AWUS01 KWNH 240212
FFGMPD
CAZ000-ORZ000-241410-

Mesoscale Precipitation Discussion 1183
NWS Weather Prediction Center College Park MD
911 PM EST Mon Dec 23 2024

Areas affected...Northern CA into Southwest OR

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

Valid 240210Z - 241410Z

SUMMARY...An atmospheric river moving into northern CA and
southwest OR will bring an increase in rainfall coverage and
intensity tonight into early Tuesday. Rainfall rates locally as
high as 0.5" to 1" in an hour may result in some flood risk.

DISCUSSION...This is a fairly progressive system, which will limit
the magnitude of rainfall totals and flood risk. However this is
also a dynamic system with a strong upper jet and deep mid level
trough moving inland. These features will help enhance ascent and
also allow for some weak instability along the front supporting
low topped convective elements locally enhancing rainfall rates.
The 18z HREF neighborhood probabilities of exceeding 0.5" in an
hour increase and expand in coverage ~04z across southwest OR and
northwest CA and peak in the 60-90% range from 06z-12z, with the
higher probabilities shifting southward with time. There are even
some 1" an hour probabilities showing up, peaking ~40% in the King
Range between 06z and 08z. The last few runs of the HRRR also
shows peak hourly rain ~0.75-1" along a narrow band from the King
Range into far southwest OR between ~04z and 08z. These rates
decrease some as the low topped convective line moves inland away
from the slightly higher instability just offshore, but both HREF
probabilities and recent HRRR runs still support localized
rainfall exceeding 0.5" in an hour inland. Despite the progressive
nature of this system, the potential for these aforementioned
higher rates does introduce some risk of rock and land slides long
with minor flooding of urban and other flood prone areas.
Antecedent rainfall has resulted in above average soil saturation
and near to above average streamflows, making the flood risk a bit
higher than it otherwise would be for such an event.=20

Guidance also suggests we may see anther uptick in rainfall rates
around 12z over the northern Sacramento Valley and adjacent
foothills. A combination of enhanced low level convergence driven
by stronger southerly flow up the valley, and slightly higher=20=20
instability values over the valley by this time, supports this=20=20
uptick in rainfall rates potentially towards 1" in an hour. These
higher rates will likely be fast moving by this time, and so the=20
heaviest rain at any one location should not last more than an=20=20
hour or so. This will help limit impacts, although rates could be
high enough to still result in some localized flood concerns.
There is a chance rainfall rates between 0.5" and 1" in an hour
could move over the Park burn scar around 12z. Rainfall rates of
this magnitude could result in debris flows, so while confidence
in rates reaching these magnitudes remains low, observational
trends will need to be monitored closely late tonight into Tuesday
morning.

Chenard

...Please see https://urldefense.com/v3/__http://www.wpc.ncep.noaa.gov__;!!=
DZ3fjg!4eSVyvBEISJtm2Pp2s5Uusl5WFNV6pSi3sVPQ6FGzsRFy5CL5U2_IJSkY2yzAZpq4KuM=
FkOxjEfRzT2GBGEDAt48HEw$  for graphic product...

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

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

LAT...LON   42842444 42652390 42122329 41652327 41392245=20
            41312189 40772164 40382141 40052118 39932108=20
            39662122 39702164 40012206 39792264 39202259=20
            38592274 38302316 38482348 38712368 38982389=20
            39782438 40242463 41152454 42102450 42642462=20
=20=20=20=20=20=20=20=20=20=20=20=20

=3D =3D =3D
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