785
FOUS30 KWBC 160036
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
836 PM EDT Sat Jun 15 2024

Day 1
Valid 01Z Sun Jun 16 2024 - 12Z Sun Jun 16 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE
UPPER MIDWEST...


Plains/Midwest...
Across KS, a shortwave will assist in the primary ascent pattern
that develop during peak diurnal heating through the overnight time
frame. A generally modest environment currently will ramp up with
the advection of richer theta-E`s and relevant instability as it
pushes north with the aid of the nocturnal LLJ. Precipitable water
values of 1.5-2" are expected across the region, which in organized
convection would allow for hourly rain totals to 2.5" and local
amounts to 5". Scattered flash flood signals are most likely the
case with this kind of setup, but any organized clusters could
promote a more significant areal extent for flooding prospects,
especially if they make their way into the suburban and urban
corridor centered over Minneapolis- St Paul. Given the above, the
SLGT risk remains in place.

The Marginal risks over North Dakota and the Southern Plains remain
justified. Changes were minimal to the outlined areas, based on
recent radar reflectivity trends and the 18z HREF guidance.
The Southern Plains and North Dakota should see a general max of
1-3" is forecast in these locations with much of it occurring in
short intervals.

Roth/Kleebauer


...Washington Cascades...
Upslope flow off the Pacific in combination with cool air
aloft/nearby upper low will lead to showers and a few
thunderstorms into western Washington over the next several hours.
Additional lift from the Cascades may result in some widely
scattered rainfall totals of 1-1.5 inches, which could result in
localized flash flooding on burn scars and any canyons and fast-
flowing creeks and streams. A Marginal Risk remains to account for this
possibility.

Wegman


Day 2
Valid 12Z Sun Jun 16 2024 - 12Z Mon Jun 17 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE
NORTHERN PLAINS AS WELL AS THE CENTRAL GULF COAST...

...20Z Update...

There were limited changes necessary to the previous SLGT risks in
places across the Northern Plains and Central Gulf coast. Guidance
remains fairly consistent in axis of enhanced QPF located over
northern NE and SD, eastward into MN, northern WI and portions of
the Michigan UP. Cold front will pivot southeast out of the Dakotas
on Sunday before hitting a wall from the expansion of ridging that
will dominate the eastern half of the CONUS. Boundary will stall
in-of the Sand Hills region and points northeast acting as a broad
surface focal point with a multitude of mid-level perturbations
ejecting northeast out of the base of the mean trough to the west.
Increasing LLJ presence across the plains will advect deep moisture
to the north and northeast with the best instability residing along
and south of the stalled frontal boundary. Scattered to widespread
convection will develop and propagate to the east and northeast
with QPF maxima relatively confined to near the front. A persistent
signal for 1-3" with locally as high as 4" is forecast across the
aforementioned corridor with the max being depicted over eastern SD
as of the latest HREF probabilities (45-70% for >2" totals). This
was plenty of confidence to maintain the previous SLGT risk over
portions of the northern CONUS.

Across the Central Gulf coast, anomalous moisture flux pivoting
around the western fringes of a prominent surface and mid-level
ridge will creep northward into the Gulf coast areas extending from
far Southeast TX through southern LA/MS/AL. PWAT anomalies will be
surging from +1 standard deviations at the beginning of the period
to as much as +3 deviations as we advance into early Monday
morning. Latest GEFS actually has a small segment of southeastern
LA within the highest climatological percentile for the forecast
PWAT indices on Monday, a tell tail sign of a well-defined tropical
airmass with origins out of the Western Caribbean. This places the
area along the Gulf coast into an environment capable of
significant heavy rainfall within any convective cores that develop
during the period in question. There`s some discrepancy on the
exact placement of where the heaviest precip will occur, but the
premise is the outlined SLGT risk area along the coastal plain up
into portions of the urban corridor along I-10 will be subject to
scattered tropical convective cells capable of putting down 2-3" of
rain in a short period of time. Overall consensus within the
ensemble output is sporadic 1-3" amounts with probability matched
means closer to 5-6" within the areas hit with multiple rounds of
heavy thunderstorms. Thankfully the area over LA is actually below
normal when it comes to soil moisture, so the grounds will be able
to soak in much of the rainfall. The areas most at risk are those
urban centers where the ability for runoff is much greater due to
the impervious surfaces. This was suitable for a SLGT risk
maintenance with agreement from the local WFOs impacted.

Kleebauer


...Previous Discussion...

...Northern Plains to the U.P. of Michigan...

