Graphic for MPD #0891

MESOSCALE PRECIPITATION DISCUSSION 0891
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
349 PM EDT WED OCT 18 2017

AREAS AFFECTED...WESTERN WASHINGTON

CONCERNING...HEAVY RAINFALL

VALID 181948Z - 190530Z

SUMMARY...AN ATMOSPHERIC RIVER WILL CONTINUE TO IMPACT WESTERN
WASHINGTON THROUGH THE MID-EVENING HOURS. HOURLY RAIN RATES SHOULD
PEAK IN THE NEXT SIX HOURS (PRIOR TO 03Z) AND COULD REACH UP TO
AROUND 0.65 TO 0.75 IN/HR ON FAVORED UPSLOPE REGIONS OF THE
OLYMPICS AND NORTHERN WASHINGTON CASCADES.

DISCUSSION...LATEST RAP ANALYSIS AND RADAR VERTICAL WIND PROFILE
FROM LANGLEY HILL WA RADAR (KLGX) SHOWS THAT THE STRONGEST PORTION
OF THE THE LOW-LEVEL JET BEGAN TO IMPACT THE OLYMPIC PENINSULA
AROUND 16-17Z. KLGX ESTIMATED 850MB WINDS INCREASED TO AROUND 60
KNOTS IN THAT TIME FRAME AND HAVE HELD STEADY SINCE THEN. SEATTLE
RADAR (KATX) HAS SHOWN A SIMILAR INCREASE IN THE PAST COUPLE
HOURS...WITH 850MB WINDS NOW CLOSER TO 50 KNOTS. COINCIDENT WITH
THE INCREASE IN FLOW IMPACTING THE COAST...RAIN RATES HAVE ALSO
INCREASED. FOR EXAMPLE...QUILLAYUTE (KUIL) HAD RAIN RATES OF
AROUND 0.2 TO 0.25 IN/HR IN THE 16-17Z TIME FRAME INCREASE TO 0.47
IN/HR ON THE LATEST OBSERVATION AT 19Z. THIS IS GENERALLY
CONSISTENT WITH THE LATEST AMSU-SSMI BLENDED RAIN RATE WHICH
SHOWED VALUES OF UP TO 0.35 IN/HR APPROACHING THE COAST OF
VANCOUVER ISLAND EARLIER IN THE MORNING (AROUND 15Z).

OVER THE NEXT SEVERAL HOURS RAIN RATES ARE LIKELY TO PEAK ACROSS
NW WASHINGTON (PARTICULARLY IN THE OLYMPICS AND NRN CASCADES).
THIS WILL COINCIDE WITH THE STRONGER LOW-MID LEVEL FLOW NOSING
FURTHER INLAND LEADING TO STRONGER OROGRAPHIC ASCENT IN FAVORED
UPSLOPE REGIONS. ADDITIONALLY...A 150+ KNOT UPPER LEVEL JET STREAK
CURRENTLY OFFSHORE (AND EVIDENT IN GOES-15 WATER VAPOR LOOPS)
SHOULD CONTINUE TO DEVELOP NORTHEAST...LIKELY PLACING THE RIGHT
ENTRANCE REGION OVER WRN WASHINGTON. THE MORE FAVORABLE JET
DYNAMICS IN THAT REGION OF THE JET STREAK COULD ADD SOME
ADDITIONAL SUPPORT FOR ASCENT THROUGH UPPER LEVEL DIVERGENCE.
THESE FAVORABLE SYNOPTIC CONDITIONS...WHEN COMBINED WITH OBSERVED
PRECIPITABLE WATER VALUES ON GPS SENSORS UP TO 1.2 TO 1.3 INCHES
ALONG THE WASHINGTON COAST...SUPPORT THE LIKELIHOOD OF RAIN RATES
PEAKING OVER THE NEXT SIX HOURS OR SO. HIGH-RESOLUTION MODEL
GUIDANCE IS IN LINE WITH THIS THINKING AS WELL.

GIVEN OBSERVED (VIA SURFACE SITES AND IMPLIED BY SATELLITE
ESTIMATES) HOURLY RAIN RATES IN COASTAL AND LOWER-ELEVATION AREAS
APPROACHING 0.5 IN/HR...IT SEEMS REASONABLE TO EXPECT SLIGHTLY
HIGHER RATES IN UPSLOPE AREAS OF TERRAIN PARTICULARLY WITH SNOW
LEVELS QUITE ELEVATED (AROUND 10000 FT). INDEED BOTH THE HRRR-TLE
AND HREF ENSEMBLES HAVE THE PROBABILITY OF EXCEEDING 0.5 IN/HR
RAIN RATES IN THE OLYMPICS AND NRN WA CASCADES OVER 90
PERCENT...WITH THE HREF BLENDED MEAN RAIN RATES PEAKING AROUND
0.75 IN/HR IN THE HIGHER TERRAIN. FROM 18Z TO 06Z...THE HREF
ENSEMBLE BLENDED MEAN DOES ALSO HAVE 12-HR RAIN TOTALS IN THESE
SAME AREAS OF ELEVATED TERRAIN COMMONLY REACHING 4-6 INCHES...WITH
A FEW ISOLATED HIGHER AMOUNTS.

AS THE CYCLONE CONTINUES TO OCCLUDE...WIND FIELDS (PARTICULARLY
THE LOW-MID LEVEL JET) SHOULD WEAKEN AND SHIFT SOUTH WITH THE
PROGRESSION OF THE COLD FRONT. THEREFORE...AFTER 00Z AND
ESPECIALLY AFTER 03Z...RAIN SHOULD BECOME FOCUSED FURTHER SOUTH IN
WASHINGTON AND PERHAPS NW OREGON WITH THE HEAVIER RATES IN THE
UPSLOPE REGIONS OF THE COASTAL RANGES AND CASCADES. HOWEVER...RAIN
RATES AT THESE TIMES ARE LIKELY TO BE SLIGHTLY LOWER.

LAMERS

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

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

LAT...LON   49202177 49202049 48772002 47982016 47132047
            45902124 45742176 45662228 45722259 46112271
            46242310 46262331 46322362 46302397 46512418
            47092428 47522451 48452495 48582447 48292352
            48472310 49072302


Last Updated: 349 PM EDT WED OCT 18 2017