AWUS01 KWNH 241325
FFGMPD
CAZ000-242200-

Mesoscale Precipitation Discussion 1184
NWS Weather Prediction Center College Park MD
825 AM EST Tue Dec 24 2024

Areas affected...Eastern Central Valley & Lower Slopes of Sierra
Nevada of California...

Concerning...Heavy rainfall

Valid 241330Z - 242200Z

SUMMARY...Quick hitting AR is starting to weaken, but 1-1.25" of
upslope along lower slopes of central and southern Sierra Nevada
will continue to pose a heavy rainfall threat into the afternoon.

DISCUSSION...GOES-W WV suite shows the very strong cyclone that
spurred the strong moisture advection and strong winds with this
Atmospheric River has lifted north and is making landfall
along/north of Vancouver island into central BC.  However, the
base of the longer wave trough remains off-shore and is pressing
eastward toward the central CA coast.  Weak sub-tropical ridging
just NE of the right entrance of the polar jet (see upstream wedge
of mid to upper level strato-cu with west-east transverse banding
signture) well south, generally parallel to Cape Conception.=20=20
CIRA LPW shows a severing of the core of the AR plume about this
location as well; combine this with slowly deminishing winds and
the AR IVT values are and expected to continue to reduce over the
next 6-9 hours, with a continued bifurcation of the moisture plume
near surface and mid-levels broadening post-frontally.   13z
surface analysis depicts the cold front starting to push though
the southern portion of the northern Valley, though low level flow
remains backed across the northern Valley.  Residual deep moisture
and steepening lapse rates will allow for increasing instability
and recent HRRR suggests a few lingering convective cores remain
possible across the Valley moving southeastward post-frontally and
intersecting the already saturated lower slopes of Tehama, Butte
and Yuba counties with potential of spotty .5-1" additional totals
to maintain ongoing flooding concerns in the area.

The main AR core along/ahead of the cold front had an excellent
broad, higher reflectivity core that resulted in .5-1"/hr rates
but that continues to weaken with the aforementioned reduction of
moisture and flux.  Still, the leading edge will have solid
moisture and fairly orthogonal intersection with the lower slopes
of the central and southern Sierra Nevada Range with .75-1" total
PWat and 35 reducing to mid-20kt flow to allow for .5"/hr reducing
to .25"/hr rates for 1-3 hours resulting in quick .75-1.25"
totals.  This may near the naturally lower FFG values (.5-.75"/hr
or .75-1"/3hr) of within the complex/steeper terrain with a very
low but non-zero risk of widely scattered exceedance; though the
probability does not rise to FF possible category and will
consider this a Heavy Rainfall discussion and likely last MPD for
this event as the front rapidly presses through the Sierra Nevada
after 21z.

Gallina

...Please see https://urldefense.com/v3/__http://www.wpc.ncep.noaa.gov__;!!=
DZ3fjg!5TIbpStOrameMt-4LqYaC6SYsiivdq3C1ktByVq2Je6Ylbz9ryj49UqPiLWrsfe4tuSL=
AmPUffAoGofCkvl6d1KZZgc$  for graphic product...

ATTN...WFO...HNX...STO...

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

LAT...LON   40842182 40332159 39872129 39402079 38882053=20
            38312021 37771978 37311927 36931884 35961832=20
            35551828 35161864 35101903 35451942 36021972=20
            36652015 37312038 37812088 38452114 39162152=20
            39702195 40092221 40392241 40692236=20

=3D =3D =3D
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