Mesoscale Precipitation Discussion 0017
NWS Weather Prediction Center College Park MD
1256 PM EST Mon Feb 03 2025

Areas affected...Northern California & San Francisco Bay Area...

Concerning...Heavy rainfall

Valid 031800Z - 040600Z

SUMMARY...AR continues though orientation will become more NE-SW
with time as weak height-falls and shortwave press slow moving
cold front southwest toward the Redwood Coast of California and
increasing rainfall potential with some embedded convective
elements possible with hourly rates of .33-.5".

DISCUSSION...18z surface analysis depicts an atypically well
defined surface front across NW NV into the northern portions of
the CA Sierra Nevada into the southern slopes of the Trinity
mountain and toward a weak surface wave near Cape Mendocino. 
Strong southerly veering to southwesterly flow through the
northern Valley continues to intersect and ascend with moderate
moisture into the upper 40s/lower 50s in Tds.  Cold air into SW OR
allows for a fairly steep isentropic ascent pattern to maintain
light to moderate rainfall across the rim of mountains across the
northern Valley with snow levels still above most but the highest
peaks of the terrain.   As such, prolonged .15-.25"/hr rates are
likely to continue for a few more hours before the orientation of
the AR/plume changes with the advance of the front; generally with
a fulcrum/pivot point of the plume centered in N CA near
Shasta/Siskyou county line.

GOES-W IR and WV suite show a defined shortwave comma
east-southeast of the 40N130W benchmark continuing to advance
rapidly northeast under the influence of the rapidly exiting 150kt
jet streak and associated right entrance region.  As the
ascent/shortwave-ridge pattern passes with this entrance region,
influence of surface to 850mb northwesterly flow will allow for
eastward progression of the well defined frontal zone.  CIRA LPW
sfc-850mb layer shows the stark moisture difference across the
boundary with solid .5" over-topped by .25-.3" at 850-700mb and
further solid core of subtropical moisture already at 700-500mb
along/downstream of the shearing shortwave to further increase
total moisture to over 1" nearing 1.25" intersecting the Redwood
Coast southeast of Cape Mendocino.   The shortwave is expected to
further elongate/shear while reaching the NW CA coast by 00-02z
period and the associated cold front intersection with the coast
will drop southeastward.  Forty-five to 60 degrees of sfc-850
25-35kt confluent flow will support IVT to increase slowly from
300-400 to 400-600 kg/m/s as the core of deepest moisture overlaps
after 00-06z.  Some mid-level CAA may support very weak vertical
development given 50-150 J/kg possible; though driving mechanism
is likely to be more low level convergence and orographic ascent
across the Coastal Range.  Typical .25-.33"/hr may occasionally
increase to .5"/hr across S Mendocino county into Sonoma county
after 00z.    Scattered showers may extend into the Bay and Santa
Cruz mountains late in the period (03-06z), though rates are not
likely to be sizable for any flash flooding with exception of most
susceptible urban locations.

...Northern Valley/Lower Slopes of Sierra Nevada...
Southerly flow will veer to more southwesterly and increase
orographic ascent toward perpendicular as the shortwave/DPVA
crosses the region after 00z.  Solid remaining moisture in the
valley will ascent and present solid potential for .25-.5"/hr
rates resulting in spotty totals of 2-2.5" by 06z.  Given recent
heavy rainfall and increased upper soil saturation increased
run-off will occur resulting in areal expansion of increased
riverine stream flows.

All locales and rates are likely to be below critical values for
any flashy style flooding and so will keep the tag on this
discussion at Heavy Rainfall but will continue to monitor for any
highly localized issues that may unfold. 

Gallina

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

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

LAT...LON   41412218 41392159 41082138 40842145 40682170
            40402166 40062151 39822115 39602081 39192055
            38772036 38372059 38532160 38292196 37802207
            37182183 36892198 36952237 37972305 38442344
            38962384 39722393 40052434 40502446 40972405
            40732365 40942346 40832288 41112243