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Excessive Rainfall Discussion
(Caution: Version displayed is not the latest version. - Issued 0010Z Jul 11, 2024)
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Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
809 PM EDT Wed Jul 10 2024

Day 1
Valid 01Z Thu Jul 11 2024 - 12Z Thu Jul 11 2024


...01Z Update...
Main change to the on-going Excessive Rainfall Outlook was to trim
off areas where rainfall has ended. Maintained the Moderate Risk
area with only a minor eastward adjustment to the boundary based on
latest radar imagery from the area. Within the moderate risk
area...though...the signals have started to become stronger for 30
to 50 percent probabilities for 100 year ARI exceedance which is
increasing the concern for occurrences of impactful flash flooding
(especially through about 11/06Z). MRMS has been showing hourly
rates of 1.5 to 2.5 inches in an hour with the potential for
similar rates persisting through the evening and into the overnight
hours...resulting in 3 to 5 inch amounts and isolated higher
totals. Refer to WPC Mesoscale Precipitation Discussion 604 for
more details on the mesoscale aspects. Overall...though...the risk
of excessive rainfall should taper off during the overnight hours
as better dynamics shift away from the area.

Across New Mexico and the adjacent southern High plains...kept the
Slight Risk focused over the Sacramento mountains given their
hydrologic sensitivity...although the expectation is that the
areal coverage and rainfall intensity will diminish with the loss
of daytime heating.


...16Z Update...

Minor adjustments were made to the MDT risk with an expansion to
cover the northeast coast off Lake Ontario near Watertown/Fort
Drum, and took the risk area to the Canadian border. The 10 and
100 year ARI exceedance probabilities off the 12z HREF were very
impressive across much of North Country through Northern VT with
>50% 10 year ARI exceedance signatures and 30-50% centered over the
Adirondacks. Totals between 2-4" are very likely for a large
portions of Upstate NY through Northern VT/NH with maxima around 6"
still expected. Further to the southwest over MI, a targeted heavy
rain threat across Southeast MI will linger through 18z before
moving out into the neighboring Ontario Province as the remnant
circulation from Beryl continues to progress northeast. The risk
areas over the Ohio Valley and MI were adjusted to conform to
current radar trends and forecasted precip next 4 hrs.

There were no changes necessary over the Southwest U.S, mainly in
NM where scattered convection will exacerbate flash flood threats
within the multitude of burn scars present from the Sacramentos up
through the north-central part of the state. Rates will top between
1-1.5"/hr at peak intensity in the strongest cells, more than
capable of causing issues within any burn scars and smaller towns
within the complex terrain.

Scattered heavy thunderstorms continue to impact the Central Gulf
coast with rates reaching upwards of 2-3"/hr in stronger pulses.
FFGs are sufficiently very high and the threat for flash flooding
will be highly localized, mainly to the southern edge of New
Orleans over to Biloxi. The threat will diminish across the area
after 00z so have maintained the targeted MRGL for continuity
purposes given the current radar depiction.


Remnant moisture and mid-level energy from Beryl will translate
northeastward out the Ohio Valley and Great Lakes creating a
heightened threat for widespread heavy rain and flash flooding
across Southeast Michigan through New York and Central/Northern
New England. At the surface, a warm front will bisect much of New
York through Central New England, marked very well by a sharp
theta-E gradient. This will be a focal point for convection prior
to the main circulation moving overhead, as well as the axis of
where the highest PWATs will be confined during the unfolding of
the synoptic scale event. Low- level instability will be highest
within the confines of the warm front and points south with the
best upper level forcing likely along and north of the boundary
creating a dynamic scenario with a zone of highest heavy rain
potential where all three characteristics overlap. The highest
heavy rain prospects lay within the Adirondacks and points east
into north-central Vermont and New Hampshire, including the Green
and White Mountains in the respective states.

