386
FOUS30 KWBC 170027
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
827 PM EDT Sun Jun 16 2024

Day 1
Valid 01Z Mon Jun 17 2024 - 12Z Mon Jun 17 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE
NORTHERN PLAINS AND UPPER MISSISSIPPI VALLEY, & PARTS OF THE
CENTRAL GULF COAST...

...Northern Plains to Michigan...
Elevated convection late Saturday night resulted in some prolific
rainfall totals, including portions of northeast NE and south-
central to southeast MN, making the region even more sensitive to
additional rainfall. Overnight tonight into Monday morning, as an
upper level jet streak traverses the Dakotas and southern Manitoba-
Ontario, upper level divergence and low-level frontogenesis in its
right- entrance region will generate new convection which should
have no trouble growing upscale given the favorable dynamical,
thermodynamic, and kinematic environment. Strengthening S-SW low-
level inflow into an increasingly stationary front will likely
equal if not exceed the magnitude of the mean 850-300 mb flow,
while becoming aligned with the mean flow by late evening/toward
midnight. This will allow for an enhanced risk for cell training
along/near the surface stationary boundary across northeast NE,
southeast SD, southern MN, and parts of western WI -- areas where
the guidance shows TPWs peaking aoa 1.75" overnight. This along
with elevated MUCAPEs of at least 1000-2000 J/Kg will likely lead
to up to 2.5" an hour rain totals and local amounts to 5" where
convection can train prior to the best deep- layer forcing shifting
east- northeast. Changes to the risk areas were minimal on this
update, with a minor nudge to the south in some locations.


...Central Gulf Coast...
Broad, weak upper level difluence east of a retrograding upper
level trough/shearline will affect portions of the Gulf Coast
overnight. Scattered weak/subtle mid- level impulses along the
western periphery of the upper ridge will lead to a rather spotty,
disorganized convective footprint, though the latest CAM
guidance/trends favor better clustering along the central Gulf
Coast (south-central to southeast LA). PWs between 2-2.25" along
with ML CAPEs between 1500-2500 J/Kg could result in fairly
prolific short-term rainfall rates very late tonight into early
Monday morning, up to 3" in an hour. The effective bulk shear has
shown some increase as of late, and could attain 25+ kts which
would lead to greater cell organization. Isolated 3-5" totals
within a couple of hours can be expected (perhaps a bit more
scattered) along the Central Gulf Coast region where the Slight
Risk is noted.


...Tennessee Valley to the Southern Appalachians...
A variety of pulse and convection with some organization is
occurring along the western periphery of the expanding upper ridge
which may lead to localized instances of flash flooding across
this area for a few more hours. Precipitable water values are
~1.75". Spotty 3" totals are possible, though such amounts should
become ever more isolated from here on out as CIN is beginning to
set in.


Roth/Hurley


Day 2
Valid 12Z Mon Jun 17 2024 - 12Z Tue Jun 18 2024

...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
CENTRAL AND SOUTHERN MINNESOTA AND FAR NORTHEASTERN SOUTH
DAKOTA...

...20Z Update...

Some shifts to the MDT risk were generated this forecast update and
are reflected within the "Upper Midwest" sub-heading below. An
additional MRGL risk was also added across the Blue Ridge and
southern Appalachians within portions of WV/VA/NC/KY/TN. More on
this located in the "Southern Appalachians" sub-section below...

Elsewhere, continuity remains across the Gulf coast and across MT.
Heavy rains from the tropical moisture advection off the Gulf will
lead to scattered instances of very heavy rainfall with hourly
rates pushing 2-3"/hr within areas under the current SLGT risk.
PWAT anomalies between 2-3 standard deviations above normal are
forecast to protrude into the southern LA coastal plain by the
second half of the period marking the beginning of the true
tropical moisture transport that will lead into the greater impacts
on D3. There has been little change in this forecast with the
greatest flash flooding focus within urban zones and within any
cells that produce rates greater than 3"/hr given the FFG`s located
across the outlined area. Over into MT, scattered thunderstorms
will generate periods of locally heavy rainfall outside the higher
terrain with the highest chance located just to the east of the
northern Rockies in western MT. Totals will generally be 1-2" at
max with rates between 0.5-1"/hr, which would be enough to satisfy
some localized flooding prospects across any urban areas and
complex terrain. There was no need for any significant adjustments
to the forecast based on the recent trends.

Kleebauer

...Upper Midwest...

