Mesoscale Precipitation Discussion 1057
NWS Weather Prediction Center College Park MD
356 AM EST Tue Dec 18 2018

Areas affected...Western OR into northwest CA

Concerning...Heavy rainfall

Valid 180900Z - 182100Z

Summary...Deep moisture associated with an atmospheric river,
combined with strong upslope flow, could result in hourly rainfall
rates of 0.50 inches or greater through 18/21z, with total
rainfall amounts of 2 to 5 inches possible.

Discussion...The GOES-15 WV loop clearly depicted a 150 knot jet
associated with an unfolding atmospheric river pointed toward far
southwest WA/OR early this morning. A 50/60 knot low level flow is
helping to focus 1.00/1.25 inch precipitable water air (per the
most recent blended TPW product) on the higher terrain of western
OR early this morning. The deep moisture plume is already
supporting hourly rainfall rates approaching 0.25 inches (based on
surface observations) over portions of the Coastal Ranges in
northwest OR.

An approaching short wave should result in increasing diffluence
over western WA/western OR into far northwest CA through the
morning hours, and the large scale ascent associated with it could
support a large area of 0.25/0.50 inch hourly rainfall rates over
the higher terrain.  Through about 18/15z, the best forcing and
strongest low level flow are expected to impact the northern and
central OR Coastal Range, with rainfall amounts through that time
of 2 to 4 inches. These rainfall amounts could result in localized
flooding issues.

As the short wave crosses northwest WA (mainly after 18/15z), the
low level flow is shunted southward, affecting the Klamath
Mountains in southwest OR and far northwest CA. By this time, the
45 to 65 knot low level flow will be almost orthogonal to the
terrain, resulting in another burst of 0.50+ inch hourly rainfall
rates. The most recent HRRR suggests that rainfall rates could
exceed 0.75 inches during this time, but a general lack of
instability (HRRR soundings showed MUCAPE values less than 100
J/KG) could make these values more difficult to attain.

In any event, rainfall totals of 2 to 5 inches are possible over
the favored upslope regions of the central and southern Klamath
Mountains through 18/21z. It should be noted that there are recent
burn scars over portions of far southwest OR and far northwest CA,
and the high rainfall rates could lead to mudslides and debris
flows.

Hayes

ATTN...WFO...EKA...MFR...PDT...PQR...SEW...

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

LAT...LON   47112207 47102181 46902172 46592156 46032142
            45362139 44842149 43802165 43142172 42812166
            42282171 42062199 42032240 42272270 42692288
            42632343 42222344 41852322 41592304 41192286
            41042324 41172392 42222434 45862400 46982381
            46962306 46752293 46442286 46222287 45982299
            45682291 45612278 45642259 45772254 46092255
            46312255 46652252 46982238