Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
239 PM EDT Sun Jun 1 2025
Day 1
Valid 16Z Sun Jun 01 2025 - 12Z Mon Jun 02 2025

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE SOUTHWEST...

16Z Update...
Incoming 12Z suite and HREF still support widespread showers and
thunderstorms developing within extremely anomalous PWs downstream
of a filling mid-level low which will pivot into AZ tonight. The
previous discussion is still reflective of today's event, and the
inherited MRGL/SLGT risks were modified just slightly to account
for the new guidance. Previous Discussion below:

Impressive closed low off the northern Baja coast will gradually
fill and start to shift northeastward today.Downstream of this
low, southerly flow will strengthen, drawing increased moisture
and instability northward into the Southwest United States. This
will support impressive thermodynamics as precipitable water values
surge to 1.25-1.5 inches, above the 99th percentile according to
NAEFS (and in some areas above anything measured in the 06Z CFSR
climatology). Given the amount of overlap with MUCAPE climbing to
250-750 J/kg, convection is expected to blossom rapidly and then
organize through 25-35 kts of bulk shear. This organization within
the favorable thermodynamics and upper level dynamics will support
at least briefly heavy rain rates that may exceed 1"/hr (10-20%
chance). With storm motions slowing as the upper low pivots
overhead later this evening/tonight, this could produce pockets of
1-2" of rain, especially in the low deserts of AZ. Farther east,
aligned Corfidi vectors with the mean flow will allow for repeating
rounds of convection to lift northward, producing other areas of
1-2" of rain despite the faster progression.


...Texas...
Low-level flow will gradually back from W to S/SW today, and
increase to 15-25 kts across portions of eastern Texas. This will
impinge effectively into a wavering warm front, providing ascent
through convergence and isentropic ascent later this aftn through
tonight. At the same time, a modest shortwave will dig southward
towards the Arklatex, amplifying the flow and driving at least
subtle height falls. The overlap of this ascent into PWs nearing 2
inches and MUCAPE as high as 3000 J/kg will support expanding
convection diving southward along the boundary, potentially growing
upscale into clusters or an MCS this evening within favorable bulk
shear. There is still quite a bit of longitudinal spread with the
placement of this MCS, but where it occurs rainfall rates of
1-2"/hr could train, leading to at least isolated instances of
flash flooding. The inherited MRGL risk was modified cosmetically
to best match the recent HREF exceedance probabilities.


...South Florida...
A cold front dropping slowly southward will serve as a focus for
showers and thunderstorms through today. This front is pushed
southward by height falls within an expanding eastern CONUS trough,
leading to some enhanced ascent, aided additionally by a weak
impulse progged to move eastward across the southern peninsula. PWs
south of the front will hover around 2 inches today, which is
above the 90th percentile of the SPC sounding climatology, which
will combine with SBCAPE reaching 2000-3000 J/kg to produce a
favorable thermodynamic environment for heavy rainfall. The HREF
and REFS probabilities indicate a 30-50% chance, and a 10-20%
chance for 1"/hr and 2"/hr rates, respectively, within blossoming
convection, and as storms train along the boundary from west to
east, or interact with subsequent outflows/sea breeze boundaries,
this could produce locally 3-5 inches of rain. This could result in
localized instances of flash flooding, especially along the urban
SW or SE coasts today.

Weiss

Day 1 threat area: www.wpc.ncep.noaa.gov/qpf/94epoints.txt