350
FOUS30 KWBC 180111
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
911 PM EDT Mon Jun 17 2024

Day 1
Valid 01Z Tue Jun 18 2024 - 12Z Tue Jun 18 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE
NORTHERN PLAINS AND UPPER MISSISSIPPI VALLEY...

...01Z Update...

For the 01Z Update, one of the bigger adjustments was to remove the
Slight Risk from the Gulf Coast. With the loss of daytime heating,
the potential for heavy rainfall rates and widespread runoff
concerns should remain low until at least tomorrow morning with the
arrival of a surge of deeper associated with Potential Tropical
Cyclone One. However, a Marginal Risk was maintained to reflect the
potential for isolated concerns through the remainder of the
evening and the overnight.

Elsewhere, made minor adjustments based on current observations and
analysis. This includes maintaining a Slight Risk across portions
of the northern Plains and the upper Mississippi Valley.
Unfortunately, the hi-res guidance does not appear to have a very
good handle on the current activity, as reflected by the difference
in the guidance and radar reflectivity across portions of the area.
However, despite the uncertainty, the environment appears to
remain favorable for continued storm development and increasing
rainfall rates. The threat for heavy amounts appears most likely
along a warm front lifting into eastern South Dakota and southern
Minnesota as storms begin to train along the boundary this evening.
Refer to WPC MPD #0456 for additional details concerning the
evening into the overnight threat for heavy rain and flash flooding
across this area.

Pereira

...Previous Discussion...

...Summary...

Per collaboration with the WFOs, notable adjustments to the
inherited ERO include dropping the Moderate Risk, which had covered
much of central MN and western WI in yesterday`s Day 2 ERO. There is
just too much spread in the guidance, including the most recent
CAMs, to support more than a Slight Risk. The transitory nature of
the warm front lifting north has a lot to do with this, limiting the
threat of persistent frontal (west to east) training. Otherwise,
have expanded the Marginal Risk areas across the Lower MS Valley and
over the Appalachians, Ohio Valley, and parts of the Northeast based
on the latest guidance trends.

...Eastern Portions of the Northern Plains into the Upper Midwest...

Stationary surface front across the Central Plains early this
morning will lift north as a warm front today and tonight as the
longwave trough drops into the Western U.S., while the broad upper
ridge builds across the eastern half of the CONUS. Elevated
convection early in the period along/north of the eventual warm
front will lift gradually northward today, with a higher likelihood
of least some cell training through early this morning given the
uptick in low-level southerly flow (nocturnal LLJ). During the day
the areas of heavy, potentially excessive rainfall will become more
spotty with the daytime (differential) heating and weakening low-
level inflow. By tonight, the upper flow will become more
meridional, with the upper level jet axis more SSW-NNE oriented vs.
W-E early today. This will shift and confine the best upper level
forcing (upper divergence and low-level frontogenesis) farther W-NW,
closer to the surface low track and associated cold front. So while
the broad Slight Risk encompasses the areas of heavy rainfall today
along and north of the warm front, the focus for heavy rainfall
later tonight will shift to the eastern Dakotas and NW-NC Minnesota,
within the region of aforementioned favorable forcing. All totaled,
the anomalous deep-layer moisture (TPW anomalies ~2 standard
deviations above normal) along with other favorable thermodynamic
and kinematic parameters (MUCAPEs increasing to 1500-2500 J/Kg) will
favor 1.5 to 2.0+ inch/hr rainfall rates underneath the strongest
cells and/or where the convection trains.

...Western to Central Gulf Coast...

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 on the western
periphery is quite subtle/transient, with small-scale convectively-
aided mid-level vortices within the COL or deformation zone riding
northward on the western edge of the ridge. The Slight Risk
continues to be aligned along the coastal regions, where the
expectation for forcing is that local Gulf breezes, outflow
boundaries, and cold pools will provide localized forcing for heavy
convection. However given the low 0-6km bulk shear (aob 20 kts),
expect predominately pulse-type convection which would become
outflow dominated rather quickly. Still, given the plentiful
moisture and instability still available, any storms that form will
be capable of heavy rainfall rates to the tune of 2 to 3 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. Within
the Slight Risk area, most of the high-res CAMs (including the ARWs
and 06Z HRRR) indicate isolated pockets of 5.00+ inches of 24hr QPF.

