Mesoscale Precipitation Discussion 1024
NWS Weather Prediction Center College Park MD
506 PM EST Mon Nov 26 2018

Areas affected...Western Washington, Northwest Oregon

Concerning...Heavy rainfall

Valid 262204Z - 271000Z

Summary...A strong atmospheric river will continue to impact
western Washington, with heavy rainfall expected in favored
upslope regions of the Olympic Mountains and the Northern
Cascades. Rainfall totals of 2-4 inches with localized amounts of
6+ inches are expected in those areas through 09Z, and that may
cause some localized runoff and flooding issues. Late in the
evening, the rain will begin to shift further down the coast,
primarily impacting the coastal ranges near the Washington-Oregon
border.

Discussion...A deep channel of 50+ knot southwesterly flow in the
925-700mb layer will continue to be focused into western
Washington through around 05Z as a front arrives along the coast.
This flow was being sampled by the KLGX radar VWP, which has been
measuring winds of over 50 knots through the depth of that layer
for several hours. This will provide another 8-9 hours of strong
orographic ascent in the Olympics and northern Cascades, embedded
within a ribbon of precipitable water values over 200 percent of
normal (per CIRA satellite analysis). Several GPS-PW observations
on the Washington coast have exceeded 1.2 inches this afternoon.
The KATX radar VWP does show southwesterly flow of 50-60 knots in
the 850mb layer to the west of the Cascades, so the potential for
heavy rain should extend further inland to those ranges as well.

Although there are likely to be some waves of rain, where the
intensity will fluctuate, 8-9 hours of even an average rain rate
of 0.25 in/hr would yield around 2.5 inches of rain in the upslope
regions. However, there is evidence that the rain rates will be
higher at times, with the 18Z HREF showing very high neighborhood
probabilities of 0.50 in/hr rain rates in the aforementioned
favored upslope areas. In fact, the neighborhood probability of 2
inches of rain in just 3 hours (from 03-06Z) in the Olympics is as
high as 60 percent. These sort of rainfall amounts in short
periods of time would be sufficient to cause some runoff issues
and localized flooding.

After 06Z, the winds in the atmospheric river will begin to veer
and weaken as the front reaches the coast, and the axis of highest
PW will shift further south. This will likewise shift the threat
of heavy rain into the coastal ranges of southwest Washington and
northwest Oregon. Some hi-res models show increasingly vigorous
convection later in the overnight period with some marginal
instability (MUCAPE around 100-200 j/kg), raising the possibility
of more isolated but focused bands of rain rates exceeding 0.5
in/hr. While the threat of flooding would not be as high as
further north (as the rain would be unlikely to be sustained for
as long a duration), it cannot be ruled out.

Lamers

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

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

LAT...LON   49142246 49002192 48762169 48512125 47992122
            47632127 47302176 47052214 46582205 46072209
            45832268 45352331 44872373 45112417 46122424
            47522467 48292503 48422461 48062375 47882312
            47752272 47752249 48032225 48392239 48962289