Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
359 PM EDT Tue Apr 15 2025

Day 1
Valid 16Z Tue Apr 15 2025 - 12Z Wed Apr 16 2025

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

Bann


Day 2
Valid 12Z Wed Apr 16 2025 - 12Z Thu Apr 17 2025

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
NORTHERN CALIFORNIA AND WESTERN NEVADA...

2030Z Update...
Except for tightening up the eastern boundary of the Marginal Risk
area...no changes made to the previously issued outlook area.

Bann

Previous Excessive Rainfall Discussion...
A slow-moving upper level low embedded within a broader positively
tilted trough will move towards the central California coast on
Wednesday. The associated upper level cold air will overspread
central California with sufficient surface heating to increase
instability along the Sierras to between 500 and 1,000 J/kg. The
upper low will also bring a bit of Pacific moisture into the region
as PWATs increase to between 0.5 and 0.75 inches, which for
central California is as much as 3 sigma above normal and exceeds
the 95th percentile for climatology. Corfidi Vectors will generally
be out of the north to northeast, which will favor south to
southwest moving convection developing on the Nevada side of the
Sierras then slowly moving over or along the range into the
foothills of the Central Valley. Given this setup, there is
potential for training convection since all of the factors that are
coming together to support heavy rainfall from the stronger storms
will be slow-moving and persist through the afternoon and evening
hours. Snow levels from the NBM Wednesday afternoon will be between
8,000 and 9,000 ft, allowing most of the precipitation that falls
to be in the form of rain except at the highest elevations of the
Sierras. Abundant snowpack also remains in place in the Sierras
below the melting level, which will allow for some contribution to
rising streams and creeks to be from snowmelt. Thus, an isolated
flash flooding risk remains in this area. The inherited Marginal
risk was trimmed on the edges, especially into western Nevada,
since the storms will generally track towards California. However,
no big changes were made with this update.

Wegman


Day 3
Valid 12Z Thu Apr 17 2025 - 12Z Fri Apr 18 2025

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
SOUTHWESTERN UTAH...

2030Z Update...
Realigned the Marginal area just a bit based on the 12Z model and
WPC deterministic QPF.

Bann


Previous Excessive Rainfall Discussion...
The upper level low causing the marginal risk of excessive rainfall
in northern California on Day 2/Wednesday will open up and become
an extension of a longwave positively tilted trough over much of
the West on Thursday. The low-turned-trough will accelerate
eastward into southern California and eventually Arizona by late
Thursday night. The energy associated with it will draw a more
substantial upper level shortwave south into the Intermountain West
Thursday and Thursday night. The advance of the associated cold
air will increase the lift ahead of the front. Some of the Pacific
moisture will draw northeastward into southwestern Utah ahead of
the front, supporting more widespread rainfall across the region,
especially Thursday evening into the overnight. While associated
rainfall rates through here are unlikely to exceed a half inch per
hour in any one area for an extended period of time, this rain
falling on very dry and hydrophobic soils are likely to convert
mostly to runoff. Typical flooding locations such as slot canyons
will be at greatest risk for flash flooding. The rainfall should
not be enough to cause widespread flash flooding, but is likely to
cause isolated flash flooding. While higher elevations of Utah may
see mostly snow from this front, once again most areas should
remain as rain.

Wegman

Day 1 threat area: www.wpc.ncep.noaa.gov/qpf/94epoints.txt
Day 2 threat area: www.wpc.ncep.noaa.gov/qpf/98epoints.txt
Day 3 threat area: www.wpc.ncep.noaa.gov/qpf/99epoints.txt