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Excessive Rainfall Discussion
 
(Caution: Version displayed is not the latest version. - Issued 0815Z Jun 14, 2024)
 
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Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
415 AM EDT Fri Jun 14 2024

Day 1
Valid 12Z Fri Jun 14 2024 - 12Z Sat Jun 15 2024

...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
FAR SOUTHERN FLORIDA...

...Florida...

A mid to upper level positively tilted trough over northern Florida
will continue to provide forcing for additional thunderstorm
activity to develop across far southern Florida, adjacent waters,
and the Keys today. CAMs consensus suggests the storms will form in
a few hours by 12Z, then persist through the morning as the line
very slowly sags southward. In addition to the upper level wave,
surface/low-level outflow boundaries from ongoing convection and
plentiful atmospheric instability will advect into the area with
these various forcings, causing the storms to form. The storms
should be both slow-moving and may backbuild with time.

00Z HREF guidance shows an over 80% chance of 3 inches of rain in
the Moderate Risk area in the 40 km neighborhood probabilities, and
over 50% chance of 5 inches of rain in the same area. This is a
significant increase from the 60% for 3 inches and 30% for 5 inches
values in the 18Z HREF.

The storms will then move over already very hard hit areas of south
Florida from the heavy rainfall of the past few days. Thus, FFGs
are very low in this area, and even moderate rain rates are likely
to cause additional flash flooding, as any rainfall will be unable
to drain anywhere, and will instead pond in place.

There remains considerable uncertainty as to where the storms will
form and low long the front will persist over the extreme southern
Florida Peninsula before the storms sag southward into the Keys and
the Straits of Florida. Due to the likelihood of additional heavy
convection through the morning and highly favorable antecedent
conditions for flash flooding, in coordination with MFL/Miami, FL
forecast office, the inherited Slight Risk was upgraded to a
Moderate Risk with this forecast update. A higher-end Slight has
been expanded to include the Keys with this update as the storms
developing along the front will push across the Keys with some of
the guidance (especially the 00Z HRRR) suggesting the storms may
stall out over the Keys. No other significant changes were made
further north as there's some possibility for typical afternoon
convection over these hard hit areas, but are highly unlikely to be
as widespread as further south.

The extremely high atmospheric moisture characteristic of this
airmass will begin a slow trek north and westward across the Gulf
starting tonight. This will effectively end the persistent rainfall
across Florida after today.

...Northeast...

A potent upper level shortwave will race from the Midwest to New
England by early Saturday morning. The associated cold front will
also move across New York State and into New England. Convection is
expected to form with these forcings from Pennsylvania north and
east into New England. Atmospheric moisture will be somewhat
limited, and the progressive nature of the storms will limit most
flooding concerns. However, given the low FFGs associated with the
urban I-95 corridor and potential for cell mergers to occur as the
storms move through NYC and into southern New England, the Marginal
Risk area was expanded east to include almost all of southern New
England except the Cape, as well as all of Long Island. Isolated
instances of flash flooding are possible in the most flood-prone
areas as the storms move across the area.

...Central Plains...

A separate upper level shortwave will track northeastward from
Arizona to the central Plains today. Leeside cyclogenesis over
eastern Colorado will tap into moisture from the LLJ with storms
developing along both the warm and cold fronts across the Marginal
Risk area. Despite the presence of an LLJ, moisture will be
somewhat limited as northeasterly flow at 850 mb over the Gulf has
largely prevented full Gulf moisture from moving into the central
Plains. Thus, moisture availability will be a limiting factor for
the storms. Nonetheless, there is some opportunity for raining
convection in the late afternoon across New Mexico and eastern
Colorado. The storms will congeal into more of an MCS overnight as
the storms move a bit more quickly eastward across Nebraska and
Kansas. With the ingredients for flash flooding largely separate
from each other, and the area not particularly flood-sensitive, the
inherited Marginal Risk remains largely unchanged with this update.

