FOUS30 KWBC 280734
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
334 AM EDT Thu May 28 2026

Day 1
Valid 12Z Thu May 28 2026 - 12Z Fri May 29 2026

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS MUCH OF
THE SOUTHEAST TO MIDDLE MISSISSIPPI VALLEY, SOUTHEAST FLORIDA, AND
FOR PORTIONS OF THE NORTHWEST...

...Central Plains to Southeast...

A bit of an omega block pattern is shaping up across the CONUS
with a broad upper trough axis fixated over the Central and
Southern Plains with ridging in place over the top. This will cause
a rather chaotic flow under the mean trough and several smaller
mid- level perturbations basically "stuck" within the overall
trough axis. Ample instability with deep moisture presence will
certainly continue to support at least a scattered heavy rain
footprint across areas underneath influence of the trough.
Scattered elevated probabilities between 30-70% exists for
localized amounts >3" from KS all the way into the Southeast U.S.
with the maxima split between eastern KS to northern AR and the
Central Gulf coast. For KS/AR, parked underneath the upper trough,
upper forcing and stagnant flow are more favorable for heavier
rainfall totals. Maintaining a marginal risk at this time as the
heaviest rainfall over the past 12 or so hours was more southwest
of where the guidance is showing today and the rest of the region
is still somewhat dry and can probably support some heavier totals.
Along the Gulf Coast and into the Southeast, instability will be
maximized in this area with weak flow supporting slow- moving
convection and heavier rainfall over those isolated areas.
Antecedent conditions across much of this region are also very wet,
so it may not take much to cause issue. Given the still fairly
isolated nature, maintaining a broad marginal risk, with any
possible upgrades today stemming more from near term radar trends.

...Florida...

An active sea breeze pattern with weakening steering flow will
lead to periods of heavy rainfall across southeast Florida this
afternoon. Greatest concern remains over the South Florida metro
corridor from West Palm Beach to Miami. Latest HREF is robust with
showing greater than 50% probabilities for 3" in 24-hours, with
even some modest, albeit localized, probabilities for 5". This
correlates well with CAM guidance, with some indicating bullseyes
of greater than 3" within the larger QPF footprint. Locally
significant rainfall is likely in this relatively classic pattern
with ample moisture and instability present. A marginal risk
remains in effect across Southeast Florida.

...Washington/Oregon...

A deep upper level low over the West will allow for a broad axis
of diffluent flow across the interior Northwest. Precipitable water
values 1-1.5" are in the 90-90th percentile, and with sufficient
instability, should support convection capable of producing heavy
rainfall. Flooding and flash flooding are a concern for mainly the
more vulnerable areas - urban, small stream, and low lying areas.
HREF 24-hr probabilities for greater than 3" are between 40-70% for
especially the Oregon cascades. Given the favorable environment
though, a marginal risk extends all the way from far northern
California northward to the eastern half of Washington state. A
previous marginal risk for parts of the northern Rockies was
trimmed out for this update given the lack of QPF signal.

Santorelli


Day 2
Valid 12Z Fri May 29 2026 - 12Z Sat May 30 2026

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PARTS OF
THE NORTHERN ROCKIES AS WELL AS MUCH OF THE SOUTHEAST TO MIDDLE
MISSISSIPPI VALLEY...

...Eastern Washington to Northern Rockies...

A marginal risk is in place across far northeast Washington to
northwest Montana due to an increase in the heavy rainfall signal
for this period as the upper low over the West on Day 1, pivots
slightly northeastward, placing this region in an area with better
upper forcing and advection of moisture. The presence of anomalous
moisture and some instability may present a threat for localized
heavy rainfall and flash flooding, especially for burn scar
locations within this region.

...Middle Mississippi Valley to the Southeast...

Strong Canadian high pressure pressing in from the north will
allow for a slow moving cold front to press south over the course
of Thursday into Friday with convection likely to spawn along and
ahead of the front and dry air advecting behind the front leading
to a quick cutoff of activity. The front will arc back into the
Tennessee Valley to Middle Mississippi Valley, supported by the
persistent troughing over the south-central Plains. A moist and
unstable environment near and south of this front will support
scattered convection, with some heavier cells likely within the
pattern. It is still too hard to pick out any discernible areas of
interest, so maintained a very broad marginal risk over this area
which is quite wet and will continue to be in the Day 1 period.

Santorelli


Day 3
Valid 12Z Sat May 30 2026 - 12Z Sun May 31 2026

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS THE
NORTHERN ROCKIES TO HIGH PLAINS AND THE SOUTHEAST...

...Northern Rockies to High Plains...

The upper level low over the West during the Day 1 and 2 periods
will weaken and shift northeastward, moving over the Northern
Rockies to High Plains region on Day 3/Saturday. Moisture anomalies
will continue to be in the 90-90th percentile, supporting moderate
to heavy rainfall, with widespread QPF greater than 0.5-1" likely
across western and central Montana. There may not be a lot of
instability to work with, so any flood or flash flood threats
should be mainly localized with particular attention to burn scar
locations. A fairly broad marginal risk is in place across much of
western and central Montana, into parts of northern Wyoming as well.

...Southeast...

Same story as Days 1 and 2, with ample moisture and instability
present near and south of a cold front that may stall over the
region on Saturday supporting convection, across much of the
Southeast. Some of this region, especially western parts of South
Carolina and into the Appalachians has been very wet as of late and
is well primed, so may be more favorable for flooding concerns.
The marginal risk though continues towards the coast due to the
presence of a weak frontal wave in the region. The Marginal Risk
extends from the Southeast coast, westward into northeast Alabama
and southeast Tennessee.

Santorelli
$$
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 * Origin: Project Scorpio TEST (618:250/6)