892
FOUS30 KWBC 280047
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
847 PM EDT Thu Jun 27 2024

Day 1
Valid 01Z Fri Jun 28 2024 - 12Z Fri Jun 28 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
THE SOUTHWEST AND NORTHERN PLAINS...

...Central Plains...

01Z update... Removed the Slight risk area due to lackluster
convective development and anticipated rainfall below FFG values.
However, did extend the Marginal risk outline into southwestern
Kansas with continued sustained convection early this evening.

Fracasso


...Northern Plains...

01Z update... Afternoon convection over eastern Montana and
isolated supercells over western ND have morphed into a QLCS over
western ND/SD this evening. This will likely be the main/sole
convective threat into the overnight hours but should maintain a
steady forward progression. Rainfall rates have been in the 1-2"/hr
range which is near FFG values, and have focused the Slight risk
area here out ahead of the line through central/southern ND and
northern/central SD.

Fracasso


16Z update... The southward trend with the higher QPF has
persisted with the latest guidance this morning. A very small
adjustment was made to the Slight Risk area across southwest North
Dakota to now cover extreme northern South Dakota. Otherwise,
everywhere else is adequately covered by the appropriate level of
risk for excessive rainfall.

Campbell

A robust shortwave trough is currently analyzed across western WA
state with sights on the Northern Plains by later this afternoon.
The aforementioned disturbance will pivot across the Northern
Rockies, taking on a closed reflection along the Canadian Border
around the longitude of Alberta and western MT. A strong diffluent
signature downstream of the mean trough will create a focus of
large scale ascent in-of the Northern Plains by Thursday afternoon
creating a scattered to widespread area of convection sweeping
across northeast MT through ND. The environment is somewhat
favorable for bouts of heavy rainfall as pattern allows for moist
air to be pulled poleward leading to PWAT anomalies between 1.5-2
standard deviations above normal centered over the northeast corner
of MT through much of ND. QPF footprint across the northern plains
to the Canadian border is stout with a swath of 1-1.5" totals
being reflected in the blend and ensemble bias corrected QPF. Local
maxima around 3" will be plausible in the setup as sufficient
shear associated with the disturbance will lead to a primary
supercellular mode upon initiation through the early evening with
some upscale growth potential at nightfall as the LLJ ramps up and
we see congealing cold pools across ND.

Recent trends in the QPF have been more pronounced further south
across the central and eastern ND plains with an axis of heavier
rainfall now steadily making progress into west-central MN by the
end of the period. The heaviest rates will likely occur further
upstream where supercell generation will provide a significant
punch in the areas they impact leading to more scattered high QPF
outputs that will eventually be smoothed as the convective scheme
shifts to more multi-cellular clusters and potential QLCS
development. Recent probabilities for at least 2"/3-hrs were
subdued over most of the region with the max around 30-40% centered
over western ND where the supercell modes will be most common.
This settled to 10-15% through much of the rest of the state as we
move into the evening. This is another capped maximum evolution,
but enough to warrant the SLGT risk from previous forecast with an
extension further south and east to cover for the latest trends in
more widespread convective coverage with embedded heavy rain
signatures.

...Southwest...

01Z update... Ongoing convection in the area continues to rebuild
over central/southern UT, and have nudged the Slight risk outline
westward to cover these regions that have already seen 1-3" today
per MRMS. Activity will slowly subside after 06Z.

Fracasso

16Z update...The Slight was expanded further west across southeast
Utah has the latest guidance continues to show increase amounts
and areal coverage of convection for this period. Elsewhere remains
in good order at this time.

Campbell

Ridge will begin breaking down across the Southwestern U.S with
the shortwave trough to the north squashing the height pattern
creating more quasi-zonal flow across the Inner Mountain West. A
litany of mid-level perturbations will ripple across the Great
Basin towards the Central and Southern Rockies creating yet another
period of convective initiation over the terrain under a
relatively moist, unstable environment. Unlike previous periods,
relative storm motions will be more prevalent, but the premise
still stands for heavy rainfall likely anchored to the terrain with
stronger cell cores capable of producing periods of steady
rainfall running upwards of 1"/hr at peak intensity, in agreement
with the 00z HREF signatures of scattered 15-20% probabilities for
the inch per hour rates. Totals being depicted are within the realm
of 0.5-1.5" with a maxima around 2" in any areas that see
persistent thunderstorm impacts within the terrain. This is
especially true for more of the interior west with the focus
shifting across UT and western CO down through a good portions of
NM. The primary concerns will be the complex terrain, slot canyons,
and any burn scars remaining over portions of the west. The SLGT
risk from previous forecast was expanded a bit to the west to
account for the latest trends in guidance showing a heftier QPF
footprint for those in eastern UT, and a touch to the east to
encompass some of the harder hit locations in NM these past couple
days, including near Ruidoso.

Kleebauer

Day 2
Valid 12Z Fri Jun 28 2024 - 12Z Sat Jun 29 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
THE CENTRAL PLAINS, MID-MISSISSIPPI VALLEY, AND MIDWEST...

...Central Plains into the Midwest...

21Z update... The latest guidance show a southward trend in the
QPF footprint across the mid-Mississippi Valley and Midwest with a
general consensus of the highest amounts focused over Missouri and
far eastern Kansas. The southern bounds of the Slight and Marginal
Risks areas were expanded southward across central and southern
Missouri to reflect this trend.

