Graphic for MPD #0937

MESOSCALE PRECIPITATION DISCUSSION 0937
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
738 PM EST SUN NOV 19 2017

AREAS AFFECTED...WESTERN WA TO NORTHWEST OR

CONCERNING...HEAVY RAINFALL

VALID 200037Z - 200930Z

SUMMARY...HEAVY RAINS OVER THE OLYMPIC PENINSULA TO SHIFT SOUTH
AND INTENSIFY OVER SOUTHWEST WA AND NORTHWEST OR OVERNIGHT AS
ROBUST ATMOSPHERIC RIVER CONDITIONS ARRIVE.

DISCUSSION...THE LATEST GOES-W IR SATELLITE IMAGERY IS SHOWING AN
AREA OF COOLING CLOUD TOPS OFFSHORE THE PACIFIC NORTHWEST IN
ASSOCIATION WITH A DEVELOPING WAVE OF LOW PRESSURE ALONG A COLD
FRONT THAT IS BEING FACILITATED BY THE APPROACH OF A
POSITIVELY-TILTED UPPER TROUGH AND ESPECIALLY THE ARRIVAL OF A
STRENGTHENING 130+ KT 250 MB JET STREAK. MEANWHILE...A SOUTHWEST
LOW LEVEL JET OF 50 KTS ALONG AND JUST AHEAD OF THE COLD FRONT IS
FOSTERING ENHANCED MOISTURE TRANSPORT AHEAD OF THE FRONT...WITH A
NOSE OF 1.1 TO 1.2 INCH PWATS FOCUSED NOW JUST OFFSHORE THE
SOUTHWEST WA AND WESTERN OR COASTLINE.

OVER THE NEXT SEVERAL HOURS...ROBUST ATMOSPHERIC RIVER CONDITIONS
ARE EXPECTED TO OVERSPREAD MUCH OF WESTERN WA...INCLUDING THE
SOUTHWEST FACING SLOPES OF THE OLYMPICS...AND THEN OVERNIGHT
BECOMING PARTICULARLY FOCUSED OVER SOUTHWEST WA AND NORTHWEST
OR...INCLUDING NOT ONLY THE COASTAL RANGES BUT ALSO THE WEST
FACING SLOPES OF THE SOUTHERN WA AND NORTHERN OR CASCADES.

RAINFALL RATES THUS FAR OVER WESTERN WA HAVE BEEN GENERALLY AROUND
0.25 TO 0.35 INCHES/HR...HOWEVER...THIS IS EXPECTED TO CHANGE THIS
EVENING. THE 850/700 MB MOISTURE FLUX ANOMALIES ARE FORECAST TO
INCREASE TO OVER 4 STANDARD DEVIATIONS ABOVE NORMAL THROUGH
06Z...AND THIS WILL BE AIDED IN PART BY THE AFOREMENTIONED LOW
LEVEL JET INCREASING TO 60 TO 70 KTS OUT OF THE SOUTHWEST AS PER
THE LATEST RAP GUIDANCE. A 21Z CIRA-LPW PASS SHOWED CONCENTRATED
MOISTURE OFFSHORE IN THE 500/700 MB LAYER THAT WILL BE JUXTAPOSED
WITH ENHANCED LARGER SCALE FORCING ASSOCIATED WITH LEFT-EXIT
REGION JET DYNAMICS...ENHANCED UPSLOPE FLOW AND SURFACE FRONTAL
CONVERGENCE. THUS...EXPECT RAINFALL RATES TO INCREASE...AND LIKELY
REACH AS HIGH AS 0.50 TO 0.60 INCHES/HR AT TIMES THROUGH 06Z AS
THE HEAVIEST RAIN AXIS GRADUALLY SHIFTS FROM WESTERN WA SOUTH DOWN
INTO NORTHWEST OR WITH THE COLD FRONT.

SOME RUNOFF PROBLEMS CANNOT BE RULED OUT GIVEN THE HEAVIER
RAINFALL RATES...AND ACTUALLY THE 18Z HREF MEAN GUIDANCE SHOWS
ELEVATED PROBABILITIES OF SEEING 3+ INCHES OF RAIN THROUGH 09Z FOR
THE FAVORED UPSLOPE AREAS OF SOUTHWEST WA AND NORTHWEST OR. WILL
CONTINUE TO MONITOR.

ORRISON

ATTN...WFO...PDT...PQR...SEW...

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

LAT...LON   48252392 47962338 46992311 46532302 46282251
            46322172 46092145 45602143 44932151 44332151
            43942187 43982228 44402257 45082267 45022313
            44392330 44442385 44662425 45312410 45912412
            46112410 47112437 47982454


Last Updated: 738 PM EST SUN NOV 19 2017