Mesoscale Precipitation Discussion 1022
NWS Weather Prediction Center College Park MD
512 AM EST Mon Nov 26 2018

Areas affected...Washington Olympics

Concerning...Heavy rainfall

Valid 261011Z - 262200Z

Summary...Rainfall intensity will oscillate up/down throughout the
morning across northwest Washington but will steadily increase
overall. Rainfall rates are expected to reach 0.5 in/hr on a more
regular basis near 18Z. Rainfall totals of 2-4 inches, local 5+,
are expected through 22Z.

Discussion...GOES West water vapor imagery showed a large closed
low south of Alaska at 10Z with multiple embedded vorticity
maxima. At the leading edge of the upper low was a strong surface
cold front identified with surface observations and 89 GHz AMSU
imagery, extending southwestward from just off of the southern
coast of British Columbia into the eastern Pacific. Blended TPW
imagery showed a broad swath of 1+ inch precipitable water values
ahead of the front, peaking just just over 1.5 inches between 33N
and 43N. At 0915Z, KLGX reflectivity showed a north-south axis of
30-40 dBZ echoes just offshore of the coastline, steadily moving
east. AMSU likely measured rainfall rates peaking over 0.25 in/hr
within this band from 0551Z when it was south of western Vancouver
Island. GOES high density winds near 250 mb matched closely to the
00Z GFS and recent runs of the RAP with a 130 kt jet core pointed
toward Vancouver Island and becoming sharply diffluent over land.

Consistent forecasts from the RAP, with a slightly faster trend,
seem to be in line with observations so far and show the cold
front slowly approaching the West Coast over the next 6-12 hours
along with increasing low level moisture transport ahead of the
boundary reaching the western coast of Washington. SSW flow at 850
mb should favor upslope enhancement of rainfall over the Olympics
and current 35-40 kt 850 mb winds are forecast to increase and
peak into the 60-70 kt range near 18Z. This magnitude when
combined with precipitable water values of 1.1 to 1.3 inches along
the coast and upslope enhancement inland are expected to allow
rainfall rates to increase to 0.5 in/hr over the Olympics near 18Z
on a more consistent basis. Rainfall of 2-4 inches, with locally
higher values are expected through 22Z. Given high freezing
levels, all precipitation will fall as rain and these totals may
cause localized runoff issues over more sensitive locations.

Otto

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

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

LAT...LON   48322463 48232428 48122399 48032367 47902327
            47802312 47632301 47432327 47212362 47112399
            47202433 47542457 47892475 48112481 48282476