Campbell

Shortwave across the Central Plains from the previous period will
eject eastward through the Midwest with a round of elevated
convection likely over portions of IA/MO into IL through Friday
morning. This is round 1 of flash flood potential as a secondary
enhancement is forecast later in the day as another shortwave
ejects east out of the Front Range and bisects the same areas that
were hit the period prior. This next setup has more potential due
to the addition of a cold front that will be progressing southeast
out of the Upper Midwest thanks to the shortwave trough to the
north perusing eastward with an associated low pressure and
trailing cold front developing and sliding south and east through
the period. The tandem of focused large scale ascent and surface
convergence along the approaching cold front will aid in a re-
development of convection across northeastern KS and northwestern
MO, tracking slowly to the east-northeast up along the frontal
boundary as the front trudges slowly through the Mid- Mississippi
Valley. The mean flow will be shifting as well thanks to the
primary trough being situated to the north. 850-700mb mean wind
vectors are oriented close to, if not parallel to the boundary
which would provide a higher threat for training/repeating
convection within the best convergence location.

This area has shifted within the last 12-24 hours with guidance
now pinpointing the convergent focus around the KS/MO line,
currently north of the Kansas City metro. The synoptic evolution
currently depicted makes sense for that location to be the main
area of interest as the nocturnal LLJ will be strongest across
eastern KS with the northern apex of the jet converging right along
the slow progressing cold front. The 850mb wind pattern between
00-06z on the latest deterministic yields a veering boundary layer
profile with south- southwest winds orienting more southwesterly,
running parallel to the height contours positioned over the Central
Plains and Mid- Mississippi Valley. This is a classic signature
for focused convergence with training potential within a surface
boundary where there could be multiple hours of repeated
development so long as the mid-level ascent pattern cooperates in
tandem. The setup would eventually vacate to the east-northeast
with some prospects further downstream, but there is more
uncertainty on whether that will transpire. The setup is certainly
there, so the SLGT risk inherited was maintained for a majority of
the previous areas outlined. The biggest change was the removal of
the Chicago urban area and surrounding suburbs as the best chance
for flash flooding is now correlated with the morning round of
convection and less so with the secondary enhancement. There is
still a possibility this gets adjusted back, but the mean QPF has
been cut back in recent ensemble depictions and was not enough to
warrant a stay of the SLGT. The other change was the SLGT moving
further west into KS to account for the initiation point of
convection where repeating is possible Friday evening.

...Southwest...

A weak mid-level perturbation will exit out of Sonora to the north
with sights on the high terrain of Southeast AZ and the NM
Bootheel. Sufficient diurnal destabilization with elevated mid-
level moisture will allow for scattered thunderstorms to form near
the Mexican border into the terrain adjacent and pivot into the
above area. This setup is coincident to a Monsoon type setup where
flash flood concerns are isolated, but non-zero and meet the low-
end MRGL criteria for the threat. Totals up to 1" are plausible,
but a few spots may see higher amounts due to overachieving
convection. The previous MRGL was maintained with no changes
necessary.

Kleebauer

Day 3
Valid 12Z Sat Jun 29 2024 - 12Z Sun Jun 30 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF THE SOUTHWEST, CENTRAL ROCKIES AND PLAINS, SOUTHERN PLAINS, OHIO
VALLEY, AND NORTHEAST...

21Z update... The Marginal was adjusted a bit further west across
north-central Ohio for this period as the latest guidance shows an
uptick in amounts in proximity to where recent heavy rains have
saturated the region. Portions of eastern Ohio, northern West
Virginia, Pennsylvania and parts of New York will have the
potential for an additional 1 to 3 inches, possibly higher, of
moderate to heavy rain rates. There is some uncertainty on exactly
where maximum amounts will setup but region may need an upgrade to
a Slight in future updates.

Campbell

Elongated MRGL risk is in place across a large chunk of the CONUS
thanks to a sprawling surface ridge over the Northern Plains
working in tandem with a surface low crossing through eastern
Canada aligning a cold front and stationary boundary extending
through the Northeast back into the Central and Southern Plains.
The boundary will be the focal point of convection in whatever area
it bisects with two main areas of interest. The first is across the
northeast where a cold front will migrate eastward scattered to
widespread showers and storms likely propagating through the Ohio
Valley, eventually making headway into the Mid Atlantic as the
surface low motions northeast into Quebec and the trailing cold
front follows. This fairly reminiscent to what transpired
yesterday with bouts of convection moving progressively through the
region along and ahead of the surface front. This will be no
different the setup will likely yield a SLGT risk somewhere if the
current depiction holds. There is still a question of timing and
location of the best large scale ascent, so decided a MRGL was
sufficient for the time being with adjustments possible as we move
closer.

Over the Central and Southern Plains, differential heating with
available moisture and a boundary overhead will be a focus of
enhanced thunderstorm risks with locally heavy rainfall possible
around the stationary boundary. A few mid-level perturbations
sliding east out of the Western U.S will likely enhance some areas
locally with a better risk of organized cell clusters and flash
flooding potential. That seems to be centered over southern KS into
northern OK currently, but that could certainly change pending the
synoptic evolution of the frontal placement, and any mid-level
energy. Another area to assess for future upgrades if the pattern
holds.

The remainder of the MRGL risk in place will be subject to a combo
of shortwave placement and relevant instability available. That
has the greatest potential over CO currently with scattered
thunderstorms likely over NM. This is bordering on Monsoon-ish,
something we will be getting used to in the coming days as the
season is right around the corner. Fairly low to mid-grade MRGL for
those areas currently, but will see if a targeted SLGT is
necessary in future forecasts.

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