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
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
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
239 PM EDT Sun Jun 1 2025
Day 3
Valid 12Z Tue Jun 03 2025 - 12Z Wed Jun 04 2025
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PARTS OF THE
CENTRAL UNITED STATES AND OVER SOUTH FLORIDA...
...Texas to the Great Lakes...
The cold front pushing southeast across the central CONUS Tuesday
may become more progressive as the Baja low from Sunday gets even
more sheared into the westerlies, producing a more amplified trough
axis pivoting east into the Plains. Embedded within this flow,
multiple vorticity maxima will rotate E/NE, leading to additionally
enhanced lift, aided significantly the latter half of D3 by
diffluence within the RRQ of a poleward arcing jet streak
downstream of the trough axis. This will result in expansive and
impressive deep layer ascent from the Southern Plains through the
Great Lakes, with return flow out of the Gulf on 30-50 kts at 850mb
drawing impressive thermodynamics northward into the front. This
suggests that convection will be widespread across the area
Tuesday, which is supported by both simulated reflectivity and
24-hr rainfall progs from the global models and ensembles, with
rainfall rates exceeding 1"/hr likely anywhere along the front. The
greatest risk for excessive rainfall will be from northern Texas
through eastern Iowa where deeper warm cloud depths will produce
more efficient rainfall rates, which will likely train SW to NE
along the front. The SLGT risk was tailored to match the highest
probabilities for 3"/24 hrs from the ECENS/GEFS, but some eastward
expansion of the SLGT was also drawn to account for the AI guidance
(AIFS, Graphcast) that is generally east of the deterministic
models.
...South Florida...
A meandering shortwave trough positioned over the Florida
Peninsula will be the impetus for another round of scattered to
widespread showers and thunderstorms Tuesday. With thermodynamics
remaining robust across the region (PWs 1.75 to 2 inches and MUCAPE
above 1000 J/kg), slow moving storms will again have the potential
to produce excessive rain rates above 2"/hr. While there is some
uncertainty into how much convective debris from Monday could limit
coverage, the general consensus among the various ensemble camps
is for another day of heavy rain, especially across South Florida,
where ECENS/GEFS/SREF all suggest at least a low end risk (5-10%)
for 3"/24 hrs. This will fall atop ground that will likely be
sensitive from heavy rain on D2, and the inherited SLGT and MRGL
risks were adjusted only cosmetically from previous.
Weiss
Day 3 threat area:
www.wpc.ncep.noaa.gov/qpf/99epoints.txt
Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
242 AM EDT Sun Jun 1 2025
the central part of the country will be wet for the start of the
extended period as a frontal system advances through the Northern
and Central Plains toward the Mississippi Valley. Scattered to
widespread convection is expected along and ahead of the front with
some of the embedded storms potentially producing heavy rainfall.
There is a growing signal for 3 to 7 inches to focus across Texas
and Oklahoma and for 3 to 5 inches for Iowa, Illinois and Missouri
WPC maintained the Slight Risk area for excessive rainfall/flash
flooding across for day 4 and added Slight Risk for the new Day 5
period for much of the same area in addition to western Arkansas.
Areas near the Iowa/Illinois border, the flash flood threat would
be primarily in urban areas. The front and associated shield of
precipitation is expected to track east/southward into the Southern
Plains and Lower Mississippi Valley/Mid-South by the end of next
week.
For the West and East Coasts drier conditions are expected under
the ridge. Florida which may see more appreciable rainfall over
the southern portions in the vicinity of a dying oceanic boundary,
where a Marginal Risk area for excessive rainfall/flash flooding
was added for day 4/Tuesday. Some mountain areas of Wyoming may see
a few inches of snow as colder air comes in behind the front.
With time, temperatures will rise to near or above normal for the
eastern half of the country which will elevate the Heat Risk threat
level to Moderate. Daily maximums in the 80s/90s may increase heat
stress for some regions-- take precautions in the heat such as
increased water intake and more time in air conditioned areas to
avoid heat exhaustion and heat stroke. Parts of South Texas will
see more days of highs into the upper 90s and low 100s with low
probabilities (20-40%) of reaching heat index values of at least
110F. Much cooler temperatures will follow behind the cold front,
starting in the Rockies on Monday then into the Plains thereafter,
but moderating with time. Much of the West will see near to
slightly above normal temperatures, especially later in the week.
Campbell/Roth
Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
242 AM EDT Sun Jun 1 2025
the central part of the country will be wet for the start of the
extended period as a frontal system advances through the Northern
and Central Plains toward the Mississippi Valley. Scattered to
widespread convection is expected along and ahead of the front with
some of the embedded storms potentially producing heavy rainfall.
There is a growing signal for 3 to 7 inches to focus across Texas
and Oklahoma and for 3 to 5 inches for Iowa, Illinois and Missouri
WPC maintained the Slight Risk area for excessive rainfall/flash
flooding across for day 4 and added Slight Risk for the new Day 5
period for much of the same area in addition to western Arkansas.
Areas near the Iowa/Illinois border, the flash flood threat would
be primarily in urban areas. The front and associated shield of
precipitation is expected to track east/southward into the Southern
Plains and Lower Mississippi Valley/Mid-South by the end of next
week.
For the West and East Coasts drier conditions are expected under
the ridge. Florida which may see more appreciable rainfall over
the southern portions in the vicinity of a dying oceanic boundary,
where a Marginal Risk area for excessive rainfall/flash flooding
was added for day 4/Tuesday. Some mountain areas of Wyoming may see
a few inches of snow as colder air comes in behind the front.
With time, temperatures will rise to near or above normal for the
eastern half of the country which will elevate the Heat Risk threat
level to Moderate. Daily maximums in the 80s/90s may increase heat
stress for some regions-- take precautions in the heat such as
increased water intake and more time in air conditioned areas to
avoid heat exhaustion and heat stroke. Parts of South Texas will
see more days of highs into the upper 90s and low 100s with low
probabilities (20-40%) of reaching heat index values of at least
110F. Much cooler temperatures will follow behind the cold front,
starting in the Rockies on Monday then into the Plains thereafter,
but moderating with time. Much of the West will see near to
slightly above normal temperatures, especially later in the week.
Campbell/Roth
