977
FOUS30 KWBC 111953
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
353 PM EDT Tue Jun 11 2024

Day 1
Valid 16Z Tue Jun 11 2024 - 12Z Wed Jun 12 2024

...16Z Update...

Changes within the respected risk areas across the Southern Plains
and Florida were minimal with minor adjustments on the northern
edges of each risk. Florida continues to carry the SLGT risk with
heavy rain potential lasting through the period with the heaviest
QPF focus in-of the western FL coast, especially across southwest
FL where local maxes of 6-9" of rainfall are plausible. The key in
limiting the flash flood threat will be maintaining the current
rates with general hourly rainfall currently between 0.5-1.25"/hr,
a far cry from what would constitute concern within the Peninsula.
Antecedent conditions leading in were very dry, so the initial wave
of moderate to heavy rain has been beneficial and will continue
through the period with some breaks at times later this evening.
The SLGT risk has the best definition over the coastal urban
centers with the primary locations from Sarasota down to Naples as
the targets for flash flooding. The sandier soils in the affected
area(s) across central and southern FL will deter major impacts
outside those urban centers that come underneath a stronger heavy
rain core. Those will be more isolated in nature and certainly not
enough consensus on when/where those would occur. The SLGT risk is
justified due to forecast totals and potential, but any upgrades
are highly contingent on near term convective evolution.

The Southern Plains continues to be impacted by a propagating MCS
this morning. More on the setup in an updated sub-heading below....

...Previous Discussion...

...Southern Florida...

A steady influx of tropical moisture, with PW anomalies up to +3.5
standard deviations will trigger scattered to widespread
convection across southern and central Florida. With the stalled
front positioned to the north in a west-east orientation the
environment will be favorable for slow moving capable of producing
several inches of rain. Some of the highest totals are expected to
focus from Tampa Bay to the Keys during this period. Areal
averages of 2 to 5 inches are forecast with very isolated
locations possibly receiving 6 to 9 inches. The Keys and southwest
portions of the peninsula will have the greater risk for excessive
rainfall (Slight Risk) and associated flash flooding, but the
threat extends to the southeast coast and into portions of central
Florida as well.

Campbell/Kleebauer

...Southern Plains...

Nocturnal MCS continues to push southeast, likely riding along a
stationary boundary analyzed from north-central TX down through the
central and eastern portion of the state as it extends near the
Gulf coast around Houston and arcs back along the remainder of the
Gulf coast to the east. Guidance has picked up on the MCS and it`s
general forward motion over the past succession of runs with some
general continuity and consensus on expected impacts. Totals over
north-central TX have been mainly between 1-2" within the zone of
impact, but a few stronger cores embedded within the bowing segment
and on the edges of the complex have aided in totals closer to 3"
with an anomalous 6+" total over Seymour, TX due to a bookend
vorticity that stalled over the area with a remnant boundary
influencing the evolution. These types of maxima are plausible in
the setup today, but will be highly localized with most of the
areas in the path of the complex likely seeing more along the 1-2"
range with locally higher in those more complex evolutions.

12z HREF was pretty bullish in the neighborhood probability fields
with some 40-45% readings for at least 5" where the MCS is
currently located and moving towards. Observations over the area
show the hi-res deterministic reading a bit hot, but still within
a reasonable margin of error on the current rates. Debated an
upgrade to a SLGT risk for central TX, but with more of an
isolated prospect for flash flooding, decided to maintain
continuity with the MRGL risk. The forward motion of the MCS is
also a deterrent on the higher potential as areas will likely see
30 min to an hour of heavier rain before settling. However, like
the areas to the north that received the higher totals this
morning, if there is a more defined concern with those types of
outcomes downstream in the MCS path, there could very well be a
targeted SLGT risk upgrade if the setup is warranted. Recent ML
output from the First Guess fields are still holding firm on a MRGL
risk in place as well, so the necessity to upgrade was
entertained, but passed on. We will continue to monitor closely.

Kleebauer


Day 2
Valid 12Z Wed Jun 12 2024 - 12Z Thu Jun 13 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR SOUTHERN
FLORIDA...

...20Z Update...

Continuity was maintained for all the risk areas with only some
minor adjustments necessary as the general pattern evolution
remains consistent from run-to-run to limit any notable changes.
The heavy rain across the southern half of the Florida Peninsula
will continue with the best flash flood threats confined to the
coastal urban corridors, especially over southwest FL from St
Petersburg down through Naples. 12z HREF signals are still fairly
robust with neighborhood probabilities for at least 5" in totals
for the period running between 70-90% within the aforementioned
corridor along the Florida Gulf Coast. Waves of heavy rain will
move inland off the adjacent Gulf with general rates running
between 1-2"/hr within heavier cells with some localized 3-4"/hr
rates possible based on the probabilistic fields of the HREF and
upper quartile of the NBM. The multi-day event will lead to
heightened concerns for flooding, but pure flash flooding will
still remain within the SLGT risk category thanks to the elevated
FFG`s that are climatologically favored over the Peninsula. The
urban areas are the most likely to see any flash flood risks due to
impervious surfaces with the sandy soils likely to absorb much of
the rain during the period, especially with any small breaks in the
precip shield.

