Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
517 PM EDT Sat Jul 10 2021

Day 1
Valid 2102Z Sat Jul 10 2021 - 12Z Sun Jul 11 2021

...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF THE MIDWEST AND OHIO VALLEY...

21z Update: We were able to trim back the western extent of the
MDT risk at this time. The MDT risk remains in place over far
southeast MO, into southern IN/IL and much of central/western KY.
It is here where the best organized risk of flash flooding exists
this evening into tonight. Some west to east training appears
likely on the south and southwest flank of the MCV currently
moving across southern IL. Convection should expand over MO and
southern IL over the next couple hours...and then exhibit some
west to east training as it moves into the MDT risk area. A swath
of 3-5" of rain appears probable...resulting in at least some area
of flash flooding.

Elsewhere, isolated to scattered flash flooding is also possible
within the broad Marginal and Slight risks areas that surrounds
the MDT. One area of focus in the near term is over southwest IA
near a stationary boundary...and another over northeast OK, far
southeast KS and southwest MO along another boundary. The
convection over this latter area will eventually become
progressive off to the southeast as the stationary front
transitions to a cold front and drops south. But in the meantime
slow moving cells will produce pockets of potentially excessive
rainfall into the evening hours.

Chenard

...Earlier Discussion...

...Midwest...
16Z Update: No significant changes with this morning update to the
D1 ERO. A potent baroclinic gradient draped across the region will
serve as a focus for convection this aftn and tonight as mid-level
impulses rotate around a deepening mid-level trough which will
drop southward and close off into D2. Steepening lapse rates will
increase instability especially along and south of the front, with
PWs surging as high as =2 standard deviations from the climo mean.
The convective nature is likely to be chaotic as multiple outflow
boundaries exist, all of which could lead to thunderstorms today,
and the HREF indicates a high probability for 1-2"/hr rates.
Training is likely along the primary front through the evening
where mean winds are aligned to the boundary, with a second or
even third round of thunderstorms moving across parts of the area
to enhance the flash flood risk. The MDT area remains unchanged
for this potential.

Surrounding this region, the MRGL risk was extended slightly SW
into OK/TX where backbuilding along the front tonight through the
impingement of the LLJ may become problematic for some isolated
flash flooding. Additionally, the MRGL risk was extended NE into
WV where enhanced westerly flow on the south side of the diving
trough may push heavy-rain producing convection into the lower FFG
of the terrain there. North of the closed low and into northern
IL/IN, instability will be limited, but the 12Z high-res suite
still suggests some heavier rainfall within a strengthening TROWAL
pivoting westward this evening, so the risk areas were unchanged.

Previous Discussion:
The combination of moist inflow from the Gulf of Mexico, decent
low-level inflow, decent instability present, and a possible 
convective TROWAL forming around a developing mid-level low across
the Midwest leads to an enhanced risk of excessive rainfall across

the region due to multiple rounds of convection.  The guidance
showed reasonable agreement mass field-wise.  Heavy rains from a
week or so ago along with ongoing heavy rain early this morning
should further saturate soils across the region before
afternoon/evening convection develops, which should be
progressive.  The Moderate Risk area was expanded
east-southeastward across southern portions of IL and IN and
western KY to account for the 00z HREF probabilities of 5"+ in 24
hours, which was 40-80% over a broad area.  A broad Slight Risk
area surrounds the moderate to account for possible convective
propagation and movement of the best low-level inflow and
instability outward from the deep layer cyclone.  Southwest MO
appeared to have the more significant uncertainty, as there could
be a couple rounds...remnant activity possibly training at the
dawn of the period and progressive storms later in the day. 
Hourly rain totals to 2" with local amounts to 6" are expected.


...Southwest... 
16Z Update: The inherited MRGL risk remains for D1. A 700mb cap of
temps 18C-21C will erode later this aftn into this evening as flow
flips to the north advecting PWs around +1 standard deviation
according to the NAEFS ensemble tables into western NM and
southern AZ. This should be sufficient in a region of elevated
instability to produce monsoonal convection after 22Z. The 12Z
high res is in decent agreement developing scattered
thunderstorms, primarily across AZ, with the HREF probabilities
showing a low chance for 1"/hr rates dropping slowly southward on
mean winds of just 5-10 kts.

