804
AWUS01 KWNH 152157
FFGMPD
WAZ000-160900-
MESOSCALE PRECIPITATION DISCUSSION 1264
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
456 PM EST MON DEC 15 2025
AREAS AFFECTED...WESTERN WASHINGTON
CONCERNING...HEAVY RAINFALL...FLASH FLOODING POSSIBLE
VALID 152153Z - 160900Z
SUMMARY...A SUBTLE INCREASE IN RAINFALL INTENSITY IS EXPECTED OVER
WESTERN WA THROUGH ABOUT 03Z WITH HOURLY RAINFALL PEAKING NEAR 0.5
INCHES. 12 HOUR ACCUMULATIONS OF 1 TO 2 INCHES WILL BE POSSIBLE
THROUGH 09Z. WHILE THESE HIGHER RAINFALL INTENSITIES SHOULD REMAIN
ISOLATED IN NATURE ACROSS THE OLYMPICS AND CASCADES, THE REGION
REMAINS SATURATED AND QUITE SENSITIVE DUE TO LAST WEEK'S HEAVY
RAINFALL EVENT.
DISCUSSION...THE LATEST GOES WEST WATER VAPOR IMAGERY SHOWED THE
APPROACH OF A MID TO UPPER-LEVEL VORTICITY MAX ABOUT 250 MILES
WEST OF THE MOUTH OF THE COLUMBIA RIVER, TRACKING TOWARD THE ENE.
TOTAL PRECIPITABLE WATER VALUES HAVE COME DOWN SINCE 12Z THIS
MORNING AS SEEN IN RECENT BLENDED TPW IMAGERY WITH 0.6 TO 0.8
INCHES OBSERVED FROM THE WASHINGTON COAST TO THE CASCADES AT 21Z.
850-700 MB MEAN LAYER WINDS WERE OBSERVED TO BE 50-60 KT VIA KLGX
VAD WIND DATA, FORECAST TO DECREASE INTO THE 30-45 KT RANGE BY 03Z
VIA RECENT RAP FORECASTS. HOURLY RAINFALL WITHIN THE UPSLOPE
REGIONS OF THE OLYMPICS AND CASCADES HAS PEAKED IN THE 0.2 TO 0.3
INCH RANGE OVER THE PAST 2 HOURS WITH 2-4 INCHES (LOCALLY HIGHER)
OBSERVED OVER THE PAST 24 HOURS IN THE OLYMPICS AND CASCADES.
THE ARRIVAL OF THE OFFSHORE VORTICITY MAX HAS BEEN PRECEDED BY
COOLING CLOUDS TOPS ON INFRARED SATELLITE IMAGERY AND RAINFALL
RATES ARE LIKELY TO INCREASE AGAIN OVER THE NEXT FEW HOURS OVER
THE OLYMPICS AND CASCADES, AT LEAST LOCALLY, DESPITE CONTINUED
LOWERING OF IVT VALUES. THIS WILL BE DUE TO INCREASED LIFT AHEAD
OF THE VORTICITY MAX AND THE LEFT-EXIT REGION OF A POWERFUL JET
CENTERED NEAR 250 MB, CENTERED ~1000 MILES WEST OF THE WA/OR
COASTLINE. RECENT GOES WEST DMVS MEASURED A FEW POINTS WITH 170 KT
(10-15 KT HIGHER THAN RAP FORECASTS). THIS JET MAX IS FORECAST TO
CONTINUE STRENGTHENING AS IT TRANSLATES EASTWARD WITH LEFT-EXIT
ASCENT MOVING ACROSS WESTERN WASHINGTON TONIGHT.
STRENGTHENING FORCING FOR ASCENT COUPLED WITH WEAK ELEVATED
INSTABILITY AND MEAN WSW FLOW COULD SUPPORT A COUPLE OF ISOLATED
HOURLY RAINFALL VALUES NEAR 0.5 INCHES AND BRIEF TRAINING OF
STRONGER ECHOES. THE GREATEST PROBABILITIES FOR HIGHER RAINFALL
RATES WILL OCCUR WITH HIGHER ELEVATIONS/OROGRAPHIC LIFT WHICH WILL
BE EXPERIENCING A TRANSITION TO SNOW AS FREEZING LEVELS FALL IN
THE WAKE OF A COLD FRONT WHICH WILL LIMIT CONTRIBUTIONS TO
ADDITIONAL FLOODING FOR THE HIGHER ELEVATIONS ABOVE 5000 FT.
HOWEVER, WHERE P-TYPE WILL REMAIN LIQUID, LIMITED POTENTIAL FOR A
FEW HOURS OF HOURLY RAINFALL IN THE 0.25 TO 0.50 INCH RANGE
THROUGH 06Z (LOCALLY HIGHER) AND ADDITIONAL RAINFALL TOTALS OF 1
TO 2 INCHES THROUGH 09Z WITHIN THE WINDWARD SLOPES COULD RESULT IN
ADDITIONAL FLOOD CONCERNS FOR THE REGION.
DUE TO THE HIGHLY SENSITIVE GROUND CONDITIONS, DUE LARGELY IN PART
TO LAST WEEKS ATMOSPHERIC RIVER EVENT, ANY ADDITIONAL RAINFALL
WILL HAVE THE POTENTIAL TO RENEW OR EXACERBATE FLOOD CONCERNS. THE
POTENTIAL WILL ALSO EXIST FOR ISOLATED LANDSLIDES/DEBRIS FLOWS
ACROSS THE MORE SENSITIVE REGIONS OF THE OLYMPICS AND CASCADES.
OTTO
..PLEASE SEE WWW.WPC.NCEP.NOAA.GOV FOR GRAPHIC PRODUCT
ATTN...WFO...PDT...PQR...SEW...
ATTN...RFC...PTR...NWC...
LAT...LON 48992144 48882113 48732105 48482127 48352141
48162144 47962132 47792125 47612136 47432143
47192137 47092151 47042160 47032170 47012180
46962186 46892188 46832185 46782179 46762164
46732161 46542156 46422167 46342213 46392236
46582254 46762263 46912265 47082312 47122349
47292387 47592402 47922419 48092409 48102367
47972318 48052273 48532226 48942185
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