Mesoscale Precipitation Discussion 1056
NWS Weather Prediction Center College Park MD
532 PM EST Mon Dec 17 2018

Areas affected...Western Washington....Western Oregon

Concerning...Heavy rainfall...Flash flooding possible

Valid 172231Z - 180900Z

Summary...An Atmospheric River will bring widespread heavy
rainfall to the coastal ranges of western Washington and western
Oregon later tonight.  Rainfall rates of 0.5 inches per hour are
possible after 18/04Z with rainfall amounts of 2 to 3 inches
possible through 18/09Z.

Discussion...An upper low in the Gulf of Alaska and its associated
110 knot to 135 knot jet streak will usher a plume of deep
moisture from the eastern North Pacific Ocean to the coastal areas
of Washington and Oregon this evening.  The RAP, HRRR and the NCEP
global models depict that south-southwest flow at 850 mb will
increase to 50 knots or by late this afternoon...with rainfall
rates picking up later in the evening as the flow becomes
increasingly directed onshore at speeds of 55 to 60 knots at 850
mb.  Blended TPW satellite imagery shows the precipitable water
values of an inch or more offshore, with the RAP bring 1 inch
precipitable water values onshore along the Oregon and southern
Washington coast around 18/05Z.

Individual runs of the 18Z RAP and the 20Z HRRR show hourly
rainfall rates in excess of 0.25 inches developing where the deep
moisture plume interacts with the coastal ranges.  Rainfall
amounts of 0.5 inches are occasionally embedded within the quarter
of an inch areas overnight...potentially leading to possible
flooding.

Bann

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

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

LAT...LON   48842455 48202275 46732237 45282248 44592366
            44802398 47232485