Previous Discussion:
A Marginal Risk of excessive rainfall remains for southeastern
Arizona. The area outlooked was based on the best overlap between
2+ standardized anomalies in precipitable water values and
sufficient instability values (varies by model but likely 1000+
J/kg for CAPE) that could force monsoonal convection causing heavy
downpours.  Temperatures at 700 hPa should be ~15C, like on
Friday, so cell coverage should be no less.  Hi-resolution
guidance was supportive of convective development in the
afternoon/evening.  Hourly rain totals to 1.75" are possible where
cells train and/or merge.


...Western Florida Peninsula...
After coordination with WFOs TBW/MFL, raised a narrow MRGL risk
along the SW coast from Marco Island to near Sarasota. Synoptic
flow from the east around high pressure is likely to drive aftn
thunderstorms westward across the peninsula, encountering the west
coast sea breeze to potentially briefly uptick through mergers and
stall. A plume of 2" PWs measured on the 12Z U/A sounding out of
MFL will advect westward in tandem, combining with sufficient
instability for rain rates to exceed 2"/hr as shown by the HREF
probabilities. Where these storms merge and slow this evening,
rainfall of 2-4" is likely in a few places, with a small chance
for 5". Parts of this area have compromised FFG due to the urban
corridor and recent rainfall from Tropical Storm Elsa, so flash
flooding is possible.

Weiss/Roth


Day 2
Valid 12Z Sun Jul 11 2021 - 12Z Mon Jul 12 2021

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PARTS OF THE
LOWER MISSISSIPPI VALLEY INTO THE TENNESSEE VALLEY, THE UPPER
MIDWEST, AND UPSTATE NEW YORK...

...Midwest...
A closed mid-level low will drop slowly southward across the
region, maintaining a vertically stacked surface low which will
move southward over IL beneath it. To the east of this low,
intensifying deep layer southerly flow will advect rich theta-e
air northward, which is then forecast to wrap cyclonically into a
TROWAL north of the surface-700mb low. During this evolution,
persistent moist advection will drive PWs above 1.5", approaching
+1 standard deviation above the climo mean, within some modest
MUCape of 250-500 J/kg. Rain rates will likely remain below 1"/hr
in the weak instability, but could still add up to 1-2" with
locally higher amounts. FFG across this region is generally
1.5"/1hr and 2"/3hrs, for which the HREF indicates has only a low
chance of exceedance. However, rainfall anticipated on D1 may
lower these thresholds and pre-condition the soils. This combined
with likely training of moderate rates within the TROWAL has
prompted a continuation of the SLGT risk on D2, primarily during
the first half of the day.

Further east towards the Ohio Valley and Mid-Atlantic, moist
advection and increasing mid-level divergence downstream of the
closed low will combine to produce scattered to widespread showers
and thunderstorms as a warm front lifts northward. The recent
guidance has lowered the rainfall over OH/MI, primarily due to a
more rapid progression eastward of the best ascent, and more
displacement from a jet streak over New England. This has prompted
the removal of the SLGT risk near Detroit, MI, but a large
marginal risk was maintained into Pennsylvania and the Central
Appalachians.


...Mississippi Valley into the Tennessee Valley...
A cold front sagging southward from the vertically stacked low
over IL will slow as it approaches the Gulf Coast and become more
zonally oriented from TX to MS, before arcing northeastward into
TN. This front will dive into an environment favorable for
excessive rainfall, with MUCape rising above 3000 J/kg during the
aftn, and PWs 1.75-2 inches, around +1 standard deviation above
the climo mean. As the front sags southward and slows, pronounced
moist confluence will develop along it to drive convective
development, with subtle RRQ jet streak diffluence aiding in
ascent. With mean winds becoming more westerly and aligned to the
front, training is expected to become a concern, with backbuilding
likely into the better instability over TX/OK later in the day.
The guidance is in good agreement focusing an axis of heavy
rainfall from eastern TX through northern AL and into parts of TN
where rain rates of 1-2"/hr are likely, and the HREF EAS
probabilities for 2"/24 hrs maximize over 20%. This resulted in a
subtle expansion of the SLGT risk from inherited despite modest
FFG across this region.


