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FOUS30 KWBC 241603
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1203 PM EDT Mon Jun 24 2024

Day 1
Valid 16Z Mon Jun 24 2024 - 12Z Tue Jun 25 2024

..THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR A PORTION OF THE
UPPER MISSISSIPPI VALLEY AND SOUTHEAST ARIZONA...

16Z Update...
Added a Slight Risk from the Twin Cities to the NW corner of IL.


...Upper Mississippi Valley/Upper Great Lakes...

A warm sector ahead of the low currently over central SD shifts
east over the Upper Midwest today, reaching Lake Michigan tonight.
Moisture anomalies in this warm sector area between 1.5 to 2
deviations above normal by this evening setting the stage for a
primed environment capable of heavy rainfall. Elevated instability
is quite extensive from eastern SD through western IA, but has a
rather strong capping inversion. The greater threat for activity is
thought to be well represented in the 12Z ARW/ARW2 with afternoon
central MN activity shifting southeast in what will become Wly deep
layer mean flow with upwind propagation vectors to the southeast,
toward northeast IA and southwest WI.

This activity may organize into a single, progressive bow like that
seen in the 12Z ARW2, but the right end interaction with the focus
of the instability and moisture presents concern, given how
sensitive this area is from 2-8" rainfall over the past 5 days.
There is a local maximum risk of 3" range considering the
available atmospheric moisture and prominent ascent pattern focused
over the region. Other members of the 12Z HREF suite, the HRRR and
FV3LAM offer disparate solutions, with activity focusing farther
east over WI. But the 13/14Z HRRRs have shifted their QPF focus
south, lending credence to this embedded SLGT upgrade southeast
from the MSP metro along the Miss River to NW IL.

The Marginal Risk remains in a similar place from southwest MN over
the northeast sector of IA and across WI/western U.P. of MI. Should
the cap be broken farther south, portions of southeast SD and
northwest IA, which are very sensitive to additional rainfall,
would see developing activity. This was noted in the Thompson
microphysics versions of the 00Z MPAS CAMS from NSSL, so this area
will continue to be monitored.


...Southwest...
16Z Update...just a note that despite morning stratus over southeast
AZ, WFO TWC agrees to leave the SLGT risk as is...


Previous Discussion...
A continued pattern of elevated deep layer moisture will preside
over much of the Western U.S as a mid-level ridge axis resides just
east of the Four Corners with remnant tropical moisture situated
on the western flank of the ridge. As a result, another afternoon
and evening of scattered convection with access to substantial
moisture across the Great Basin to the Mexican border during peak
diurnal heating. Destabilization pattern is forecast during the
typical climatological period between 17-01z before we see a decay
in the overall environment. PWAT indices are running incredibly
high across the aforementioned area with 2-4 deviations located
over the central Great Basin and western CO interior with
deviations closer to +4-5 within AZ leading to enhanced rainfall
within any convective cells that develop. The best opportunity for
more general storm clusters and organized thunderstorm development
will be across Southeast AZ within the topography comprised of the
Huachucas down into nearby Sonora. A pronounced theta-E ridge will
extend north out of Mexico into the Southeast AZ terrain leading to
targeted areas of convection that will be tied to the mountains in
the vicinity. Considering the ambient environment, this will allow
for local maxima exceeding 2" in some of the more prevalent
storms, easily capable of causing flash flood concerns given the
more complex terrain and flashy soils. The threat will extend close
to the Tuscon urban center where any heavy rain threat could turn
quickly into flash flooding within the urbanized setting. This
prompted the continuation of the previous SLGT risk as the setup
has deviated very little from the previous forecast.

Further north and west, the focus for convection is a little more
scattered, but the premise stands with such a moisture laden
environment. Locally heavy rainfall could spell trouble for
locations across the Desert Southwest with the best potential lying
within the terrain centered around the Mogollon Rim into Northwest
NM. Totals of 1-2" will be plausible in those affected by slow-
moving convection tied to the terrain, enough to cause a flash
flood concern within the terrain and surrounding locales.


...Northern New England....

Previous Discussion...
A digging mid-level shortwave analyzed over Quebec will shift
south into Northern New England, eventually closing off into an
upper low centered across NH into ME by the afternoon hours.
Focused mid- level ascent within an destabilized environment under
the low will allow for a generation of convection across northern
NH into ME during peak diurnal heating leading to scattered
thunderstorms capable of heavy rainfall under any cell development.
Storm motions will be weak thanks to being situated under the
closed upper reflection, so cell cores will be able to espouse
locally heavy rainfall that could bring totals upwards of 2-3" in
the most prominent cell structures. Surface low pressure
development over ME will drift into the Gulf of Maine creating an
easterly low-level moisture feed inland which is part of the
expected convergence pattern centered on the leeside of the higher
terrain in central and western ME. This corridor is being depicted
as the target for the heaviest precip potential with 00z HREF
neighborhood probabilities for at least 2" signaling values between
60-80% encompassing the above area. The signal for 3" drops off a
bit, but still has a central point of 25-35% situated across
central and south-central ME. Considering the setup and convective
mode for rainfall, a MRGL risk was kept from previous forecast.

Kleebauer/Jackson


Day 2
Valid 12Z Tue Jun 25 2024 - 12Z Wed Jun 26 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
THE MIDWEST, OHIO AND MID-MISSISSIPPI VALLEYS...


...Midwest and Mid-Mississippi Valley...

