Subj : HVYRAIN: Excessive Rainfa
To   : All
From : Mike Powell
Date : Fri Dec 29 2023 08:26:00

FOUS30 KWBC 290821
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
321 AM EST Fri Dec 29 2023

Day 1
Valid 12Z Fri Dec 29 2023 - 12Z Sat Dec 30 2023

....THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL FOR PORTIONS OF
CENTRAL AND NORTHERN CALIFORNIA THROUGH SOUTHWEST OREGON...

A highly amplified trough will move into the West Coast Friday.
The base of the trough will move into California today, immediate
downstream of which the flow will abruptly turn southerly upwind
of a large amplified ridge over the Rockies. In fact the southerly
flow will reach as far north as the coast of the Arctic Ocean near
the Yukon/Nunavut border. The result will be a strong cold front
that will only slowly move east, but all of the associated
rainfall will be directed almost due south to north in the broad
southerly flow. The strong forcing will be the driver for some of
the heavier rainfall rates, with orographic uplift the other
primary contributing factor. Thus, the mountains of the coastal
ranges will be where the greatest flash flooding concerns will be,
especially those that were hit hard in recent weeks. The Marginal
Risk was expanded a bit down the coast to include portions of the
Southern Coast Ranges.

As the front pushes inland into the Central Valley, the south
facing slopes of the Klamath Mountains will be the primary focal
point for the heaviest rains and hence the best chances of flash
flooding. The Marginal Risk for this area remains unchanged with
this update.

Wegman


Day 2
Valid 12Z Sat Dec 30 2023 - 12Z Sun Dec 31 2023

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Wegman






Day 3
Valid 12Z Sun Dec 31 2023 - 12Z Mon Jan 01 2024

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Wegman

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