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Excessive Rainfall Discussion
 
(Latest Discussion - Issued 1549Z Jul 05, 2022)
 
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Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1149 AM EDT Tue Jul 05 2022

Day 1
Valid 16Z Tue Jul 05 2022 - 12Z Wed Jul 06 2022

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
EASTERN MONTANA EAST SOUTHEAST INTO THE CENTRAL APPALACHIANS...

16Z Update...
A complex setup for this afternoon as a series of convective
complexes makes their way across northern portions of the US. Our
Slight Risk area outlined from eastern Montana into West Virginia
has been trimmed off for the western sides as we follow the
convective complex that is already formed and pushes
east-southeast this morning. Hi-res guidance has this complex
continuing its trek eastward through the afternoon into evening
hours across our Slight Risk area heading into a region of higher
instability as it reaches Iowa. Training storms with enhanced
rainfall rates are going to be the main concern this period as
well as antecedent conditions for much of the region from
overnight storms. There was some talk of targeted Moderate Risk
areas within the Slight, but felt this heightened threat will
mostly be through urban/smaller areas and can be addressed with
MPDs as the day progresses. HREF probs for 10 year ARI exceedance
is quite high throughout the outlined Slight Risk area reaching
upwards of 50-60% for some areas. As such, only minor updates were
made for this region mostly to incorporate latest hi-res and
current radar.

Elsewhere, minor tweaks were made to the other Slight Risk through
portions of southeast Louisiana and Mississippi based on latest
HREF probs for 3-hr FFG exceedance and hi-res models. The
outlining Marginal Risk areas for this area as well as the Slight
mentioned above were also adjusted accordingly. No changes were
made to the Marginal Risk through southwest Florida at this time.

Chiari

Previous Discussion...

...Northern Plains through the Mid Atlantic...
A messy and uncertain convective forecast today from MT/ND/SD
southeast across the OH Valley and into the Mid Atlantic. We are
likely to end up with multiple forward propagating convective
clusters today into tonight along this corridor...but exact timing
and placement is uncertain. At 12z this morning expecting a squall
line will be ongoing across portions of IN/OH. While not for
certain, there does seem to be a pretty good chance this activity
maintains as it moves southeast across the OH valley and into the
Mid Atlantic through the day. The line should be fairly quick
moving, which should limit the magnitude of the flash flood risk.
However enough moisture/instability to support a quick 1-2" of
rain...which could cause some localized flooding issues across any
more susceptible areas. The southwest to western flank of this
activity would have the best chance of hanging up some and
allowing for a bit of training and enhanced rainfall totals.

In the wake of this initial MCS, a narrow corridor of high to
extreme instability is forecast, generally centered from SD into
northern IA and eventually towards northern IL/IN and OH...with
upwards of 4000+ j/kg of CAPE forecast. Plentiful moisture is also
forecast along this corridor, with PWs approaching and locally
exceeding 2". This is an impressive overlap of very high CAPE and
anomalous PWs, and certainly an environment that would support
very heavy rainfall rates. The exact timing and location of
convective initiation along this corridor remains unclear. Some
chance ongoing convection over MT survives its trip southeast into
this instability axis, and some chance it does not. Either way it
appears likely that we will eventually see the development of
multiple convective clusters along this instability/moisture
axis...each with the potential for some upscale growth as they
track east southeastward. These clusters should tend to forward
propagate...which would tend to limit the duration of heavy rains.
However the flow is pretty unidirectional across this corridor out
of the west...and Corfidi vectors end up quite weak along the
corridor. This would seem to support the potential of some
backbuilding and/or training of convection on the tail of any
forward propagating MCS. Also, with storm motions parallel to the
instability/moisture axis...need to worry about downstream
development ahead of any convective clusters...which would
increase the chance of cell merging and training.

With all that said, it would appear like a scattered flash flood
risk exists today anywhere from eastern MT into ND/SD, northern
IA, southern MN, southern WI, northern IL/IN and much of OH and
WV. Again, details on the exact timing and placement of potential
flash flooding is low...but the moisture/instability overlap in
place, combined with the likelihood of several convective clusters
along this axis, supports an elongated Slight risk area. Unlikely
all of this corridor sees scattered flash flooding...but too
difficult at the moment to pin down exact areas...so the elongated
risk area encompassing the best ingredients seemed like the best
option. Can not rule out the need for a focused MDT risk if it
becomes more clear that training convection will overlap areas
hard hit over the past 24 hours...with northern IL being one area
to keep an eye on.

