FOUS30 KWBC 160821
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
421 AM EDT Tue Jun 16 2026

Day 1
Valid 12Z Tue Jun 16 2026 - 12Z Wed Jun 17 2026

...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE TEXAS GULF COAST THROUGH THE LOWER MISSISSIPPI VALLEY...

An impressive and dynamic setup with multiple mesoscale features
is expected to develop through the day today from the Texas Gulf
Coast through the lower Mississippi Valley. The most rainfall for
the day will occur through this morning. A tropical disturbance
over Deep South Texas will support the strengthening of the typical
diurnal low level jet out over the Gulf, driving a plume of
impressively deep tropical moisture northward into the Texas and
Louisiana Gulf Coasts. Meanwhile a stationary front currently over
southern Arkansas and northern Mississippi will advance southward
as a cold front due to a disturbance and push of cooler drier air
south behind the front. This clash of air masses will result in
continued upscale development of storms that have broken out over
northern Louisiana through the overnight and into Tuesday morning.
FFGs are already lower in many spots west from the western tip of
Mississippi. This nearly stationary line of training storms is
likely to persist well into the afternoon, which could make for a
narrow but nevertheless severe area of very high totals of storm
total rainfall, especially into western Mississippi. Guidance has
suggested this narrow corridor, highlighted in much of the guidance
since yesterday along the I-20 corridor has now shifted a few
counties south, extending from Alexandria, LA east almost to
Hattiesburg, MS. The inherited Moderate Risk for this area was
trimmed on the north side both with the current start of convection
in this corridor and the southward trend in the guidance.

Meanwhile, a number of other pieces of CAMs guidance suggest that
the showers and storms developing along the immediate coast roughly
from Galveston/Trinity Bays east to Lafayette, LA will be the
primary and dominant line of showers and storms impacting the
northwestern Gulf Coast today. This line would from largely from
sea breeze/frictional convergence of the strong onshore southerly
flow into the coast. The front to the north may play a role in
maintaining the storms where they are, but would likely play a
secondary role. Should this line develop robustly, it will force
the aforementioned line of storms over northern LA/southwest MS
eastward, resulting in less rain over northern/central LA. Most
likely there will be a combination of the two, with the east-
northeastward moving storms along the southwest Louisiana coast
continuing with the line to the northeast over western Mississippi.
Once again the Moderate Risk here remains in place with few
changes.

Finally, further south along the Texas Gulf Coast, the larger
synoptic level disturbance, responsible for the heavy rains and
flash flooding over Deep South Texas yesterday will likely eject
east into the Gulf today. As it does so, it will create localized
bands of heavy rain along the Middle Texas Coast, which will then
advect northeast to join the line east of Houston. This area from
Corpus Christi to Houston remains the most uncertain portion of the
Moderate Risk area, as a small south/east shift in the track of the
disturbance will take the lion's share of the associated rainfall
ahead of the low track out over the open Gulf, and greatly limit
any rainfall today along the coast. The inherited Moderate Risk was
trimmed completely south of Corpus Christi, with just a Slight
along the immediate lower Texas Gulf Coast to account for ongoing
rains in the area. The threat will shift off the coast by late this
afternoon.

With all three areas of heavy rain above, they will all diminish
and dissipate as a significant heavy rain threat by this evening.
Occasional showers and a storm or two may impact the immediate Gulf
Coast through the overnight, but for the most part the heavy rain
threat through tonight should be minimal.

Wegman


Day 2
Valid 12Z Wed Jun 17 2026 - 12Z Thu Jun 18 2026

...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR THE UPPER
TEXAS GULF COAST THROUGH SOUTHWESTERN LOUISIANA...

...Gulf Coast...

An upper level shortwave disturbance over the northwestern Gulf
will likely contribute to the development of a small surface low
or surface trough along the Texas Gulf Coast on Wednesday. This
disturbance will gradually track parallel to the coast towards the
northeast, and move into southwestern Louisiana this evening. The
exact track of the center of circulation will remain the
distinctive factor determining how much rain penetrates inland
along the Texas coast, including into the Houston Metro. Some of
the CAMs guidance has trended west with the track of the low, which
will prolong a period of heavy rain into Houston as cells ride
northwestward ahead of the low in a small but very potent band of
heavy rain. It will be potent not just because of the multiple
sources of forcing for the heavy rain, but also the sheer amount of
atmospheric moisture that will be available for the storms to tap
into. PWATs will remain exceptionally high, likely peaking in the
upper 2s as the low reaches its peak intensity this afternoon.
While the predominant southerly flow will advect some instability
into the low, extensive cloud cover will work against achieving
heavy rainfall rates in many cases. The hydrology will become
increasingly favorable for flash flooding even with less than
optimal rainfall rates as prior days' rains combine with the new
rainfall to continue lowering FFGs. The Moderate Risk was nudged
west in coordination with HGX/Houston, TX forecast office to
include all of the Houston Metro. Prolonged northwestward moving
cells in a narrow band could easily start causing flash flooding
problems over the Houston metro should the track optimize.

Further east, there is more certainty with heavy rainfall rates
impacting the area from Beaumont east into southwestern Louisiana.
Southerly flow exactly orthogonal to the coast, frictional
convergence, and then the low itself impacting the area from late
in the day Wednesday through Wednesday night will all contribute to
additional heavy rains over an area of Louisiana already hard hit
from the current rain and expected rain in the area on Day 1. The
Moderate Risk area was little changed from inherited in this area.

Further north and east, the heavy rain threat will concentrate over
the Wednesday night timeframe. As the low and associated upper
level disturbance move into Louisiana, a leading band of heavy
rain, oriented southwest to northeast, will overspread portions of
central and eastern Louisiana and continue into western
Mississippi. There is considerable uncertainty how far into
Mississippi the heavy rain gets. But it will likely extend from the
coast south of Lafayette northeast through Baton Rouge and into
southwestern Mississippi. Here too, favorable hydrology due to wet
soils from heavy rains from today/Day 1 will contribute to any
resultant flash flooding. For now, this appears to stay west of New
Orleans, but will likely be too close for comfort, and the New
Orleans Metro could see occasional heavy rain from passing cells.

...Midwest...

Very few changes were needed to the large Slight Risk across much
of the Midwest on Wednesday. A strong and impressively potent low
driving eastward from the Plains will run into increasing amounts
of deep tropical moisture as it moves into the Great Lakes. This
will contribute to increasing coverage of showers and storms, first
with its warm front from Chicago east along Michigan's southern
border, then followed very quickly in some areas by stronger storms
associated with the low's cold front. Favorable hydrology from
several bouts of heavy rain over the past week will support flash
flooding development. However, this low and its associated storms
will be racing eastward. Thus, duration of heavy rain will be very
limiting in many areas. For the corridor forming the northern half
of the Slight Risk area, heavy rain duration will be longer, as the
warm front and associated elevated convection quickly followed by
stronger convection with the cold front could allow for some areas
to see multiple hours with interspersed heavy rain through the day.
Instability may be more limited in this area however, so that will
work against heavy rains.

For the southern half of the Slight, there will be significantly
more moisture and instability for the storms to work with as they
race southeastward, so rainfall rates should be higher than further
north. However, it's likely just the cold front's storms that will
impact most areas, so it's one shot and done. Training and
backbuilding won't be a concern until you get back into Missouri
and Kansas. Once again, the soils remain wet in these areas, so it
may be enough to get widely scattered instances of flash flooding,
particularly in urban areas. As is typical this time of year, the
cold front will hang up on its southern and western periphery, as
it runs into the southwesterly low level jet. This will greatly
extend the potential for heavy rain from southeastern Kansas
through southern Missouri Wednesday night. These areas have also
been hard hit with heavy rain in recent days, as well as the
topography of the Ozarks, so the Slight remains in place there with
few changes.

Wegman


Day 3
Valid 12Z Thu Jun 18 2026 - 12Z Fri Jun 19 2026

...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE LOWER MISSISSIPPI VALLEY THROUGH CENTRAL ALABAMA...

...Lower Mississippi Valley through Central Alabama...

The tropical low and its associated moisture are expected to start
the Day 3 period over northern Louisiana or western Mississippi.
The leading band of heavy rain will be ongoing from eastern
Louisiana into southern Mississippi. The low will slowly drift
eastward across Mississippi through the day, then gradually
increase its forward speed Thursday night as it tracks east across
Alabama and ends the period in northern Georgia. A very persistent
heavy band of rain will remain south and east of the low center
throughout the Day 3 period. This will focus the heaviest rains in
the Moderate Risk corridor from central Louisiana through central
Alabama. Prolonged southerly flow of the deep tropical moisture
with PWATs to 2.5 inches will support development and redevelopment
of convection ahead of the main band associated with the low. Thus,
heavy rain with multiple inches of rain per hour rain rates are
expected through the Moderate Risk area. Soils in Louisiana and
Mississippi are already saturated now even before Days 1 and 2's
rains. While some parts of central Alabama have dry soils, that
should change as the soils are primed today/Day 1. Thus, by Day 3,
the entire Moderate Risk area should have well wetter than normal
soils. Thus, hydrology will play a big role in flash flood
development. For western areas, nearly constant heavy rain in Days
1-2 will make flooding into D3/Thursday very easy. Further east
into Alabama, the southerly flow of deep tropical moisture will
last longer as the low tracks eastward, so there will be a longer
period where heavy rain may be occurring over Alabama. In
coordination with BMX/Birmingham, AL forecast office, the Moderate
Risk was expanded east across central Alabama with this update.
Even once the low center passes, the air mass will be largely
unchanged behind the low, as the tropical air mass will still be in
place into the lower Mississippi Valley, thus, the low's cold front
could continue to produce storms into southern Alabama into
Thursday night.

