Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
800 PM EDT Sat Jun 03 2023
Day 1
Valid 01Z Sun Jun 04 2023 - 12Z Sun Jun 04 2023
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
THE SOUTHERN HIGH PLAINS...
...Southern/Central Plains...
Convection is expected from the Central Plains to the Mexico/Texas
border into the Red River Valley of the South has generally had
more diffuse organization than previous days. Mid-level troughing
elongated across the Plains combined with modest left-exit jet
level diffluence and a residual surface trough has driven deep
layer ascent through the day. With effective bulk shear of 30-35
kts, this could result in some storm interactions/more organized
clusters moving generally 10-15 kts as progged by the layer
Corfidi vectors. Although the signal for training overall is
modest today, with rainfall rates possibly reaching 2.5"/hr at
times, any short term training or repeated rounds could quickly
result in runoff issues, especially where soils are most saturated
from 14-day rainfall that has been more than 300% of normal across
much of west/central Texas into western OK and southern KS.
The Slight Risk area in this region remains unchanged, as
convection in the risk area has been slow to move. Although the
model signal is vague for any more organized areas of convection
leading to more focused heavy rainfall, heavy rainfall remains in
the cards. The Slight Risk is strongly driven by antecedent
conditions from recent rainfall leading to compromised FFG as low
as 1.5"/3hrs which should be exceeded in an isolated to widely
scattered basis. Any storm that moves across this region could
quickly lead to additional runoff concerns.
While the rest of the area remains in a broad MRGL risk, that
thunderstorms near the Rio Grande have congealed into a slowly
forward propagating MCS with rain rates exceeding 2"/hr and push
E/SE into the RGV or western TX Hill Country tonight. Some cells
appear to be forming ahead of the line which could merge in. This
convective area is moving south of yesterday's heavy rainfall
footprint. Have left the risk area there Marginal but Slight Risk
impacts cannot be ruled out.
...Northern Rockies/Intermountain West...
Convective coverage has been fairly sparse across the region. The
Slight and Marginal Risk areas across the Northern Rockies have
been removed since we're passing the peak of the afternoon
convective activity.
...Florida...
Post-Tropical Cyclone Arlene is turning almost due east, and is
expected to scrape the Cuban coast overnight. This subtly enhanced
ascent combined with the continued modest diffluence aloft and a
surface trough/convergence axis/old frontal zone will provide
sufficient ascent for convection for a little while longer across
the southern Florida peninsula. Overnight, convection with heavy
rainfall is possible in the Keys as Arlene's remnant low moves by.
Hourly rain totals to 2.5" and local amounts to 5" cannot be
ruled out, so looped the Marginal Risk area around the FL Keys to
cover the possible threat.
Roth/Weiss
Day 1 threat area:
www.wpc.ncep.noaa.gov/qpf/94epoints.txt
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
326 PM EDT Sat Jun 03 2023
Day 2
Valid 12Z Sun Jun 04 2023 - 12Z Mon Jun 05 2023
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
THE SOUTHERN HIGH PLAINS AND NORTHERN ROCKIES...
...Southern Plains through the Northern Rockies...
Weakening expansive trough positioned NW to SE from the Northern
Rockies through the Southern Plains will continue to provide an
environment favorable for scattered to widespread aftn convection.
This trough will become even more elongated with time on Sunday,
while aloft an axis of deformation shears from the Central Rockies
through Texas, providing additional deep layer ascent. Once again,
the simulated reflectivity from the guidance suggests primarily
scattered showers and thunderstorms, but weak vorticity maxes
embedded within the flow could locally organize more substantial
convection somewhere across the area.
This region has been quite wet the past two weeks noted by AHPS
14-day rainfall that is generally 150-300% of normal, with some
areas >600% near the TX Panhandle, or less than 50% for eastern
KS/OK/TX. Where rainfall has been above normal, FFG is compromised
to widespread 1-1.5"/3hrs, but in some places is as low as
0.25"/3hrs. This suggests that any scattered storm which ==falls
over a sensitive area could produce runoff/flash flooding, which
is reflected by a large but scattered signal for 3-hr FFG
exceedance in the HREF, driving the expansive MRGL risk area.
