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Excessive Rainfall Discussion
 
(Latest Discussion - Issued 2032Z Jul 04, 2022)
 
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Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
431 PM EDT Mon Jul 04 2022

Day 1
Valid 16Z Mon Jul 04 2022 - 12Z Tue Jul 05 2022

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PARTS OF THE
FAR WESTERN GREAT LAKES REGION, EASTERN MONTANA AND PORTIONS OF
ARIZONA AND NEW MEXICO...

...Upper MS Valley into the Great Lakes...
16Z Update...
MCS moving across the region this morning is a bit south of most
available guidance, suggesting the inherited SLGT risk which is on
the southern edge of the greatest HREF probabilities continues to
be supported. This MCS is associated with a mid-level impulse
embedded within modest divergent flow downstream of a longwave
trough across the Intermountain West and Pacific Northwest. This
shortwave will continue to shift east through the evening,
coincident with a veering of the 850mb LLJ from SW to W, driving
more westerly moisture transport into the SLGT risk area. While
the impulse should remain progressive, there is a continued threat
for backbuilding later this aftn/eve as Corfidi vectors begin to
turn into the veering moist flow, while MUCAPE climbs above 2000
J/kg. This indicates the likelihood for storms to develop into the
greater instability then train east ahead of a cold front. PWs
above 2", more than 2 sigma above the climo mean, combined with
this instability will support rain rates of 1-2"/hr, creating
modest HREF exceedance probabilities for 3-hr FFG. There continues
to be a lot of spread amongst the high-res, and in general the
guidance has backed off slightly on the heavy rain threat.
However, the HREF EAS probabilities suggest at least a moderate
threat for 3" of rain focused near the IL/WI border, and the SLGT
risk was only modestly adjusted for flash flood potential in any
training and across urban areas.

Previous Discussion:
The morning activity will generally be progressive off to the
east...however as the day progresses we will see an increasing
axis of low level moisture transport pointing into the area of
convection from the southwest...along with an increasing plume of
upstream instability. Thus there is some potential we see
backbuilding upstream development into this low level
jet/instability axis on the west or southwest flank of the
eastward moving convection. Given the forecast location of this
enhanced low level moisture transport...somewhere from southeast
MN into central WI seems most favored for this potential
development through the day into tonight. If this upstream
development does occur, Corfidi vectors become quite weak
underneath this low level jet axis suggesting the potential for
continue backbuilding and an eventual southeastward propagation.

Quite a bit of uncertainty remains with this, but the inherited
Slight risk looked to be in pretty good shape given the above
mentioned ingredients. If there does end up being a more organized
flash flood risk it will probably be somewhere from southeast MN
into central/southern WI and eastern IA to northern IL. This
aligns well with the 00z HREF 3"+ rainfall probabilities too.
Would say this is a below average confidence forecast for a day 1
ERO...made even more so by the rather poor model handling of
ongoing convection early this morning (given it's likely role in
how things play out later today into tonight). Thus we will
continue to monitor and make any needed adjustments with our 16z
update today.


...Southeast U.S. and northern Gulf Coast...
16Z Update...
Another day of slow moving thunderstorms is expected across much
of the Southeast. A mid-level ridge which will slowly build
overhead may limit the coverage compared to previous days, but
what does develop will be very slow moving as noted by 0-6km mean
winds of just 5-10 kts. The thermodynamics will remain very
supportive of heavy rainfall, with PWs over 2", 1.5-2 sigma above
the climo mean, and MLCAPE approaching 2000 J/kg. While a lack of
share suggests generally pulse type convection with limited
temporal duration, outflow boundaries, sea breeze, and storm
mergers could produce locally longer duration of heavy rainfall
which could reach 3-4" in some locations due to rainfall rates
which are forecast by the HREF to exceed 2"/hr. While pinpointing
the greatest, but still isolated, flash flood risk is challenging
in this setup, there may be two areas of slightly greater risk.
The first is along the central Gulf Coast where recent heavy
rainfall has driven 40cm soil moisture above the 90th percentile
according to NASA SPoRT. The other is across the Carolinas where
subtly enhanced upper divergence will increase ascent, leading to
more coverage of convection which could merge with the sea breeze
at times. However, any location within the MRGL risk area could
experience a local flash flood instance beneath any of the most
intense slow moving storms.