...Ohio Valley...

The northern edge of the deep tropical air mass supporting the
tropical low to the south will bump up against a strong cold front
that will be set up along the Ohio River on Thursday. As the
plentiful moisture on southwesterly flow runs into the front,
training thunderstorms are expected through much of Kentucky,
particularly the southern half, Thursday afternoon and evening.
This could lead to a prolonged period of heavy rain due to training
and backbuilding thunderstorms over the region. For this reason, an
internal higher-end Slight was issued for the southern 2/3 of the
state. The Slight into the mid-Appalachians was trimmed on the
northern side around Pittsburgh, PA due to less moisture making it
that far north and east, and with the cold front over Kentucky
concentrating the storms further west, coverage of storms into West
Virginia also appears to have diminished in the guidance. Thus, the
Slight was dropped to a lower-end Slight.

At the western end of the front into southeastern Oklahoma and
parts of Arkansas, an MCS may develop late Thursday night as the
nocturnal jet intensifies. A Slight Risk was drawn in from far
north Texas along the MO/AR border to the Mississippi/Ohio River
confluence for expected slow-moving and training storms along much
of the front, though the highest concentration of storms should be
into southeastern Oklahoma.

Wegman


Day 4 and Day 5

Valid 12Z Fri Jun 19 2026 - 12Z Sun Jun 21 2026

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE SOUTHERN PLAINS, MID SOUTH, AND SOUTHERN APPALACHIANS ON DAY
4...

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER THE PARTS OF
THE MIDWEST ON DAY 5...

Day 4's (Friday) Excessive Rainfall threat remains fixed across
much of the South. The combination of the lingering disturbance,
that may become a more formidable surface cyclone, will reside
within an environment that contains >2.0" PWs over much of the
Mid-South. Sufficient instability aloft will continue to support
thunderstorms that can produce highly efficient rainfall rates.
Much of the region will also have received a steady dousing of
heavy rainfall during the work-week as well, priming soils for more
potential flash flooding. Farther west, a shortwave trough over TX
and the return of low-level SErly flow will introduce will
reintroduce more Gulf moisture over central TX and as far west as
southwest NM. The TX Hill Country is most prone to flash flooding,
although some localized flash flooding over the Guadalupe and
Sacramento mountains could also transpire.

By Day 5 (Saturday), a potent 500mb shortwave trough over the Great
Basin will race east towards the Northern Rockies around midday and
reach the northern High Plains by Saturday night. As a cold front
races south from Canada, a warm front lifting north through the
Central Plains will deliver a plume of anomalous moisture and
foster an increasingly unstable environment over the Midwest by
late Saturday. While the exact placement of the developing
thunderstorm activity varies by model, they all show a deepening
surface low over the Central Plains and a strengthening LLJ that
results in a >500 kg/m/s IVT field over the Midwest. Storms should
be able to produce efficient rainfall rates thanks to a growing
shield of >1.5" PWs over the MO River Valley, which does surpass
the 90th climatological percentile. For these reasons, a Day 5
Slight Risk was introduced. Farther south, a weakening frontal
boundary and residual anomalous moisture aloft will help trigger
additional storms from South TX on east across many of the Gulf
Coast States. Given the South's increasingly sensitive soils and
the presence of PWs hovering around 2.0", thunderstorms could
produce localized flash flooding Saturday afternoon and evening.

Mullinax


Day 1 threat area: https://www.wpc.ncep.noaa.gov/qpf/94epoints.txt
Day 2 threat area: https://www.wpc.ncep.noaa.gov/qpf/98epoints.txt
Day 3 threat area: https://www.wpc.ncep.noaa.gov/qpf/99epoints.txt



$$