The primary upper level evolution has not changed much since the
previous forecast issuance, however there are some differences in
the location of the areal QPF max due to some adjustments in the
surface reflection across the Northern Plains and Upper Midwest. An
unusually strong low level jet (LLJ) and broad southerly flow off
the Gulf will continue to advect abundant moisture and instability
into the Northern Plains and Upper Midwest through the day Monday
and into Monday night. Heavy rainfall is likely to be ongoing
across southern portions of the Slight and Moderate risk areas from
eastern SD into southwestern and south-central MN on Monday
morning. A stalled out cold front turned stationary front will be
draped across the region with the current forecast positioning
located south a line from Sioux Falls through Minneapolis to north
of Green Bay. The front will provide the focus for the heaviest
convection as an airmass characterized by PWATs up to 1.75 inches,
or 2 sigma above normal is lifted by the front. Cooler, drier air
north of the front will start out the day moving into northern MN
from the west. This influx of air will prevent the front from
moving north very quickly to start out the day. This will allow for
a more concentrated area of heavy rain with likely initiation over
southern SD by late Monday morning, moving northeast along the
front as the flow remains parallel to the boundary. Additional
lift will be provided in the right entrance region of a 120 kt jet
over southern Manitoba and western Ontario creating a scenario for
widespread heavy rains extending from eastern SD through the
central part of MN.

On Monday afternoon, the cooler drier air north of the front will
begin retreating to the north as the southerly flow south of the
front begins to push the front north as a warm front. The
convective coverage will diminish for a short time Monday
afternoon, but with daytime heating, some heavy rain from the
invigorated storms will continue across the middle of Minnesota and
into northern Wisconsin.

On Monday night, a low ejecting out of the Plains will rapidly lift
northeast towards the ongoing convection across central and
northern Minnesota, perhaps extending as far west as northeastern
SD and southeastern ND. At the same time, a strong shortwave trough
will move into the western Dakotas. The shortwave will support the
intensifying low as it moves into the eastern Dakotas. The low will
concentrate the convection along its warm front as the supporting
LLJ intensifies above 50 kts. This will result in a
reintensification of the heavy rain mover much of northern
Minnesota.

By the end of the period, a widespread area of 1-3" will
be forecast from eastern SD and south-central MN up through
northern MN with a maxima located across central and east-central
MN where 3-6" with locally as high as 10" are plausible if there is
considerable training in any given location within the confines of
the warm front. The upper threshold of the setup near 10" is
supported by the latest 12z HREF neighborhood probability fields
where a bullseye of 20-25% for at least 8" exists north of the
Minneapolis-St Paul metro. General probabilities for totals between
1-3" are well supported via the hi-resolution ensemble with a
swath of 60-80% probabilities for at least 2" across all of central
MN and over 90% for the same area for at least 1" within 40km of
any point. The reason for the shift was due to the location of the
warm front which is well-defined within the theta-E fields off
model guidance. Trends have shown the higher theta-E`s to be
situated a bit south of yesterday leading to the QPF maximum being
displaced about 50-75 miles further south than 24 hours ago. This
is beginning to correlate more with the latest ML guidance which
has actually trended a bit north and is more in-line with ensemble
QPF footprint and the relevant probability fields. As a result, the
MDT risk was expanded on the southern edge to include the
Minneapolis-St Paul metro area and the southern suburbs to account
for some potential the setup could shift a bit more south. The
northern edge was consolidated along with the northeastern extent
into the Arrowhead with the mainly the area around DLH remaining to
reflect the converging signal near the area. The probability of
locally significant flooding is still very much in the cards
considering the setup, antecedent conditions, and anomalous deep
layer moisture being pulled poleward into the region.

Wegman/Kleebauer


...Previous Discussion...

...Central Gulf Coast...

Guidance across the region for the Monday and Monday night period
has come down quite a bit from previous forecasts. A plume of
impressive Caribbean and Gulf moisture characterized by PWATs as
high as 2.25 inches is still expected to move into the central
Gulf Coast, but forcing for heavy rain has become much less. Thus,
the rainfall across mostly southern Louisiana, but also the
Mississippi and Upper Texas Gulf Coasts has become much more spotty
and less well defined. Given this downward trend, the Slight Risk
in the area has been shrunk to focus on coastal regions...where the
expectation for forcing is that local sea breezes, outflow
boundaries, and cold pools will provide localized forcing for heavy
convection. However, the convection now looks to not be as
widespread. Given the plentiful moisture and instability still
available...any storms that form will be capable of heavy rainfall
rates to 2 inches per hour, which could still cause widely
scattered instances of flash flooding. This would especially be the
case should these heavy rainfall rates occur over an urban or
flood sensitive area.