The cold front associated with a low over Canada will stall out as
the parent low moves into northern Canada. Meanwhile a new leeside
low will develop over Colorado. This will drag abundant Gulf
moisture over the nation`s midsection back to the west into the
low. After sunset Sunday night, the strengthening LLJ will run into
the stalled out front, turning it into a warm front. The front will
act as a forcing mechanism...lifting the abundant moisture and
instability along it, resulting in widespread convective initiation
across South Dakota and Nebraska. Due to the abundant moisture, the
storms will be capable of heavy rainfall rates as they track
northeastward across southern South Dakota and into southwestern
Minnesota in the predawn hours Monday morning. The strong parent
low over northern Canada will keep strong westerly flow into the
front from the cold side. The result is despite impressive moisture
influx from the south with the LLJ, the front will struggle to
move. This will turn the front into a "train track" of sorts,
allowing numerous rounds of convection to develop and move
northeast along it, with the LLJ fostering new convective
backbuilding to the west. The flow will turn northeastward upon
running into the front, resulting in largely unidirectional flow,
supporting training and backbuilding. This will set this stage for
a more impressive flooding event possibly into Monday. More on
that in the Day 3 discussion.

Portions of the Slight Risk area across SD and MN will have been
hit with multiple rounds of heavy rain from now through Sunday
morning. Thus, the start of the potentially bigger flooding event
across SD/MN may have some support of already wet soils as the
storms move through, resulting in widely scattered instances of
flash flooding. The Slight Risk area was largely unchanged from
inherited.

...Central Gulf Coast...

The other area of increasing concern will be along the Gulf Coast.
A nothing short of impressive plume of very deep tropical moisture
straight from the Caribbean will track northwestward across the
Gulf and into the central Gulf Coast on Sunday. PWATs associated
with this air mass will be between 2.25 and 2.5 inches, exceeding
the 95th percentile compared with climatology and as high as 4
sigma above normal. This deep moisture plume will result in
convection with daytime heating during the afternoon on Sunday,
generally from New Orleans south...but the more substantive
rainfall will come Sunday night as atmospheric moisture continues
to build. Widely scattered convection all of which capable of very
heavy rainfall will move across the Central Gulf...generally
between Mobile and New Orleans, where the Slight Risk remains.

The Slight was shifted eastward with this update, reflecting a
slowing of the westward movement of the moisture plume. This
slowing would further enhance the potential for flash flooding, but
a lack of forcing should keep convection rather limited in
coverage. One piece of good news is that this area has been rather
dry in the recent past, with the last heavy rain episode a week or
more ago. Thus, for the first part of this event Sunday night,
instances of flash flooding should be isolated to widely scattered,
with the greatest threat over flood sensitive urban areas from
Mobile to New Orleans.

Wegman

Day 3
Valid 12Z Mon Jun 17 2024 - 12Z Tue Jun 18 2024

...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
NORTHEAST SOUTH DAKOTA, CENTRAL MINNESOTA UP THROUGH THE ARROWHEAD,
AND NORTHWEST WISCONSIN...

...20Z Update...

A Moderate Risk has been forecast across portions of the Upper
Midwest and adjacent Northern Plains for Monday into early Tuesday
morning. More on this potential in the "Upper Midwest" sub-heading
below...

Elsewhere, the previous forecasts across the Gulf coast and MT were
relatively unchanged with some minor adjustments further west with
the SLGT risk into southeastern TX near Houston as recent ensemble
mean QPF field has shifted slightly to the west with influence from
the recent 12z deterministic suite showing the nudging of the
anomalous moisture flux on the western flank of the ridge.

Totals across the Gulf coast are still fairly aggressive within
the confines of the 3-4 deviation above normal PWAT anomaly that
advects north- northwestward out of the Gulf with origins based out
of the Western Caribbean. IVT signatures from the GFS/ECMWF are
both stout with rooted connections basically correlating from the
ITZ located from Central America through the Caribbean. This is a
profound setup to extract the deepest tropical airmass possible for
the mid- latitudes creating an environment conducive for not only
waves of heavy rainfall to protrude the Gulf coast areas, but to
maximize efficiency given the source of the moisture in question.
Despite some deviations in the outcome within the sense of the QPF
footprint, ensemble means still suggest an area of 2-5" with
locally higher likely situated from Beaumont along the I-10
corridor back into Lake Charles towards New Orleans. After the
previous period of rainfall, this would be a secondary round that
would push 2 day totals above 5" for some within the southern 1/3
of LA, a setup consistent for flooding potential. In coordination
with the local WFOs over TX/LA/MS, decided against a MDT risk at
this time as the prior period will play a pivotal role in the
potential upgrade. Expect further evaluation in subsequent forecast
periods with a possibility for an upgrade over the next 24 hours.

Setup across MT maintains general continuity with no changes
necessary from the previous MRGL issuance.

...Upper Midwest...

A potentially high-impact flash flooding event may develop across
portions of Minnesota on Monday and especially Monday night. A very
impressive 130 kt jet for June will be nearly stationary across
Manitoba and northern Ontario provinces. The jet exists because
unseasonably cold air will be digging south across the
Intermountain West. Meanwhile, an unseasonably hot and humid air
mass associated with a strong upper level high over the East Coast
will pump impressive amounts of Gulf moisture up the entire
Mississippi River and into Minnesota. A impressively strong
shortwave trough will eject out of the Intermountain West and into
the northern Plains Monday night. The warm moist air mass will run
into the front that will be over central and northern Minnesota
Monday and Monday night. Additional lift from the right entrance
region of the upper level jet streak, the strong shortwave, and the
stationary front will result in nearly continuous convection across
this region from Monday morning straight through Monday night.
While convection typically wanes through the day due to competing
heating sources, the front and ample additional lift will keep the
rainfall going. Then as the LLJ intensifies to over 50 kt after
sunset Monday night, the associated storms will also increase in
coverage. Since the front, acting as the primary forcing mechanism
will be either stationary or very slow moving, the convection will
continuously train and redevelop over Minnesota into Monday night.