Embedded within the MRGL risk there is an indication for two
smaller regions with a targeted higher excessive rain threat. The
first is across the Panhandle of TX and into western OK where
72-hr rainfall on MRMS has been as high as 5-8", resulting in
antecedent streamflow that is above the 90th percentile according
to USGS. In this area, even a disorganized heavy rain producing
cell could result in flash flooding, and the HREF exceedance
probabilities peak around 30-40%. The other region is invof WY/MT
border where recent rainfall has also been excessive resulting in
more favorable conditions for rapid runoff. This area also has a
narrow overlap of aligned mean winds/Corfidi vectors with 25-35
kts of bulk shear and 1000 J/kg MUCAPE, suggesting the potential
for some storm organization and training, and where the PQPF first
guess field indicates the need for a narrow SLGT risk.
...Florida...
Another day of scattered to widespread afternoon thunderstorms is
likely across the southern half of the Florida Peninsula on
Sunday. The residual vorticity associated from the remnants of
Tropical Storm Arlene will be exiting to the east early in the
forecast period, but a secondary shortwave embedded within the
larger scale trough may move across the region during the evening.
This will combine with a persistent but weakening upper jet to
provide deep layer ascent. Although PWs are progged to slowly dry
out during D2, there is still likely to be an extended period of
MUCAPE > 1000 J/kg with PWs > 1.5 inches to support aftn/eve
thunderstorms with rain rates of 1-2"/hr drifting slowly northeast
on 5-10 kts of 0-6km mean winds. Although the simulated
reflectivity for Sunday features lesser coverage of storms, the
bulk shear magnitudes are progged to climb to 25-30 kts, at least
marginally supportive for some organization/clusters to support
these efficient rain rates. If any of these heavy rates fall atop
soils that are primed from several recent days of heavy rain, it
could result in isolated runoff issues leading to flash flooding.
Weiss
Day 2 threat area:
www.wpc.ncep.noaa.gov/qpf/98epoints.txt
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
327 PM EDT Sat Jun 03 2023
Day 3
Valid 12Z Mon Jun 05 2023 - 12Z Tue Jun 06 2023
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
THE SOUTHERN HIGH PLAINS...
...Southern Plains through the Northern Rockies...
Broad but weakening elongated trough will persist across the High
Plains and into the Northern Rockies Monday, continuing a pattern
that has changed little during the past week. This will continue
to support scattered afternoon convection from Texas through
Montana, with ascent provided via subtle impulses moving through
the flow and weak jet level diffluence. Thermodynamically, the
environment will remain favorable for heavy rain rates as PWs of
+1 to +2 standard deviations above the climo mean coincide with at
least modest MUCAPE of around 500 J/kg. While the ensemble signals
for heavy rain rates on D3 are generally weaker and less
widespread, it is likely some areas will still receive more than 1
inch of rain, which could fall atop saturated soils from the
recent heavy rainfall. However, the signals are modest enough to
preclude much more than a MRGL risk for most areas.
The exception is across the High Plains of NM into the TX
Panhandle where the SREF indicates at least a 5-10% chance of 3"
of rain, which will fall atop soils saturated from 3-day rainfall
of 5-8" resulting in FFG that is 0.75-1.5"/3hrs. Confidence in
scattered flash flooding is not extremely high, but the inherited
SLGT risk was maintained primarily for the antecedent soil
conditions, as even the CSU first guess field has just a MRGL over
part of the area, and this was coordinated with WFOs AMA/LUB/ABQ.
Should the model signals continue to temper, a removal or trimming
of this SLGT risk area may be required with later updates.
...Sierra Nevada...
A closed mid-level low approaching from the Pacific will move
onshore central CA late Monday evening. Downstream of this
feature, height falls and mid-level divergence will intensify
across the Sierra, with ascent being aided by modest upper
diffluence. Through this evolution, moisture will increase across
CA and Great Basin noted by PWs rising to +1 to +2 standard
deviations above the climo mean according to NAEFS, coincident
with a corridor of MUCAPE exceeding 1000 J/kg. This should support
afternoon convection, with weak mean winds supporting terrain tied
development with slow storm motions. Although recent rainfall here
has been very low, USGS streamflow is still above the 90th
percentile in many locations implying still saturated soils. This
suggests that as storms move slowly across the area with 0.5"/hr
or greater rates, rapid runoff could occur.
