FOUS30 KWBC 250800
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
300 AM EST Sat Jan 25 2025

Day 1
Valid 12Z Sat Jan 25 2025 - 12Z Sun Jan 26 2025

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF THE TRANSVERSE/PENINSULAR RANGES AND LOS ANGELES BASIN OF
SOUTHERN CALIFORNIA...

...Southern California...

A developing closed low over central California will foster an 
area of surface low pressure across the Central Valley which will 
then refocus offshore of the coastal ranges tonight into Sunday 
morning. This will help set the stage for weak and relatively moist
onshore flow to aim into the Transverse Ranges (and eventually 
into the Peninsular Ranges as well). This onshore flow coupled with
DPVA and steepening mid-level lapse rates will allow for the 
development and expansion of moderate to locally heavy shower 
activity, and especially by Sunday morning when some pockets of 
locally stronger convection may be possible. This will include 
areas from Santa Barbara County southeast through San Diego County,
and also some western portions of San Bernadino and Riverside 
Counties. Generally as much as a 0.5" to 0.75" of rain is forecast
by Sunday morning (with highly localized stronger orographic 
ascent/upslope flow possibly facilitating isolated convective 
activity capable of 0.25"+/hr rainfall rates and totals of 1"+).

Given the ongoing wildfire/burn scar sensitivities over the region,
these rains may be heavy enough to result in some localized debris
flow and related flash flooding activity. Therefore, the Marginal 
Risk has been maintained over portions of the Transverse Ranges and
the Los Angeles Basin. While these conditions are unlikely to
manifest (as the rainfall will largely be beneficial for fire
fighting activities), sub-hourly rates up to 0.25" would be the
primary driver of the threat. 

Churchill/Orrison


Day 2
Valid 12Z Sun Jan 26 2025 - 12Z Mon Jan 27 2025

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF THE TRANSVERSE/PENINSULAR RANGES AND LOS ANGELES BASIN OF
SOUTHERN CALIFORNIA, AS WELL AS FOR PORTIONS OF THE TEXAS GULF
COAST INTO THE LOWER MISSISSIPPI VALLEY...

...Southern California...

Localized areas of moderate to heavy shower activity look to
continue into Sunday and Sunday night for upslope portions of the
Transverse and Peninsular Ranges (and to a lesser extent the
adjacent Los Angeles Basin and coastal areas), though snow levels
will be falling and that should help to mitigate runoff issues at
higher elevations. An additional 0.75" to 1.50" is generally 
forecast for much of the same areas as Day 1, and concerns are 
greatest for isolated convection with 0.25"+/hr rates interacting 
with recent burn scar areas resulting in the potential for 
localized flash flooding and potential debris flows/mudslides
(driven primarily by sub-hourly rates of 0.25", if able to
manifest with very limited instability). While the bulk of the 
rainfall should be beneficial for the region, interests in the burn
scars (and more sensitive flash flooding areas) should stay tuned 
to the forecast. 

...Texas Gulf Coast into Lower Mississippi Valley...

As a shortwave trough departs eastward by Saturday, low-level
return flow will return to the western Gulf Coast along with a wave
of low pressure, resulting in areas of moderate to heavy rainfall 
by Sunday morning. While areal average QPF is generally expected to
be capped near 1" over a 3-6 hr period, 24-hr totals could add up 
to be locally as high as 3-4" (mainly now indicated by the ECMWF
and ECENS suite). While instability will be rather limited, MU 
CAPE up to 500 J/kg could support localized embedded convection 
(with high-res guidance providing more insight in future cycles). 
For now, uncertainty remains high given differences in the ensemble
systems and how the deterministic solutions handle the mass fields
(with the more aggressive EC suite suggesting the potential for a
future targeted upgrade for the Houston metro region, should hi-res
guidance support this with future updates. The inherited Marginal 
Risk area was maintained with no adjustments necessary, as the
guidance remains fairly consistent spatially with depiction of QPF. 

Churchill


Day 3
Valid 12Z Mon Jan 27 2025 - 12Z Tue Jan 28 2025

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

Churchill

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