...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 1/Monday-Monday
night 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.

...Lower Great Lakes-Ohio Valley to the Appalachians and Parts of
the Northeast...

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 2-4+ inches, which despite the relatively dry soils
initially in some areas (relatively high FFGs), could nevertheless
lead to localized runoff issues, especially over urban areas or more
sloped terrain.

Hurley

Day 2
Valid 12Z Tue Jun 18 2024 - 12Z Wed Jun 19 2024

...THERE`S A MODERATE RISK OF EXCESSIVE RAINFALL FOR MUCH OF THE
UPPER TEXAS COAST...

...20Z Update...

Some minor adjustments were made to the previous MDT risk across
the Texas and LA coastal plain. The primary change was an
incremental shift to the west based on the mean QPF field relaying
the maximum further south along the TX coast with a core centered
between Rockport and Galveston. This correlates well with the
recent ML output with the 24-hour precip forecast noted north of
CRP and just south of HGX. The setup is still pointing at a
significant tropical moisture advection regime with the center of
circulation passing south of the International Border, but plenty
close enough for a prominent stream of elevated, deep moisture
advection along a steady easterly flow off the Gulf of Mexico.
Forecast PWATs are expected to reach above 2.5" for a large area
encompassing southwestern LA down through Deep South TX with the
plume maxima located along the central TX coast. GFS/ECMWF output
are particularly bullish with deterministic forecasts indicating
PWATs just above 2.9" at peak, a standard deviation closing in on
+4 compared to June climatology. This makes sense considering an
IVT pulse exceeding 1000 kg/ms protruding the TX coast by the end
of the period within the global model consensus currently, a value
typically reserved for the deeper tropical connections involved in
any given setup in the month of June.

All this will lead to significant rainfall on the order of 2-4"
areal average along the TX coast from the Deep South TX area up
through the Upper Texas coast to the northeast of Houston. QPF
maximum between 5-8" is anticipated within the hardest hit areas,
much of the heaviest precip falling in the 12 hour span from 00-12z
Wednesday. General rates between 2-3"/hr at peak are very likely
within that time frame of interest with 1-2"/hr prior to the main
IVT pulse moving onshore. This would allow for localized flooding
early in the period transitioning to more scattered to widespread
flash flood concerns overnight into Wednesday, especially in the
urban corridors from Beaumont down to Corpus Christi. A MDT risk
was maintained given the current signals.

Across the Central Plains, the setup for widespread heavy rain
remains steadfast from previous forecast issuance with the
convergence signal along the stalled front draped across KS/NE up
through the Upper Mississippi Valley leading to locally significant
rainfall in the above corridor. The area of concern continues to
be the Central Plains from southwestern KS up through eastern NE
and western IA within the Corn Belt with current QPF footprint
indicating a vast area of greater than 2" precip totals with local
amounts exceeding 5" in the hardest hit areas. Considering the
strong deep moist convergence pattern in the region with mean flow
aligned parallel to the boundary, training signals will be
prominent and will likely be the primary cause for the excessive
totals breaching 4" in any given areas. 12z HREF probability fields
depict a relatively stout reflection for at least 3" with a
widespread area of 30-50% probabilities for the targeted 3" marker
with bullseyes of up to 70% located across west-central KS up to
the NE border. The 5" probabilities are still fairly significant as
well with an areal average of 20-40% located over the same areas
mentioned above, and low-end probabilities of at least 8" centered
over southwest KS. The threat within that zone remains on the
higher-end of SLGT risk with the potential for a targeted MDT
pending the expected convective evolution and relevant probability
signals prior to the event. We will continue to monitor to see if
an upgrade is necessary in future packages.

Kleebauer


...Previous Discussion...

...Central Plains...

