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

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