AWUS01 KWNH 140501
FFGMPD
CAZ000-141500-

Mesoscale Precipitation Discussion 1175
NWS Weather Prediction Center College Park MD
1200 AM EST Sat Dec 14 2024

Areas affected...Northern and Central California....

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

Valid 140500Z - 141500Z

SUMMARY...Initial surge of deep moisture/typical Atmospheric River
will given way to approaching stronger cyclogenesis/flux
convergence with potetial of .75-1"/hr localized showers that may
induce localized flash flooding particularly in/near urban
locations around San Francisco Bay after 11z.

DISCUSSION...GOES-W WV suite depicts a mature strong closed low
along 130W near 46N that has driven an occluded/cold front
through the coastal range of W WA/OR. Solid shortwave ridge
within the upper-level cirrus canopy denotes the left exit of the
130kt 3H jet streaking northward, while the right entrance exists
at the trailing edge of the cold front resulting in a weak surface
to 850mb wave/inflection along/just north of Cape Mendocino. CIRA
LPW denotes this feature with an enhanced moisture pocket of .6"
and .4" with the respective sfc-850 and 850-700mb layers before
connecting back to the main core of the SW to NE oriented warm
conveyor/AR plume. This plume currently intersects the Redwood
Coast from Cape Mendocino to near the entrance to the San
Francisco Bay. Enhanced convergence and weak cooling aloft has
seen some steepening of lapse rates near the surface inflection
back near the Cape, with RADAR indicating some enhanced linear
filaments of convective still remaining but ushering themselves
ashore likely with .5"/hr rates resulting in best opportunity for
short-term flooding concerns. However, the main core of the AR
will continue with solid 45-50kt fairly orthogonal ascent
resulting in 700-800 kg/m/s of IVT that will slowly drift
southward over the next 4-6hrs resulting in average .33 to .5"/hr
rates across the Redwood Coast toward the Napa Region. Additional
totals of 2-3" are likely through 12z as the core of the AR shifts
ashore into the central Valley and lower slopes of the Northern
Sierra Nevada range.

Possible Flash Flooding after 10-15z in Central California...
GOES-W WV suite also depicts the core of a strong mid to low level
cyclone developing just west of 130W about 36-37N quickly
approaching. Upstream strong digging of the trough is
strengthening the descending branch of the mid to upper level jet
rapidly deepening the cyclone. The strong vorticity advection is
expected to peak over the next 3-4hrs just west of the central CA
coast with near negative tilt as a 130-140kt jet streak rounds the
base of the larger scale trough from 09-12z. Low level flow will back/strengthen and flux will steadily increase. Additionally,
CAA will help to steepen mid-level lapse rates and potential for
250-500 J/kg of MUCAPE will accompany the enhanced moisture flux.
Hi-Res CAMs continue to suggest convective streets/elements with
capacity of .75-1"/hr rates likely to focus where the cold front
sags/flattens which looks to be trending near Sonoma county, but
with height-falls/forward propagation along/ahead of the DPVA will
enhance through San Francisco Bay toward 12z. 00z HREF
probability of 1"/3hr is nearly 100% while 1"/hr peaks around
50-60% at 12-13z near the mouth of the Bay; providing enhanced
confidence for possible incidents of flash flooding in/near the
urban locations surrounding the Bay (initially north side before
15z). Localized totals of 1-2.5" are possible with convective
areas near the Bay though spots of 3-5" are also likely along SW
facing orographic peaks in the Trinity Range, Coastal Range of
Mendocino/Lake and Napa counties and lower slopes of Tehama/Butte
county by 15z.

Gallina

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

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

LAT...LON 41372412 41312379 40962356 40822328 40672266
40702194 39892160 39332095 39012071 38642080
38232113 37302130 36662155 36422192 36792243
37392272 38002312 38592356 39072384 39742401
40012425 40322450 40762436 41032427

$$
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