858
FOUS30 KWBC 300830
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
430 AM EDT Sun Jun 30 2024

Day 1
Valid 12Z Sun Jun 30 2024 - 12Z Mon Jul 01 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PARTS OF THE
SOUTHWEST US...

...Southwest U.S...
Moisture will continue to serge northward into parts of Arizona and
New Mexico in broad anticyclonic flow around a mid- and upper-level
ridge over the Southern Plains. Convective chances will be aided by
mid-level height falls from the Pacific Northwest extending into
the central Great Basin by evening. Any convection that develops
should tap into an airmass characterized by precipitable water
values in excess of an inch from eastern Utah and western Colorado
to values approaching 2 in near the international border in
southeast Arizona by evening (2 to 4 standardized anomalies above
climatology and greater than the 95th climatological percentiles).
The resulting instability should be enough to support some
localized intense rainfall rates that pose a flash flooding
risk...especially in complex terrain and over burn scars and
normally dry washes.

...Eastern Seaboard to Florida Peninsula...
A cold front associated with low pressure north of the
international border will be moving into an anomalously moist
airmass from the Mid-Atlantic region southward with precipitable
water values expected to be as high as the 2.0 to 2.3 inch range in
a 50 to 100 mile wide corridor from the Carolinas to the Florida
panhandle by 01/00Z (approximately 3 standardize anomalies greater
than climatology and near the 95th climatological percentile). The
HREF focused its highest probability of 1- and 2-inch per hour
rates and low-end probability of 1- and 3-hour rainfall exceeding
flash flood guidance near the front given the moisture flux
convergence. There was a similar region of higher probabilities
extending along the Florida peninsula...presumably along sea breeze
boundaries and interaction with a weak shortwave seen on water
vapor imagery early this morning.

...Plains of Northeastern Montana...
Convection capable of producing locally heavy rainfall is expected
to develop later today as a shortwave moves from Washington/Oregon
early in the period and eventually emerges east of the Rockies.
Between falling surface pressures approaching from the west and
surface high pressure over southern Canada moving south and
east...surface dewpoints over the plains of northeast Montana
should climb through the 50s in response to increasing moisture
transport once a low level jet forms. Given expected rainfall
amounts from what should be progressive storms...a Marginal is
still sufficient.

Bann

Day 2
Valid 12Z Mon Jul 01 2024 - 12Z Tue Jul 02 2024

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

...Upper Midwest/Northern Plains...
The system that helps foster a risk of excessive rainfall on Day 1
will continue to propagate eastward during the period and a risk of
excessive rainfall on Day 2 over portions of the Upper Midwest.
With upwards of 12 hours of southerly winds drawing deep moisture
into region...some storms will be able to produce locally intense
rainfall rates and locally heavy rainfall amounts. The
amounts...while actually fairly modest...could fall in areas
recently soaked by heavy rainfall which has made some places
hydrologically sensitive. The previously issued Slight Risk still
captures the area and only minor changes were needed.

...SOUTHEAST US...
Introduced a Slight Risk area along and near the coast from South
Carolina into parts of Georgia. With the cold front sagging
southward acting to focus convection and a fetch of post-frontal
on-shore flow persisting into the afternoon...felt some enhanced
chances for flooding/run-off problems existed. Elsewhere in the
Southeast...deep moisture and weak flow south of the front will
open the potential for disorganized convection to form along any
boundary/outflow...so left a unfocused Marginal surrounding the
Slight.

...Southwest US...
Monsoonal moisture will be lingering over the Southwest
(especially Arizona and New Mexico) that is drawn back north and
east around an upper level ridge. Maintained a Marginal Risk at
this point given the uncertainty of cloud-cover lingering from
convection on Day 1 and its ramifications for when and where
convection fires later in the day and how much subsidence there is
in the wake of a wave moving north and east of Arizona...and the
magnitude of dry air in the sub-cloud layer. Will be able to
refine this later.

Bann

Day 3
Valid 12Z Tue Jul 02 2024 - 12Z Wed Jul 03 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE UPPER MIDWEST TO WESTERN GREAT LAKES...

...Upper Midwest/Northern Plains to Western Lakes...
Numerous showers and thunderstorms should be on-going ahead of an
advancing cold front and associated impulse/wave aloft. There
should be enough instability of support local downpours that may
fall on ground still saturated from heavy to excessive rainfall
recently. In addition to new or re-newed flooding from this batch
of rainfall is the concern for on-going flooding to be aggravated.

...Southwest...
Maintained the Marginal Risk area over the Southwest US on
Tuesday...with some southern suppression of this moisture shown in
line with the global guidance and decreased areal coverage of 1-
and 2-inch contours from the QPF spaghetti plots. Will keep an eye
open for trends in upcoming model cycles.


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