGIN OVERSPREADING PORTIONS OF THE
CENTRAL AND SOUTHERN CALIFORNIA COASTAL RANGES THIS MORNING. FLASH
FLOODING WILL BE AN INCREASING CONCERN WITH TIME.

DISCUSSION...THE LATEST GOES-WEST IR/WV IMAGERY SHOWS A RAPIDLY
AMPLIFYING UPPER TROUGH OFFSHORE OF CALIFORNIA APPROACHING 130W AS
THE NOSE OF A POWERFUL 130 TO 150+ KT UPPER JET MAX UPSTREAM DIGS
SOUTHEAST IN BETWEEN ABOUT 30N AND 35N. DOWNSTREAM OF THIS 250 MB
JET CORE IS AN AREA OF RAPIDLY STRENGTHENING LEFT-EXIT JET
DYNAMICS AND AN EXPANSION COOLING CLOUD TOPS. THIS IS RESULTING IN
MORE ORGANIZED AND HEAVIER OFFSHORE RAINS AS EVIDENCED BY THE LAST
COUPLE OF AMSU PASSES AND ESPECIALLY THE 0546Z PASS WHICH SHOWS
SOME POCKETS OF 0.50+ INCH/HR RAINFALL RATES.

OVER THE NEXT SEVERAL HOURS...THE UPPER TROUGH WILL CONTINUE TO
DEEPEN AND LOW PRESSURE WILL BE ENCROACHING ON CENTRAL AND
SOUTHERN CA. LIGHT TO MODERATE WARM ADVECTION RAINS INITIALLY WILL
BECOME STEADIER AND HEAVIER THROUGH THE MORNING...AND BY
MID-MORNING...THERE SHOULD BE ORGANIZED HEAVY RAINFALL
OVERSPREADING THE COASTAL RANGES. THE LATEST HIRES GUIDANCE LED BY
THE 00Z ARW/NMMB AND CONSECUTIVE RUNS OF THE HRRR OVER THE LAST
FEW HOURS INDICATE RAINFALL AMOUNTS THROUGH 1800Z OF UP TO 2 TO 3
INCHES WITH ISOLATED HEAVIER AMOUNTS.

HOWEVER...BY 1800Z...IT IS EXPECTED THAT SOME PARTICULARLY HEAVY
RAINFALL RATES THAT MAY APPROACH 1 INCH/HR MAY BEGIN TO ARRIVE
BASED ON THE LATEST AMSU TRENDS AND HIRES MODEL OUTPUT...AND WITH
A EMPHASIS ON THE SOUTH-CENTRAL CA COASTAL RANGES. MORE
SPECIFICALLY...AREAS IN AND AROUND SANTA BARBARA COUNTY ARE
EXPECTED TO BE VULNERABLE AS IT IS HERE WHERE A STRENGTHENING LOW
LEVEL JET THAT WILL BE APPROACHING 50 KTS WILL BE ORIENTED
ORTHOGONAL TO THE TERRAIN INCLUDING THE SANTA YNEZ MOUNTAINS. THIS
WILL HELP TRANSPORT PWATS OF 1.25 TO 1.4 INCHES TOWARD THE COAST
AND RESULT IN HEAVY OROGRAPHICALLY ENHANCED RAINFALL RATES. SOME
CONVECTIVE ELEMENTS MAY ALSO BEGIN TO APPROACH THE COAST AS WELL
GIVEN STEEPENING MID/UPPER LEVEL LAPSE RATES BY THAT TIME.

GIVEN THE INCREASED RAINFALL RATE POTENTIAL BY LATE
MORNING...THERE WILL BE A GROWING CONCERN FOR FLASH FLOODING WITH
TIME AND ESPECIALLY OVER THE PACIFIC-FACING COASTAL
RANGES/COUNTIES WHERE THERE ARE ALSO BURN SCAR CONCERNS. MUCH
HEAVIER RAIN IS EXPECTED JUST BEYOND THIS PERIOD FOR THE REMAINDER
OF SOUTHERN CA WHERE A SIGNIFICANT FLASH FLOOD THREAT EXISTS FOR
LATER TODAY. STAY TUNED FOR UPDATES.

ORRISON

ATTN...WFO...HNX...LOX...MTR...STO...

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

LAT...LON   37452216 37412155 36892097 36452055 35922022
            35242003 34831945 34691905 34411864 34041874
            34201949 34292006 34432066 34812080 35112090
            35512120 35802147 36772224 37182247


Last Updated: 336 AM EST FRI FEB 17 2017