Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
748 PM EST Thu Dec 18 2025
Day 2
Valid 12Z Fri Dec 19 2025 - 12Z Sat Dec 20 2025

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL MAINLY OVER
PORTIONS OF NORTHWEST CALIFORNIA...

20Z Update: Little deviation in the 12z model suite leading to
general run-to-run continuity across both New England and the
Western U.S for the respective ERO risks in place. Small
adjustments were made based on the QPF distribution and HREF
neighborhood probabilities from the latest numerical suite, but
changes were not enough to warrant any considerable detail in
either area. Best chance will likely lie within the coastal terrain
of northwestern CA where flow orthogonal to the terrain will
generate efficient upslope enhancement over the bulk of the period.

Kleebauer

..Previous Discussions..

...Pacific Northwest...

The atmospheric river that came on-shore in the Day 1 period will
be on-going across Oregon...although the event should be winding
down in terms of rainfall rates and additional amounts fairly early
in the outlook period. A well-defined plume of deep layer moisture
will continue to be drawn from the eastern North Pacific Ocean
eastward into parts of the northern Great Basin by fast westerly
flow aloft. IVT values at or above 700 units are forecast to cover
much of Oregon initially...then weaken as the axis of the moisture
plume and associated precipitation settles into northern
California. Maintained a large amount of continuity in terms of
placement of the Slight Risk area clipping northwest California and
a portion of adjacent southwest Oregon.

Saw little reason to make any significant changes to the expanded
coverage of the Slight Risk area made yesterday to cover more into
the Trinity Mountains given latest QPF.

...Southern New England...
A trough crossing the eastern United States will draw increasing
amounts of moisture northward into parts of New England...helping
set the stage for locally heavy rainfall when flow aloft becomes
increasingly diffluent and a surface warm front approaches from
the south early on Friday. WPC Deterministic QPF in the 1.0 to 1.4
inch range is forecast along the track of the low. Offsetting the
potential of excessive rainfall by limiting rainfall rates/amount
is the meager instability expected to be in place.

Bann


Day 2 threat area: www.wpc.ncep.noaa.gov/qpf/98epoints.txt