Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
407 PM EDT Sat May 31 2025
Day 1
Valid 16Z Sat May 31 2025 - 12Z Sun Jun 01 2025
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
NEW ENGLAND...
...New England...
Impressive almost-June low pressure system will continue to track
northeast across Maine and get pulled NW into Canada as the parent
shortwave gets absorbed into a large closed low across Quebec. The
model trends have continued to pivot just a bit farther west with
the impressive deformation that will continue to support heavy rain
rates, especially across Vermont, through this evening. Although
instability will remain modest, generally less than 500 J/kg,
impressive PWs around 1" (near the 75th percentile) will support
rainfall rates within the strong ascent that could reach 1"/hr at
times (although will generally be around 0.5"/hr). Still, this rain
will be persistent, and Corfidi vectors becoming anti-parallel to
the mean flow suggest training to support total rainfall for which
the GEFS/ECENS/HREF probabilities all suggest will exceed 3 inches across
the sensitive terrain/soils of Vermont, supporting the continued
SLGT risk.
Farther east, the high-res CAMs have become a bit more aggressive
with rainfall rates around 1"/hr lifting north across eastern Maine
within higher PWs (above the 90th percentile) and greater
instability. While training here is less of a concern, clusters of
convection organized through 30-40 kts of bulk shear could still
produce local flash flood concerns, so the MRGL risk was pulled all
the way into Downeast Maine.
...Southern Plains...
Broad NW flow on the upwind side of an expansive trough centered
over Ontario will persist over the Plains as an embedded but
potent shortwave tracks SE tonight. This shortwave will push a
wave of low pressure and weakening cold front south into Oklahoma
tonight, where it will interact with a slowly advancing warm front
out of Texas on the intensifying LLJ. This LLJ, progged to reach at
least a modest 20-25 kts from the SW, will transport more robust
thermodynamics (PWs to 1.5 inches and MUCAPE above 1000 J/kg), to
support expanding convection along this decaying cold front. With
bulk shear expected to be in the 30-40 kt range, convection will
likely grow upscale into clusters or an MCS tonight as it races
southeast on 20-25 kts of 0-6km mean winds. This this will limit
the duration of heavy rain, rates that will likely reach 1-2"/hr
could result in isolated instances of flash flooding, especially
during any short-term training that may develop on the periphery of
this MCS. The guidance, as expected, features considerable
longitudinal spread of the MCS path, which makes the cosmetic
adjustments to the inherited MRGL risk the best approach for the
risk area tonight.
Weiss
Day 1 threat area:
www.wpc.ncep.noaa.gov/qpf/94epoints.txt
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
407 PM EDT Sat May 31 2025
Day 2
Valid 12Z Sun Jun 01 2025 - 12Z Mon Jun 02 2025
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE SOUTHWEST...
Impressive closed low off the northern Baja coast (NAEFS 500mb
heights near -2 sigma at 12Z Sunday) will gradually fill and shift
northeast through D2, reaching Arizona by Monday morning.
Downstream of this low, southerly flow will become more intense,
drawing increased moisture and instability northward into the
Southwest. This will support impressive thermodynamics as PWs 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), which will overlap with MUCAPE climbing to 250-750
J/kg, an increase from the past few model cycles. As ascent
increases through height falls and PVA, convection is expected to
blossom rapidly and then organize through 25-35 kts of bulk shear.
This organization within the favorable thermodynamics 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 late D2, 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. Despite dry soils from a lack of
recent rain, this could produce instances of flash flooding,
including atop sensitive terrain or burn scars, and after
coordination with WFOs TWC and PSR, a small SLGT risk was added for
south-central AZ.
Weiss
Day 2 threat area:
www.wpc.ncep.noaa.gov/qpf/98epoints.txt
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
407 PM EDT Sat May 31 2025
Day 3
Valid 12Z Mon Jun 02 2025 - 12Z Tue Jun 03 2025
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE CENTRAL HIGH PLAINS AND CENTRAL PLAINS...
...Central High Plains and Central Plains...
Baja mid-level low which will be opening over Arizona to start the
period will continue to weaken and shear out as it becomes embedded
within pinched westerlies south of a trough diving out of Montana.
This will result in expanding deep layer lift through height falls
and modest deformation expanding from the Four Corners through the
Northern Plains, with additional ascent occurring as the northern
stream trough drives a cold front southeastward and a jet streak
pivots meridionally into Canada leaving favorable RRQ diffluence
overhead, and a surface low is also forecast to develop along this
boundary.
This synoptic setup will produce an expansive area of lift into
increasingly robust thermodynamics, and it is likely that a large
area of showers and thunderstorms will develop much of D3.