...Central Gulf Coast...
A plume of anomalous PWs as high as 2.25" will remain entrenched
beneath a strong mid-level ridge on Tuesday. At the same time, a
modest shortwave is apparent on satellite imagery, and will help
focus convective development with daytime heating today. Despite
the impressive ridge aloft, high instability and these robust PWs
will produce an environment supportive of scattered thunderstorms
with rain rates of greater than 2"/hr. Individually, these storms
pose a limited flash flood threat due to their pulse nature.
However, cells will tend to be very slow moving, which combined
with the pulse nature of convection suggests boundary collisions
and storm mergers are likely. Where this occurs, the heavy rain
could persist much longer, and the HREF suggests a high potential
for areas of 3"+ rainfall, with locally more than 5" possible in a
few locations. Soil saturation across this area is above average
for early July, and thus FFG is also a bit lower than normal.
Given this, it seems probable that the coverage of FFG exceedance
may get up over 15% today, and thus decided to go ahead with an
upgrade to a Slight risk over portions of northeast LA, southern
MS and far southwest AL.

...Southwest U.S....
Maintained a Marginal risk with the monsoonal moisture over the
Southwest. PWs are generally not as anomalous as yesterday, or
other days this season, but still high enough to support isolated
rain rates of 0.5"/hr or locally higher. Weak mid level impulses
rotating northward across the area, along with periods of modest
upper level divergence, should support at least scattered
convective coverage today. Antecedent hydrologic conditions are
more sensitive than normal over much of the area from recent heavy
rainfall...thus where any heavy rain rates can occur and train
today, especially atop recent burn scars, flash flooding is
possible.

...Southern FL...
A weak tropical wave moving SE to NW will lift across the southern
Peninsula Tuesday. Not the strongest feature by any means, but
still enough to push PWs up over 2" and produce a modest uptick in
low level easterly flow. Subtly stronger synoptic flow from the
east will likely limit the temporal duration of any heavy rainfall
across South Florida except on the SW coast where merging with the
sea breeze could stall storms, at least briefly.

Chenard


Day 2
Valid 12Z Wed Jul 06 2022 - 12Z Thu Jul 07 2022

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE UPPER MIDWEST INTO THE OHIO VALLEY...

An expansive ridge centered across the Southern Plains will leave
high heights across a large portion of the country. Along the
periphery of this ridge, generally from the Desert Southwest
northward to the Northern Plains and then dropping east into the
Mid-Atlantic, conditions will be favorable for periodic convective
clusters and potential heavy rainfall.

Th greatest risk for excessive rainfall appears to be from
portions of eastern NE into IA, northern IL, IN, OH and portions
of KY and WV. PW anomalies are forecast to be even higher on day 2
than on day 1...with plentiful instability again forecast. Thus
where convection forms heavy rainfall rates will be likely. Much
like day 1, the pattern will favor forward propagating convective
clusters...however unidirectional westerly flow is resulting weak
Corfidi Vectors...suggesting some potential backbuilding/training
potential. Exact details with regards to convective timing and
placement remain uncertain...but there is much better certainty
that areas within this corridor will see heavy rainfall. Some
indications in the GFS and EC that a slightly stronger mid level
shortwave will be riding across the region Wednesday compared to
Tuesday. Thus certainly some chance the excessive rainfall risk
ends up even more pronounced this day compared to the day 1
forecast.

Used the 24 hr GEFS/ECENS/GEPS 1" rainfall probabilities as an
indication of where the ensemble supports highest chances of
organized convection. When placing the Slight risk area accounted
for the likelihood that the actual convection should probably end
up on the southern edge of these higher probabilities where the
greater instability exists (typical northward bias in global
guidance for organized convection). These probabilities aligned
pretty well with where the environmental ingredients appear to
support heavy rainfall...and thus continue to feel confident
enough to depict a Slight risk area. Given the overlap in
ingredients from day 1 to day 2, can not rule out the need for an
embedded MDT risk as the event nears, if it becomes more clear
that training convection may overlap areas that were hard hit
already on day 1.

