Mesoscale Precipitation Discussion 0800
NWS Weather Prediction Center College Park MD
622 PM EDT Fri Jul 25 2025

Areas affected...Northern California and far Northwest Nevada

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

Valid 252221Z - 260415Z

Summary...Showers and thunderstorms will continue to pivot slowly
across portions of northern CA through this evening. Rainfall
rates within convection could reach 1-1.5"/hr, leading to 1-2" of
rain or slightly more in some areas. This may cause isolated
instances of flash flooding.

Discussion...The GOES-E day-cloud phase microphysics RGB this
afternoon shows rapid growth of updrafts into Cb along the
northern Sierra Nevada and into the other high terrain of northern
California including the Shasta/Siskiyou and Klamath regions.
These deepening updrafts are leading to numerous thunderstorms
which are depicted via the regional radar mosaic. This activity is
building in response to steepening lapse rates ahead of an upper
low clearly indicated on WV imagery pivoting northward from near
San Francisco, leading to a region of impressive upper level
directional diffluence. At the same time, light onshore flow is
pushing PWs to above 1 inch along the immediate coast, with a
gradient falling to around 0.6 inches across the Sierra, both of
which are still well above normal and approaching the 90th
percentile in some areas. This elevated moisture combined with
modest, but sufficient, MLCAPE of 250-500 J/kg are providing
favorable thermodynamics to support heavy rainfall which has been
measured via MRMS to be locally above 1"/1hr and as much as
0.4"/15 mins.

As the aftn progresses, the northward advance of the upper low
should provide continued synoptic ascent, while additionally
supporting increased instability potentially surging as high as
750 J/kg. In this environment storms should continue to develop,
expanding in both coverage and intensity as reflected by simulated
reflectivity in the high res CAMs. Additionally, the HREF
neighborhood probabilities for 1"/hr rates peak above 40% from
around 23Z to 02Z, further indicating the continued enhancement in
the next few hours. These storms will move slowly in the vicinity
of this upper low as well, noted by 0-6km mean winds of just
around 5 kts, and similarly weak and chaotic propagation vectors
which could indicate short-term training at times. While storms
should remain of the pulse variety due to a lack of meaningful
shear, the coverage of cells will likely lead to mergers and
collisions, locally enhancing the intensity and duration of heavy
rainfall to produce 1-2" of rain, or locally higher (HREF PMM
nearly 3 inches) in a few areas.

In general, the 0-10cm soil percentiles from NASA SPoRT are quite
dry, indicating a high filtration capacity which should be able to
absorb much of this rain. However, these intense rates, especially
during any mergers, or where duration lengthens due to collisions,
could cause runoff concerns. This will be most likely atop any
more vulnerable terrain or sensitive burn scars where isolated
flash flooding instances could occur.

Weiss

ATTN...WFO...EKA...LKN...MFR...REV...STO...

ATTN...RFC...PTR...RSA...NWC...

LAT...LON   42112125 42082006 41811920 41391895 40911896
            40561914 40181945 39551957 39141966 38681959
            38641979 38832020 39142059 39522097 39922129
            40322155 40622185 40592224 40252234 39932236
            39522272 39532326 39942338 40472353 41332347
            41712299 41992219