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QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
357 AM EDT Tue Apr 21 2026

Day 1
Valid 12Z Tue Apr 21 2026 - 12Z Wed Apr 22 2026

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
CALIFORNIA AND TEXAS...

...California...

A cutoff large upper level low moving into California this morning
will continue to spread low-elevation rain and high elevation snow
into much of the West Coast states through tonight. Southwesterly
onshore flow ahead of this low is driving a plume of rainfall
northward up the coast. Some instability will allow for convective
elements to support cells with heavier rainfall within the broader
rain plume. This instability may allow the heavier rains to impact
urban areas such as San Francisco, Sacramento on up towards
Seattle. In addition to these urban concerns, steep upslope against
the Sierra Nevada range will support multiple inches of
precipitation into the mountains. While the highest elevations
could see as much as 2 feet of snow, much of the lower elevations
could pick up 2-4 inches of new rainfall, which will quickly flow
into the Central Valley. The inherited Marginal captures the
isolated potential for flash flooding due to both urban concerns
and heavy rainfall being quickly directed into steep valleys
draining the Sierra Nevada. No changes were needed to the inherited
Marginal Risk area.

...Texas...

A plume of rain moving across eastern Texas is the result of deep
Gulf moisture advecting northward off the western Gulf ahead of a
series of weak upper level disturbances moving across Texas. These
disturbances are supporting the broader plume of light rain and
where the forcing is greatest, supporting locally heavier rainfall
rates. Guidance has come into enough agreement about the potential
for locally heavy rain associated with slow-moving cells impacting
portions of eastern Texas as far north as the Metroplex. Thus,
given the lower FFGs associated with the large urban area
constituting the Metroplex, the Marginal has been expanded north to
cover the Triangle to include the Dallas-Fort Worth metro.
Nonetheless, with more moisture and a bit more instability, the
threat for heavy rain capable of producing isolated instances of
flash flooding should hold closer to I-10.

Wegman


Day 2
Valid 12Z Wed Apr 22 2026 - 12Z Thu Apr 23 2026

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE UPPER TEXAS COAST AND SOUTHWESTERN LOUISIANA...

Only minor tweaks were made to the inherited Marginal Risk area
across far southeastern Texas (upper Texas Coast) and southwestern
Louisiana. Steady southerly flow off the Gulf will continue to
advect a plume of Gulf moisture northward across the Marginal Risk
area. The flow will not be much to write home about, generally
between 5-15 kts at 850 mb. In addition to the rather weak
advection, it's notable that unlike Day 1/Tuesday, the upper levels
will become decidedly less favorable for widespread rain across the
region as ridging and associated subsidence builds in. Thus, there
is good agreement that any heavy rains will be disorganized and
widely scattered in nature, as compared with the rains further west
across Texas the past couple days. Urban concerns remain a driving
factor for maintaining the Marginal, which appears to be a lower-
end risk.

Wegman


Day 3
Valid 12Z Thu Apr 23 2026 - 12Z Fri Apr 24 2026

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE LOWER MISSOURI AND MID-MISSISSIPPI VALLEYS...

A plume of moisture originating from the Gulf will advect moisture
and instability northward into the Missouri and Mississippi Valleys
on Thursday. Meanwhile, a strong cold front emanating from a deep
low over the northern Plains will sweep eastward across the same
region. Where these two competing air masses meet, most likely in
the Marginal Risk area, expect widespread development of showers
and thunderstorms. The greatest clashes are expected after sunset
Thursday night, as the low level jet advecting the moisture and
instability strengthens with the normal diurnal cycle. As that
warm, moist air collides with the cold front, it will stall the
latter's southward progress, creating an east-west boundary, likely
starting around the Kansas/Missouri border well south of Kansas
City, then expanding eastward with the front. Ozarks topography
will likely also support locally heavier rainfall on the south-
facing slopes as well. Guidance is in very poor agreement as to
exactly where and when this clash of air masses will occur, with
the above-described scenario seemingly the most likely solution
based on an ensemble of the guidance. Should the guidance come into
better agreement with strength and timing of the heaviest rainfall
with future updates, a Slight Risk may need to be considered. The
still full rivers and saturated soils across southeast Kansas and
southwest Missouri from recent heavy rainfall events will
contribute to more widely scattered instances of flash flooding
should the heaviest rainfall occur over this region.

Wegman
$$

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