757
AWUS01 KWNH 211731
FFGMPD
CAZ000-220400-

Mesoscale Precipitation Discussion 1163
NWS Weather Prediction Center College Park MD
1230 PM EST Thu Nov 21 2024

Areas affected...Northern California...

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

Valid 211730Z - 220400Z

SUMMARY...Remarkably static and intense AR moisture flux
continuing to compound rainfall totals.  Spots of add`l 3-6"
totals expected through 04z ahead of next enhanced surge
associated with deepening cyclone.  Soils are nearly fully
saturated so greater run-off is expected.  Flash flooding remains
possible.

DISCUSSION...GOES-W Airmass and RAP analysis shows post-frontal
deep dry air mass with red hues converting quickly to yellows and
greenish-blues along and south of the front depicts strength of
the front and ability to increase thermal wind along and south of
the boundary though a large depth of the atmosphere.  This
unidirectional flow through depth is expected to remain though the
next 9 to 12 hours directed at the Redwood Coastline and
continuing to infiltrate well into northern California and the
northern Sierra Nevada and Trinity Ranges...continuing similar
evolution to unfold.

This will be changing into the 03-06z period...GOES-W WV suite
denotes a short-wave rounding the base of the global trough just
east of 40N140W, which will be spurring rapid cyclogenesis over
the next 12-24hrs.  This will strengthen and amplify the flow
nearing the coast, but that is for subsequent MPD issuance.  Its
influence, however, is starting to be felt in other downstream
fields; including upper-level jet starting to split/ridge a bit
across coastal OR and peripherally N CA.  This will provide broad
right entrance ascent from the exiting jet streak across the
region and a very weak surface wave(s) are noted near Cape
Mendocino into SW OR; low level winds appear to be responding with
slightly backed flow especially along and east of the coastal
range, further enhancing deep moisture flux through the northern
Sacramento Valley and increasing orographic ascent (while further
increasing freezing levels above nearly all peaks minus Shasta by
the end of the valid time).

CIRA LPW analysis shows a small split in the low level moisture
further upstream, likely associated with the approaching shortwave
DPVA/cyclogenesis.  This enhanced area however, has solid
isentropic slope through the 700-500mb layer that is currently
intersecting far NW CA and overlaid with the leading surface to
850 (and 850-700mb) slug of moisture from Cape Gorda through the
San Francisco Bay (and up through the Sacramento Valley). This is
resulting in solid core of 1.25-1.5" with embedded spots just over
1.5".  The strength of the moisture flux in the sfc-850mb (about
.6-.75") on steady 45-50kt winds is solidly within the 95-99th
percentile of CIRA LPW moisture flux values.  This matches with
consistent and remarkably broad 600 kg/m/s ticking up to 700
kg/m/s in the core directed at S Mendocino/N Sonoma county.  This
will result in consistent .33"/hr rates with occasional .5"+/hr
spurts in the southwest facing topography of the Coastal Range.
Spots of additional 3-6" totals are expected and given 6-12" that
have fallen in spots, soil saturation has reached 90% with spots
over that.  This will result in increased run-off and may spur
mudslides and debris flows, especially in/near recent burn scars.
As such, will continue to tag this MPD as flash flooding
possible...though it remains more the duration of the rate that is
driving the flooding conditions.

A bit less consistent with a slow northward intersection with the
northern Sierra Nevada Range of 500-600 kg/m/s, but slightly
increased moisture flux convergence due to cyclonic acceleration
through the gap of the Bay, should see similar rates and totals in
strongest orographic ascent (Butte/Plumas county). OAK 12z RAOB
showed near daily record of 1.25" TPW and that continues to
increase with approaching core of moisture through the gap to
indicate how anomalous the deep moisture is within the central
valley.  Slow sfc to 850mb backing of the flow will redirect
orientation of the plume toward the eastern Trinity Range toward
00z, reducing orthogonal flow into the Sierra Range.  Still,
swaths of 3-5" from Butte to Tehama and 2-3" into Shasta county
are expected.

Gallina

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

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

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

LAT...LON   41582391 41562347 41322293 41382250 41362231
            41262177 40722153 40212113 39462076 39072106
            39122139 39462177 39692222 39332243 38902229
            38412237 38212291 38522340 38822370 39272392
            39742400 40212441 40592447 41092422