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FOUS30 KWBC 160825
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
425 AM EDT Sun Jun 16 2024

Day 1
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 AND UPPER MISSISSIPPI VALLEY, ALONG WITH PARTS OF
THE CENTRAL GULF COAST...

...Summary...

Relatively minor adjustments were made to the inherited Slight Risk
areas across the Northern Plains-Upper MS Valley and over the
Central Gulf Coast region. We did expand the Marginal Risk along the
Gulf Coast, which now includes the Upper TX Coast along with much of
the FL Gulf Coast.

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

Elevated convection along/ahead of the surface warm front late
Saturday night had resulted in some prolific rainfall totals,
including portions of northeast NE and south-central to southeast
MN. Part of the reshaping of the Slight Risk was with this in mind,
i.e. with the notable increase in 0-40cm relative soil moisture/soil
moisture percentiles and subsequent lowering of the FFG over these
areas. The other reason was with the model trends, particularly the
high-res CAMs. The outlook areas here will see a break in the more
organized, heavier rainfall today as the warm front pushes northeast
and is followed by flat shortwave ridging aloft. Overnight, 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 the quasi-stationary boundary 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 1.5-2.0/hr
rainfall rates where convection can train prior to the best deep-
layer forcing shifting east-northeast.

The Slight and surrounding Marginal Risk areas do have support from
the CSU ERO first-guess fields (UFVS-verified), though as with
yesterday`s Day 2 ERO, will keep the Slight Risk out of the
Sandhills.


...Central Gulf Coast...

Broad, weak upper level difluence will affect much of the Gulf Coast
downstream of the weak upper level trough over eastern TX. 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), along with western portions of the FL Peninsula where the deep-
layer easterly flow will push the Gulf-East Coast Sea Breeze
boundary a bit farther west than what would otherwise would be. PWs
between 2-2.25" along with ML CAPEs between 1500-2500 J/Kg during
the day will result in fairly prolific short-term rainfall rates,
likely 2+ inches within an hour or less, based on the latest HREF
neighborhood probabilities. 0-6km bulk shear values generally 20
kts or less will favor more pulse type convection and thus more
short- lived intense rainfall rates, however based on the
deterministic CAMs and HREF probabilities, isolated 3-5" totals
within a couple of hours can be expected, again especially (perhaps
a bit more scattered) along the Central Gulf Coast region where
the Slight Risk is noted.

...Tennessee Valley to the Southern Appalachians and Parts of the
Southeast...

Mainly pulse-variety convection (with 0-6km bulk shear aob 20 kts)
along the western periphery of the expanding upper ridge may lead to
localized instances of flash flooding across this area today. This
as MLCAPE values climb between at least 1000-2000 J/Kg by the
afternoon, while at the same time TPWs reach ~1.75". High-res CAMs
show spotty 1.5-2.0+ inch/hr rainfall rates during peak diurnal
heating this afternoon, then continuing (though becoming more
isolated) into the evening hours. Most of the CAMs show isolated QPF
totals of 3-5+ inches, which despite the relatively dry soils
initially (relatively high FFGs), could nevertheless lead to
localized runoff issues, especially over urban areas or more sloped
terrain.

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 NORTHERN MINNESOTA AND FAR NORTHEASTERN SOUTH DAKOTA...

...Upper Midwest...

Little has changed with the overall synoptic setup with this
morning update. 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 central
MN on Monday morning. A stalled out cold front turned stationary
front will be draped across the region. 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,
likely over southwest/central Minnesota Monday morning. Additional
lift will be provided in the right entrance region of a 120 kt jet
over southern Manitoba and western Ontario.

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 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. It`s possible the heaviest rain will concentrate over
west-central Minnesota, while the heaviest rain will continue
drifting northward with the front over the Arrowhead. This may
concentrate the heaviest storm total rainfall across western
Minnesota, with somewhat lighter but still significant amounts over
northeastern MN. There remains some uncertainty as to where the
"pivot point" will be, though the general consensus is west central
MN near the ND/SD/MN tripoint.

The Moderate Risk was trimmed out of most of the Arrowhead of
Minnesota as it appears both a fair bit of rain associated with
this event will occur after 12Z Tuesday, and will be more
progressive to the north, resulting in lower rainfall totals than
further west. The Moderate was expanded a row of counties further
west in northeastern SD with this update. Consistent signal for
storm formation in this area before moving northeast suggests that
Moderate level impacts are probable into far northeastern SD as
well.

...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

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...

...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