698
FOUS30 KWBC 230031
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
831 PM EDT Sat Jun 22 2024

Day 1
Valid 01Z Sun Jun 23 2024 - 12Z Sun Jun 23 2024

...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE UPPER MIDWEST/UPPER GREAT LAKES...

...01Z Excessive Rainfall Discussion...
Main focus continues to be across the Upper Midwest and Upper Great
Lakes region through the night. Convection capable of producing
downpours has been growing in areal coverage late this afternoon
and early evening with cell motion and the mean flow aligning in
such a way to result in training of storms and enhancing the risk
of flash flooding from southern Wisconsin into portions of eastern
Iowa and northern Illinois. The HREF may be too far south with
placement of the heaviest rainfall...but felt the threat of
heavy to excessive rainfall is high enough to warrant a southward
extension of the Slight risk area given the precipitable water
values and the moisture transport into the region that may
encounter outflow boundaries. Refer to MPD 0491 for additional
details.

Farther to the east...one wave of convection has pulled
off shore from New England but could not entirely remove the risk
of excessive rainfall due to additions storms forming in eastern
Pennsylvania ahead of the next impulse. The expectation is that
the convection will eventually weaken with the loss of daytime
heating.

Maintained the Marginal Risk area in proximity to the swirl near
the South Carolina coast and southward into northern Florida along
the low level vort field shown by the RAP and NAM-NEST.

Only changes in the Western United States were minor and made based
on short term radar and satellite imagery.

Bann


...Previous Excessive Rainfall Discussion...


...Upper Midwest into the Upper Lakes...
Early morning activity continues to progress eastward across
Wisconsin and the Upper Great Lakes associated with a wave of low
pressure along a gradually lifting warm front. Rain rates have
remained tame so far this morning and generally under an inch per
hour, but with increasing instability due to daytime heating a few
embedded thunderstorms should contain at least 1-2" per hour rates
across the northern mitten of Michigan this afternoon. PW values
expected to remain much above average in the vicinity of this west
to east front extending westward to northern Iowa, 2 to 3 standard
deviations above the mean, supporting the likelihood of heavy
totals spreading from the U.P. of Michigan/northern L.P. west
southwestward into southern Wisconsin/northern Illinois and
northeast Iowa. In the wake of the initial band of potentially
training convection, additional convection may form along and north
of the stationary front in response to additional upstream height
falls forecast to push east across South Dakota, southern Minnesota
and into Wisconsin. There is potential for overlap of the early
day 1 convection with the late afternoon convection from from far
northeast Iowa into southern Wisconsin. Across this overlap area,
the Excessive Rainfall Outlook remains a Moderate Risk. This risk
area was shifted to the south and west with this update to
incorporate latest CAMs further south development of afternoon
convection. This corresponds to where the latest HREF neighborhood
probabilities are modest (40-60%) for 3"+ totals day 1, with these
probabilities also extending southward into northern Illinois as
convective progresses southward overnight.


...Southern New England and the Interior Northeast...
Another round of organized convection possible Saturday afternoon
along and to the south of the stationary frontal boundary forecast
to stretch in a west northwest to east southeast direction from
northern NY state into southern New England. There is potential for
overlap of where heavy rains have fallen over the past 24 hours
with new rains this afternoon. The Slight Risk was maintained and
stretched a little westward to incorporate latest CAMs potential
for numerous rounds of convection lingering into the early
overnight period. PW values nearing 2" (in the 90th climatological
percentile) and surface dew points in the upper 60s and low 70s
when combined with SBCAPE of 1500-2000 J/kg will support the
potential for maximum rainfall rates up to 3" per hour in localized
spots. This matches well with where the new 12z HREF neighborhood
probabilities show high chances(60-80%) for 2"+ amounts in 6-hours
(ending 00z tonight). Low probabilities (10-20%) of 3"+ amounts
within an hour (ending 22z) exists across northwestern CT.
Scattered convection is also forecast elsewhere due to convergent
flow and increasing instability south of the main frontal boundary,
with thunderstorms potentially containing efficient rainfall rates
from northern PA to NJ. This prompted a subtle expansion of the
Marginal across parts of PA.

...Southern Georgia into the Florida Peninsula...
Only changes made were to extend the Marginal Risk to incorporate
more of western Florida, where 12z HREF probabilities highlight 50%
chances for over 5" of total rainfall. The weak surface low along
the GA coast is forecast to linger over the region with maybe some
gradual northwestward motion, accompanied by 2 to 2.25"+ PW values.
Slow moving cells near the center and diurnal convection southward
into northern to central FL will support locally heavy totals.
HREF neighborhood probabilities are high for 1", 2" and 3"+ totals
in the marginal risk area, supporting isolated runoff issues. A few
CAMs do highlight the potential for 5"+ totals in very localized
areas just to the south of the low pressure center, but would have
to occur in an urban area for any elevated impacts.

...Far South Texas...
Models are consistent in showing the next surge of high PW values,
to the north of low pressure moving east to west across the
southwest Gulf of Mexico/Bay of Campeche, moving into South Texas
day 1. Model consensus continues to be for the heaviest precip to
remain across northeast Mexico, with far South Texas on the
northern edge of this precip. A Marginal Risk was maintained from
the previous issuance given potential for locally heavy rainfall
amounts across regions that received heavy rains recently from
Alberto.

