280
AWUS01 KWNH 172204
FFGMPD
ORZ000-WAZ000-181000-
MESOSCALE PRECIPITATION DISCUSSION 1177
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
503 PM EST TUE DEC 17 2024
AREAS AFFECTED...WESTERN WA INTO NORTHWESTERN OR
CONCERNING...HEAVY RAINFALL
VALID 172202Z - 181000Z
SUMMARY...AREAS OF HEAVY RAINFALL WILL CONTINUE TO AFFECT WESTERN
WA INTO NORTHWESTERN OR THROUGH TONIGHT. WHILE HIGH RAINFALL RATES
WILL NOT NOT BE CONSTANT OVER THE NEXT 6-12 HOURS, RATES WILL
OCCASIONALLY SURPASS 0.5 IN/HR AND LOCALLY APPROACH 1 IN/HR INTO
THE FAVORED TERRAIN LEADING TO AN ADDITIONAL 2-5 INCHES OF RAIN
THROUGH WEDNESDAY MORNING.
DISCUSSION...GOES WEST WATER VAPOR IMAGERY AT 21Z SHOWED A
VORTICITY MAX EMBEDDED WITHIN A BROADER MID-UPPER LEVEL TROUGH
AXIS NEAR 41N 141W, TRACKING TOWARD THE EAST AND PRECEDED BY A
130-150 KT UPPER LEVEL JET. A COLD FRONT WAS ANALYZED AT THE
SURFACE OUT AHEAD OF THE UPPER TROUGH WITH A SURFACE WAVE NEAR 42N
135W. AN ATMOSPHERIC RIVER WAS NOTED WITH PEAK PWAT VALUES OF 1.2
TO 1.3 INCHES ON CIRA ADVECTED TPW IMAGERY AND RAP ANALYSIS DATA
AHEAD OF THE COLD FRONT TO THE WEST OF WA/OR. PWAT VALUES ALONG
THE COAST, NEAR THE MOUTH OF THE COLUMBIA RIVER, WERE JUST BELOW
1.2 INCHES VIA GPS DATA, BUT WHEN COMBINED WITH SOUTHWESTERLY 850
MB WINDS OF 40-50 KT, IVT VALUES WERE ESTIMATED TO BE NEAR 700
KG/M/S ALONG THE SOUTHERN WA/NORTHERN OR COAST. DOWNSTREAM
RAINFALL RATES INTO THE NORTHERN OR COASTAL RANGES HAVE ALREADY
BEEN OBSERVED IN THE 0.5 TO NEAR 1.0 IN/HR RANGE FROM EARLIER
TODAY, BUT RADAR/SATELLITE IMAGERY INDICATED THE HEAVIEST RAIN WAS
SHIFTING NORTH INTO WESTERN WA.
WATER VAPOR IMAGERY DOES NOT SUGGEST THE BASE OF THE UPPER TROUGH
IS AMPLIFYING ANY MORE TO THE SOUTH AND MAY EVEN BE BEGINNING TO
LIFT SLIGHTLY NORTH. A GENERAL E TO ENE MOTION TO THE UPPER TROUGH
IS FORECAST BY RAP GUIDANCE THROUGH THE OVERNIGHT WHICH WILL
CONTINUE TO TRANSLATE THE GREATEST MAGNITUDE OF IVT VALUES
NORTHWARD ACROSS WESTERN WA, WITH THE PEAK AXIS FOCUSING PRIMARILY
INTO VANCOUVER ISLAND JUST AFTER 00Z, PERHAPS CLIPPING FAR
NORTHWESTERN CLALLAM COUNTY IN WA. PEAK RAINFALL RATES ALONG THE
WA COASTAL RANGES INTO THE OLYMPICS WITHIN THE NORTHWARD SHIFTING
IVT AXIS SHOULD REACH INTO THE 0.5 TO 0.8 IN/HR RANGE, PERHAPS
ISOLATED VALUES NEAR 1.0 IN/HR. A TEMPORARY LULL IN HEAVY RAIN IS
EXPECTED FOR MUCH OF THE PACIFIC NORTHWEST JUST AFTER 00Z FOLLOWED
BY ANOTHER SURGE AS THE UPPER TROUGH AND SURFACE COLD FRONT
APPROACH OVERNIGHT. FROM ROUGHLY 06Z ONWARD, IVT VALUES ARE
FORECAST TO SURGE TO NEAR 1000 KG/M/S ALONG THE WA/OR COAST VIA
RECENT RAP GUIDANCE, BUT ONLY LAST FOR AN HOUR OR TWO AT THAT
MAGNITUDE. THE OVERNIGHT SURGE IS LIKELY TO PRODUCE RAINFALL RATES
IN THE 0.5 TO 0.8 INCH RANGE WITH LOCALIZED VALUES NEAR OR IN
EXCESS OF 1.0 IN/HR.
ADDITIONAL RAINFALL TOTALS OF 2-4 INCHES ARE LIKELY TO OCCUR
THROUGH 09Z FOR THE WA COASTAL RANGES INTO NORTHWESTERN OR, ALONG
WITH THE UPSLOPE REGIONS OF THE WA CASCADES. ISOLATED TOTALS IN
EXCESS OF 5 INCHES CANNOT BE RULED OUT, ESPECIALLY IN ANY FAVORED
SOUTHWEST FACING SLOPES OF THE OLYMPICS WHERE THE DURATION OF
HIGHER RAINFALL RATES IS EXPECTED TO BE THE LONGEST. GAUGES SHOWED
24 HOUR TOTALS OF 1-2 INCHES FOR THE COASTAL RANGES AND CASCADES
SO FAR, WITH LOCALIZED MAXIMA NEAR 3 INCHES. EARLIER COLLABORATION
WITH THE NATIONAL WATER CENTER AND NWM OUTPUT SUGGESTS FLOODING
SHOULD BE ISOLATED AT BEST, PARTIALLY DUE TO DRY ANTECEDENT
CONDITIONS, BUT 48 HOUR RAINFALL TOTALS OF 5-7 INCHES THROUGH
WEDNESDAY MORNING WILL LIKELY SUPPORT NOTABLE RISES ON AREA
CREEKS/STREAMS.
OTTO
..PLEASE SEE WWW.WPC.NCEP.NOAA.GOV FOR GRAPHIC PRODUCT
ATTN...WFO...OTX...PDT...PQR...SEW...
ATTN...RFC...NWRFC...NWC...
LAT...LON 49102238 49072164 48862117 48662101 48372094
48102088 47782096 47562096 47322103 47122107
46952103 46782107 46562129 46332135 46172137
45982159 45782159 45592169 45472197 45512216
45702243 45852246 46072267 46182270 46422248
46632258 46742255 46872228 46962223 47042238
47012275 46822305 46672311 46502308 46332320
46272340 46112344 45962345 45672333 45552337
45382341 45282347 45172354 44982347 44792356
44722384 44982433 45492455 46732462 48242509
48442453 48282397 48302321 48622290 48872289
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