Flash Flood Guidance
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000
AWUS01 KWNH 201613
FFGMPD
CAZ000-ORZ000-210411-

MESOSCALE PRECIPITATION DISCUSSION 0939
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1112 AM EST MON NOV 20 2017

AREAS AFFECTED...SW OREGON...NW CALIFORNIA

CONCERNING...HEAVY RAINFALL

VALID 201611Z - 210411Z

SUMMARY...ANOTHER SURGE OF ATMOSPHERIC MOISTURE INTO THIS
AFTERNOON ALONG THE COASTS OF NORTHERN CALIFORNIA AND OREGON
SHOULD LEAD TO A ROUND OF HEAVY RAIN WITH RAINFALL RATES POSSIBLY
REACHING UP TO 0.5 TO 0.7 IN/HR IN FAVORED UPSLOPE REGIONS.
ISOLATED 12-HR RAIN TOTALS IN EXCESS OF 3 INCHES ARE EXPECTED IN
UPSLOPE REGIONS OF THE KLAMATH MOUNTAINS THROUGH THE EVENING.

DISCUSSION...REGIONAL RADARS SHOW A RELATIVE LULL IN PRECIPITATION
ACROSS PORTIONS OF SOUTHWEST OREGON AND NORTHERN CALIFORNIA AS OF
15Z. HOWEVER...ADDITIONAL MODERATE TO OCCASIONALLY HEAVY RAIN IS
LIKELY TO DEVELOP THIS AFTERNOON WITH ANOTHER SURGE OF MORE
SUBSTANTIAL ATMOSPHERIC MOISTURE INTO THE REGION. CIRA BLENDED TPW
PRODUCT SHOWS A LARGE RIBBON OF 1.5 INCH PW SITUATED JUST
OFFSHORE...EXTENDING FROM NEAR POINT ARENA ON THE NORTH COAST OF
CALIFORNIA...TO AROUND 36N/135W OFFSHORE. LOW-LEVEL CYCLOGENESIS
APPEARS LIKELY TO COMMENCE OVER THE NEXT 6-12 HOURS NEAR THE
INFLECTION POINT IN THE CURVED BAND OF DEEPER CLOUD COVER (GREATER
BRIGHTNESS TEMPERATURES ON GOES-15 WATER VAPOR LOOPS) AND THE
ENTRANCE REGION OF AN ANTICYCLONICALLY CURVED 150+ KNOT UPPER
LEVEL JET STREAK. NUMERICAL MODELS ARE IN GOOD AGREEMENT WITH
THIS...AND ALSO SHOW INCREASING LOW-LEVEL FLOW DOWNSTREAM AND TO
THE EAST. BY 00Z A RIBBON OF ENHANCED LOW-LEVEL MOISTURE TRANSPORT
SHOULD BE NOSING INTO THE COASTLINE OF SW OREGON AND NW CALIFORNIA
WITH HIGHER PWS APPROACHING 1.5 INCHES (EXCEEDING THE 95TH
PERCENTILE CLIMATOLOGICALLY FOR THIS TIME OF YEAR).

MODELS INDICATE ONLY A BRIEF PERIOD (AROUND 6-12 HOURS) WHERE THE
LOW-LEVEL FLOW WILL BE MORE FAVORABLY ALIGNED IN A WEST-EAST
FASHION FOR STRONGER OROGRAPHIC LIFT AND PRECIPITATION
ENHANCEMENT. AFTER THAT...FLOW SHOULD BECOME MORE ALIGNED IN A
SOUTHWESTERLY DIRECTION AND PIVOT FURTHER UP THE COAST.
THEREFORE...AT ANY GIVEN LOCATION ALONG THE NW CA AND SW OR
COAST...THE DURATION OF HEAVY RAINFALL MAY NOT LAST MORE THAN A
FEW HOURS WITH THIS PARTICULAR EVENT. HOWEVER...HI-RES MODELS DO
INDICATE THAT 0.5 IN/HR RAIN RATES WILL BE LIKELY IN PORTIONS OF
THE REGION...PARTICULARLY IN THE KLAMATH MOUNTAINS CLOSER TO THE
COAST...BUT POSSIBLY EXTENDING INTO PORTIONS OF THE OREGON
CASCADES. 12Z HREF PROBABILITIES OF 0.5 IN/HR RAIN RATES REACH
50-70 PERCENT BETWEEN 21Z AND 03Z IN THE AFOREMENTIONED FAVORED
UPSLOPE REGIONS OF THE KLAMATH MOUNTAINS. THESE RAIN RATES COULD
BE HIGH ENOUGH TO CAUSE SOME FLOODING...PARTICULARLY NEAR BURN
SCARS AND OTHER TYPICALLY FLOOD-PRONE AREAS.

LAMERS

...PLEASE SEE WWW.WPC.NCEP.NOAA.GOV FOR GRAPHIC PRODUCT...

ATTN...WFO...EKA...MFR...PDT...PQR...STO...

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

LAT...LON   45002152 44262147 43872155 43332183 42792194
            42262200 41872242 41352275 41062287 40542298
            39862274 39512295 39252369 39472405 39842416
            40382464 40992449 41582431 42232466 43142472
            43582451 43732398 43532353 43692290 44272249
            44832190



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