From Mike Powell@618:250/1 to All on Mon Nov 17 09:35:04 2025
AWUS01 KWNH 171014
FFGMPD
CAZ000-171600-
Mesoscale Precipitation Discussion 1227
NWS Weather Prediction Center College Park MD
514 AM EST Mon Nov 17 2025
Areas affected...portions of northern and central California
Concerning...Heavy rainfall...Flash flooding possible
Valid 171001Z - 171600Z
Summary...A stout mid-level wave west of Eureka was prompting
scattered shower and thunderstorm activity across a broad part of
northern and central California. These trends will continue for
the next few hours, and areas of 0.5-0.75 inch/hr rain rates are
possible. A few instances of flash flooding could occur in
sensitive areas and near burn scars.
Discussion...A vigorous mid-level wave was providing ascent for
several areas of convection 1) just west of Eureka over open
Pacific waters and 2) along an axis from Redding south-southwest
to near Monterrey. Over the past hour, more robust convective
activity (with lightning) developed near/south of San Francisco
and moved inland. Both of these regimes were responsible for
areas of 0.5-0.75 inch/hr rain rates. The Redding/Monterrey
convective band was being forced by a strong front moving
southeastward across the region. PW values of 1 inch along and
ahead of the front near the coast was aiding in the heavier rain rates.
Over time, as the aforementioned front shifts southward across the
state, strong upslope flow will aid in continued moderate to heavy
rainfall and a few instances of flash flooding especially along
upwind areas of the Sierra. The front will also aid in occasional
bursts of heavier rainfall along coastal areas from Monterrey
southward toward the Transverse Ranges/Santa Barbara County
through 16Z/8a Pacific time. Sensitive areas/burn scars could
experience another round of debris flows and excessive runoff.
Another more conditional risk for heavier rainfall will occur near
the center of the mid/upper trough near Eureka. Slow-moving
convection was being supported by ascent from the trough and very
cold temps aloft (-20 to -24C @ 500 mb). Heavier rainfall could
occur between Eureka and Santa Rosa. Flash flooding is also
possible with this activity.