12z HREF EAS probabilities were much more aggressive in the signals
for at least 2" and 3" across the aforementioned areas with a
50-80% probability for at least 2" within the Adirondacks to just
south of the Champlain Valley, a strong signal for higher totals
given the necessary overlap of CAMs to exhibit such a larger
probability. Historically, when an EAS signal is above 70%, the
expectation for widespread coverage of that value of rain or more
is very certain and regardless a higher risk consideration if
the areal FFG indices allow. HREF EAS for 3" managed to come in
more aggressive with a large coverage of 25-40% with the highest
potential across the Adirondacks and northern Mohawk Valley
characterized by a targeted 45-50% signature. This is right within
the inflection of where the warm front is forecast to reside,
creating a zone of higher confidence for heavy rainfall.

Heavy rain will be possible all the way into Maine where the
elevated PWAT anomalies between +2 to +3 standard deviations will
be recognized leading to a higher end SLGT risk residing from
Southeast Michigan all the way into western and central Maine. Totals
of 2-4" are anticipated with local maxima to 6" plausible within
the above areas in the Moderate Risk. 1-3" will be possible as far
south as central and northeastern PA up through the NY Capital
District due to convection developing along and ahead of the
trailing cold front moving through the area tomorrow afternoon and


...Previous Discussion...

...Central Rockies and portions of the Southwest...

A Slight Risk for excessive rainfall remains in effect across the
higher terrain of central New Mexico. During this period monsoonal
moisture and diurnal heating will maintain convection across
portions of the Central and Southern Rockies and southwest New
Mexico. Some of the highest forecast QPF is expected to focus in
the vicinity of the Sacramento Mountains where there have been a
few recent wildfires. he burn scar complex
(Blue-2, South Fork, Salt and McBride) are highly sensitive to
rainfall and can easily lead to flooding and debris flows,
especially with the 0.25 to 1 inch QPF that is forecast.

...Central Gulf Coast...

Training convection expected to continue along the stalled frontal
boundary. Very high FFGs within the area highlighted by a Marginal
Risk which will limit flash flooding to very isolated and mainly
within larger urban zones like New Orleans and towns based in the
southeastern Parishes. Given the limited threat, should the
convection shift further south offshore there may be the need to
downgrade in future updates.


Day 2
Valid 12Z Thu Jul 11 2024 - 12Z Fri Jul 12 2024


...20z Update...

Previous MRGL risk across the east coast was broken into two
different areas; Northern ME and the coastal Mid-Atlantic down
through the eastern Carolinas. An additional MRGL risk was issued
across the Central Midwest (More on this threat in the "Midwest"
sub-heading below). There were no necessary changes to the
orientation of the MRGL risk across the Southwest as guidance
remains steadfast on a highly isolated convective regime with the
primary areas of interest being the burn scars in NM and small
urban threat of towns within complex terrain across Southeast AZ
through much of NM.

...Northern Maine...

Remnants of Beryl will finally eject out into the Canadian
Maritimes, but some lingering convective threat across Northern ME
could induce some local flash flood concerns, mainly within the
Caribou CWA. Ensemble QPF remains heaviest across Downeast ME where
the highest PWAT anomalies remain leading to some deterministic
output indicating 1-2" of rainfall within the first 3-6 hrs of the
D2 period. This is subject to some variability as some guidance has
the bulk of the precip out of the region by 12/13z tomorrow which
would constitute a nil for the ERO proposal. Considering the
environment and ensemble output, decided to maintain continuity in
the outlined areas, but removed areas further south where dry air
advection is more likely and flash flooding threat is next to zero.

...Coastal Mid Atlantic...

Stalled frontal boundary will bisect portions of the eastern sea
board with elevated PWATs mirroring the alignment of the front to
points east. Local environment will be primed for convective
potential, but will have some assistance in upper level support as
a meandering wave off the Southeast coast lifts northwest on the
western fringe of the ridge in the Atlantic, acting as a beneficial
forcing mechanism to enhance regional rainfall potential. The best
chance will lie from the Southern Delmarva through the Virginia
Tidewater down into the Eastern Carolinas. Latest HREF
probabilities are fairly aggressive in their signature for local
totals between 2-5" as neighborhood probs for 2-3" totals are
running between 60-90% with an area of 5" probabilities between
50-70% across Eastern NC. This area has been very dry as of late,
and is well reflected within the FFG indices present for 1/3/6 hr
markers. Sandier soils located within the area encompassed by the
MRGL risk will deter a higher threat for flash flooding, however
isolated urbanized flooding is still plausible considering the
potential for 2-4"/hr rates as per the latest HREF mean hourly QPF
output. The MRGL risk was maintained in this update for the
aforementioned area, but cut out the extent of NNJ and NYC/LI due
to drier air advecting overhead, limiting the threat of flash
flooding to basically zero.


