367
AWUS01 KWNH 220401
FFGMPD
CAZ000-ORZ000-221600-

Mesoscale Precipitation Discussion 1164
NWS Weather Prediction Center College Park MD
1100 PM EST Thu Nov 21 2024

Areas affected...Northern California...Southwest Oregon

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

Valid 220400Z - 221600Z

SUMMARY...Strong atmospheric river activity will continue to
impact areas of northern CA overnight and into early Friday while
gradually refocusing farther north back into portions of southwest
OR. Widespread areal flooding concerns will continue, and there
will also still be a possibility for some localized burn scar
flash flooding impacts.

DISCUSSION...The latest GOES-W GeoColor satellite imagery is
showing the rapid deepening of a new area of low pressure offshore
of the West Coast as strong shortwave energy rounds the base of
the persistent larger scale upper-level trough. This low pressure
center will strengthen over the next 6 to 12 hours and move to a
position southwest of Vancouver Island by later Friday morning. As
this occurs, the long duration, multi-day atmospheric river that
continues to impact northern CA will advance a bit farther north
and edge back into areas of southwest OR which will be the result
of a backing of the deeper layer flow across the region and the
northward advance of a warm front. By very early Friday morning, a
cold front will then begin to arrive across coastal areas of
southwest OR and northwest CA.

Enhanced low to mid-level southwest flow out ahead of this cold
front will continue to drive areas of persistently heavy rain
given the level of moisture transport and warm air advection
coupled with upslope flow/orographic ascent over the higher
terrain. The 850/700 mb moisture flux anomalies are forecast to
peak between 3 and 5 standard deviations above normal by around
12Z (4AM PST) across coastal areas of northwest CA and southwest
OR and the IVT magnitudes are forecast to locally peak in between
750 and 1000 kg/m/s. While the highest values of IVT will be along
the immediate coast, some of these elevated IVT magnitudes are
forecast to spread inland including the northern Sacramento Valley.

The 00Z HREF guidance suggests rainfall rates will occasionally
reach 0.50" to 0.75"/hour, with potentially some spotty rates in
the 0.75" to 1.0"/hour late tonight as peak IVT magnitudes arrive
ahead of the cold front. Additional rainfall totals going through
early Friday morning are expected to reach 3 to 5 inches, with
isolated heavier amounts not out of the question. Given the
persistence of the rainfall, and level of soil saturation and high
streamflows already from this multi-day atmospheric river event,
there will continue to be concerns for widespread flooding going
through Friday morning. This may again include concerns for some
burn scar flash flooding impacts at least locally.

Orrison

...Please see www.wpc.ncep.noaa.gov for graphic product...

ATTN...WFO...EKA...MFR...MTR...REV...STO...

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

LAT...LON   43542413 43432352 42482323 41812272 41672224
            41692162 41402136 40982140 40402082 39812057
            39322086 38962179 38232215 38022260 38282320
            39062390 39812426 40672459 42002459 43102467