Wegman

Day 2
Valid 12Z Sat Jun 15 2024 - 12Z Sun Jun 16 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE
UPPER MIDWEST...

...Upper Midwest...

As a strong upper level shortwave tracks northeast out of the
central Plains and into the Upper Midwest, cyclogenesis occurring
over the Plains will work to slow the eastward movement of the
storms, while Iowa and Minnesota are on the nose of an LLJ that
will be tapping increasingly deep moisture. The result will be the
potential for training storms along a slow-moving warm frontal
boundary over Iowa and Minnesota, with cold-frontal storms moving
through overnight Saturday night having the potential for heavy
rain as the nocturnal jet strengthens and PWATs associated with the
LLJ increase to over 1.75 inches ahead of the cold front. Thus,
there is potential for backbuilding storms as the LLJ shifts east,
but Corfidi vectors will be perpendicular (westerly) to the
southerly flow within the LLJ.

Soils across much of Minnesota and northwestern Wisconsin are more
saturated than normal for this time of year, so they will be a bit
more flood prone than normal. Further, the potential for heavy rain
with backbuilding convection being heightened necessitated the
continuation of the inherited Slight Risk. The area was shifted a
bit to the north with the latest guidance, but only nominally. This
is likely to be the first of several consecutive days with threats
for heavy rain for the northern Plains and upper Midwest.

...Florida...

Plenty of lingering moisture and weak steering flow should allow
the typical afternoon convection to develop across portions of the
Florida Peninsula Saturday afternoon. Guidance is in fair agreement
that the center of the Peninsula, generally north of Lake
Okeechobee will be favored for the heaviest rainfall on Saturday,
but it will be significantly diminished in both coverage and
intensity from the rain over the Peninsula over the past few days.
The inherited Marginal was left largely unchanged with this update,
but it's a high end Marginal and any increase in forecast rainfall
further south and west may require a targeted Slight Risk upgrade.
At this point, the heaviest rain should be sufficiently offset from
the heaviest hit areas of the past few days, so flash flooding
should be generally isolated.

Wegman

Day 3
Valid 12Z Sun Jun 16 2024 - 12Z Mon Jun 17 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE
NORTHERN PLAINS AS WELL AS FOR PORTIONS OF SOUTHERN LOUISIANA...

...Northern Plains...

The cold front following the heavy rainfall over the northern
Plains on Saturday will stall out across the Slight Risk area
around the SD/NE/MN/IA border region. Highly diffluent upper level
low will increase lift and as the typical nocturnal jet kicks in
Sunday night, convection is expected to develop over northeastern
Nebraska through the evening, then track northeastward into
southwestern Minnesota through the overnight. Slower moving storms
and higher potential for training is expected across northeastern
Nebraska, meanwhile southern Minnesota will have the benefit of a
couple previous days of heavy rain over the region. The stalled out
front will serve as a focus for convection, increasing the
likelihood of training as the storm complex tracks northeast. With
plenty of moisture available for heavy rain, a Slight Risk upgrade
was introduced with this update to account for both higher
potential for training over this region and favorable antecedent
conditions into Minnesota.

...Southern Louisiana...

A portion of the same deep tropical air mass that has haunted south
Florida over the past few days will have trekked from south Florida
to the central Gulf Coast by Sunday. Flow straight out of the
Caribbean will increase PWATs to near 2 inches by Sunday afternoon.
While convection is not expected to be too widespread, a nearly
unlimited supply of moisture will allow any storms that form to be
capable of very heavy rainfall rates. Both Baton Rouge and New
Orleans will be threatened by storms that may move over those
respective cities. Soil moisture levels are about normal for this
time of year, so there has been quite a bit of recovery from the
last several weeks of occasional heavy rain. Thus, this is a
lower-end Slight, as more widespread convection is likely in coming
days. Flood sensitive urban areas are at greatest risk of flash
flooding Sunday afternoon.

Wegman


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