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FOUS30 KWBC 151552
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1152 AM EDT Sat Jun 15 2024

Day 1
Valid 16Z Sat Jun 15 2024 - 12Z Sun Jun 16 2024

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

...16Z Update...

The previous forecast was generally maintained across the Central
Plains and Upper Midwest in conjunction with the SLGT risk. A minor
adjustment was made to the southern periphery of the SLGT across
northeast KS where a strong convective cluster this morning put out
a local maximum as convection became anchored to the warm front and
produced some totals between 1.5-2" within a few hours. This area
has been primed for the next wave of convection expected later as
indicated in the discussion below...

Across KS, mid-level WV satellite shows our next shortwave making
progress to the northeast which 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.
Guidance is consistent on the heaviest precip focus within eastern
NE up through northern IA and southern MN with the QPF maximum
confined within that corridor in question. 12z HREF neighborhood
probabilities are still fairly robust for at least 3" across the
aforementioned corridor with percentages around 35-50% for the 3"
threshold with even some low-end probabilities for >5" settling
between 10-20%. 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 was more than valid for the period.

The MRGL risks over North Dakota, the Southern Plains, Florida, and
Washington state were all justified within the latest forecasted
QPF fields on all 12z deterministic. There were no changes to the
outlined areas considering the general continuity from guidance.
South FL will see their chances dwindle after 21z with north-
central FL seeing their best chance after 19z with the introduction
of the seabreeze driven convection. Washington state will remain in
a low-end flash flood concern through the period across portions of
the Cascades where remnant burn scars within the terrain will allow
for a localized threat with the elevated convective pattern
pivoting through the area. The Southern Plains and North Dakota
will have their MRGL risk generally after 23z with isolated flash
flood threats bordering around 5-10% given the environment in place
for the respective locations. General max of 1-3" is forecast in
these locations with much of it occurring in short intervals.

Kleebauer


...Previous Discussion...

...Upper Midwest to the Texas Panhandle...

A potent upper level shortwave currently over western Nebraska and
northeastern Colorado is spurring along a strong squall line across
the Missouri River Valley between Nebraska and Iowa and eastern
Kansas this morning. An impressive low level jet (LLJ) is
increasing moisture ahead of the squall line with PWATs locally
exceeding 1.75 inches and 850 flow of up to 45 kts. On the northern
end of the line, a warm front will remain nearly stationary across
southern/central Minnesota. This will allow for training of mostly
light rain, but with embedded convection for much of the day. The
heaviest rain is likely to develop well after sunset as the
nocturnal LLJ strengthens further and advects instability to 3,000
J/kg into the Slight Risk area. This will allow for backbuilding
convection into western Iowa and extreme eastern Nebraska, which
will then track ENE across northern Iowa, southern Minnesota, and
into western Wisconsin. The embedded convection with heavier
rainfall rates will increase the flash flooding potential across
the Slight risk area as the cold front continues pushing east. Much
of the area has about average to slightly above average soil
moisture content, which will support the development of widely
scattered instances of flash flooding through tonight.

The trailing cold front and associated moisture plume will allow
some limited convection to develop across the Texas Panhandle north
into Kansas today. Since coverage will be more widely scattered,
any flash flooding will be isolated, and the Marginal Risk for
these areas remains in place with no changes.

A second cold front associated with a low over Saskatchewan and
Manitoba tonight will spur its own convection across western North
Dakota. This convection too will have plenty of instability to work
with with values between 2,000 and 3,000 J/kg. Isolated flash
flooding is possible as a result.

...Florida Peninsula...

Typical afternoon convection is expected to redevelop again today
across the Florida Peninsula. It will continue the downward trend
as far as coverage is concerned, but may impact more northern areas
of central Florida, which remain in a drought. While coverage will
be greater across northern Florida, even isolated convection is
possible across southern Florida, where the Marginal risk was
extended to account for even isolated convection resulting in
additional flash flooding as many areas of the urban I-95 corridor
from Ft. Lauderdale south through Miami remain flooded with soils
and reservoirs still full of water and unable to handle even the
brief heavy downpours typical of this time of year in the area and
are possible today. Today should be the last day where even a
modest degree of organization is possible with the convection.

...Washington Cascades...

Upslope flow off the Pacific will continue to uplift showers and a
few thunderstorms into western Washington. 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 was introduced with this update 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...

...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 SLIGHT RISK OF EXCESSIVE RAINFALL FOR MUCH OF
MINNESOTA AND PORTIONS OF THE EASTERN DAKOTAS AND WESTERN WISCONSIN
AS WELL AS FOR THE MISSISSIPPI DELTA REGION...

...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, but as would be expected, there is still
disagreement as to whether the front will slowly move north or
remain stationary. This will ultimately be the difference between
more widespread instances of somewhat less severe flash flooding
over a larger area, or a more concentrated area of high-impact
flash flooding over a smaller area. Due to this uncertainty and in
coordination with DLH/Duluth, MN; MPX/Minneapolis, MN; and
FGF/Fargo, ND forecast offices, the Slight Risk area was expanded a
bit, and is a very high-end Slight over the aforementioned areas. A
Moderate Risk is very likely with future updates, and should the
guidance suggest the latter scenario, a High Risk is not out of the
question. The potential is there for a flash flooding event
unrivaled in the area for at least the last 10 years. Stay tuned.

...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