660
FOUS30 KWBC 242029
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
429 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
LOWER MISSOURI AND MID-MISSISSIPPI VALLEYS...


...Central Plains through the Midwest...

An overnight organized thunderstorm complex is likely to be over
portions MI by 12Z Tuesday with outflow/ongoing convection over
northern IL into IA. However, the 12Z consensus is for this early
morning activity to be progressive from Chicago and east with an
afternoon/evening round focusing ahead of a cold front associated
with the low currently over the Dakotas. That places later day
development over southern IA into MO/the KC metro. Therefore, the
Slight Risk that was over northern IN/southern MI and Chicago has
been shifted southwest of these areas. Uncertainty remains with
this organized development tonight, as evidenced by the 18Z HRRR
which is farther east with its track (over WI) compared to mid-
morning runs like the 15Z which were move in line with a farther
west solution. So positioning of the Slight Risk for Tuesday
retains a fair amount of uncertainty.


A plume of 1.75-2" PW from the central Plains into the Mid-Miss
Valley is about 2 sigma above normal and the presence of the cold
front should allow capping concerns (present today over the
northern Plains) to be alleviated a bit. Please stay tuned for
further updates on the area of greatest heavy rain focus, but as of
now the risk areas look to be south of the northern tier areas hard
hit by heavy rain in recent days.


...Southwest...

Mid-level ridge across the Southwestern US will amplify up the
northern Rockies from a high centered over the AZ/NM border
Tuesday afternoon before drifting east through Wednesday. This will
expand the convective focus area farther north in UT. PWAT
anomalies are still exceptional, 2-4 deviations above normal from
SoCal through the southern Great Basin, across AZ to central NM and
much of UT/western CO. Slow storm motions under the ridge should
allow 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. Given the broad coverage of
mostly isolated activity, the Marginal Risk is maintained for now.
continuity.

Jackson


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

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR TERRAIN IN
CENTRAL NEW MEXICO...

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

The focus for diurnally-driven scattered thunderstorms shifts a bit
east Wednesday as the high center drifts over central NM with the
axis to the central High Plains. Elevated moisture accompanies this
eastward shift with PWs 1-3 standard deviations above normal across
these areas. Terrain will continue to be the convective focus with
the broad Marginal Risk maintained. In coordination with WFOs ABQ
and EPZ, a Slight Risk is raised for central NM terrain from the
Sangre de Cristos to the Sacramento Mtns where ongoing concerns
with burn scars persist.

A more organized convective scheme is likely over the portions of
the Central High Plains as a mid-level shortwave trough approaches
from the Northwest, allowing moisture to surge up the Plains later
Wednesday. Organized thunderstorms are possible over the central
High Plains with the focus a bit north from the previous forecast,
mainly covering western Neb/KS into SD. Will continue to assess
whether the Marginal Risk needs to expand north of the Nebraska
Sandhills.


...Northeast through lower Mississippi Valley...

A cold front progresses east and south from the Central Plains
into the Ohio Valley Wednesday with organized thunderstorms likely
across the Ohio Valley up through the Northeast and down over the
Mid-South to the central Gulf Coast. This expansive area features
elevated moisture and plentiful surface instability to allow
heavy rainfall within any convective cores along and ahead of the
approaching cold front.

Over the Northeast, there is a combination of the approaching front
and a broad area ahead of right-entrance region of an amplifying
jet over Southeast Canada which will create a widespread
convective setup by Wednesday afternoon into the overnight. Heavy
rain with localized amounts over 2" can be expected in places
where convection is most prevalent/repeating.

Guidance did tick north a bit which allow the Marginal Risk to be
removed for portions of the central Mid-Atlantic (which are
experiencing a rapid onset drought). However, urban area
sensitivity to heavy rain is not much affected by drought, so the
Washington DC metro area may yet warrant a Marginal Risk. Also,
progressive activity down to the central Gulf Coast warranted
expanding the Marginal to the central Gulf Coast, which is in
agreement with AI models such as the ECMWF- AIFS which generally
handle heavy QPF footprints well (with the magnitude better
depicted in global deterministics.


Jackson


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