FOUS30 KWBC 270753
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
353 AM EDT Thu Jun 27 2024

Day 1
Valid 12Z Thu Jun 27 2024 - 12Z Fri Jun 28 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
THE SOUTHWESTERN U.S, AS WELL AS THE CENTRAL AND NORTHERN PLAINS...

...Central Plains...

A cantankerous mid-level shortwave will eject east out of the 
central Rockies providing enhanced large scale forcing in-of the 
Front Range out through the Central Plains later this afternoon and
evening. Return flow pattern on the backside of a surface ridge 
extension east of the Mississippi will allow for the advection of a
moist, unstable airmass across much of KS/NE and points east with 
the help of a budding LLJ developing after 00z. Convective 
initiation across the Front Range through the western Sand Hills of
NE will propagate eastward with upscale growth expected as it 
moves across the Plains, creating a swath of elevated QPF 
signatures in wake of the eventual complex. Recent QPF trends have 
favored a threat of 1-2" areal coverage with a maxima around 4" 
possible considering the environment in place. The primary focus is
centered over north- central KS through the intersection of 
southeast NE and northeast KS where the current ensemble QPF 
footprint shows the heaviest precip axis, aligning well with the 
mean placement of the mid-level vorticity maxima as it propagates 
east through the above areas. 

Recent probabilities have showed a favorable axis of heavy precip
potential, but capped in the higher potential due to the overall
progressive nature of the expected complex. 00z HREF neighborhood
probabilities for at least 2" were a general 20-40% with a max of
50% located around North Platte, NE and points southeast towards
the NE/KS line. The 20-40% stripe runs west to east along the state
line until out into northeast KS before the probabilities fall
rapidly towards the converging state lines. There was less
consensus in higher totals with a drop off to 10-15% for upwards of
3", so the threat will likely hit a relative maximum of 4" in those
harder hit locations along the state lines. Some guidance also
indicates another cell cluster forming across southern KS near
Dodge City which could spell a secondary maxima if convection
breaks properly. This is not shared amongst all CAMs, but was
noticeable in the probability fields to warrant an extension of the
SLGT further southwest, just to the north of DDC. The SLGT risk to
the north was adjusted further west in its axis with the eastern
fringes now just crossing the NE/IA/KS/MO state lines as more
progressive deterministic members still hold the potential for the
complex to outpace current thinking and make it towards the quad-
state region prior to the end of the period.

...Northern Plains...

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...

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...

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
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