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Excessive Rainfall Discussion
(Latest Discussion - Issued 1549Z Jul 25, 2021)
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Abbreviations and acronyms used in this product
Geographic Boundaries -  Map 1: Color  Black/White       Map 2: Color  Black/White

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1148 AM EDT Sun Jul 25 2021

Day 1
Valid 16Z Sun Jul 25 2021 - 12Z Mon Jul 26 2021


1600Z Update...
Made minor adjustments to the previous outlook based on the 12Z
hi-res guidance.


As the mid-level low continues to track west across AZ, anticipate
this feature will again focus convection from central AZ into far
southern NV, eastern CA and southwestern UT.  However, with the
moisture feed starting to wane and gradually lift northwest (as
noted by decreasing precipitable water values within the region)
the convective coverage will start to diminish.  Precipitation
activity will start to advance west and lift farther into NV/UT
through the period with convection blossoming along the terrain
(thanks to differential heating). This activity will eventually
move into the desert by late afternoon/evening.  With precipitable
water values around 1.25 inches and pockets of instability
climbing above 1000 J/kg, anticipate rain rates of 1-2
inches/hour.  As the convection grows upscale, the storm motion
should pick up from northeast to southwest with enough steering
flow aloft.  It appears the best overlapping ingredients (of
moisture and instability) highlights western AZ into southwestern
UT where areal average precipitation will range between 1-2 inches
with locally higher amounts closer to 3+ inches possible. 

Given the wet antecedent conditions, complex terrain (including
slot canyon and washes), and residual burn areas, this region will
be sensitive to additional heavy rainfall thus becoming more
susceptible to flash flooding.  Therefore, the Slight Risk was
maintained and extended to cover the area of greatest concern. 
With monsoonal moisture extending from southeastern CA into much
of CO and NM, there is an isolated flash flood threat across the
rest of the region, especially focused along the terrain and
pockets of well above normal precipitation observed over the last
several days.  

Southern FL will be in the vicinity of a surface boundary with a
wave of low pressure off the Southeast coast supplying a tropical
airmass across the region. While model spread remains with respect
to the aforementioned surface wave, low confidence continues in
the evolution of convection. Regardless, there is enough anomalous
moisture and instability under deep layer southerly flow to
anticipate widespread convection across the southern peninsula.
Areal average precipitation is expected to range between 1-2+
inches. Given the wet antecedent conditions, anticipate localized
flooding to occur, especially if high rain rates occur over the
urban corridor. Therefore, a Marginal Risk was maintained and
refined based on the latest model guidance.

...Central Plains/Mid-MS Valley/Lower Oh Valley...
A cold front will continue to advance to the south and east
through the forecast period, eventually becoming draped across the
High Plains into the Mid-MS/OH Valleys. This will help focus
convection as moisture and instability pool just south of the
boundary. With precipitable water values surging over 2 inches
just south of the front, instability climbing above 3000 J/kg, and
weak dynamical forcing aloft, expect scattered to widespread
convection to blossom through the afternoon.  With sufficient
ingredients in place, anticipate efficient rain rates with any
storms that does develop. This is supported by the high HREF
probabilities of 1-2 inches per hour rain rates.  In addition,
there is some indication that mid-level shortwaves will help focus
and expand the convective coverage across portions of the Central
Plains later tonight and again later this afternoon/evening. 
While much of this region has fairly decent FFG values (2-3
inches/3 hours), there is enough evidence to suggest slow moving
convection, with cell mergers resulting in 3 hourly rainfall
totals of 2+ inches in some locations.  Therefore, localized flash
flooding may occur within this region. 

With the approaching trough and associated cold front moving east,
there will be convection that develops ahead of the boundary
within the anomalously high moisture and instability pool. 
Precipitation water values will surge to over 1.75 inches through
the afternoon aided by 20-30 knot low level southerly flow.  In
addition, instability will climb to over 2000 J/kg. This combined
with enough forcing aloft should promote scattered convection
dropping 1-1.5 inches of rain in an hour.  While these are fairly
decent rain rates, the precipitation activity should be scattered
and progressive helping to limit any flooding impacts.  However,
there is some model signal to suggest that multiple rounds and/or
brief back-building may occur. Given the modest HREF probabilities
for rain rates over 1 inch per hour over the same general area for
multiple hours, decided to highlight the I-95/urban corridor in a
Marginal Risk which was well collaborated with the local offices. 


