FOUS30 KWBC 240846
QPFERD
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
446 AM EDT Sat Aug 24 2024
Day 1
Valid 12Z Sat Aug 24 2024 - 12Z Sun Aug 25 2024
...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF THE SOUTHWEST...MIDWEST...AND SOUTH FLORIDA...
...Southwest...
Another round of daytime convection is expected in the Southwest
again on Saturday, although the primary threat area should be
relatively narrow, focused in a plume of deeper moisture along the
periphery of an advancing deep trough. The Marginal Risk was
generally maintained in the same area as the previous outlook
issuance with only minor adjustments around the southern periphery.
Localized heavy downpours will remain possible given the plume of
moisture still being drawn northward out of Mexico and northward
around the western side of a sprawling upper level ridge. However,
the mid-level flow will be much stronger than usual for this time
of year helping to mitigate the flash flooding concern by providing
a strong and persistent steering flow. Nevertheless, around an
inch of rain falling in a short period of time may still lead to
flash flooding, and the Marginal Risk was maintained.
...Midwest.
A complicated forecast exists with several conditional threat
areas that still need to be sorted out. Models are generally in
good agreement in showing several rounds of elevated convection
along the periphery of a pronounced elevated mixed layer, with
700mb temperatures approaching record highs in the sounding archive
and ERA reanalysis dataset. One of the rounds is focused early in
the Day 1 period in or near Missouri. Satellite and radar imagery
from the overnight hours was showing thunderstorm development along
the Kansas/Nebraska border moving westward around the 1000-700 mb
thickness gradient and being fed by south to southwesterly winds of
35 to 50 kts per area VAD Wind profilers upstream over Kansas and
Oklahoma. The 06Z run of the HRRR continued to generate a narrow
stripe of moderate to heavy rainfall in western Kansas...its areal
coverage apparently being in check from the warm mid level
temperatures noted above but providing support to maintain the
Marginal risk area without much need to adjust the placement. The
other round looks to be focused after 00Z farther to the north from
southeast Iowa into northwestern Illinois. Ingredients favoring
heavy to excessive rainfall extend to the north and northwest in
so those solutions extending as far north and northwest into
Minnesota or southeast South Dakota cannot be ruled out yet.
Multiple hi- res models also show much of the northern sections of
the region remaining almost entirely dry. However, the risk should
an inch of rain falling in a short period of time may still lead to
flash flooding, and the Marginal Risk was maintained.be assumed to
be non- zero in the areas described above, and above 5 percent in
the Marginal Risk areas. Further updates and expansion of the
Marginal Risk is possible.
...South Florida..
Incoming deep moisture with roots from the Caribbean will make
headway into South Florida today with area precipitable water
around the 90th percentile, which is well above 2 inches. The
increasing moisture will be in proximity to a quasi-stationary
front to the north...with a sharp delineation point of where the
moisture pooling will reside. A Marginal risk area was maintained
across the Southern FL Peninsula.
Bann
Day 2
Valid 12Z Sun Aug 25 2024 - 12Z Mon Aug 26 2024
...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF NEW MEXICO AND COLORADO...AND THE MIDWEST...
...New Mexico and Colorado...
As the western periphery of the Southern Plains upper level high
begins to erode, the heavy rain threat area should expand eastward
on Sunday into more of New Mexico and Colorado. Scattered daytime
convection should develop in a plume of deeper moisture and
moderate instability, and the thunderstorms will be capable of producing
flash flooding within complex terrain and especially around any
burn scars. Hourly rain rates may locally exceed 1 inch. The
Marginal Risk was mostly maintained in the same areas.
...Midwest...
The latest guidance shows potential for for organized convection
and an associated heavy rainfall threat to persist into the Day 2
period over portions of southeast Iowa into western Illinois and
northeast Missouri. The latest CSU machine learning Excessive an
inch of rain falling in a short period of time may still lead to
flash flooding, and the Marginal Risk was maintained.
Rainfall Outlook still appeared to be displaced north and west of
where the ensembles favored heaviest rainfall...and the 00Z runs
did little to resolve the considerable degree of mesoscale
uncertainty both from the overall setup across the region. The
model consensus was best on the idea that heavy rainfall was
possible but they varied widely on placement. Therefore, the
Marginal Risk should still be viewed as the most likely placement
for heavy rain on Sunday morning, but future updates are possible.
Lamers/Bann
Day 3
Valid 12Z Mon Aug 26 2024 - 12Z Tue Aug 27 2024
...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE SOUTHWEST US AND THE NORTHERN PLAINS/UPPER MIDWEST ON MONDAY...
...Southwest US...
The threat of excessive rainfall will linger into Monday across
portions of the Southwest US as precipitable water values of 0.75
to 1.00 inches remain in place from extreme southeast Arizona into
New Mexico. Scattered daytime storms remain possible within the
plume if deeper moisture...but strengthening steering flow should
help limit residence time to mitigate some of the flash flooding
concern. As mentioned earlier...though...even brief heavy rainfall
rates in a short period of time may still lead to flash flooding,
and a Marginal Risk was maintained in the region.
...Northern Plains...
An upper trough over the Intermountain region will begin lifting
northeastward across the High Plains helping to sharpen a front
across parts of the Upper Midwest/Northern Plains. Precipitable
water values are forecast to approach 1.75 inches over portions of
the eastern Dakotas prior to the arrival of the surface front and
increasing divergence/difluence associated with the upper level
system. The expectation is that moisture flux convergence along the
front will focus convection moving into an unstable environment
driven by diurnal heating that leads to locally heavy downpours.
Farther west where instability is more driven by cold mid level
temperatures...rainfall should be less focused and less intense.
Bann
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