Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
239 PM EDT Sun Jun 1 2025
Day 2
Valid 12Z Mon Jun 02 2025 - 12Z Tue Jun 03 2025

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE CENTRAL PLAINS AS WELL AS THE SOUTHEAST FLORIDA PENINSULA...

...Central Plains...
Mid-level low over Baja Sunday will fill and eject northeast from
Arizona, reaching the Central Plains by the end of D2. This
evolution will be driven by the amplification of a northern stream
trough digging across Montana, which will help shear out the closed
low and absorb it into the pinched westerlies. The interaction of
these two features will result in widespread convection on Monday
from the Four Corners all the way to the Upper Midwest as ample
moisture from the tropical Pacific streams northward, and impinges
into a cold front being pushed southward by the northern stream
trough.

As the front sags southeast through the day, it will serve as the
primary focus for developing thunderstorm activity Monday aftn and
Monday evening, generally from Minnesota southwest into Colorado.
While there is still considerable spread in the longitudinal
placement of the heaviest rain due to timing differences of the
models, there is good agreement that heavy rain producing
thunderstorms will be widespread along the boundary. This is due to
pronounced ascent through low-level convergence, upper divergence,
and isentropic ascent as the LLJ ramps up and pushes PWs to above
1.5 inches, potentially as high as 1.75 inches across NE/SD, well
above the 90th percentile. This elevated PW will combine with a
ribbon of CAPE above 2000 J/kg to fuel thunderstorms with rain
rates that have a high probability (60-70% chance) of exceeding
1"/hr. With mean winds expected to be 10-15 kts aligned to the
front, but Corfidi vectors becoming increasingly anti-parallel,
some backbuilding into the higher instability and training is
likely. This will support total rainfall that could exceed 3 inches
in some areas, and the SLGT risk was adjusted eastward to account
for the latest guidance from the high plains of CO eastward towards
the Coteau des Prairies.

To the south and west of this SLGT risk, the MRGL risk remains
expansive as far back as the slot canyon region of UT and across
much of CO and northern NM where afternoon thunderstorms could
produce 0.5+"/hr rates atop sensitive terrain leading to instances
of flash flooding. Additionally, a wave of low pressure developing
in the lee of the Southern Rockies may push additional moderate
rainfall across eastern CO late D2, and while this will likely be
less intense due to weaker instability, could produce at least
localized additional flash flooding if the rain occurs atop of
primed soils from earlier convection.


...West Texas...
Convection blossoming along the dry line Monday aftn/eve may become
widespread, although there is quite a bit of spread both in
location, coverage, and intensity, among the various 12Z CAMs.
While the coverage may be somewhat modest, any storms that develop
will be capable of producing intense rainfall rates exceeding
2"/hr as they track to the E/NE at 15-20 kts. Storms on the dry
line will likely organize through 25-35 kts of bulk shear, forming
clusters which could briefly enhance rain rates even further, and
lead to multiple rounds of heavy rain in some areas. FFG from the
Rolling Plains southward towards Big Bend is only 2-2.5"/3hrs due
to 7-day rainfall according to AHPS that has been more than 300% of
normal. This suggests that the inherited MRGL risk is still
warranted for any clusters of storms that move atop these more
sensitive soils.


...South Florida...
An active day of thunderstorms is again expected across South
Florida on Monday as an impressive shortwave tracks overhead and
interacts with a residual front draped across the region. During
the period of peak heating /late aftn and eve/ when SBCAPE will
climb towards 2000 J/kg and PWs surge to nearly 2.2 inches,
approaching the daily record for MFL. The simulated reflectivity
from the available high-res members have become much more
aggressive, likely owing to a more pronounced tail of a jet streak
to the east helping to drive ascent. With mean storm motions (using
0-6km mean winds as proxy) expected to be less than 10 kts within
modest bulk shear of 20-25 kts, clusters of storms that merge with
outflow/sea breeze boundaries could lengthen the duration of
rainfall rates that are expected to reach at least 2"/hr, and
possibly exceed 3"/hr at times. This will result in heavy rain
accumulating to 3-5", with locally higher amounts likely as
reflected by both HREF and REFS 5"/24hr probabilities rising above
60% along the Gold Coast. The inherited SLGT risk was modified only
modestly for the new guidance, and instances of flash flooding
appear likely in the urban areas D2, even moreso if heavy rain
falls atop areas impacted by rainfall on D1 as well.


Weiss


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