Elsewhere in the broad Marginal risk, areas of heavy rain leading
to isolated flash flooding is possible within the Monsoon in the
Desert Southwest, as well as across the Mid-Atlantic where
decaying MCS cresting the ridge in NW flow could trigger
additional convection during peak heating (eastern NC and
southeast VA currently most favored for this). Both of these areas
could experience rainfall rates in excess of 1"/hr, with any
training leading to isolated flash flooding, especially across
burn scars in the Southwest, or within urban areas with lower FFG
in the Mid-Atlantic.

...Southern FL...
The weak tropical wave from Tuesday will continue to trek westward
in the vicinity of the southern FL Peninsula on Wednesday. This
will continue to be trailed by high PWs above 2" and enhanced
ascent overlapping sufficient instability to drive scattered
thunderstorms with coverage a bit beyond a typical July FL day.
Rainfall rates of 2"/hr are again likely, with the highest
probabilities for 3"/24hrs focused along the Sun Coast once
again...as the easterly low level flow favors the west coast of FL
for enhanced sea breeze convergence. Model QPFs are actually
higher Wednesday than Tuesday over this area...so a continuing of
the Marginal risk seems warranted.

...Central Gulf Coast...
Similar setup as day 1...just shifted a bit west in location. A
plume of anomalous PWs as high as 2.25" will remain entrenched
beneath a strong mid-level ridge, with a modest shortwave in the
area helping focus convective development with daytime heating.
Despite the impressive ridge aloft, high instability and these
robust PWs will produce an environment supportive of scattered
thunderstorms with rain rates of greater than 2"/hr. Cells will
tend to be very slow moving, which combined with the pulse nature
of convection suggests boundary collisions and storm mergers are
likely. Where this occurs, the heavy rain could persist longer,
resulting in localized areas of excessive rainfall. Overall the
setup does look to be on a downward trend compared to day 1
(Tuesday), so a Marginal risk looks to suffice (compared to the
Slight on day 1 to the east). Overall the risk this day should be
confined to any more sensitive urban areas across the Marginal
risk.

Chenard

Day 3
Valid 12Z Thu Jul 07 2022 - 12Z Fri Jul 08 2022

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE OHIO VALLEY AND CENTRAL APPALACHIANS...

Similar pattern as day 2 across the CONUS, with a continued heavy
rain risk on the periphery of the expansive ridge centered over
the southern U.S. At least a localized flash flood risk looks to
exist from NM to MT, east across the central Plains and Mid MS
Valley and into the OH Valley and Mid Atlantic...with this
corridor again seeing above average PWs/instability, along with
conditions favorable for scattered convective development.

The large scale forcing this period does seem to be a bit stronger
from the OH Valley into the Mid Atlantic and Northeast, with both
the EC and GFS indicating a more pronounced mid level wave, upper
jet and low level frontal passage across this corridor. Seems
likely that another day of organized convection will result,
leading to areas of heavy rainfall and potential flash flooding.
The 00z EC made a decent northward shift with it's QPF axis, with
the UKMET quite north as well. With that said, still think the
better excessive rainfall risk ends up on the southern periphery
of these model solutions, where the better PW/CAPE overlap is
situated. Not uncommon to see a northward QPF bias in the global
models at this lead time in cases of organized convection...so not
jumping on to the northern shift a this point. Instead opted to
focus a Slight risk across portions of the OH Valley into the
central Appalachians....which aligns fairly well with the 00z GEFS
and 12z ECENS 1" QPF probabilities. By this time portions of this
corridor may have saturated ground conditions, given the
convective forecast on both days 1 and 2. The continuous nature of
this risk over the next 3 days does indicate some higher end flood
potential could evolve at some point. So can not rule out the need
for Moderate risk areas by day 3 (possibly even day 2), but
certainly not enough confidence at this point to pin down that
threat area. But we will continue to closely monitor convective
trends over the coming days from IL/IN to WV.

Chenard


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