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FOUS30 KWBC 211559
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1159 AM EDT Fri Jun 21 2024

Day 1
Valid 16Z Fri Jun 21 2024 - 12Z Sat Jun 22 2024

...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL ACROSS SOUTHEAST
SOUTH DAKOTA... NORTHEAST NEBRASKA... NORTHERN IOWA... SOUTHERN
MINNESOTA... AND WESTERN IOWA...

...Mid Missouri Valley and Upper Mississippi Valley...

The primary change with this outlook update is to shift the
Moderate and Slight Risk contours south about 30-40 miles. An
ongoing band of elevated convection from northern Nebraska (near
VTN) to southern Minnesota (near MKT) seems likely to be a
reasonable proxy for the northward extent of potential extreme
rainfall today. The atmosphere to the south is rapidly becoming
unstable, with abundant sunshine, and the dichotomy should
reinforce the surface front. This afternoon, new convection is
expected to focus closer to the surface front. Overall, the front
is slightly further south than models were projecting in some of
their previous runs. The new data seems to have led to convergence
in the hi-res model solutions with reasonably good consistency on
the projected location of the highest totals.

There was some consideration given to a High Risk upgrade within
the Moderate Risk area. The pattern over the region is classic for
flash flooding with an extensive WSW-ENE oriented front, strong
instability and anomalous moisture available to the south, and
increasing low-level inflow from a southwesterly direction to favor
backbuilding and training. One key lingering open question for a
High Risk upgrade is the degree to which today`s heaviest rainfall
will overlap the areas most sensitive from previous rainfall; most
of the hi-res models are just to the southeast, but it is very
close. The area of greatest concern is from YKN-EST, or from
Yankton County, SD to Emmet County, IA, and about 25mi either side
of that line. These areas are in sunshine at the moment (indicating
the surface front may be nearby or even just north) and overlap
sufficiently with the extreme rainfall footprint from the past 24
hours. So it is possible the 12Z hi-res models have slightly
overestimated the cold pool from ongoing convection; if this is the
case, a High Risk may be issued on an unscheduled update this
afternoon. Convective trends will be monitored closely.

Either way, the potential exists for significant and life-
threatening flash flood impacts in the region, and people in the
area are advised to closely monitor for warnings later today.


...Four Corners Region into New Mexico...

The Slight Risk area was expanded to the west into portions of the
slot canyon region of Utah. Model consensus is for another round of
afternoon convection to develop in a fairly unstable air mass ahead
of an advancing shortwave in portions of UT, W CO, and N AZ.
Generally, one of the biggest differentiating factors between
active flash flood days and inactive or less active days in the
Southwest U.S. is instability, and models are showing fairly
widespread areas of CAPE above 1500 j/kg by this afternoon. Despite
some pockets of morning convection, there is enough sunshine to
support widespread destabilization, and precipitable water values
should be above the 95th percentile for this time of year.
Convection may also be more organized than is typical, with some
backbuilding, as southerly low-mid level flow will be relatively
strong. The IVT is above the 99th percentile for this time of year.

Further southeast into New Mexico, there is more cloud cover
associated with a deep moisture plume from the remnants of T.S.
Alberto, which has since dissipated. However, where breaks in the
cloud cover can occur, enough instability should support renewed
convective activity this afternoon and evening, and the deep
moisture (as seen in the CIRA ALPW product) should support
efficient rainfall. One of the better signals is, unfortunately, in
the vicinity of the Sacramento Mountains where there are several
large burn scars and the HREF has reasonably high probabilities of
1 inch per hour rain rates. GOES day cloud phase RGB does show some
low level cloud cover beginning to scatter out, so some instability
nearby may be able to develop.


...New York and northern Pennsylvania into New England...

A Slight Risk was introduced over southern New York and the lower
Hudson Valley into southern New England -- particularly
Massachusetts, Connecticut, and Rhode Island. Precipitable water
values approaching 2 inches (above 95th percentile) will support
very high rain rates in scattered convection that is already
developing across the region. For more mesoscale detail, please
reference MPD #480 about to be issued. Across the Slight Risk area,
the 12Z HREF probabilities of 2 inch per hour rain rates peak at
about 20 to 40 percent in any given hour, and this would be enough
to generate some flash flooding, particularly in urban areas.