...Upstate New York...
A warm front extending from the low pressure in IL will lift
gradually northward through the Mid-Atlantic and into the
Northeast Sunday evening. This front will be accompanied by
increasing low-level WAA driving PWs towards 2" with a sharp
MLCape gradient accompanying this surge in moisture. These
increasingly favorable thermodynamics will lift into New York late
in the period, with convection blossoming in response to enhanced
ascent through the isentropic upglide of the low-level flow atop
the front, aided by mid-level divergence downstream of the closed
low, and the RRQ of an upper jet streak. Together, these are
likely to create widespread thunderstorms with rain rates in
excess of 1"/hr. While storm motions may be quick to the east,
there is a signal for training from west to east as mean winds
align to the boundary, and the 12z guidance has increased its
rainfall across this region. The HREF indicates a low-end
potential for 3"/6hrs both 00Z-06Z and 06Z-12Z. This occurs atop
saturated soils where FFG is compromised to just 1-1.5"/3hrs,
leading to HREF exceedances for 3-hr FFG approaching 50%. A
consensus among the guidance combined with these favorable
antecedent hydrologic conditions led to the introduction of a SLGT
risk late D2.


...Southwest...
Maintained the MRGL risk with only subtle changes to account for
new 12Z guidance suite. Monsoonal moisture will remain in place
across parts of the Southwest as an upper ridge weakness develops
to the northwest, allowing for an increase in Gulf of Mexico
moisture to stream northward up the Rio Grande Valley. PWs Sunday
are progged to exceed 1.5", 1-1.5 standard deviations above the
climo mean which could support rain rates of 1"/hr or more as
MLCape climbs to 1000-2000 J/kg. There was some coordination with
EPZ about potentially needing a small SLGT risk for the Sacramento
Mountains area where the high res has a signal for higher QPF due
to persistent Gulf moisture and some upslope enhancement
increasing both the intensity and coverage of thunderstorms with
heavy rainfall. However, mean 0-6km wind from the north
approaching 20 kts may preclude a long enough duration of any
storm for the upgrade, so maintained the MRGL. However, if the
signal increases in this area or progs slow more storm mergers and
slower motions, a small SLGT risk may be needed in later updates.

Weiss


Day 3
Valid 12Z Mon Jul 12 2021 - 12Z Tue Jul 13 2021

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
THE MID-SOUTH AND NEW YORK/NEW ENGLAND... 

...Midwest/Tennessee Valley/Southeast...
A front progresses ahead of an opening up mid-level trough that
begins to move eastward.  The combination of high amounts of
moisture and convergence in and near the front should spur heavy
rainfall during the afternoon and evening hours.  Some portions of
the Slight Risk area have had above average rainfall over the past
couple weeks, which has partially saturated soils noted by NASA
SPoRT soil moisture that is in most places above the 90th
percentile, with additional heavy rainfall forecast on D2 to even
further lower the FFG. Favorable thermodynamics ahead of and along
this front include PWs rising to 1.75" in the confluent moisture
ahead of the front, and MLCape exceeding 1500 J/kg. Mean winds
parallel to the front suggest short-term training of rain rates
which may reach 1-2"/hr is possible, and GEFS/ECENS probabilities
show a low-end potential for 3", highest in parts of AL/MS.


...Upstate New York and New England......
Showers and thunderstorms may be ongoing at the start of D3 as a
warm front lifts slowly into the region. This front is likely to
stall across Southern New England during Monday, with persistent
moist southerly flow maintaining a resupply of moisture and
instability northward. MLCape may reach 2000 J/kg in conjunction
with PWs over 1.75", more than 2 standard deviations above the
climo mean. While the morning thunderstorms may somewhat limit
insolation, enough transport of the better instability southward
on persistent southerly low-level flow should allow for
thunderstorms to regenerate through the aftn. Training of these
cells is expected with mean winds paralleling the boundary,
leading to an axis of higher rainfall along the northern MA border
and into Upstate New York. Rainfall of 1-3" is possible, which
will fall atop soils that are above the 98th percentile according
to NASA SPoRT measurements. The inherited SLGT risk was adjusted
just slightly and expanded westward and southward to account for
updated guidance.


...Southwest...
Higher than average moisture across AZ noted by PW anomalies
forecast to reach +2 standard deviations is expected to take
advantage of a weakening Western Ridge and lead to scattered
shower and thunderstorm activity during the afternoon and evening
hours.  Weak 0-6km bulk shear suggests storms will generally be of
pulse variety, but slower storm motions and storm
mergers/interactions could enhance the rainfall threat. Warm cloud
depths on Monday are progged to be slightly lower than previous
days so rain rates could be a bit less than on Sunday, but where
mergers occur these could be enhanced, and locally 1-2" of
rainfall in 1 hour is possible.

Weiss/Roth


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