MESOSCALE PRECIPITATION DISCUSSION 0106
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
545 PM EDT THU APR 06 2017

AREAS AFFECTED...FAR SOUTHWEST OR...NORTHERN CA COASTAL
RANGES...FOOTHILLS OF NORTHERN SIERRA-NEVADA

CONCERNING...HEAVY RAINFALL

VALID 062145Z - 070745Z

SUMMARY...STRONG UPPER TROUGH OFFSHORE WILL APPROACH THE WEST
COAST AND BRING HEAVY RAIN TO FAR SOUTHWEST OR AND ESPECIALLY
NORTHERN CA THIS EVENING.

DISCUSSION...THE LATEST GOES-W IR SATELLITE IMAGERY SHOWS AN
AMPLIFYING SHORTWAVE TROUGH OFFSHORE THE WEST COAST WHICH WILL BE
MOVING EAST THIS EVENING AND WILL BE TRANSPORTING A DEEP SURGE OF
PACIFIC MOISTURE AND HEAVY RAIN INTO THE COASTAL RANGES OF FAR
SOUTHWEST OR AND ESPECIALLY NORTHERN CA. A DEEPENING AREA OF LOW
PRESSURE ACCOMPANYING THE UPPER TROUGH WILL BE LIFTING NORTHEAST
TO OFFSHORE WESTERN OR BY 06Z...BUT THIS WILL HELP DRIVE A 50 TO
60 KT SOUTH-SOUTHWEST LOW LEVEL JET INTO THE COASTAL RANGES WHICH
WILL FOSTER SUFFICIENTLY STRONG ENOUGH OROGRAPHIC FORCING FOR
ENHANCED RAINFALL RATES. VERY STRONG UPPER JET FORCING WILL BE
NOTED TOO AS THE NOSE OF A 130-150+ KT 250MB JET ARRIVES...WITH
EXCELLENT LEFT-EXIT REGION JET DYNAMICS/ASCENT.

AN 1828Z AMSU PASS ALREADY INDICATES OFFSHORE RAINFALL RATES OF UP
TO 0.50 TO 0.60 INCHES/HR WHICH APPEAR TO BE CONNECTED TO SOME
CONVECTIVE ENHANCEMENT ASSOCIATED WITH STEEPENING MID LEVEL LAPSE
RATES. THIS IS SUPPORTED BY GOES-SOUNDER LI/S OF -2 TO -4 NEAR THE
CORE OF THE DEEPENING MID LEVEL LOW CENTER. THESE HEAVIER RAINFALL
RATES WILL APPROACH NORTHERN CA OVERNIGHT AS PART OF THE STRONGER
WARM ADVECTION PATTERN AND STRENGTHENING ONSHORE/UPSLOPE FLOW.
PWATS CORRESPONDINGLY WILL INCREASE AND ARE FORECAST TO INCREASE
TO NEAR 1.25 INCHES ALONG THE IMMEDIATE COAST BETWEEN 00Z AND
06Z...AND THIS WILL BE A SOLID 3 STANDARD DEVIATIONS ABOVE NORMAL.

THE LATEST HRRR GUIDANCE SUPPORTS RAINFALL RATES REACHING AS HIGH
AS 0.75 TO 1 INCH/HR OVER SOME OF THE MORE OROGRAPHICALLY FAVORED
TERRAIN INVOLVING SONOMA...MENDOCINO AND HUMBOLDT COUNTIES OF
NORTHERN CA...WHICH WILL BE BETTER ALIGNED WITH THE CORE OF THE
STRONGEST MOISTURE TRANSPORT. STORM TOTALS THROUGH 06Z OF 2 TO 4
INCHES CAN BE EXPECTED FOR THE COASTAL RANGES WITH ISOLATED
HEAVIER TOTALS. SLIGHTLY LESSER AMOUNTS WILL BE NOTED FARTHER
INLAND INCLUDING THE FOOTHILLS OF THE NORTHERN SIERRA-NEVADA AND
SHASTA/SISKIYOU RANGES. A COLD FRONT WILL BE MOVING INLAND BY
06Z...BUT SOME ADDITIONAL MODERATE TO HEAVY RAIN WILL BE LIKELY
AFTER 06Z AND EXTENDING THROUGH 12Z WITH PERHAPS UP TO ANOTHER 1
TO 1.5 INCHES OF RAIN LOCALLY. GIVEN THE ELEVATED RAINFALL RATE
POTENTIAL IN THE NEAR-TERM THROUGH THE EVENING HOURS...ENHANCED
RUNOFF IS POSSIBLE ALONG WITH SOME DEBRIS FLOW/MUDSLIDE CONCERNS.

ORRISON

ATTN...WFO...EKA...MFR...MTR...STO...

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

LAT...LON   42792422 42672344 42232306 41522298 41222290
            41122259 41192210 41012180 40882169 40792162
            40432156 39762118 39332098 38962095 38982121
            39472159 39812182 40212199 40262235 39922254
            39342233 39092222 38732213 38022183 37232120
            36832159 36822201 37032242 37602275 37962296
            38512340 39082401 39832417 40332455 41172430
            42272458