...North Florida and Coastal Southeast...

A Marginal Risk was introduced for the potential of localized heavy
rainfall around the tropical disturbance nearby in the Atlantic.
Hi-res models show localized rainfall in excess of 2 inches per
hour in some of the convection today.


Lamers


Day 2
Valid 12Z Sat Jun 22 2024 - 12Z Sun Jun 23 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE
UPPER GREAT LAKES AND UPPER MISSISSIPPI VALLEY...

The day 1 height falls supporting additional heavy precip across
the Upper Mississippi Valley/Mid Missouri Valley will be pushing
into the Upper Great Lakes day 2. Hi res models suggest a well
defined MCV will persist into early day 2 as it pushes from the
Upper Mississippi Valley into the Upper Lakes. With PW values
expected to remain anomalous ahead of this MCV, 2-3 standard
deviations above the mean, heavy rains are likely across the Upper
Lakes on Saturday. This is reflected in the 12 hr HREF neighborhood
probabilities ending 0000 UTC Sun that are high for 1 and 2"+
totals from northeast Wisconsin into the northern L.P. of Michigan.
Convection also likely to become increasingly organized late
Saturday afternoon in an axis of increasing instability, MUCAPE
values 1000-1500, PW values 2-2.5" and area of enhanced upper
difluence along the trailing cold front moving across northern
Illinois, Iowa, into northeast Kansas and northern Missouri. Heavy
rains are possible along and ahead of this trailing front, with the
slight risk drawn southward into northern Illinois to cover areas
of relatively lower ffg values.

...Portions of the Southwest...
The upper trof over California into the Southern Great Basin during
day 1 will weaken day 2 as the eastern upper ridge expands
westward. The anomalous area of PW values will, however, expand
farther to the north and west, encompassing areas from southeast
California, into central to southern Arizona and western New
Mexico, with values reaching 3 to 4 standard deviations above the
mean. Not a lot of confidence on any scattered convection in this
high PW axis, but locally heavy rainfall amounts and isolated
runoff issues are possible.

...Far South Texas...

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 will move into South Texas day 2. Model consensus is 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 southern Georgia into North Florida...

The weak surface low approaching the Southeast coast day 1 will
weaken further day 2 as it pushes into south central Georgia.
Simulated radars from the NAM NEST and FV3LAM show potential for
some slow moving cells in the vicinity of this weak low level
circulation, which will be accompanied by an axis of above average
PW values. Locally heavy precip totals possible near this weak low
level center, southward into North Florida where a marginal risk
was maintained from the previous issuance.

Oravec


Day 3
Valid 12Z Sun Jun 23 2024 - 12Z Mon Jun 24 2024

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

The mid to upper level height falls pushing heavy precip potential
into the Upper Lakes day 2 will continue to press eastward day 3
into the Northeast. The anomalous axis of PW values 2 to 2.5+
standard deviations above the mean across the Upper Lakes day 2
will also push into northern NY State and Northern New England day
3 ahead of surface low pressure moving eastward across these areas.
The NAM is an outlier with a farther southward surface low track
and subsequently farther south heavy precip axis compared to the
remainder of the global guidance. Consensus is for an axis of heavy
rains from far northern NY State into Northern New England, where
a slight risk was maintained.


...Portions of the Southwest...

Not a lot of large scale changes day 3 across the Southwest from
day 2 with a broad marginal risk area depicted across areas from
far southeast California, through much of Arizona and into
northwest New Mexico.   PW values will continue to remain 3 to 4
standard deviations above the mean in a region of generally weak
forcing. Additional scattered convection likely across these
areas, with locally heavy totals and isolated runoff issues
possible.

...South Texas...

PW values 2-2.5" and sustained east southeasterly low level flow is
expected to persist for most of the upcoming day 3 period across
northeast Mexico into South Texas. Similar to the day 2 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.


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