...Montana and Central Idaho...

The comma-head region of a developing low over Wyoming will impact
much of Montana and Central Idaho throughout the Day 2/Monday
period. The low will form in the divergence region between 2
different jet streaks...one to the south over Wyoming, and a second
over southern Saskatchewan. Unseasonably cold air over western MT
and ID will clash with moisture tracking westward to the north of
the low from the excessive rainfall over Minnesota to support the
rain (and snow at higher elevations) across the region. Upslope
flow may enhance total precipitation in the mountains in and around
Great Falls, MT. Cooling from the upslope should support a good
amount of that precipitation falling as snow at the higher
elevations, which should cut any resultant flash flooding at the
lower elevations to just isolated instances (due to lack of runoff
from the higher elevations). While the amounts of up to an inch of
liquid equivalent is a wetter rainfall event for this area, they
should generally stay below FFGs, keeping any flash flooding
confined to flood-prone areas. No major changes were made, with the
portion of the Marginal in the mountains of western MT and ID
emphasizing isolated valley flash flooding, as snow is expected at
the higher elevations.

Wegman

...Southern Appalachians...

Significant differential heating within the terrain will occur
under the strengthening mid-level ridge centered over the southern
Mid Atlantic. Seasonal PWATs between 1.25-1.5" along with
sufficient surface based instability around 2000-2500 J/kg SBCAPE
will help initiate pulse variety convection with very little
motion due to the steering pattern aloft. The combination of
complex terrain and very slow to nearly stationary cell motion will
create pockets of localized flash flooding concerns within the
mountains and adjacent valleys. Probability for flash flooding is
within the lower end of MRGL thresholds which was enough to warrant
the addition of a MRGL extending down the Appalachian front from
northeast WV down through much of western NC and eastern TN.

Kleebauer

Day 3
Valid 12Z Tue Jun 18 2024 - 12Z Wed Jun 19 2024

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

...20Z Update...

Little to no change was necessary for the previous MDT risk area
across portions of the central and western Gulf coast as the
overall ensemble QPF footprint and magnitudes remained very
similar from the previous forecast. Deterministic output has some
variance with the location of the heaviest precip and associated
SLP signature in the western Gulf that has allowed for a large
coverage of at least a SLGT risk at the immediate coast. There is
more consensus with the heaviest rainfall being positioned within
the TX Gulf coast in the ensemble output as well as the ML output
off the latest Graphcast and AIFS. Those specific ML outputs are
slightly more southwest down the TX coast, so would not be
surprised in some minor adjustments based on that proposition as ML
guidance tends to do well with SLP tracks in the short and medium
range. Didn`t want to deviate too much, however with the current
ensemble blend and NBM probabilities still highlighting that upper
TX coast. As a result, maintained continuity for heavy rains to
impact the outlined area with totals pushing upwards of 3" with
locally much higher based on the tropical rainfall efficiency and
deep moist convection expected.

Over the central plains, the signal for heavy rain late-Tuesday
afternoon and evening has increased with deterministic signals now
suggesting localized areas exceeding 5" as plausible considering
the strong convergence pattern and mean flow running parallel to
the frontal boundary in place. The pattern is a textbook way to get
a long axis of training convection with a widespread QPF footprint
that would produce an areal extent of heavy precip totals. Latest
24-hour mean QPF off the NBM is up near 2" over an area extending
from northern KS through eastern NE. The 75th percentile, upper
quartile is closer to 3" in the same location which is a pretty
aggressive signature at these leads without the addition of the
CAMs. The current SLGT risk remains and will be worded as a high-
end SLGT for the time being, but there is a potential for this to
be upgraded in future periods considering the growing consensus.

Kleebauer


...Previous Forecast...

...Upper Texas Coast and Southwest Louisiana...

A Moderate Risk upgrade was introduced with this update in
coordination with LCH/Lake Charles, LA and HGX/Houston, TX forecast
offices.

The northern side of a developing tropical low in the western Gulf
is expected to begin impacting the Upper Texas Coast and southwest
Louisiana on Tuesday. The north side of the low will continue the
broader southeasterly flow from the Caribbean across the Gulf and
into the Upper Texas and southwest Louisiana Gulf Coasts. PWATs
with this air mass are very impressive, at times approaching 3
inches. Unlike on Day 2/Monday, by Tuesday the low itself and any
associated upper level energy will provide the forcing for
convection, so there`s higher confidence on more widespread heavy
rainfall and resultant high storm total rainfall amounts. Where
confidence remains lacking is where the forcing
tracks...particularly how far north it gets. This will play a
critical role as to where the heaviest rainfall amounts will be.
For now there appears to be fair consensus on heavy rainfall
impacting the Gulf Coast from as far east as Pecan
Island/Vermilion Bay, LA to as far west as Corpus
Christi/Brownsville, TX. There`s better agreement highlighting the
section of coast from Cameron, LA to Matagorda, TX. This includes
Lake Charles, LA and Houston, TX.

