Graphic for MPD #0042

Mesoscale Precipitation Discussion 0042...Corrected
NWS Weather Prediction Center College Park MD
435 PM EST Wed Jan 31 2024

Corrected for Typo in third paragraph

Areas affected...Northern and Central California

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

Valid 312122Z - 010502Z

Summary...A shallow convective line with 0.50-1"/hr rainfall rates
is traversing through northern California along a cold front
associated with strong low pressure offshore. This southward
translating line may lead to instances of flash flooding going
into tonight, focused along the northern and central California
coastline.  

Discussion...The KBHX (Eureka,CA) radar this afternoon continues
to track a shallow convective line gradually marching inland along
a cold front associated with a strong 968 mb cyclone in the
Eastern Pacific.

As highlighted in MPD 041, this activity is embedded within a
plume of widespread stratiform rainfall as a long fetch (3000+
miles) of Pacific moisture streams northward ahead of the cold
front, with precipitable water values 200-350% of normal noted
across northern California and the Pacific Northwest per blended
TPW. For much of today, rainfall rates have hovered in the
.25-.50"/hr range, generally focused in favorable upslope areas of
the California Coastal Range. However, the passage of the frontal
band over the last hour ushered in observed rainfall rates of
.60-.90"/hr in the vicinity of KHBX, along with several reports of
flooding along Highway 254.

This narrow cold frontal convection is expected to expand and
translate to the south along the coast as strong frontogenetic
forcing focuses the activity in the presence of 600-700 kg/m/s IVT
ahead of the front, and low (but non-negligible) 100-200 J/kg of
MUCAPE. The very favorable upper-jet dynamics highlighted in MPD
041 will also persist into tonight, including strong left-exit
forcing and widely diffluent 350-250 mb flow.

This scenario is reflected by the 12z HREF suite, which maintains
rainfall rates of .5-1"/hr along the front going into tonight. The
progressive nature of the cold front will likely limit a more
robust flash flooding threat. However, 1 HR FFGs are low
(.75-1"/hr), and increasing stratiform rainfall ahead of the
convection will precondition soils for flash flooding aside from
vulnerable urban population centers. Thus, several instances of
flash flooding remain possible going into tonight.

Asherman

ATTN...WFO...EKA...HNX...LOX...MTR...STO...

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

LAT...LON   41772400 40892325 38832227 36032064 35492148
            37952328 39872429 41172457
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Last Updated: 435 PM EST Wed Jan 31 2024