FOUS30 KWBC 240811
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
411 AM EDT Mon Jun 24 2024

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

..THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PARTS OF
SOUTHEAST ARIZONA...

...Southwest...

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. 

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

Flow will remain slightly backed across the Midwest ahead of an
approaching front from the northwest. Moisture anomalies between
1-2 deviations above normal will encompass much of the Upper
Midwest and adjacent Great Lake areas by this evening setting the 
stage for a primed environment capable of heavy rainfall. As the 
front approaches, a small mid-level perturbation will eject out of
the Dakotas and help enhance large scale ascent somewhere over the
Upper Mississippi River Valley. Steering flow aloft will take any
convection and focus it southeast as it grows upscale and
progressively moves into nearby WI. The progressive nature of the
anticipated complex will limit the severity for flash flood
concerns, however the local maximum is likely to be within the 3"
range considering the available atmospheric moisture and prominent
ascent pattern focused over the region. There is also some
discrepancy in guidance in exactly where the forecasted complex
will track as the deviation in proximity ranges from as far north
as Green Bay to as far south as northern IL by the end of the
period. This is also addressed within the probability fields as the
1" HREF EAS signature is uncharacteristically low around 25-30% for
a large area between MSP down towards Milwaukee. Usually in a
favorable setup like this, we would see some higher values
reflected due to agreement within the CAMs. 

This is a red flag when it comes to this part of the ERO equation 
as the prospects for somewhere in the Upper Mississippi Valley to 
get flash flooding is decent, but specifically where is a question.
This is also reflected in the latest SPC D1 Outlook and has been a
factor in their higher risk potential. The area across southern WI
is the most susceptible to flash flood concerns thanks to the 
previous periods of heavy rain that caused significant flood 
concerns all across that area. If there is growing consensus for 
that area in general, there will be opportunities for an upgrade. 
Until then, maintained the MRGL risk with emphasis on the prospects
for a targeted upgrade somewhere across MN/WI/Northern IL. 

...Northern New England....

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


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
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