Since the forcing is highly concentrated, there`s likely to be a
sharp gradient on the northern end as to who sees the heaviest
precipitation. It appears likely that areas along a Leesville, LA
west to College Station, TX line and points north will remain with
either no or light, unimpactful rain. Those details too will need
to be hashed out with better guidance agreement in the coming days.
For now, therefore, the Moderate Risk largely stays along and south
of the I-10 corridor.

The heaviest rain will slowly track westward with time, so the back
edge of the rain should move west along the Louisiana coast with
time. Houston looks to get periods of heavy rain throughout Tuesday
and Tuesday night as the storms track westward in waves. The
heaviest rain rates are likely Tuesday night from Houston south
along the coast as the strongest forcing associated with a front
north of the low moves ashore from the Gulf into mainland Texas.
PWATs approaching 3 inches would support highly efficient warm rain
processes given the very high freezing levels associated with this
fully tropical air mass. Any storms that track across this region
will be capable of 3 inch per hour rain rates that would quickly
overwhelm all but the sandiest/most flood resistant soils in a
hurry. Thus, despite prior dry weather and dry soils from Houston
south along the coast, these prodigious rainfall rates would easily
overwhelm those soils, leading to local flash flooding. Urban and
flood prone areas would be particularly susceptible. Interests in
and around Houston should monitor forecast updates closely.

The surrounding Slight Risk was shifted west with guidance trends,
and includes nearly the entire Texas Gulf coast now, including into
Brownsville in deep south Texas. Any rain that far south is likely
to start just prior to the 12Z Wed cutoff and continue into D4/Wed.
There remains significant uncertainty as to how far north the core
of heaviest rain, and latitudinal adjustments are likely with
future updates.

...Central Plains to Upper Midwest...

Broad southerly flow continues across much of the nation`s
heartland at the start of the period at 7am Tuesday morning. An
unusually moist air mass characterized by PWATs around 1.75 inches
will continue streaming north up the entire Mississippi River
Valley. Rain across northern Minnesota will be ongoing at the start
of the period with a stalled out/slow moving front. A surface low
developing along this front should help to keep convection ongoing
into the Arrowhead through the morning. Instability between 1,000
and 2,000 J/kg will ensure convection will be sustained through the
period. The Slight Risk was expanded north ot the Canadian border
and the Arrowhead to cover the likelihood of rain continuing from
Day 2 period into Tuesday morning.

Meanwhile, a burst of cooler, drier air associated with the cool
air mass will track southward on gusty northerly winds over the
High Plains of eastern MT, the western Dakotas and Nebraska, and
eastern Colorado. This air mass will create a new frontal gradient
between the drier, cooler air to the west and the hot and humid air
mass across the eastern Plains and upper Midwest. An intensifying
low over northern MN and eastern Ontario through Tuesday morning
will push a cold front across the Dakotas and Minnesota. This will
push the convection in the form of a line of storms from Minnesota
south and east into Wisconsin and Iowa. The storms will likely form
over hard hit areas of Minnesota, so additional flash flooding is
likely Tuesday afternoon before it moves into a bit less hard-hit
portions of WI. Given the eventual progression of the storms
contrasted with the antecedent conditions, a Slight remains across
much of Minnesota and western Wisconsin.

Further south, convection across Kansas and Nebraska is likely to
largely hold off until the early evening, and then continue through
Tuesday night. A slow-moving almost stalled front in that area is
expected as the southerly flow of hot and humid air south of the
front causes the surge of drier air north of the front to stall,
resulting in a clashing of air masses of southerly flow to the
south and northerly flow becoming weak to the north. With ample
moisture running into the front, the storms that form are likely to
backbuild and train as they move northeastward along the front.
Corfidi Vectors are southwesterly, parallel to the front, which
will support training convection. Expect a swath of storm total
rainfall through 12Z Wednesday of 1-2 inches from north central
Kansas to extreme western Iowa, with locally higher embedded totals
likely. The Slight Risk area was expanded southwestward to cover
all of southwestern Kansas with this update, due to increasing
numbers of the guidance suggesting storms will form and train into
this part of Kansas as well.

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