Trailing shortwave trough pivoting southeast around the backside of
the mean trough over the Great Lakes and Ohio Valley will
strengthen as it moves across IA into IL by tomorrow afternoon with
large scale forcing increasing across the Central Midwest and
adjacent Mid-Mississippi Valley. Scattered convection will develop
over central and southern IL, MO, over into IN by late tomorrow
afternoon through the evening with a few stronger cores possible
given some modest buoyancy and favorable mid-level ascent focused
in-of the region. 12z HREF signatures for heavy rainfall point to a
more locally impactful setup with less organized convection, but
rates of 1-2"/hr possible, heavy enough to breach the current FFG
indices located over the area after getting impacted from Beryl.
Antecedent moisture in the top layer of the soils is well-defined
with NASA SPoRT moisture percentiles relaying the 0-10cm layer
running between 75-90% leading to higher runoff capabilities.
Typically a setup like this wouldn't cause too much concern in the
area of focus, but the recent rains push this setup over into the
low-end MRGL threshold thanks to the expected rates in convection
and primed soils. A MRGL risk was introduced across portions of the
Central Midwest into MO.


...Previous Discussion...


The persistent monsoonal pattern of diurnal convection will
maintain an elevated threat for localized heavy rainfall and flash
flooding concerns. Local totals of up to 1" signal a lower-end
threat, however it remains within the Marginal Risk threshold.


Day 3
Valid 12Z Fri Jul 12 2024 - 12Z Sat Jul 13 2024


...20Z Update...

A SLGT risk was maintained from previous forecast as a second round
of convection along a stalled frontal boundary will lead to
scattered instances of flash flooding, mainly within urbanized
areas and areas that see repeat heavy rains from the period prior.
Updated CSU First Guess fields maintain the alignment of a SLGT
risk across the Eastern Carolinas with a MRGL up through the
Delmarva into South Jersey. The favorable surface and mid-level
ascent pattern will continue through the D3 time frame with the
ridge flexing back further west from the Atlantic leading to
elevated moisture and mid-level ascent being pushed further inland
of the Mid Atlantic. It's possible the northern extent of the SLGT
could be scaled back given some small trends in lower QPF focused
north of the Virginia Tidewater. This will be the area to monitor
for future adjustments. The Eastern Carolinas are still the primary
target for heavier rainfall with some areas in the two day period
potentially receiving over 5" from the evolving event.

Wanted to also make mention of monitoring the progression and
convective evolution within a migrating inverted trough axis
through South TX into the Rio Grande. Current ensemble forecast is
relatively tame within the realm of QPF, however the environment is
more than favorable for higher convective impacts, including heavy
rain within the Rio Grande and points just inland. First Guess
fields do pinpoint a small MRGL located within the corridor from
Eagle Pass down through Laredo, so perhaps if guidance ramps up the
QPF signal in future updates, there could be a targeted MRGL risk
area added to the central RGV. For now, decided against the
addition as the threat remains non-zero, but outside the 5%
threshold for issuance. Another area we'll be monitoring in future


...Previous Discussion...

The training of thunderstorms capable of producing several inches
of rain will continue through this period. Rain from the day 2
period will have helped to raise soil saturation along the Mid-
Atlantic and Carolina region thus lowering FFG. Most of the
guidance is suggesting additional amounts of 1 to 3 inches from
South Carolina to southern New Jersey with a couple showing local
maximums up to 5 inches along this corridor. A Slight Risk was
maintained for this part of the East Coast. A Marginal Risk spans
from South Carolina northward to southern New hampshire.


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