696
AWUS01 KWNH 221645
FFGMPD
CAZ000-230400-

Mesoscale Precipitation Discussion 1165
NWS Weather Prediction Center College Park MD
1144 AM EST Fri Nov 22 2024

Areas affected...cenral to northern CA

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

Valid 221643Z - 230400Z

SUMMARY...The atmospheric river will continue to gradually shift
southward across north-central CA today and early tonight. While
the magnitude of moisture flux will be lower than earlier today,
locally significant impacts will remain possible where areas of
training allow rainfall rates in excess of 0.5 in/hr. Additional
rainfall through 04Z of 3 to 7 inches for the northern Sierra
Nevada and localized totals of 2-4 inches for the Coastal Ranges
are anticipated.

DISCUSSION...16Z surface observations showed that a northward
moving surface low continued to deepen roughly 250 miles west of
the northern OR coast, with an attached occluded/cold front now
inland across northern CA. GOES West water vapor imagery showed
the strengthening mid to upper-level reflection of this surface
low with the associated upper low moving inland across the Pacific
Northwest. IVT values have weakened from near 1000 kg/m/s earlier
this morning near the coastal OR/CA border but remained of a
moderate intensity in the 600-700 kg/m/s range per short term
model forecast data. While the intensity of the atmospheric river
has decreased, there are indications upstream in water vapor
imagery that short periods of stalling may occur with the moisture
axis, along with a temporary increase in IVT values later this
afternoon and early evening. While subtle inflections were noted
in water vapor imagery within the southwesterly flow pointed at
CA, which could allow for brief training, a more notable shortwave
impulse was observed near 35N 140W which could result in a longer
period of training potential.

As this mid-level shortwave impulse near 140W continues to advance
toward the east, RAP guidance indicates some amplification within
the base of the broader parent closed low and slowing/stalling of
mid-upper level height falls across the central CA coast. At the
surface, a weak surface wave along the cold front is forecast to
develop and approach the CA coast which will result in some minor
increase in IVT values (up to 800 kg/m/s) and slowing/stalling of
the plume over the San Francisco metro region with 850 mb winds
intensifying to near 60 kt along the coast. The best indication
for stalling will be in the 21Z-03Z time frame.

Downstream across the Great Valley into the Sierra Nevada, 700 mb
winds up to ~70 kt are expected later today with upslope
enhancement favoring the greatest likelihood of 0.5 to 1.0 inch of
rain in an hour or less. Additional rainfall totals through 04Z of
3-7 inches are expected for the Sierra Nevada along with localized
2-4 inch totals for the Coastal Ranges. Urban flooding impacts
will be possible as well given overlap with the San Francisco and
Sacramento metro regions and the addition of 1 to 2 inches, though
rates are likely to remain below 0.5 in/hr outside of terrain due
to negligible forecast instability.

Otto

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

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

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

LAT...LON   41662174 41602146 41102119 40502072 39572017
            39262006 38712026 38232078 37692133 37022163
            36822200 37132258 37822319 38812384 39602360
            40532302 41182266 41562216