Flash Flood Guidance
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000
AWUS01 KWNH 191739
FFGMPD
CAZ000-ORZ000-200400-

MESOSCALE PRECIPITATION DISCUSSION 0893
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
138 PM EDT THU OCT 19 2017

AREAS AFFECTED...SWRN OR INTO NWRN CA

CONCERNING...HEAVY RAINFALL

VALID 191737Z - 200400Z

SUMMARY...AN INCREASE IN RAIN RATES PEAKING IN THE 0.5 TO 0.6
IN/HR RANGE IS EXPECTED TO REACH THE SOUTHWEST OREGON COAST INTO
PORTIONS OF NORTHWESTERN CALIFORNIA AFTER 21Z. MAXIMUM 12 HR
RAINFALL TOTALS OF 3-4 INCHES ARE EXPECTED THROUGH 06Z ACROSS
THESE LOCATIONS.

DISCUSSION...AN ONGOING ATMOSPHERIC RIVER EVENT WAS REPRESENTED BY
A PLUME OF PRECIPITABLE WATER VALUES PEAKING IN THE 1.3 TO 1.4
INCH RANGE OFFSHORE OF THE OREGON COAST. BLENDED TPW IMAGERY HAS
SHOWN A DOWNWARD TREND IN PEAK PRECIPITABLE WATER VALUES OVER THE
PAST 12 HOURS WITHIN 300 MILES OF THE PACIFIC NORTHWEST COASTLINE
AND GPS SITES SHOWED 1.1 TO 1.2 INCHES AT 16Z INTO CENTRAL OREGON
WEST OF THE CASCADE RIDGE. AVAILABLE OBSERVED RAINFALL RATES
ENDING AT 16Z WERE SEEN TO MAX OUT IN THE 0.3 TO 0.4 IN/HR RANGE
OVER THE SOUTHERN WASHINGTON CASCADES...A DECREASE COINCIDENT WITH
A WEAKENING OBSERVED THE 700 MB VAD WIND PLOT AT THE PORTLAND
RADAR...KRTX... SHOWING NEAR 40 KT AT 17Z. THIS WEAKENING HAS BEEN
WELL FORECAST BY THE RAP AND RECENT HI-RES MODEL GUIDANCE.

CHANGES TO RAINFALL INTENSITY ARE EXPECTED LATER THIS AFTERNOON IN
ADVANCE OF A MID-UPPER VORTICITY MAX IDENTIFIED IN WATER VAPOR
IMAGERY NEAR 44N 133W AT 17Z...AND DOWNSTREAM OF A 12Z
GFS-FORECAST 180+ KT UPPER LEVEL JET NEAR 150W. THE UPPER TROUGH
PRESENTLY WEST OF WASHINGTON/OREGON IS EXPECTED TO AMPLIFY AND
SHARPEN THROUGH 00Z AS THE TROUGH AXIS NEARS THE OREGON COAST.
THIS AMPLIFICATION WILL ALLOW FOR INCREASING DIFFLUENCE AND
DIVERGENCE ALOFT ALONG WITH A DEPARTING 130 KT JET STREAK
CURRENTLY OVER THE OLYMPICS.

RAP FORECASTS HAVE BEEN CONSISTENT FROM RUN TO RUN SHOWING A
CONTINUED WEAKENING OF THE PRECIPITABLE WATER AXIS THROUGH
00Z...AND NO APPRECIABLE INSTABILITY UNTIL AFTER THE COLD FRONT
PASSES THROUGH...BUT THE INCREASING DYNAMIC LIFT AHEAD OF THE
UPPER TROUGH COMBINED WITH PRECIPITABLE WATER VALUES PEAKING IN
THE 1.1 TO 1.2 INCH RANGE SHOULD BE ENOUGH TO SUPPORT RAINFALL
RATES NEAR 0.5 IN/HR AT TIMES WHICH MAY POSE LOCALIZED FLASH
FLOODING ACROSS BURN SCAR LOCATIONS. THE MOISTURE PLUME WILL
SLOWLY BUT STEADILY DROP SOUTH DOWN THE NORTHERN CALIFORNIA COAST
BEYOND 00Z WHICH SHOULD LIMIT MAXIMUM RAINFALL IN THE 3-4 INCH
RANGES...WITH MAY LOCATIONS RECEIVING LESS THAN 3 INCHES BY 06Z.

OTTO

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

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

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

LAT...LON   43222408 42932362 42412319 42162267 41812259
            41422259 41132254 40652285 40322294 40012290
            39592271 39302275 39162321 39532395 39922441
            40982464 42532466 43142443


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