FOUS30 KWBC 290834
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
434 AM EDT Sat Jun 29 2024

Day 1
Valid 12Z Sat Jun 29 2024 - 12Z Sun Jun 30 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE SOUTHERN HIGH PLAINS AND THE LOWER GREAT LAKES/OHIO VALLEY...

...Northeast US...
The better chance for organized heavy ton potentially excessive 
rainfall looks to be the the Northeast US as deep moisture advects 
into the region ahead of a shortwave trough propagating eastward 
across the international border. Backing flow aloft ahead of the 
trough will draw deep moisture into the region...as shown by 
precipitable water values approaching 2 inches over the eastern 
Great Lakes region early in the period and values at or above 1.75 
inches as far north as upstate New York by late afternoon helping 
to foster an environment supportive of some downpours and cell 
training. By later in the day as the deep moisture arrives in 
upstate New York...there is still some question as to whether or 
not the convection will be elevated or rooted closer to the 
surface...but enough mesoscale forcing in an atmosphere with 
precipitable water values at or above 2 inches around time of 
maximum heating suggests sufficiently intense rainfall rates to 
maintain both Slight Risk areas.

...High Plains of New Mexico into Colorado...
Model signal persists for isolated to scattered convection later
this afternoon and evening as low level flow turns upslope behind a
cold front. Above normal moisture as shown by surface dewpoints
temperatures around 60 degs should be in place beneath steep
lapse rates...supporting local downpours and at least some 
potential for flash flooding especially from burn scars and in 
normally dry washes.


Bann

Day 2
Valid 12Z Sun Jun 30 2024 - 12Z Mon Jul 01 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE SOUTHWEST US AND SOUTHERN HIGH PLAINS...

...Southwestern US...

Moisture will continue to be drawn northward around the western
flank if a mid level high located over the Southern Plains. Model
guidance brings precipitable water values in excess of 1.75 inches
back into southern Arizona by Sunday afternoon. Expanded the
Marginal risk area to the west of the previous outlook to fit
better with placement of the highest precipitable water values
shown by the NCEP guidance by 01/00Z...although thinking is that
the southeastern part of Arizona has the higher risk of excessive
rainfall. The extent and placement of cloud cover that lingers from
convection on Saturday will influence where convection fires on
Sunday and where the greater risk of excessive rainfall occurs. So
largely maintained the Marginal in Arizona with a focused Slight
Risk in the southeastern part of the state. The plume of moisture
will be pulled north and eastward into parts of New Mexico and 
Colorado with a corresponding risk of excessive rainfall in the 
terrain given the steepening lapse rates at the time of maximum 
heating. 

...Northern Plains...
A mid- level trough will be moving out of the Northwestern US and
emerging from the northern Rockies by Sunday afternoon helping to 
focus late day convection. Any convection will be able to tap
deeper moisture over the far eastern Montana and the western
Dakotas...especially given increasing moisture transport into the 
region by a strengthening low level jet during the evening. with at
least some concern for convection to persist into the night given 
the model shear profile.

Eastern U.S....
Maintained a Marginal Risk of excessive rainfall along and south of
a front extending from off-shore into the Mid-Atlantic region into
the Tennessee Valley. The GFS and NAM both focus the axis of 2.0 to
2.3 inches precipitable water values and nearly 1500 J per kg of
CAPE along and immediately south of the front. The resulting
updrafts should be able to result in some intense rainfall rates.
Given modest flow...at best...cell motion should be slow enough
that isolated problems due to run off and flooding of poor drainage
areas is a concern despite high flash flood guidance.

Bann

Day 3
Valid 12Z Mon Jul 01 2024 - 12Z Tue Jul 02 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER THE UPPER
MIDWEST...

...Upper Midwest...
The area of low pressure the brings a risk of excessive rainfall to
parts of the northern plains on Day 2 will continue eastward and
bring a risk of excessive rainfall to portions of the Upper Midwest
on Monday and Monday night. Low level winds will back in response
to height falls and result in 1.75 to nearly 2 inch precipitable
water values into the central and eastern Dakotas early Monday.
Despite the fact that overall forecast rainfall totals are 
currently expected to be in the 1-2 inch range, it is expected to
fall on ground that is highly saturated in many cases, which would
be an aggravating factor when it comes to flooding potential. 
Therefore, the Slight Risk remained in place. 

...Southwest...
Monsoonal flow will support another round of late day and evening
convection on Monday. Continued uncertainty in the extent of cloud
cover lingering from convection on Sunday and what impacts that has
in terms of where convection sets up limits confidence in hoisting
a Slight Risk at this point although there is not much uncertainty
as to the supply of deep moisture being drawn up into Arizona and 
then northeastward around the upper high into New Mexico and
Colorado. 

...Southeast US...
While most of the showers and thunderstorms will be shunted into
the Atlantic as a mid-level trough continues to push the eastern US
cold front eastward...models still suggest the front will be
hugging the Carolina coast early on Monday with lingering risk of
some heavy rainfall. Maintained the Marginal Risk here from the
medium range period without much change.

Bann
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