FOUS30 KWBC 240825
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
325 AM EST Fri Jan 24 2025

Day 1
Valid 12Z Fri Jan 24 2025 - 12Z Sat Jan 25 2025

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

Churchill


Day 2
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...

A developing closed low over central California by late Saturday
will foster an area of surface low pressure across the Central
Valley which will then refocus offshore of the coastal ranges
Saturday night and 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. A somewhat stronger and 
wetter solution continues to be advertised by much of the latest 
model guidance, and 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 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 Saturday night and Sunday morning 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. Some
local adjustments were made based on the addition of high-res
guidance (primarily the NAM-nest) focusing on upslope regions most
at risk for highly localized 1"+ totals. 

Churchill/Orrison


Day 3
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). An additional 0.75"
to 1.50" is generally forecast for much of the same areas as Day 2,
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 debris flows. 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 as more high resolution model data 
becomes available in the next 24-48 hours to better hone in on the
threat and specific areas. 


...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 resulting in
areas of moderate to heavy rainfall by Sunday morning. While areal
average QPF is generally expected to be capped at 1"/6-hr period,
24-hr totals could add up to be locally as high as 3-4" (per
ensemble bias corrected guidance and GEFS/ECENS probabilities for
3" exceedance of 1-5%). 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, the inherited Marginal Risk area was maintained and
adjusted based on the latest guidance (and is in line with the
GEFS-driven machine learning first guess guidance, depicting a
Marginal Risk area). 

Churchill

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