...Southwest U.S....
16Z Update...
The monsoon continues today across the Southwest. PWs have climbed
a bit higher today, forecast to reach more than 1.5 sigma above
the climo mean by this aftn, highest in AZ and NM. This moisture
combined with CAPE forecast to reach at least 1000 J/kg this aftn
will support scattered to widespread convection with rain rates
reaching 0.5"/hr according to the UA WRF, possibly locally as high
as 1"/hr as progged by the HREF probabilities. General storm
motion will be 10-15 kts from the SW, with some training possible
through aligned Corfidi vectors. Additionally, especially early in
the aftn, some convection may become tied to the upwind side of
terrain leading to slower storm motions that could be nearly
stationary at times. While the high-res is modest overall in its
depiction today, the inherited SLGT risk was maintained with just
subtle cosmetic adjustments to account for the latest HREF
probabilities atop extremely saturated soils.

Previous Discussion:
Additional showers and thunderstorms are expected to fire once
again over portions of the Southwest U.S. given the rather
continuous presence of a plume of monsoonal moisture. PWs look
even higher today over portions of southeast AZ into western
NM...and also note potential shortwave energy moving north into
the region and some right entrance region enhanced upper
divergence as well. So some potential we end up with a bit more of
an organized heavy rainfall threat today over portions of
southeast AZ into western NM. Given this potential, combined with
saturating soil conditions in the area and some sensitive burn
scars...we went ahead and upgraded to a Slight risk for this
region...with the flash flood risk today appearing a bit higher
than the typical Monsoon day (assuming cloud cover this morning
can given way to some heating and destabilization).


...Northwest to North central U.S....
16Z Update:
Closed low centered over the Pacific Northwest will maintain
downstream divergence across the Intermountain West and into the
Northern High Plains today and tonight, with weak mid-level
impulses embedded within the flow rotating overhead. Additionally,
a modest jet streak lifting northeast will leave favorable RRQ
diffluence for ascent, which combined with the mid-level forcing
should produce scattered shower and thunderstorm development. PWs
increase to 1-1.25" as the low-level flow turns easterly
supporting better moisture transport into the region. Each
mid-level impulse could lead to clusters of convection, with a
possible MCS this evening/tonight. While these storms should
remain progressive, rain rates will likely exceed 1"/hr as
forecast by the HREF, and there exists at least a low-end training
threat tonight. This region has been wet recently with pockets of
200% of normal 7-day rainfall, and the SLGT risk was adjusted only
cosmetically, with similar adjustments for the MRGL risk atop
drier soils with less pronounced forcing.


Chenard/Weiss


Day 2
Valid 12Z Tue Jul 05 2022 - 12Z Wed Jul 06 2022

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE OHIO VALLEY TO THE CENTRAL APPALACHIANS AND OVER EASTERN MT
INTO THE WESTERN DAKOTAS...

...Ohio Valley to Upslope Regions of the Appalachians...
A cold front slowly sinking southward through the day will lay up
east to west as it settles into the Ohio Valley and Central
Appalachians. This evolution will be driven by an expanding ridge
to the south but flattening flow to the north, causing the front
to stall the latter half of D2. Convergence along this front will
drive ascent, aided by broad upper diffluence in the tail of a
northern stream jet streak. This will drive veering 850-500mb
winds, becoming more aligned to the west, and parallel to the
boundary. Convection developing along the front will be fueled by
MUCAPE climbing to 3000 J/kg and PWs of 1.75-2 inches, approaching
+2 sigma above the climo mean. As the flow turns more westerly, it
will not only drive a better chance for training, but resupply
more favorable thermodynamics to persist heavy rainfall and limit
convective overturning to exhaust instability. This suggests an
increasing flash flood risk despite mean storm motion of more than
20 kts, as training could lead to multiple rounds of heavy rain
across the area. While the guidance has backed off a bit on the
coverage of thunderstorms, lowering the confidence in flash flood
potential, the HREF still has a 20-40% chance for 3" in 24 hours,
with rain rates above 1"/hr moving atop compromised FFG of
1.5"/3hrs, especially in WV. The inherited SLGT risk was trimmed
slightly, but maintained for continuity across the lowest FFG. It
is possible this SLGT risk could be removed all together if the
future high-res continues this decline in potential.