A cold front pushing east across the northern Plains is expected to
stall out over the central Plains Tuesday and Tuesday night.
Surface high pressure over Montana will dive southeastward into the
Dakotas Tuesday night. The high will provide a north to
northeasterly flow of cooler and drier air into the front.
Northeasterly flow at 850 mb will increase to 30-40 kt Tuesday
night. Meanwhile...broad southerly flow of deep Gulf moisture will
stream northward across all of the southern Plains and into the
central Plains and the front. The southerly LLJ will peak at 45-55
kts across southern Kansas Tuesday evening. The two highly
contrasting air masses will slam into each other at the front into
north central Kansas and south central/southeast Nebraska. The
storms will track northeastward with the deep moisture and
convergence into northeastern Nebraska...an area that has been very
hard hit with rain in recent days, resulting in very low FFGs.
Thus, despite the lesser convergence over northeast Nebraska, the
risk is higher due to more favorable antecedent conditions. Corfidi
Vectors south of the front are out of the north at 5-10 kts. This
will strongly favor backbuilding and training storms since they
will be anti-parallel (opposite) the prevailing south to
southwesterly flow of moisture into the front. The two air masses
colliding head on into each other near the KS/NE border will
prevent much if any movement of the front...allowing the storms to
repeatedly impact the same areas as plentiful atmospheric moisture
replaces that lost from rainfall.

Further north, rainfall across northern MN will be ongoing through
the morning from the overnight rainfall event. While some
convective enhancement is expected due to an abundance of moisture
availability for the convection, as PWATs will be around 1.75
inches. The warm front the storms will be tracking along will be
moving north quickly enough that any flash flooding should continue
with the rain after 12Z Tuesday.

...Upper Texas Coast...

No significant changes were made to the ERO Risk areas. A plume of
deep tropical moisture originating in the Caribbean will track
northwest across the Gulf of Mexico with a developing tropical low
in the western Gulf. The southeasterly flow around the low will
begin to spread convection with deep tropical moisture and
potential for high rainfall rates into the immediate coast through
the day Tuesday. However, the better forcing and thus, more
widespread heavy rainfall likely holds off at the coast until
Tuesday night. There has been a small westward shift, as has been
common with this rainfall event. Thus, a bit less storm total
rainfall is expected into southwest Louisiana...with a bit more now
expected into the middle and lower Texas coasts. This should
overall limit the flooding potential a bit as southern Louisiana
has been much wetter in recent weeks than south Texas. With
continued westward shifts in the guidance it`s likely the Moderate
Risk area can be shifted accordingly out of Louisiana and perhaps
expanded a little along the Texas Gulf Coast. Regardless...most of
the heavy rainfall event in Texas will be on Day 3/Wednesday except
for the upper Texas coast.

Wegman

Day 3
Valid 12Z Wed Jun 19 2024 - 12Z Thu Jun 20 2024

...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR THE MIDDLE
TEXAS COAST INTO PORTIONS OF SOUTH-CENTRAL TEXAS...

...20Z Update...

Previous MDT risk issuance was maintained with confidence growing
for at least a high-end MDT risk located somewhere over south-
central TX with the impending tropical moisture flux. Totals based
off the latest blend and ensemble bias corrected QPF indicate
upwards of 5-8" with locally higher in a large area encompassing
interior Deep South TX up through Central TX, including parts of
eastern Hill country and the I-35 corridor south of Waco. There`s
still some spread in the overall deterministic output with the
consensus indicating a maximum generally focused over south-central
TX, including the Austin/San Antonio corridor. Ensemble forecast
QPF is moving towards very similar output with the ECENS and GEFS
pinpointing the maxima within the currently outlined MDT risk over
central TX back to the TX coast near and south of Houston. If we
get a general agreement within the next succession of updates,
there is some opportunity for targeted upgrade, if the current QPF
presentation holds firm. Stay tuned for future updates....

Across the Central Plains, remnant convection from the previous
period will linger across areas of KS/NE/IA with the current QPF
maximum focused over eastern NE into IA. There`s still some
discrepancy on the exact location of the where the heaviest QPF
will occur with the blend generally modest with widespread 0.5-1"
totals centered over the current SLGT risk in place. A lot of the
potential is contingent on the convective evolution the period
prior. Once there is better consensus, the focus could shift a bit
from current location, but the ensemble and ML output currently
situates the heaviest precip within the SLGT inherited. Only minor
adjustments were made to the risk area based on the areal extent of
the heavier precip within the NBM and ensemble bias correction in
conjunction with where the heavier precip is expected the prior
period.

Kleebauer


...Previous Discussion...

...Southern and Central Texas...