A complex from the previous period will likely enter into portions
of northern IL into MI by start of the D2 time frame with potential
for heavy rainfall across these areas with the progression of the
expected MCS. Rainfall totals of 1-3" will be possible within the
impact zones in the complex`s path, much of the area comprised
being urban focused corridors like Chicago, Southern Lake Michigan,
extending towards northern IN. Considering the uncertainty of where
the complex will motion, there was enough reason to have a broad
MRGL, at minimum extend into Lower MI all the way out towards Lake
Erie as some guidance suggests. When there is greater consensus on
the eventual path of the complex, there could be some targeted
changes in the risk areas, so stay tuned for those proposals in
future updates.

In the grand scheme, a cold front pressing from the north will
begin to shift orientation more west to east as it crosses into the
central Midwest and adjacent Ohio and Mississippi Valleys. Surface
flow ahead of the front will allow for a pooling of moisture ahead
of the boundary with a strengthening convergence signal located
along the front. PWAT deviations around +1-2 will remain situated
across the aforementioned areas with sufficient low-level buoyancy
aligned within the same areas encompassed by the elevated moisture
field. A round of convection will fire Tuesday afternoon and
evening across IL and northwest MO into IN with locally enhanced
rainfall capable of flash flooding as rates reach upwards of 2"/hr
or greater in some of the better cell structures. Recent ensemble
QPF output signals a widespread region of 1-2" of precip with
associated deterministic signaling opportunities for totals greater
than 4" in spots. These areas are situated as far north as the
Chicago Suburbs, as far west as northern MO, and as far east as
Northwest OH. Recent ensemble trends in the QPF with the expected
setup and environment have led to enough confidence to expand the
previous SLGT risk further west and east, aligning with the
expected frontal positioning on Tuesday evening. The SLGT risk into
southwest MI is for the prospects of the morning activity, as well
as some chance for convection later in the period. Some guidance is
also indicating a quick moving mid-level shortwave progressing
southeast out of IA with a secondary enhancement of convection
overnight into Wednesday across southeast IA into northern MO. The
precip trends were much more pronounced from previous model output,
so the SLGT was also expanded into that section of IA to account
for the opportunity.

...Southwest...

Mid-level ridge across the Southwestern US will strengthen with a
more focused ridge axis centered over the AZ/NM border come Tuesday
afternoon and beyond. This will shift the focus of the convective
pattern across the west a bit further east with scattered storm
development basically highlighting the Great Basin and much of
AZ/NM into southwestern CO. PWAT anomalies will still be on the
high side with a general 2-4 deviations above normal situated
within the aforementioned areas of convective focus. Any storm
development will exhibit weak storm motions with locally heavy
rainfall upwards of 1"/hr in the most substantial updrafts. Slot
canyons, arroyos, burn scars, and complex topography over the
interior will continue to be the zones of highest opportunity for
flash flooding with any urbanized areas also residing in the
threat window. Considering a less organized threat with limited
large scale ascent accompanying, a MRGL risk for the region was
sufficient and maintained for continuity.

Kleebauer


Day 3
Valid 12Z Wed Jun 26 2024 - 12Z Thu Jun 27 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS MUCH OF
THE NORTHEAST, OHIO VALLEY, AND LOWER MISSISSIPPI VALLEY, AS WELL
AS THE SOUTHWEST U.S...

...East of the Mississippi River...

A cold front located from the Central Plains into the Ohio Valley
will make progress to the south and east through the course of
Wednesday with scattered instances of thunderstorms likely across
much of the region east of the Mississippi. The most prominent
areas of precip will reside over the Lower Mississippi Valley where
a combination of elevated moisture and plentiful surface
instability will allow for heavy rainfall within any convective
cores that permeate along and ahead of the approaching cold front.
The second area of focus will reside over the Mid Atlantic and
Northeast where the approach of the front, as well as the region
positioned under the right-entrance region of an amplifying jet
over Southeast Canada will create a widespread convective setup by
Wednesday afternoon carrying through the remainder of the period.
Recent NAEFS PWAT anomalies center the best deep layer moisture
presence within the Mid Atlantic from WV up through the Delmarva,
including the Northeast Megalopolis from DC to Boston. A MRGL risk
is currently forecast across much of the Northeast and central Mid
Atlantic with potential for a SLGT risk upgrade in future updates
as the pattern evolves and details become more concrete on
convective specifics. The current setup is more than capable of
dropping some localized amounts over 2" in places where convection
is most prevalent, especially in any areas that see repeating
cells before the cold front passes. The MRGL risk extended
southwest through the Ohio and Lower Mississippi Valleys to account
for the scattered storm threat with isolated flash flood concerns
within more urbanized settings and/or training convective
signatures along the approaching cold front.

...Southwest into the Central High Plains...

Scattered to widely scattered thunderstorms will persist across the
Southwestern US, expanding into the Central Rockies and adjacent
High Plains as our mid-level ridge orients more southwest to
northeast with an extension over the above areas. PWAT indices will
still be elevated as noted by the vast expanse of 1-2 standard
deviations above normal PWATs within the latest NAEFS and ECENS
output. This is a far cry from what we have experienced recently
however, so the threat for organized convection will be subdued
across the interior Southwest with a general flash flood threat
relegated to the usual complex topography, burn scars, and any
urban settings as convection will be slow-moving in nature. A more
organized convective scheme is plausible over the portions of the
Central High Plains as a mid-level shortwave is forecast to round
the northern extent of the ridge and enter Southwest WY into
western NE by Wednesday evening. A focused ascent pattern under the
guide of the mid-level energy will help spawn a complex of
thunderstorms across the High plains with a forward propagation to
the south and southeast as it scoots into portions of the Front
Range and adjacent parts of NE/KS. Some guidance is more bullish on
the potential and was enough of a signal to allow for the previous
MRGL inheritance to be expanded northeast. The threat is currently
on the lower end of MRGL for the setup, but will bear watching as
we get closer with CAMs support.

Kleebauer


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