Increasing low-level flow as much as 40-50 kts at 850mb will emerge
from the Gulf, pushing PWs to 0.75 to 1.25 inches which is
extremely anomalous and above the 97th, or even 99th percentile,
in some areas. This will overlap with MUCAPE rising to 250-1000
J/kg, sufficient to produce rainfall rates of 0.5"-1"/hr. While
there is still some uncertainty in the placement of the heaviest
rainfall, the guidance is in general good agreement in a maximum of
2-3", or locally higher as reflected by <10% probability for 3"/24
hrs from the ECENS/GEFS, from eastern CO and WY through eastern
NE, which is where the SLGT risk was drawn. Some of this area is
the Sand Hills of NE which do not typically flood and have high
FFG, but for potential and after coordination with the affected
WFOs, the inherited SLGT risk was adjusted just slightly.
Farther to the north from SD through MN, fast moving storms on
20-30 kts of 0-6km mean winds will have the potential to train
where Corfidi vectors are aligned to the mean wind and the cold
front. This will offset, at least marginally, the faster motion,
allowing for an isolated flash flood risk across this region.
Farther SW, the MRGL risk was expanded into western CO and the
southern half of UT where impressive PWs and slower moving storms
could organize to produce areas of heavy rainfall exceeding 1 inch.
This region is sensitive due to the variety of slot canyons and
other sensitive terrain, which feature FFG that is as low as
0.5"/1hr. After coordination with WFO SLC and GJT, the MRGL risk
was expanded across these areas.
...South Florida...
The inherited MRGL risk was maintained, but trimmed to just the SE
portions of the peninsula to focus on the more urbanized regions of
the state. As an upper trough digs south along the FL Panhandle and
into the peninsula, it will push a weakening cold front and
shortwave southward, enhancing ascent across the area. With PWs
approaching 2" overlapped with MUCAPE above 2000 J/kg, this will
support scattered to widespread heavy-rain producing convection on
Monday. Mean storm motions will be generally west to east, aligned
to the front, and this should focus storms along the SE coast where
sea breeze interaction occurs. With rain rates expected to exceed
2"/hr at times, this could produce above 3" of rain as reflected by
5-20% probabilities from the SREF and ECENS.
Weiss
Day 3 threat area:
www.wpc.ncep.noaa.gov/qpf/99epoints.txt
Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
252 PM EDT Sat May 31 2025
The beginning of the week starts with a frontal system moving
through the Northern and Central Plains toward the Mid-Mississippi
Valley. This front will be the focus for scattered to widespread
showers and thunderstorms, with some produce heavy rainfall. WPC
added Slight Risk areas for excessive rainfall/flash flooding
across for days 4-5/Tuesday and Wednesday for portions of
Oklahoma and northern Texas where roughly 3-7" of areal averaged
rain is explicitly forecast, and in and near the Iowa/Illinois/
Missouri border junction for day 4/Tuesday in a region where local
amounts in the 3-5" range are possible. For areas near the
Iowa/Illinois border, the flash flood threat would be primarily in
urban areas. The front advances east/southward along with the heavy
rain threat (into the Southern Plains/Lower Mississippi
Valley/Mid- South) by the end of next week. Drier conditions are
expected along both the East and West Coasts with the exception of
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.
Near normal to above normal temperatures are expected to spread
across the central and eastern states. With daily temperatures in
the 80s/90s a Moderate HeatRisk threat is expected to develop 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.
Roth
Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
252 PM EDT Sat May 31 2025
The beginning of the week starts with a frontal system moving
through the Northern and Central Plains toward the Mid-Mississippi
Valley. This front will be the focus for scattered to widespread
showers and thunderstorms, with some produce heavy rainfall. WPC
added Slight Risk areas for excessive rainfall/flash flooding
across for days 4-5/Tuesday and Wednesday for portions of
Oklahoma and northern Texas where roughly 3-7" of areal averaged
rain is explicitly forecast, and in and near the Iowa/Illinois/
Missouri border junction for day 4/Tuesday in a region where local
amounts in the 3-5" range are possible. For areas near the
Iowa/Illinois border, the flash flood threat would be primarily in
urban areas. The front advances east/southward along with the heavy
rain threat (into the Southern Plains/Lower Mississippi
Valley/Mid- South) by the end of next week. Drier conditions are
expected along both the East and West Coasts with the exception of
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.
Near normal to above normal temperatures are expected to spread
across the central and eastern states. With daily temperatures in
the 80s/90s a Moderate HeatRisk threat is expected to develop 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.
Roth
