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FOUS30 KWBC 261621
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1221 PM EDT Wed Jun 26 2024

Day 1 Valid 12Z Wed Jun 26 2024 - 12Z Thu Jun 27 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
NEW MEXICO AND EASTERN OHIO VALLEY INTO THE NORTHEAST...

...Southwest...

16Z update... Minor adjustments were made to the Marginal Risk area
across west-central Arizona and near the Texas and Oklahoma
Panhandles to reflect current observations and new QPF trends.

Campbell

..Southwest...

Fairly broad ridge axis across the Southwest U.S will slowly
truncate with the center of the ridge shifting more to the east
over NM by the afternoon today. This will lead to the theta-E ridge
being displaced more through NM with remnant deep layer moisture
still trapped underneath the mean ridge. With ample heating in the
morning and afternoon allowing for destabilization, another round
of scattered convection will initiate across the high desert with
the an initial terrain focus likely after 18z with lower deserts
seeing impact a few hours after as convection drifts off the
mountains/hills. We`ve seen a plethora of flash flood warnings in
recent days across the Southwest given the very moist environment
and slow storm motions prompting flood concerns within the
topography and adjacent towns. A few remnant burn scars and ongoing
wildfires are not helping the situation either across NM leading to
heightened risks for flash flooding in those locales. Rates will be
generally capped at 1-1.5"/hr at the peak of any storm impact, but
that is plentiful to cause issues over areas in the interior. Given
the latest QPF footprint and positioning of the theta-E ridge, the
best chance for flash flooding will occur on the eastern extent of
the Mogollon Rim and points north and east with much of NM seeing a
better threat of heavy thunderstorms capable for flooding concerns.
In coordination with the local WFOs (FGZ/ABQ), decided to expand
the SLGT risk to encompass parts of eastern AZ out through the
northern half of NM with a southern extent into parts of the
Sacramento`s. A MRGL risk expands around the SLGT risk area with a
northern extension into northeast NV through northern UT.

...Southeast...

Overnight complex out of the Missouri Valley will move south-
southeast into AR and portions of eastern OK by morning with a
swath of heavy rainfall affecting those areas for the first few
hours of the period. Guidance is keying on the remnant outflow from
the complex being a focal point for more convective development
later in the afternoon leading to some heavy rain chances over the
Lower Mississippi Valley into the Arklatex. General rates between
1-2"/hr will be the forecast during the period leading to a lower
end MRGL risk for the outlined area, but certainly non-zero as some
deterministic remains bullish on the prospects of isolated pockets
over 3" within the stronger cells in the region. Further north, a
cold front will continue to push south into the area with pooling
low-level moisture across the Deep South leading to scattered
heavy convection along and ahead of the front. The signals for
precip over 3" in any given cell is weak, but non-zero with the
probabilities for 1-2" most common within the neighborhood
probability scheme (Areal average of 35-60% for at least 2" within
40km spacing). This is indicative of a capped potential and settled
on the lower end of the MRGL risk category.

...Ohio Valley and Northeast...

16Z update... Upgraded to a Slight Risk area that cover
northeast/eastern Ohio, northern West Virginia, western/central
Pennsylvania and southwest New York. There has been a growing
signal for upper level support during the afternoon/evening hours
to enhance forcing/lift over the region to boost rainfall
intensity, especially from eastern Ohio to Pennsylvania. Training
of storms in the west-east orientation will be capable of upwards
of 3 inches per hour rainrates that will more than like reach or
exceed the local FFG.

Campbell

Shortwave trough analyzed over the Upper Midwest will rapidly move
eastward through the Great Lakes during the afternoon today with
the current forecast showing the mean trough exiting off Southern
New England at the end of the period. A secondary shortwave across
the Midwest will eject northeast as it gets caught up in the mid-
level flow south of the stronger shortwave trough to north
providing a focus of ascent across portions of the Ohio Valley and
Mid Atlantic later this afternoon and evening. At the surface, a
cold front will be pressing eastward fairly quickly with surface
convergence maximized along and just ahead of the front as it
progresses through the above areas. Current forecast is the
development of scattered to widespread showers and storms migrating
eastward out of OH/KY with sights on the Mid Atlantic and Northeast
during peak diurnal heating. The progressive motion of the
convection across the region will help curb the worst of the flash
flood threat, but with an environment of 1-2 standard deviations
above normal for PWATs advecting overhead prior to the frontal
passage, the threat for localized flash flooding remains privy
within the complex terrain and any urban zones located across the
Ohio Valley through most of New England. Current 3-hr FFG
exceedance probabilities were hovering between 10-15% for much of
the Northern Mid Atlantic into New England with rates generally
capped between 1-1.5"/hr at peak intensity from any convection.
This coincides well with a MRGL risk with a lean towards the higher
end of MRGL (10-14%) chance for flash flooding within the current
setup.

Kleebauer


Day 2 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 robust 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. Return flow pattern on the
backside of a surface ridge extension east of the Mississippi will
allow for a return of a moist, unstable airmass across much of
KS/NE and points east with the help of a budding LLJ come Thursday
evening. Convective initiation across the Front Range 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
1-3" areal coverage with a maxima around 5" possible considering
the environment in place. The primary focus is centered over
north-central KS through the intersection of NE/IA/MO where the
current ensemble QPF footprint shows the heaviest precip axis,
aligning well with current ML output and the mean placement of the
mid-level vorticity maxima as it propagates east through the above
areas. A SLGT risk was maintained from previous forecast and
expanded westward to note the higher QPF signature located across
northern KS.

...Northern Plains...

A robust mid-level shortwave 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 central and northern 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 expected upscale growth at nightfall as the LLJ ramps
up and we see congealing cold pools across ND. A SLGT risk was
maintained with only a small deviation in the previous forecast to
align with trends in the QPF.

...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. 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 1-1.5"/hr at peak intensity. This is especially true for
more of the interior west with the focus shifting across UT,
mainly within the Wasatch and adjacent higher terrain over south-
central UT through northern 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 account for some areas that could see repeated
impact from heavy rainfall in successive periods across northwest
NM. The remainder of the forecast was sufficient in placement.

Kleebauer


Day 3 Valid 12Z Fri Jun 28 2024 - 12Z Sat Jun 29 2024

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

...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 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 eastern KS and northern MO, tracking east-
northeast up along the frontal boundary as it slowly moves 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. That is currently
positioned over the Quad Cities area and general surrounds with QPF
forecasts already upwards of 2.5" within some of the ensemble
suite. This area has been hit recently with heavy thunderstorms
leading to lower FFGs prior to the setup evolving. As of now, the
SLGT risk was maintained and expanded to the west to account the
recent QPF increase located over southeast NE and northeast KS.
This setup has the potential for future upgrades, especially within
the aforementioned corridor that may see a multi-round impact over
the course of the forecast, along with inherited low FFGs leading
up to the threat. We will be monitoring the evolution closely.

...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 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