The other areas of concern across the Midwest and Texas are
generally MRGL with local rainfall totals likely within the 1-2"
range in the affected areas with maxes more around 3" for each
region in question. Convection over the Midwest will have the most
dynamical support with the approach of a shortwave trough and
associated cold front that will enhance surface forcing along with
the favored upper pattern. The progressive nature of the precip
will deter higher end impacts, however some backbuilding is
plausible over southern MN into northern IA that could enhance
local totals towards that upper threshold mentioned previously.
This is noted on a few hi-res deterministic and falls within the
90-99th percentile of outcomes in the blend. Considering this is
more localized, the threat remains on the MRGL level with a chance
at a targeted upgrade pending short term convective evolution.
Further south, pulse convection will occur with the passage of a
weak mid-level vorticity maximum that will swing southeast around
the eastern flank of a mid-level ridge that will shift eastward
from the Southwestern U.S. A few spots could see a quick 1-2" of
rainfall, but will be highly localized. Considering the rainfall
from the previous period, some places could still be wet and more
conducive for flooding concerns. This is highly contingent on what
falls this evening and there`s a chance this risk area is voided,
or shrinks in size.

Kleebauer


...Previous Discussion...

...Southern Florida...

The stalled front will sag a little to the south during this period
but the heavy rain will persist across South Florida. PW values of
2 inches will hold steady across the peninsula and will maintain
very efficient rain and rain rates across the nearly saturated
rural and urban areas. Areal averages will again be 2 to 4 inches
with local maxes up to 8 inches possible. A Slight Risk for
Southern Florida and the Keys remains in effect along with a
Marginal Risk for Central Florida. The nature of this multi-day
event with storm total rainfall amounts potentially rising into the
double digits, there may be the need for an upgrade in future
updates.

...Texas...

Convection will continue to shift south and east while reducing
areal coverage during this period. Most of the thunderstorms will
focus across eastern portions of the Hill Country and points east.
While amounts are on the lower end the recent rains will have
increased soil saturation thus lowering the amount of rain needed
to become problematic. The Marginal Risk was maintained for an
isolated threat for flooding concerns for portions of central and
eastern Texas toward the Mid and Upper Coast.

...Upper Mississippi Valley...

A Marginal Risk was introduced for portions of Minnesota, Wisconsin
and Iowa where soils have been nearly saturated from multiple
instances of recent rainfall. Showers and thunderstorms are
expected to accompany a passing cold front through the northern
tier states, potentially spreading 1 to 3 inches over the sensitive
area and may lead to isolated areas of flooding.


Campbell


Day 3
Valid 12Z Thu Jun 13 2024 - 12Z Fri Jun 14 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR SOUTHERN
FLORIDA...

...20Z Update...

No changes were necessary for the previous D3 issuance as model
continuity maintains the areas over the central Midwest and Florida
as the primary areas of note for flash flood concerns. Recent bias
corrected QPF across the southern FL Peninsula remains aggressive
with totals approaching 2-4" across the southwest urban corridors
with 1-3" expanding further inland. This will likely lead to some
areas in FL seeing over 12" of precipitation in the span of 3-days,
a substantial amount even with a consideration for it being in
Florida. Rates are still a factor in maintaining the area in a SLGT
risk as the FFG`s, while expected to decrease, will likely remain
within the 2.5-3"/hr range for any expected flash flood threat
keeping the threat tied to the coast for greater impact. As of now,
there was not enough of a signal to warrant an upgrade.

The central Midwest through IL/IN will see the pattern evolve
upstream to overhead by Thursday afternoon and evening with a
shortwave progression bisecting the region along with a cold front
moving through on the base of a low moving through the northern
Great Lakes in to Ontario. Local totals of 2-3" are possible across
parts of the aforementioned area with the heaviest rain likely
tied to convection along the frontal boundary. The threat is still
a question mark on the latitudinal extent of the precip field as
much of the setup is contingent on the placement of the front.
Ensemble guidance is generally impacting northern IL down into the
central portion of IL/IN, so there was no reason to deviate from
the previous forecast. Thus, the MRGL was maintained over the same
general area from the past forecast.

Kleebauer


...Previous Discussion...

...Florida...

Little change in the environment and pattern across FLorida.
Thunderstorms will continue to focus across South Florida and the
Keys bringing another 1 to 3 inches to the area. WPC has 3-day
forecast accumulation of 2 to 12 inches from north to south across
the peninsula with isolated higher amounts. A Slight Risk is in
place from Tampa Bay to Melbourne metros and south to the Keys. A
Marginal Risk area covers central Florida.

...Midwest...

Thunderstorms will pass from the Upper Mississippi Valley into the
Midwest/western Ohio Valley as the cold front progresses eastward.
Areal averages of 1 to 3 inches will be possible across this part
of the country where recent rains have left soils nearly saturated.
Rain may become excessive and lead to isolated instances flooding.
A Marginal Risk was maintained from the previous day 4 forecast and
spans from eastern Iowa/northeast Missouri to southwest Michigan
and northern Indiana.

Campbell

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