FOUS30 KWBC 252048

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
348 PM EST Sat Jan 25 2020

Day 1
Valid 2045Z Sat Jan 25 2020 - 12Z Sun Jan 26 2020


...Far Southwest Oregon into far northwest California...
In the wake of height falls/mid-level shortwaves moving into the
Pacific Northwest early this period, an upstream area of northeast
Pacific height falls/mid-level troughing will be amplifying and
moving quickly toward the Pacific Northwest/Northern California
coast Saturday.  Strengthening southwest low level flow ahead of
the amplifying northeast Pacific system will raise precipitable
water values to 2 to 2.5 standard deviations above the mean
Saturday afternoon into Saturday night across northern California
into much of Oregon.  The onshore low level southwest flow will
produce strong orographic lift from far northwest California into
far southwest Oregon where 850-700 mb moisture flux anomalies
reach 2 to 2.5 standard deviations above the mean.  Periodic
rainfall rates approaching 0.5" in an hour will be possible in the
favored terrain of northwest CA and southwest OR.  Model consensus
is for widespread areal average amounts of 1-2" with isolated max
totals of 3-5" in the favored upslope areas.  This is reflected in
the HREF neighborhood probabilities which are high for 2"+ amounts
this period 90%+ and 50-70% for 3"+ across the marginal risk area.
 With stream flow anomalies above to much above as per the
National Water Model...isolated runoff issues may occur given the
expected heavy rains.


Day 2
Valid 12Z Sun Jan 26 2020 - 12Z Mon Jan 27 2020

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.


Day 3
Valid 12Z Mon Jan 27 2020 - 12Z Tue Jan 28 2020


...Southwest Washington to northwest California...

...20Z Update...
The below discussion remains relevant, however, did increase QPF
across the coastal ranges of Washington and Oregon as the next
storm system approaches lifting a warm front across the region. 
While this is a weak atmospheric river at best, there is still
strong energy and moisture that will be directed into the Pacific
northwest, especially later Monday into early Tuesday. And given
this region has encountered a series of impulses for weeks now,
the antecedent conditions will help promote a higher potential for
localized flash flooding. This region has observed >150% percent
of normal precipitation resulting in saturated soils with
additional rainfall quickly becoming runoff. In addition, with the
potential for weak instability moving onshore, rain rates may
approach 0.50 inches/hour. The question remains where will the
strongest moisture flux occur with the low level flow becoming
nearly orthogonal to the terrain. It appears most of the model
guidance and their subsequent ensembles are hinting at a more
southern solution. Thus, maintained the Marginal Risk with the
notion that the higher rainfall amounts and rates may actually
occur along the spine of the coastal ranges from Oregon into far
northern California with better forcing and moisture.

...Previous Discussion...
Another wet period is in store for the Northwest coastal areas
Monday as an approaching surface low pressure system pushes
onshore, ushering with it a deep plume of moisture. Widespread
rainfall will range from moderate to heavy as the cold front
advances inland into the interior Northwest; meanwhile, the trough
aloft will become reinforced by a secondary system tracking across
the North Pacific Ocean toward the Pacific Northwest which will
prolong the wet period beyond the day 3 forecast. Multiple pieces
of guidance are suggesting areal averages of 1 to 3 inches all
along the coast with local maxes of 4 to 6 inches. Portions of
California and far southern Oregon have had recent rains to help
bring soil saturation levels up; however, further north up the
coast parts of Oregon and southern Washington have been drier for
the season. While this region could accommodate a period of
moderate rain for a few hours before becoming problematic,
rainfall rates 0.50 inch per hour or greater could quickly lead to
an increased risk for flash flooding, or debris flows in sensitive
areas such as steep terrain or burn scars. A Marginal Risk for
excessive rainfall was hoisted for southwest Washington to
northwest California.   


Day 1 threat area: https://www.wpc.ncep.noaa.gov/qpf/94epoints.txt
Day 2 threat area: https://www.wpc.ncep.noaa.gov/qpf/98epoints.txt
Day 3 threat area: https://www.wpc.ncep.noaa.gov/qpf/99epoints.txt