...Southwest U.S....
The inherited MRGL risk was adjusted slightly to the NW, but
overall features minimal changes. A monsoon plume of moisture with
PWs 0.75-1", about 1.5 sigma above the climo mean, but lower than
on D1, will persist across the Southwest D2. Nearly unidirectional
SW to NE mid-level flow around the expansive ridge to the east
will continue to surge northward, within which weak impulses will
rotate northward to drive locally enhanced ascent, aided by
periods of modest upper divergence. The mean storm motions are
progged to bee 10-15 kts to the NE, weaker to the east closer to
the ridge axis. This could lead to short term training of rain
rates of 0.5"/hr or locally higher. Soils across the region are
saturated above the 98th percentile from 600% of normal 7-day
rainfall, leading to low FFG. Where any heavy rain rates can occur
and train, especially atop recent burn scars, flash flooding is
possible.


...MT into the Upper Midwest...
A stationary boundary wavering across MT Tuesday will combine with
upper divergence downstream of a trough over the Pacific Northwest
and RRQ diffluence in the tail of a departing jet streak to
produce pronounced deep layer ascent across the High Plains.
Impulses embedded within this flow will each have the potential to
produce clusters of convection /possible MCS/ within favorable
overlap of PW and instability. Convection that develops will
likely maintain fast forward progression noted by 0-6km mean winds
of 20-30 kts. However, impressive rain rates of more than 1"/hr
have the potential to train west to east with multiple rounds of
thunderstorms possible. This heavy rain will fall atop soils that
are already saturated from 7-day rainfall that is 150-200% of
normal, leading to FFG that is just 0.75-1.5"/3hrs. The recent CSU
first guess field indicates a high-end SLGT for eastern MT where
soils are most saturated, and after coordination with GGW and BIS,
the SLGT was modified just cosmetically for updated guidance.


...Southern FL...
A weak tropical wave moving SE to NW will lift across the southern
Peninsula Tuesday. The guidance has backed off on the strength of
this feature, but this wave moving westward will still surge PWs
above 2", coincident with MUCAPE climbing to 2000 J/kg or higher.
Thunderstorms developing in response to the high instability and
within the enhanced ascent downstream of the wave will likely have
rainfall rates of 2"/hr or more as reflected by HREF
probabilities. 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. The HREF
probabilities for 3"/24 hrs have increased to more than 50% across
this area, but fallen elsewhere, with the FFG exceedance following
a similar pattern. After coordination with MFL and TBW the MRGL
risk was trimmed to just the southern half of the Sun Coast and
far western South Florida.


...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 rotating beneath the ridge will lift onshore the
Central Gulf Coast, lifting into Louisiana late D2. 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,
storm motions are progged to be just 5-10 kts with even weaker
Corfidi vectors, 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 3" of rainfall,
with locally more than 5" possible in a few locations. This could
lead to isolated instances of flash flooding.

Weiss

Day 3
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...

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, lower heights, pinched flow, and robust moisture
transport will provide a large area of favorable conditions for
heavy rainfall.

The greatest risk for heavy rainfall and flash flooding appears to
focus in the Upper Midwest to the Western Great Lakes. Here,
impulses embedded within the westerly flow atop the ridge could
produce multiple MCS, one decaying early D3 and another Wednesday
night. Each of these MCS has the potential to produce excessive
rain rates above 1"/hr with training potential along the southern
flanks of these features. PWs are forecast to be +1.5 to +2 sigma
above the climo mean, with ample instability surging northward on
the LLJ. While there is still uncertainty into the latitudinal
placement of these convective clusters, the SLGT risk was drawn a
bit south of the highest ECENS/GEFS 24-hr probabilities for 3" to
account for the potential southward displacement into the better
instability. This area was favored for the SLGT risk due to the
potential for two rounds of heavy rainfall on D3, atop pockets of
lower FFG and pockets of above normal antecedent streamflow
according to USGS.

Elsewhere in the broad MRGL 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 training convection during peak heating. 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 wave shifts westward there may be some subsidence and lower
moisture in its wake, so the MRGL was again trimmed to just the SW
coast of the FL Peninsula.

Weiss


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