Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
406 AM EDT Sun Jun 04 2023

Day 1
Valid 12Z Sun Jun 04 2023 - 12Z Mon Jun 05 2023

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
THE SOUTHERN HIGH PLAINS...

...Southern to Central Plains into the Central to Northern
Rockies...
No significant changes anticipate to the blocking pattern across
much of the nation during the day 1 period.  A closed upper high
will remain centered across the Northern Plains with an elongated
area of troffing extending northwest to southeast on the western
side of this ridge from the Northern Rockies into the Central to
Southern High Plains. PW values in this elongated trof axis will
remain above average, especially from northeast Colorado into
Montana where PW anomalies will run 2+ standard deviations above
the mean.  Weak vorts in the high PW axis will again support
another day of widespread scattered convection stretching from the
Southern to Central Plains into the Central to Northern Rockies.
This entire region has seen much above average precip over the
past few weeks, with many areas having observed values 300 to 600%
of normal.  This will continue the risk of isolated flash flooding
where any additional day 1 rains fall across areas that have
received recent heavy amounts.  With confidence low with respect
to where any heavy day 1 totals will occur, much of this region
was kept in marginal risk.  The one exception continues over the
Southern High Plains from far northeast New Mexico, across the
Texas/Oklahoma Panhandle region and southwest Oklahoma, where a
slight risk area was maintained.  This region has seen heavy rains
over the past two days in the general same region and has a larger
region of high 1"+ HREF probabilities for the upcoming day 1
period.

...Florida...
The remnants of Arlene expected to pass to the south of South
Florida early Sunday with any associated heavy rains on the north
side likely having already pushed to the southeast of the southern
portion of the peninsula by the beginning of the day 1 period.   
In its wake, another day of scattered to widespread afternoon
thunderstorms is likely across the southern half of the Florida
Peninsula as the upper trof that the remnants of Arlene have been
rotating through pushes across the Florida peninsula.  HREF
probabilities are high across the southern half of the Florida
peninsula for 1 and 2"+ amounts.  This may lead to isolated flash
flooding, especially in urbanized areas where hourly rate of 1 to
2" are possible.

Oravec


Day 2
Valid 12Z Mon Jun 05 2023 - 12Z Tue Jun 06 2023

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
THE SOUTHERN HIGH PLAINS...

...Southern Plains through the Northern Rockies...
With little in the way any significant changes to the overall
large scale flow across the central portion of the nation day 2,
the day 2 Excessive Rainfall Outlook is very similar to the day 1
outlook.  PW values are forecast to remain above average from the
Southern to Central Plains through the Central to Northern
Rockies.  Similar to the day 1 period, shortwave energy embedded
in this high PW axis will support another day of widespread
scattered convection across these areas.  A slight risk area was
continued from northeast New Mexico into the Texas/Oklahoma
Panhandle region.  There continues to be somewhat better agreement
that this area will see a better chance of organized heavy precip
than other areas in the high PW axis.  As stated in the day 1
discussion, isolated flash flooding is possible across the entire
area given recent rainfall amounts that have been as much as
400-600% of normal. 

...Sierra Nevada...
One of the biggest changes to the large scale flow across the U.S.
during the day 2 period will be the closed mid to upper level low
approaching the central to southern California coastal region. 
Increasing upper difluence Monday afternoon to the north and
northeast of this closed low will support scattered convection
through the Sierra, northern California and portions of the
Northern Great Basin.  With PW anomalies rising to 1 to 2+
standard deviations above the mean in this upper difluent region,
locally heavy rainfall amounts of up to an inch possible.  With
stream flows remaining much above average through the Sierra,
isolated flash flooding is possible Monday afternoon into Monday
evening.

Oravec


Day 3
Valid 12Z Mon Jun 05 2023 - 12Z Tue Jun 06 2023

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS
NORTHEASTERN NEW MEXICO...

The story remains the same for the upcoming day 3 period with
little changes to the overall stagnant blocked pattern across the
nation.  Another day of widespread scattered convection likely
from the Southern to Central High Plains, northward through the
Central to Northern Rockies into the Northern High Plains.  There
is model consensus for a westward shift in the axis of heaviest
rains over the Southern High Plains day 3. This will remove
portions of central Texas from the marginal risk area.  A slight
risk area was maintained over northeast New Mexico where model
consensus is for a max precip area.  This is a region where upper
difluence strengthens day 3 well to the east of the upstream
coastal California mid to upper level low.   This will bring three
days of potential heavy rains over this region, warranting the
continuation of a slight risk.   Similar to the day 1 and 2
periods, given recent heavy rains, 400-600% of normal through
portions of this area, isolated flash flooding will again be
possible from the additional day 3 heavy rain potential across the
remainder of the broad marginal risk area extending into the
Central to Northern Rockies and Northern High Plains.  There is
also a model signal for a precip max along the western Gulf coast
from the middle to upper Texas coast into southwest Louisiana. 
With recent rains not as anomalous across this area as other areas
in the Southern Plains, the threat was kept at marginal for the
time being.

...Northern Sierra/Northern California into portions of the Great
Basin...
The closed low that approached the central to southern California
coast day 2 is not expected to move much day 3.  This will keep a
favorable upper difluent pattern in place to the north and
northeast of this system and support another day of scattered
convection.  Consensus is for a broader region of upper difluence
compared to previous days, resulting in a broader area of
potentially locally heavy rains in what will again be an axis of
above average PW values stretching across Northern California into
the Great Basin.  This is reflected in a larger marginal risk area
than on day 2 extending into northern California, southern Oregon,
northern Nevada and far southwest Idaho. Stream flows are above
average across much of the marginal risk region with isolated
flash flooding possible.

Oravec


Day 1 threat area: www.wpc.ncep.noaa.gov/qpf/94epoints.txt
Day 2 threat area: www.wpc.ncep.noaa.gov/qpf/98epoints.txt
Day 3 threat area: www.wpc.ncep.noaa.gov/qpf/99epoints.txt