The combination of all the above ingredients could result in a
high-impact flash flooding event across Minnesota. Guidance is in
above average agreement that this will occur over central to
northern Minnesota with the latest guidance now coming into better
agreement on the placement of the stalled frontal boundary that
will be the focal point of where the heaviest precip will occur.
With a growing consensus on the downstream ridge pattern blossoming
east of the Mississippi and the progression of a stout shortwave
trough across MT, there is more confidence in the overall synoptic
evolution that will create a prolific convergence pattern centered
across the Upper Midwest and Northern Plains.

Latest ensemble bias corrected output for the area encompassing
eastern SD up through central and northwest MN is fairly prolific
with the QPF output a general 2-5" with a max of 6.5" located
across west-central MN near the SD border. This is consistent with
the current ML output from both the Graphcast ECMWF/GFS and the
ECMWF AIFS with heaviest QPF footprint located that aforementioned
vicinity. This is likely due to the increasing influence of the
nocturnal LLJ with the nose of the 850mb jet core aimed right into
the frontal boundary in that general location maximizing boundary
layer convergence. This is textbook for some of the more
significant flash flood episodes involving this type of regime, so
the prospects have merit. Widespread totals of 3-6" with maxima of
10" will likely be the range of outcomes based on the latest
synoptic and thermodynamic evolution with the evolving pattern.
According to the GEFS climatological percentile for mean QPF, this
would rank towards the 99th percentile of feasible outcomes for the
date relaying the type of precedence for this pattern. The
potential is there for a flash flooding event unrivaled in the area
for at least the last 10 years.

Considering all the data at hand, increasing confidence, correlating
factors for significant rainfall, and in coordination with a
multitude of WFOs (DLH/MPX/FGF/ABR) a Moderate Risk for excessive
rainfall has been issued for the corridor extending from far
Northeast SD through western and central MN, up into the Arrowhead
and the Northwest section of WI. This is an evolving scenario for
significant flooding, so please stay tuned for updates over the
next several days.

Wegman/Kleebauer


...Previous Discussion...

...Mississippi Delta Region...

Concurrent with the potential flooding in the Upper Midwest, the
plume of moisture moving into the central Gulf Coast from the Gulf
will continue to intensify through the day Monday. PWATs will be
nothing short of impressive, as they eclipse 2.5 inches along the
Louisiana coast. Storms are likely to be ongoing across the area
from the overnight period as the Day 3 period starts at 12Z/7am CDT
Monday morning. Continuous southerly flow of deep tropical moisture
will support continuous development and redevelopment of storms
capable of very heavy rainfall rates across Louisiana and
Mississippi on Monday. Corfidi Vectors will be both weak (under 10
kts, and out of the northwest, against the flow. This will support
persistent training convection and backbuilding when Corfidi
vectors are against the prevailing southerly flow. A subtle and
weak disturbance in the upper levels may further support
convection, but forcing should largely be low levels driven from
outflow boundaries and cold pools that are constantly recycled with
deep tropical moisture out of the Caribbean.

In coordination with LIX/Slidell, LA; LCH/Lake Charles, LA; and
MOB/Mobile, AL forecast offices, the Slight was greatly expanded
with this update to central Louisiana and Mississippi, with a
higher-end Slight introduced for southern Louisiana and the
Mississippi Gulf Coast. This area is the most likely to be
upgraded to a Moderate Risk with future updates, especially if Day
2 rainfall over this area overperforms or increases in future model
guidance. Given rivers in some of these areas of far eastern Texas
and western Louisiana are still in flood from events over more than
a month, soils and reservoirs have not had sufficient time to
recover from those events for another rainfall event of this
magnitude. Further east into New Orleans and the Mississippi Gulf
Coast, FFGs from urbanization are already lower. With potential
for 3 inch/hour rainfall rates from convection, perhaps repeatedly,
even flood resistant bayous will be unlikely to handle a deluge of
that magnitude, which will be possible.

...Montana...

Much of Montana will be on the "cold side" of the system, with
high-elevation snow expected. The rainfall across the state will be
much more reminiscent of a cold-season event, with ample upper
level forcing involved and some Gulf moisture. Essentially some of
the Gulf moisture from further east into Minnesota will curl back
to the west as another leeside low that forms over CO early in the
period tracks north into the northern Plains. Rainfall amounts
should remain generally under 1 inch, but will be widespread enough
across Montana that the inherited Marginal remains in place with no
changes.

Wegman


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