370
FOUS30 KWBC 171556
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1156 AM EDT Mon Jun 17 2024

Day 1
Valid 16Z Mon Jun 17 2024 - 12Z Tue Jun 18 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 AND WESTERN GULF COAST...

...16Z Update...

General continuity from the previous forecast. There were some
minor adjustments based on observed trends and hi-res output that
constituted some adjustments in the risk fields. Across the
Northern Plains and Upper Mississippi Valley, light to moderate is
ongoing as the initial vorticity maxima from last night continues
to press northeast through WI with a trailing vorticity maxima
currently located over northeast SD into southwestern MN.
Trajectory of the secondary vort max will take the energy through
southern MN, eventually coinciding with the stalled frontal
boundary analyzed across southeastern MN, southeast of
Minneapolis. Recent trends in hi-res guidance indicate a
redevelopment of heavier convection in-of the above area within the
tight theta-E gradient associated with the frontal boundary. That
small area will be an area of focus later this afternoon with some
indications of a localized maximum exceeding 2" over a span of
3-hrs within the confines of the front. This looks to extend a bit
into WI as the convective pattern bisects the Mississippi River.

Further north, expectation is for another larger, more potent
shortwave to eject northeast through the Dakotas allowing more
backed flow to take precedent across the northern Midwest with the
stalled frontal boundary getting a push from an invigorated LLJ
leading to a transition to a warm front as it propagates northward.
Two areas of interest will include the area along the boundary as
it lifts north as convection fires across eastern SD and moves to
the north within the confines of the front. The second area of
interest is over central and eastern ND where large scale ascent
is maximized under the influence of our shortwave trough lifting
overhead. This will lead to local maximums in QPF with spotty
neighborhood probabilities from the 12z HREF indicating areas of
20-40% for at least 3" across the aforementioned areas with a 60%
bullseye located across southwestern MN, likely with some
contribution from this mornings heavy rainfall. Flash flood risks
will be highest within more urban corridors and areas that see
prolonged 2+"/hr rates which will be more widely scattered in the
spatial depiction of the 2"/hr probability fields. In this case,
the SLGT risk remains in effect with still some locally significant
flooding within the three main zones of southeastern MN, southwest
and central MN, as well as eastern ND later this evening.

Across the Gulf Coast, regional moisture flux over the Gulf will
continue to push northward on the western fringe of the expansive
surface trough over the western Atlantic, as well as the
strengthening mid-level ridge positioned east of the Mississippi.
Scattered convection will develop near the Gulf Coast and just
onshore throughout the period with locally heavy rainfall exceeding
2"/hr probable over the southern half of LA and coastal MS. Recent
probability fields indicate some opportunity for localized QPF
maxima around 5" (35-45%) over parts of southeastern LA, including
near New Orleans which would cause some concern for flash flooding
within the urban corridor. Given the airmass becoming even more
anomalous through the period with a deeper moisture presence, the
prospects for flash flooding are on the higher end of the SLGT
risk, especially within those urban settings. The previous forecast
was maintained as a result.

Other minor changes included the expansion of the MRGL risk through
the Mississippi Valley from the south as a weak area of mid-level
vorticity moves northward out of MS and sets sights on the Mid-
Mississippi Valley this afternoon. Isolated flash flood potential
will exist within the zone encompassing the IL/MO/KY/TN border as
inferred  by the recent hi-res deterministic and HREF blended mean
QPF output. This was enough to solidify the MRGL expansion. No
changes were made to MRGL across MT and the Ohio Valley/Mid
Atlantic as the pattern remains favorable for isolated flash flood
concerns with very little variance in the previous model QPF output
for the respective locations.

Kleebauer


...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...

...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...

...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