MESOSCALE PRECIPITATION DISCUSSION 0933
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
120 PM EST WED NOV 15 2017

AREAS AFFECTED...N COASTAL CA...NORTHERN SIERRAS

CONCERNING...HEAVY RAINFALL

VALID 151820Z - 160600Z

SUMMARY...APPROACH OF WAVE BY 22Z AND PERSISTENT PERPENDICULAR
MOIST ONSHORE FLOW TO SLOWLY DRIFT SOUTH IN COASTAL RANGES AND
NORTHERN SIERRAS THROUGH 06Z.  FLASH FLOODING AND DEBRIS FLOWS ARE
LIKELY AFTER 00Z DUE TO RECENT BURN SCARS. 

DISCUSSION...GOES-W WV LOOP SHOWS VERY ANOMALOUS LARGE SCALE
CYCLONE OVER THE NORTHEAST PACIFIC WITH 120KT JET CROSSING SW-NE
OVER OR ATTM PROVIDING LARGE SCALE ASCENT ACROSS SW OR/N CA LIKELY
TO PERSIST AND STRENGTHEN (130+KTS) AS MAIN +PV CORE SLIDES
SOUTHEAST WITH TIME.  WV LOOP ALSO DEPICTS A HEALTH SUBTROPICAL
CLOSED LOW NEAR 31N/142W...WHICH HAS BEGUN STRETCHING AND SHEDDING
SUBTLE S/W FEATURES THROUGH THE INTERFACE WITH THE NORTHERN
STREAM.  AS SUCH THE SUBTROPICAL JET STREAK OF 100 KTS IS ALSO
DETECTED ANGLING TOWARD SAN FRANCISCO BAY...LEAVING A SMALL REGION
OF DUAL JET ENHANCEMENT.

THIS AREA IS DENOTED BY RECENT CONVECTIVE ACTIVITY WITH 14Z AMSU
PASS RAIN RATES UP TO .35" WHICH IS QUITE IMPRESSIVE GIVEN THE
LARGE FOOTPRINT. EXTRAPOLATION OF THIS FEATURE WILL BRING ASHORE
BETWEEN 21-22Z CAPE MENDOCINO/PUNTA GORDA.  A DRAPED COLD FRONT TO
THE NORTH OF THE WAVE HAS BEGUN TO STALL ITS SOUTHWARD PROGRESS
OVER SW OR...AND WHILE THIS WILL ALLOW FOR A FEW ADDITIONAL HOURS
OF 25-35KT ONSHORE FLOW INTO THE SW OR COASTAL RANGE...THE AREA IS
GENERALLY MOISTURE STARVED AND LIKELY TO RESULT IN RAIN RATES LESS
THAN .25" OVER THE NEXT 3HRS AND ENDING JUST AFTER 00Z AND LOWERED
TOTALS. WHILE THERE MAY BE CONVECTIVE ELEMENTS SUPPORTED BY UP TO
250 J/KG MUCAPES NEAR THE SHORTWAVE/SFC WAVE...MAY BE NEAR .5"/HR
AT TIMES IN THE SISKIYOU/TRINITY RANGES... THE TRANSLATION OF THE
WAVE THROUGH NW CA SHOULD ALSO LIMIT TOTALS TO 1-3".

COASTAL RANGE AND NORTHERN SIERRA RANGES...AFTER 00Z UPPER LEVEL
FLOW WILL CONTINUE TO STRENGTH WITH 130KTS OF JET ENTRANCE REGION
BROAD SCALE ASCENT...BUT THE FRONTAL BOUNDARY AND ONSHORE FLOW
WILL BE MORE PARALLEL WITH VERY LIMITED SOUTHWARD PROPAGATION. 
850-7H FLOW WILL SLACK A BIT AS THE WAVE PASSES BUT WILL REMAIN IN
THE 35-50KT RANGE RESPECTFULLY AND WILL BE GENERALLY PERPENDICULAR
TO THE COASTAL RANGE AND THE DOWNSTREAM NORTHERN SIERRAS (E TEHAMA
TO PLACER COUNTIES).  THE DEEPENING UNIDIRECTIONAL FLOW IN THE
DEFORMATION/SHEARING ZONE SOUTH OF THE FRONT WILL ALSO MOISTEN
WITH TPWS INCREASING TO 1.25" PERHAPS REACHING 1.5" BY 06Z IN THE
NORTH BAY REGION.  THIS WILL ALLOW FOR RAIN RATES IN THE TERRAIN
TO OCCASIONALLY REACH .75"/HR AND PERHAPS 1"/HR IN THE LOWER
FLANKS (BELOW FREEZING LEVEL) IN THE NORTHERN SIERRAS WITH
PERSISTENT .5"/HR RATES MORE LIKELY AND TOTALS OF 3-5" THROUGH
06Z.

THESE RATES/RAINFALL MAGNITUDES IS OF PARTICULAR CONCERN FOR FLASH
FLOODING AND POTENTIALLY HEAVY DEBRIS FLOWS GIVEN THIS IS THE
FIRST SIGNIFICANT RAINFALL EVENT OVER THE RECENT BURN SCARS
(TUBBS/NUNS/ATLAS/PARTRICK) THIS SUMMER. CURRENT HI-RES GUIDANCE
SUGGESTS THAT THE MAIN CORE IS LIKELY TO BE NORTH OF THE BURN
SCARS BUT THE GUIDANCE HAS HAD A SLIGHT SLOW/NORTH BIAS OVER THE
PAST FEW DAYS. GIVEN THE DENSITY OF POPULATION IN PROXIMITY TO THE
BURN SCARS CONCERN OVER POTENTIAL IMPACTS IS HEIGHTENED. 

GALLINA

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

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

LAT...LON   41882415 41632315 41382284 41362203 40822151
            40592097 39972052 39312033 38641996 38231973
            38062017 38292041 38332079 38212120 38352157
            38952201 38982162 39492168 39912178 40512216
            40532265 39802265 39372245 38832211 38312199
            38102218 37612201 37072205 37032222 37252272
            37942313 38462363 39022390 39622396 39952425
            40282452 40782441 41212429 41732431