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Excessive Rainfall Discussion
(Caution: Version displayed is not the latest version. - Issued 0125Z Jun 22, 2024)
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Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
925 PM EDT Fri Jun 21 2024

Day 1
Valid 01Z Sat Jun 22 2024 - 12Z Sat Jun 22 2024


A primary area of concern for numerous flash floods during the
evening and overnight remains across portions of the mid Missouri
and upper Mississippi Valleys. A mid-level shortwave and right-
entrance upper jet forcing is helping to organize convection along
a well-defined, slow-moving front extending across the region. A
deep moisture pool (PWs 1.75-2 inches), replenished by strong
southwestern inflow, will support heavy rainfall rates, with
redeveloping/training cells elevating the threat for heavy
accumulations. Recent runs of the HRRR and HREF guidance show the
greatest threat for heavy amounts centered over northwestern Iowa.
HREF neighborhood probabilities indicate that overnight
accumulations of 5 inches or more are likely across this region.
This forecast axis is displaced slightly to the south of the
previous night's very heavy rainfall, however the most recent HRRR
has shifted a little further north and current radar returns show
convection once again developing over some of the same areas
previously impacted, accentuating additional flash flooding
concerns. Overall, the updated outlook is similar to the previous;
however, the northern gradient was tightened some given the recent
RAP analysis that shows much more stable air to the north.

The Slight Risk was removed from southern New England. Storms that
produced locally heavy rainfall and flash flooding across CT/RI are
now moving east, with rainfall rates diminishing as they move into
a more stable air mass. Scattered storms developing upstream ahead
of the front dipping south across Upstate NY and southern New
England have shown the potential to produce heavy rainfall rates
across northeastern PA. Therefore, a Marginal Risk was maintained
to reflect the threat for additional isolated runoff concerns.

Elsewhere, made small adjustments based on recent hi-res guidance
and observation trends.


Previous Discussion...
...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.


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


...Upper Midwest and Western Great Lakes...
It appears likely that organized convection will be ongoing at the
start of the forecast period across the Upper Midwest, most likely
from Central Wisconsin into Northern Iowa. The initial round of
convection should continue to lift into northeast Wisconsin and
northern Lower Michigan through the morning, with a secondary round
of convection developing in the afternoon in central Wisconsin
closer to an advancing surface low, and eventually progressing into
northern Lower Michigan overnight. The multiple rounds of
organized convection are producing a clear heavy rain signal in the
hi-res guidance, particularly in northern Lower Michigan where the
12Z HREF has a maximum probability of 55 percent for over 5 inches
of rainfall during this forecast period in the vicinity of Alpena.
That is rare for the area, with a limited number of cases of 5
inches of rain in 24 hours, and an average annual recurrence
interval of around 100 years. The Slight Risk was maintained across
the region, but could be thought of as a "high end" Slight Risk at
the present moment, and a Moderate Risk upgrade may need to be
considered depending on convective evolution.

Within the broader Slight Risk area, a second corridor of enhanced
potential exists near the Wisconsin-Illinois border. Hi-res models
are in reasonably good agreement about the placement of a band of
thunderstorms in advance of the trailing cold front. The primary
limiting factor for rainfall amounts and flash flood potential may
be that this cluster or line of thunderstorms could be more
progressive, limiting the amount of time it spends in any one
location. Some hi-res models, such as the 12Z WRF-ARW2, showed a
period of greater training, but that is only a possibility.

...New England...
Portions of the region were upgraded to a Slight Risk, primarily in
New Hampshire, Massachusetts, eastern Connecticut, and Rhode
Island. The persistence of a plume of high precipitable water
values approaching 2 inches (above the 95th percentile) and
moderate to strong instability should support organized convection
with relatively high rain rates. These could exceed 2 inches per
hour at times, and the best signal for that is where the Slight
Risk was drawn. However, flash flooding cannot be ruled out in the
broader Marginal Risk area which encompasses more of the region.

...Portions of the Southwest...
The upper-level trough over California into the Southern Great
Basin will weaken as a ridge expands westward. 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. The primary
uncertainty will be how much heating can occur, as there can often
be thicker mid-high level cloud cover with such a deep moisture
plume. Given that instability tends to be a big differentiating
factor for flash flood activity in the region, this is not a
trivial uncertainty. Therefore, the Marginal Risk was generally
maintained for now.

...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 surface low approaching the Southeast coast should weaken
further as it pushes into south central Georgia. Simulated radars
from hi-res models 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.


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


...Northeastern US...
A shortwave trough, with corresponding height falls and a deepening
surface low, will advance toward the Northeast U.S., shifting the
plume of anomalous precipitable water values more squarely into the
region as well. Model consensus is strongest for heavy rainfall in
northern New England and Upstate New York closer to the nose of the
LLJ and northern periphery of the expanding warm sector. The Slight
Risk was maintained in these areas, with a Marginal Risk existing
elsewhere further south along the cold front. One limiting factor
overall may be a tendency for forward progression for the
convection, but enough instability and deep moisture should exist
to support 1-2 inch per hour rain rates in some of the stronger and
more organized convection.

...Portions of the Southwest...
The Marginal Risk was maintained across the region on Day 3,
largely due to very anomalous precipitable water values that will
remain above the 99th percentile in some areas. One notable change
for Sunday relative to Saturday would be continued advancement of
the higher PWs into southern California. At this point, the
Marginal Risk mostly does not include California due to a weak QPF
signal, but this may need to expand into those areas in future
outlooks. The primary uncertainty continues to be with the
potential for some areas of mid-high level cloud cover that may
restrict instability, overall thunderstorm strength, and therefore
put a ceiling on potential rain rates.

...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.


Day 1 threat area:
Day 2 threat area:
Day 3 threat area: