FOUS30 KWBC 262043

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
442 PM EDT Mon Jul 26 2021

Day 1
Valid 16Z Mon Jul 26 2021 - 12Z Tue Jul 27 2021


...Southwest/Central Rockies/Southern Rockies...
A weakening mid level feature will continue to track west and
eventually northwest with its energy becoming somewhat sheared.
However, given the anomalously high moisture within the column and
thermal profile, it will not take much lift to produce convection
and thus heavy rain across portions of Southeastern CA, Southern
NV into far Southwestern UT as seen in CA this morning. Observed
precipitable water values are as high as 2.20+ inches in western
AZ and anticipate this to advect to the west through tonight. This
is over 3 standard deviations above the mean and with enough
mid-level ascent and building instability today, expect convective
coverage to blossom again in the afternoon with rain rates ranging
between 1-2 inches/hour. Areal average precipitation will range
between 1-2.5+ with locally higher amounts possible. This type of
rain over pockets of wet antecedent conditions, complex terrain
(including slot canyons and washes), and residual burn areas could
cause more scattered flash flooding, some of which could result in
significant impacts. The Slight Risk area was maintained.

Farther to the east, anticipate convection to develop along the
terrain with differential heating as breaks in the clouds occur
through the morning hours. More scattered precipitation activity
is anticipate under lower precipitable water values. Thus, expect
rain rates to also be less, around 0.5-1+ inches/hour.  Therefore,
isolated flash flooding is possible across more vulnerable soils. 

...Lower MS Valley/TN Valley/Southeast...
The residual surface boundary will slowly sink south toward the
Southeast and Deep South helping to promote convection along and
south of the front, especially focused across portions of NC/VA
where better 700mb lift resides associated with the sharpened
trough axis. Precipitable water values will surge to over 2 inches
aided by moist southerly flow. In addition, instability will also
climb above 2000 J/kg through the afternoon, aiding in the
production of scattered to widespread convection. With better
dynamical forcing (also noted by enhanced divergence within the
right entrance region of the upper jet), anticipate more
precipitation coverage across the Southern Appalachians northeast
into the Piedmont of NC/VA and southeast VA. Given the warm cloud
depths within the entire risk area (both Marginal/Slight),
efficient rain rates are expected with hourly totals surpassing 2
inches in some locations. Storm motions will be quite nebulous
across the Lower MS/TN Valleys. However, slightly higher storm
motions of 10+ knots with the potential for cell mergers should
occur across NC/VA and may lead to higher rainfall totals overall.
12Z HREF continues to show moderate to high neighborhood
probabilities for accumulations of 3-inches or more within the
Slight Risk area, including the the Richmond and Hampton Roads

While antecedent soil conditions vary quite a bit within this
region, the Slight Risk area captures the location where more
organized convection will likely occur over soils that may be more
susceptible to flash flooding. Meanwhile, very saturated soils
exist across much of GA/AL/MS, though convection may be more
scattered/less organized. There is a modest signal for more
organized convection across southern TN/northern AL through the
afternoon given a weak impulse aloft, but confidence on this
remains too low to hoist a Slight Risk.

Bumped the Marginal Risk north a bit over the Delmarva Peninsula
due to increasing instability this evening with lingering moisture.

...Upper Great Lakes...
A mid-level impulse currently over southern Saskatchewan will
shift southeast along the northern periphery of the ridge and
across the Upper Great Lakes tonight. Ahead of this wave,
southerly flow will increase allowing precipitable water values to
surge to over 1.50 inches. Instability will rise above 1000 J/kg
through tonight which should support thunderstorms capable of
dropping 2 inches per hour rainfall. Anticipate scattered
convection to develop in the late afternoon over northern MN to
become better organized as shear improves. With the mean wind
aligning with the cold front, expect convection to potentially
train across some locations, especially as the nocturnal low level
jet strengthens on the nose of the 850mb front around northern
Lake Michigan. Therefore, expanded the Marginal Risk east from
northern WI and the UP of MI to across the northern LP of MI.

...Coastal Southeast...
Convection associated with a low moving inland along the FL/GA
line will allow local rainfall of 2-4+ inches across northeast FL
toward coastal GA. More isolated to scattered convection will
surge inland with onshore moist flow into far southern SC through
the afternoon. Rain rates associated with this system will range
between 1-2+ inches. Maintained the Marginal Risk for isolated
flash flooding potential. 