Weiss
Day 3 threat area:
www.wpc.ncep.noaa.gov/qpf/99epoints.txt
Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
158 PM EDT Sat Jun 03 2023
A daily diurnal risk for heavy showers and thunderstorms will be
maintained through the period from the southern Plains to the
northern Rockies and westward into the Great Basin as tropical
Pacific moisture is funneled northward in between a strong central
U.S. ridge and an upper low parked over California. For now, the
days 4 and 5 Excessive Rainfall Outlooks continue to depict just
one large Marginal risk area for this region, however embedded
Slight risk areas may become necessary if heavy rains focus over
antecedent wet areas. For parts of central Montana, a cold front
looks to drop into the region supporting an increased risk for
heavy rainfall and flash flooding by days 6 and 7 (this area has
been very wet and is more susceptible anyways).
Elsewhere, a cold front through the Great Lakes/Upper Midwest
early in the week should focus showers and storms ahead of it
across the Northern Plains/Upper Mississippi Valley. With decent
moisture and instability, a marginal risk remains on the day 4
Excessive Rainfall Outlook. In the Northeast, the deepening upper
trough/lows should favor pockets of rainfall into Friday as
moisture gets wrapped back into especially northern Maine from a
lingering low pressure system near the coast. In Florida, a
lingering weak front should support unsettled conditions over the
Sunshine State with better heavy rain chances in southern Florida
late week as deeper moisture gets pulled north.
Central U.S. ridging aloft will maintain an axis of above normal
temperatures from the northern Plains south-southeast to the
Mid-South, with plus 5-10F anomalies for highs possible most days.
The Northwest will be the other focus for very warm temperatures,
with highs 10-20F or so above normal and moderation later in the
week as troughing begins to push into the region. Expect below
normal temperatures over the Northeast and at times into the
Mid-Atlantic as upper troughing becomes established. Clouds and
periods of rainfall should keep highs 5-10F or so below normal
over the southern half of the Rockies/High Plains for most of the
period and the upper low reaching southern California Monday will
also bring a cooling trend into the southwestern U.S. next week.
Kebede/Santorelli
Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
158 PM EDT Sat Jun 03 2023
A daily diurnal risk for heavy showers and thunderstorms will be
maintained through the period from the southern Plains to the
northern Rockies and westward into the Great Basin as tropical
Pacific moisture is funneled northward in between a strong central
U.S. ridge and an upper low parked over California. For now, the
days 4 and 5 Excessive Rainfall Outlooks continue to depict just
one large Marginal risk area for this region, however embedded
Slight risk areas may become necessary if heavy rains focus over
antecedent wet areas. For parts of central Montana, a cold front
looks to drop into the region supporting an increased risk for
heavy rainfall and flash flooding by days 6 and 7 (this area has
been very wet and is more susceptible anyways).
Elsewhere, a cold front through the Great Lakes/Upper Midwest
early in the week should focus showers and storms ahead of it
across the Northern Plains/Upper Mississippi Valley. With decent
moisture and instability, a marginal risk remains on the day 4
Excessive Rainfall Outlook. In the Northeast, the deepening upper
trough/lows should favor pockets of rainfall into Friday as
moisture gets wrapped back into especially northern Maine from a
lingering low pressure system near the coast. In Florida, a
lingering weak front should support unsettled conditions over the
Sunshine State with better heavy rain chances in southern Florida
late week as deeper moisture gets pulled north.
Central U.S. ridging aloft will maintain an axis of above normal
temperatures from the northern Plains south-southeast to the
Mid-South, with plus 5-10F anomalies for highs possible most days.
The Northwest will be the other focus for very warm temperatures,
with highs 10-20F or so above normal and moderation later in the
week as troughing begins to push into the region. Expect below
normal temperatures over the Northeast and at times into the
Mid-Atlantic as upper troughing becomes established. Clouds and
periods of rainfall should keep highs 5-10F or so below normal
over the southern half of the Rockies/High Plains for most of the
period and the upper low reaching southern California Monday will
also bring a cooling trend into the southwestern U.S. next week.
Kebede/Santorelli
