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FOUS30 KWBC 020829
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
429 AM EDT Sun Jun 2 2024

Day 1
Valid 12Z Sun Jun 02 2024 - 12Z Mon Jun 03 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE CENTRAL AND SOUTHERN PLAINS AS WELL AS PORTIONS OF THE PACIFIC
NORTHWEST....

...Northern and Central Plains...
The emergence of a low amplitude shortwave trough from the
northern Rockies out over the adjacent western high plains this
morning will set the stage for convection that produces locally
heavy rainfall across parts of the northern and central plains
later. Showers and thunderstorms should become more numerous across
parts of the eastern Dakotas as a cold front becomes better
defined and propagates across Minnesota and Iowa later. Model QPF
shows convection building along the front as far south as Nebraska
and Kansas during the evening. Accelerating southerly flow ahead
of the cold front will draw deeper moisture into the area with
precipitable water values ranging from roughly 1.3 inches along the
international border to values approaching 1.75 inches across
western Iowa/eastern Nebraska and northeast Kansas by 03/00Z should
be aided aloft by divergence associated with a developing jet
streak over the upper midwest late tonight/early Monday morning.

,,,Southern Plains...
Introduced a Slight Risk across portions of the Southern Plains
where increasingly deep southeast flow develops and interacts with
a poorly defined boundary moving that move northward. Confidence in
placement is below average but there are signals in various model
runs that point to a better than Marginal chance for excessive
rainfall given the moisture laden atmosphere and the passage of an
upper level wave ejecting eastward from the southern Rockies during
the afternoon.

...Pacific Northwest...
There will be a growing threat for excessive rainfall over along
the Coastal Range and Cascade Range from Washington and Oregon
southward into the mountains of far northwest California as an
anomalously strong mid and upper level jet usher in deep moisture
throughout the day. While not normally a concern in the summer
time...satellite imagery shows the moisture plume has its roots
well into the subtropics in the central/western Pacific. With the
arrival of strong onshore flow and precipitable water values that
approach 1.5 inches along the immediate coast of Oregon by late
this afternoon...IVT values at or above 700 kg per meter per second
that is directed normal to the mountains for a rather prolonged
period. Only change to the previous outlook was to bring the Slight
Risk areas southward into the Coastal and Cascade ranges based on
latest deterministic QPF amounts with isolated spots in the west
facing terrain potentially receiving 4+ inch amounts due to the
terrain influences and at least some possibility of enhanced rates
from embedded convective elements. The combination of rainfall and
snow-melt run-off from higher elevation has the potential for
flooding or flash flooding. Latest numerical guidance brings an
area of height falls...leading to the development of a trough
axis...which reaches the west coast towards the end of the outlook
period at 03/12Z. No modification was done to the changes
coordinated farther inland by the previous shift.

...Southeast Florida...
With precipitable water values nearing 2 inches ahead of an
approaching shortwave trough...expectation is for increasing
coverage of showers and thunderstorms capable of producing locally
heavy rainfall in a short period. The areas most vulnerable to
problems with run-off or ponding will be the urban corridor along
the southeast Florida coastline. Model QPF is not blockbuster in
magnitude but the some higher amounts lurking offshore suggests
some possibility for a few showers with very heavy rainfall rates
making it onshore.

Bann

Day 2
Valid 12Z Mon Jun 03 2024 - 12Z Tue Jun 04 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL MAINLY ACROSS
PORTIONS OF THE CENTRAL/SOUTHERN PLAINS AND THE NORTHERN ROCKIES...

...Plains...
The front which provided focus for showers and thunderstorms
capable of producing heavy rainfall will continue eastward on
Monday...extending from the western Great Lakes region southward
into the central and southern plains by Monday afternoon. The front
outruns better upper support and becomes less of a driving force
for heavy to excessive rainfall...especially across the northern
tier of states. The southern portion of the front should still be
able to provide a mechanism for convective initiation. Exactly
where continues to be a big challenge...with numerical guidance
offering a wide range of possibilities, Introduced a Slight Risk
area mainly across the central and southern Plains where an upper
level trough moving out of the Southern Rockies should provide some
upper level support. The vertical depth of this wave has been
fairly limited but models were beginning to show increase its
reach. Any convection that forms will be in a region with
precipitable water values over 1.75 inches being drawn towards the
front by southerly flow in excess of 30 knots. The GFS and CMC
guidance fits the Slight risk area best but nothing to preclude the
northern/western solution offered by the NAM.

...Rockies,,,
Saw little reason to make too many changes to the outlook area
from Washington eastward Idaho and far western Montana as the flow
of deeper moisture associated with the atmospheric river continues
to push inland. The moisture fetch should eventually become
disrupted by low amplitude ridging which leads to decreasing
intensity and coverage from west to east. Until that happens...the
combination of rainfall run-off...perhaps enhanced by snow
melt...will continue.

Bann

Day 3
Valid 12Z Tue Jun 04 2024 - 12Z Wed Jun 05 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS MUCH OF THE
NORTHERN AND CENTRAL PLAINS...

As one cold front weakens as it progresses eastward...a second
front takes shape and pushes southward/eastward on Tuesday in
response to the sharpening of a northern stream trough on Tuesday.
Low level flow is forecast to back once again and draw deeper
moisture northward ahead of the on-coming front. It appears to be
two mechanism for heavy rainfall...with the southern portion of the
Slight Risk being aided by weak but important shortwaves
supporting convection along the old front or other outflow boundary
while the northern area was driven by the approaching front and
upper level divergence/support from the developing synoptic scale
system. Given the proximity of the two areas...confidence in
placement was not high enough to warrant two separate areas so one
broad Slight Risk was indicated.


Bann


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