...Portions of the Southwest...
Latest models continue to show the upper-level trough over
California into the Southern Great Basin weakening day 1 as upper
ridging expands westward from the central to eastern U.S.. An area
of anomalously high precipitable water values will, however, expand
farther to the north and west, encompassing much more of southern
and central Arizona and continuing in New Mexico, with values 2 to
4+ standard deviations above the mean. There may likely be a lot
of clouds in this high PW axis, limiting instability potential.
However, there will still likely be at least scattered convection
beginning over higher terrain across southern to central Arizona
into southwest NM. HREF neighborhood probabilities are showing
scattered 40-60% chances for 1"+ totals, from the Mogollon Rim
southward into southeast AZ, and lower chances extending into
southeastern UT with this region depicted in the marginal risk area
where isolated runoff issues are possible.

Snell/Oravec


Day 2
Valid 12Z Sun Jun 23 2024 - 12Z Mon Jun 24 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
NORTHERN NEW ENGLAND...


..Northeastern US...
The latest model suite is in fairly good agreement on the eastward
push of a well defined surface low across far southeast Canada
into far northern NY state and northern New England. The plume of
anomalous PW values ahead of this low will continue to lie across
much of the Northeast day 2 with values 2+ standard deviations
above the mean. Strong isentropic lift ahead of this low in the
anomalous PW plume will support potential for an axis of heavy
rains from far northern NY state, southern QB and across northern
New England. As CAMs come into range the expected QPF amounts have
come up in parts of New England where more organized convection is
likely, which prompted the Slight Risk to be extended southward
into the Portland metro and much of New Hampshire. 12z HREF
neighborhood probabilities are also modest (40-60%) in this region
for at least 2" of rain in 6-hours (ending 00z Sun night). Storm
motions for the remainder of New England and the Northeast are
expected to be too fast for widespread flash flooding concerns even
with the potential for organized cells containing rainfall rates up
to 2" per hour. The Marginal Risk was mostly maintained with this
update outside of an extension southward along the I-95 corridor to
far northeast MD. Convection is likely to be fairly progressive
within the MRGL given the expected fast movement of the front, but
with PW values above average along and ahead of the front as well
as sufficient daytime heating and instability in place, locally
heavy totals and isolated runoff issues possible.

...Portions of the Southwest...
The axis of much above average PW values, as high 5 standard
deviations above the mean across southern Arizona and southwest
California (per the 12z GEFS), will persist across much of the
Southwest day 2. Another round of scattered convection likely over
much of Arizona into portions of New Mexico. Expanded the Marginal
Risk into southern California given the amount of anomalous
moisture entering the region and scattered qpf now showing up in
the CAMs. There remains low confidence in where any isolated
runoff issues may occur, but low 12z HREF neighborhood
probabilities (10-30%) for 2" of rain have started to increase
across central and southern Arizona.

...Far South Texas...
PW values 2-2.5" and sustained east southeasterly low level flow
is expected to persist for most of the upcoming day 2 period
across northeast Mexico into South Texas. Similar to the day 1
period, model consensus is for the heaviest totals to be across
northeast Mexico with far South Texas on the northern edge of this
precip. Given no big changes overall to the pattern, the marginal
risk was maintained over far South Texas for isolated runoff
issues.

Snell/Oravec


Day 3
Valid 12Z Mon Jun 24 2024 - 12Z Tue Jun 25 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF THE UPPER MISSISSIPPI VALLEY, UPPER GREAT LAKES AND SOUTHWEST...


...Upper Mississippi Valley/Upper Great Lakes...
The surface frontal boundary oriented northwest to southeast from
the Northern Plains into the Mid Missouri Valley at the start of
day 3 will begin to push back to the northeast toward the Upper
Mississippi Valley/Upper Lakes region. Strengthening south
southwesterly low level flow into this front will raise PW values
to 1.5-2" along and to the northeast of the front. Shortwave energy
pushing eastward across the Northern Plains toward the Upper
Mississippi Valley will help strengthen isentropic lift over this
front and support for overrunning convection. There is not a lot
of agreement on placement of any heavy precip totals, but overall
some potential given the favorable MCS set up. A broad marginal
risk was maintained and trimmed somewhat on the western edge to
match model qpf uncertainty and 90th percentile NBM higher amounts.

...Southwest...
No significant changes expected day 3 to the large scale pattern
across the Southwest. PW values will remain highly anomalous from
southern California into the Southwest with a continuation of weak
forcing under the upper ridge axis stretching from the
Southern/Central Plains into the Southwest. Models continue to show
potential for widely scattered diurnal convection, with continued
low confidence with locations of any isolated heavy totals and
isolated runoff issues. This update expanded the Marginal Risk to
match the updated day 2 outline, which encompasses the broad
isolated convection possible, sensitive terrain and burn scars, as
well as the reasonable worse case scenario depicted by 90th
percentile NBM.

Snell/Oravec


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