...Florida Peninsula...
A surface wave along the FL/GA line this morning, supplies a
tropical airmass across much of the region through the forecast
period. In addition, a ridge to the Southeast will continue to
feed moist southerly flow into the FL Peninsula. With precipitable
water values above 2.25 inches, diurnal instability, and sea
breeze convergence, expected scattered to widespread convection to
develop this afternoon and linger through the evening hours. Given
weak steering flow and the potential for converging sea breezes,
conditions look favorable for heavy rain to develop inland or
across the western peninsula (given a strong Atlantic sea breeze
could pin against the GOM sea breeze). Hourly rainfall totals will
range from 1.5-2.5+ inches. This could become problematic,
especially over the urban corridors.Therefore, a Marginal Risk
area was maintained for localized flash flooding across much of
the peninsula.


Day 2
Valid 12Z Tue Jul 27 2021 - 12Z Wed Jul 28 2021


With a surface low tracking along a surface boundary that
stretches from central GA up the Carolina Coast, raised a Slight
Risk for southern GA and southern SC (up to Charleston).
Widespread convection is expected with precipitable water values
exceeding 2.25 inches, which is 2.5 standard deviations above the
mean. With low level convergence from a front coming from the
north and light westerly deep layer mean flow, expect heavy
activity to be slow moving and potentially repeating. With hourly
rainfall locally exceeding 2 to 3 inches in the enhanced diurnal
convection, decided a Slight Risk was warranted. There is an
isolated flash flood risk this convergent axis, from southeast VA
to east TX, so the Marginal Risk was expanded as well.

...Great Basin through the Southern Rockies...
Monsoonal moisture will bring the next round of showers and
thunderstorms across the Intermountain West. Areas west of the
ridge axis will continue to see well above normal moisture,
particularly over the Great Basin and over far north CA into
central and eastern OR where precipitable water values of 1" to
1.25" are 2 to 3 standard deviations above normal. Largely
maintained the expansive Marginal Risk over terrain from central
OR to southern AZ by way of UT in accordance with the 12Z

A northern stream trough will pivot through the region Tuesday
night with an associated cold front becoming oriented northeast to
southwest opening the window for training convection. Ahead of the
trough axis, southerly flow will increase ushering in higher
precipitable water values which will exceed 1.75 inches aided by
35+ knot 850mb west-southwesterly winds. This low level flow will
become aligned with the frontal zone allowing for multiple rounds
and/or training convection over any given location. With
instability climbing above 1000 J/kg and enough mid/upper level
lift, convective coverage should be widespread. With the available
moisture, instability, and large scale lift, expect hourly rain
rates to exceed 1.5 inches in some locations. Due the progressive
nature of the activity, only areas of repeating convection should
have excessive rain, so the flash flood risk should be isolated.
The Marginal Risk area was maintained and expanded a bit to
include all of the eastern Great Lakes. 


Day 3
Valid 12Z Wed Jul 28 2021 - 12Z Thu Jul 29 2021


...Great Basin/Intermountain West...
Monsoonal moisture continues to surge north across the region with
weak mid-level impulses helping to focus convection farther north
than has been observed in weeks. This becomes no more evident than
by the precipitable water values remaining above 1.25 inches in
some locations under broad southerly low level flow. With plenty
of diurnal heating, anticipate MUCAPE to climb above 1000 J/kg
allowing for differential heating across the terrain. Therefore,
afternoon convection will largely be tied to the mountains and
eventually shift off into the valleys/deserts through the late
afternoon/evening. Hourly rain rates could reach between 0.5-1
inches, though isolated higher amounts are possible, especially
with slow moving convection. Given the complex terrain, wet
antecedent conditions across the Southwest/Central-Southern
Rockies, and burn scars, localized flash flooding could occur.
Therefore, the Marginal Risk maintained.

...Upper Midwest/Great Lakes...
The next shortwave trough will cross the region through the
forecast period with enough synoptic scale lift, moisture, and
instability to promote widespread organized convection capable of
producing localized flash flooding. Ahead of this trough, deep
layer moisture will surge north on strong return flow. In
addition, instability will climb well above 3000 J/kg through the
afternoon hours across the Midwest into the Upper Great Lakes
region. This will prime the region ahead of the strong upper level
forcing for ascent and surface cold front. Anticipate scattered
convection in the afternoon across MN/WI (and perhaps even as far
west as the Dakotas) to become better organized overnight as the
nocturnal low level jet strengthens. Most of the uncertainty is
related to the existing boundaries from mesoscale convection that
will occur over the region tonight and Tuesday. Given the northern
half of WI has observed 200% of normal precipitation over the past
week with another shortwave expected to move through
today/tonight, the soils within this region will likely be more
sensitive to additional rainfall. Therefore, the Marginal Risk was

A boundary should stall near the FL/GA line extending west to the
central Gulf Coast. Convergence of tropical air along this
boundary should allow heavy enough rainfall to locally exceed the
currently high flash flood guidance. Therefore, a Marginal Risk
was raised at this time.


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