Tropical moisture associated with a strengthening low over the
western Gulf will continue pushing west and inland across much of
Texas on Wednesday. The moisture plume will have a direct
connection to the Caribbean and moving over the entirety of the
Gulf will mean the air mass will have incredible amounts of
moisture associated with it. Multiple pieces of guidance suggest
PWATs could approach 3 inches and will be broadly above 2.5 inches
across all of south Texas Wednesday and Wednesday night. The low
will provide the forcing adding lift to the atmosphere. The result
will be a prolonged period of very heavy rain across much of south
Texas. Embedded convection within the broader precipitation shield
will be capable of rainfall rates to 3 inches per hour. This level
of rainfall should easily exceed FFGs in all but the most flood
resistant portions of Texas. Outside of the heaviest rainfall
rates, much of the rainfall should consistently exceed an inch per
hour rates given the tremendous amount of atmospheric moisture
available for the storms to efficiently convert via warm rain
processes to heavy rainfall. The heaviest rainfall totals will be
along the coast, with the Gulf supplying abundant moisture and
instability to the rain. As the precipitation shield presses
inland, amounts will gradually come down, but storm total rainfall
is likely to be in the 2 to 4 inch range well up the Rio Grande
Valley. Meanwhile near the coast, storm total rainfall into
Thursday of 6 to 10 inches is expected, with locally higher amounts
where training convection adds to the rainfall total.

Fortunately, much of south Texas has been very dry of late, so
there will be plenty of room for the rivers to handle the deluge of
moisture. The problem will be that the rain will likely be falling
so heavily locally that there will be local exceedance of FFG
despite the antecedent dry conditions. If the soils had been wet
prior to this event, then it`s very possible that a High Risk area
would`ve been needed. Fortunately, High Risk level impacts are not
yet expected, at least on a widespread basis.

There remains considerable uncertainty regarding the surrounding
Slight (and Marginal) risk areas. Starting in the east...the
guidance has been trending west with time, so amounts may continue
to come down around the LA/TX border and points east of Houston.
Into central and north central Texas, there is about a 50/50 split
in the guidance as to whether significant tropical moisture and
rainfall tracks north into this region, or continues pressing west
into Mexico and the Rio Grande Valley. Guidance the past several
days has trended on the west side, but in deference to the guidance
suggesting the heavy rain moves north with time, the Slight Risk
has been expanded to the DFW Metroplex with this update. That said,
it`s very low confidence in this area and a southward shift in the
guidance would have this area downgraded. Meanwhile, further west,
rainfall amounts in the 2 to 4 inch range is still a very
significant rainfall event for the Rio Grande Valley to as far west
as the Big Bend. The Slight risk area was nudged west to the
Amistad Reservoir with this update, and continued westward shifts
may require additional westward expansions with future updates. Of
course, this would also imply faster motion of the rainfall, which
in turn would subsequently begin to reduce total rainfall and
therefore the flood threat, so it`s unlikely the risk areas will
need to be expanded much going forward, at least for Slight and
higher.

...Central Plains into the Upper Midwest...

In coordination with GID/Hastings, NE and OAX/Omaha, NE forecast
offices, a Slight Risk area was introduced with this update for
much of eastern Nebraska. Heavy rainfall from the Day 2 period will
continue into Day 3/Wednesday, albeit most certainly on a downward
trend in intensity Wednesday morning. Unfortunately, with continued
moisture advection into the area and a small area of high pressure
to the north preventing much northward motion of the convection for
at least the daylight hours...scattered convection is expected
across eastern Nebraska throughout the period. The storms will
trend toward moving north with time Wednesday night as the front
forcing the rainfall begins to also move north with the retreat of
the high. Regardless...forcing for convection will be greatly
reduced in this area on Wednesday as compared with Tuesday, so a
significant portion of the confidence in Slight level impacts from
flash flooding is contingent on heavy rain having occurred on Day
2/Tuesday. Thus, adjustments to the Slight Risk are likely once the
rainfall pattern from Tuesday becomes more clear.

Further east, a cold front moving across Minnesota and Wisconsin
from Day 2/Tuesday will continue into the morning hours of
Wednesday. The front will likely stall somewhere near the IA/MN
border and southern WI, becoming a warm front with the
strengthening moist southerly flow into the front Wednesday night.
This will support training storms across the area, though they
should be generally widely scattered in coverage. Thus, the
inherited Marginal risk for this area remains largely unchanged.

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