Day 2
Valid 12Z Mon Jul 26 2021 - 12Z Tue Jul 27 2021


As the weakening mid-level low continues to track west the energy
will split migrating off the CA coast and also north into NV.  As
this occurs, the moisture plume will expand into CA and much of
NV/UT under the western periphery of the upper level ridge. This
will allow convection to become more widely scattered across these
regions. The aforementioned shortwave may be too weak to really
overcome any cloud debris that could linger within this region
during the morning/early afternoon hours.  Therefore, uncertainty
still exists through the forecast period with respect to more
focused convection. Based on the moisture feed from the Baja
region into eastern CA, southern NV and western AZ, it would
appear that the highest rainfall totals may occur along the
southern slopes of the terrain under the influence of 10-15 knot
low level moist southerly flow. Otherwise, anticipate convection
to develop along the terrain from the Southwest into the Central
Rockies with better differential heating by late afternoon.  Rain
rates will likely not be as efficient (as compared to the last
several days) with decreasing precipitable water values through
the afternoon/evening.  Hourly rain rates should be closer to 1
inch per hour with quicker storm motions.  This will lead to lower
areal average precipitation across this region with 0.5-1.5+
inches forecast. 

Given some pockets of wet antecedent conditions, complex terrain
(including washes and slot canyons), and residual burn areas,
additional heavy rainfall could lead to localized flash flooding.
Therefore, a Marginal Risk was retained and expanded to cover the
areas of greatest concern.

...Lower MS Valley/TN Valley/Southeast...
The residual surface boundary will slowly sink south helping to
promote another round of convection across the region, especially
focused across portions of NC/VA where there is better 700mb lift
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, aided in the production of scattered to widespread
convection. Again, with better dynamical forcing, anticipate
better convective coverage across NC and VA.  Regardless, given
the warm cloud depths within the risk area, 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. More erratic storm motions, with the potential for
cell mergers, was also noted across NC and VA which may lead to
higher rainfall totals overall.  While this region has not
observed an abundance of precipitation over the past week, the
rain rates may be enough to result in localized flash flooding,
especially across the urban corridors. 

...Upper Great Lakes Region...
Another shortwave will cross the region with the potential for
training convection that may lead to localized flash flooding.
While broad ridging will dominate the pattern across the central
portion of the country, mid-level impulses will ride along the
northern periphery of the ridge axis from the Upper Midwest into
the Upper Great Lakes region late Monday through the overnight. 
In response, precipitable water values will surge to over 1.50
inches aided by strong southerly flow.  Lingering instability
around 1000 J/kg should be enough to support strong thunderstorms
capable of dropping 2 inch per hour rain rates.  This especially
true given the mean wind will become aligned with the surface
boundary to produce training convection with the nocturnal low
level jet strengthening on the nose of the 850mb front. 

After coordinating with the local offices, conditions across MN
are far too dry to worry about flash flooding. Therefore, the best
potential for flash flooding, albeit limited, appears to be across
northern WI into the UP of MI where soils are a bit more saturated
with recent rains.


Day 3
Valid 12Z Tue Jul 27 2021 - 12Z Wed Jul 28 2021


Monsoonal moisture will bring yet another round of showers and
thunderstorms to portions of the Central/Southern Rockies and
Southwest Tuesday afternoon/evening.  While much of the region is
still under a ridge of high pressure, precipitable water values of
0.5-1+ inches will continue to stream into the region from the
south.  With enough differential heating along the terrain,
anticipate convection to develop along the slopes, especially
across the UT/CO/NM and eastern AZ where the best forcing for
ascent is present.  The coverage will be scattered at best, but
given much of the region has observed 300-600% of normal
precipitation over the past 2 weeks, heavy rainfall could lead to
localized flash flooding, especially over complex terrain and/or
burn areas.  Therefore, a broad Marginal Risk area was introduced.

With a residual surface boundary draped from the coast of NC into
central GA and a surface low moving onshore near the FL/GA border,
anticipate widespread convection across the region.  Precipitable
water values will be quite high, exceeding 2.25 inches, which is
over 1.5 standard deviation above the mean.  With summer type
diurnal instability expected, heavy rain rates are expected with
some storms dropping over 2 inches/hour.  There is still a bit of
uncertainty with respect to the aforementioned surface wave in
terms of both its evolution and track.  This will heavily
influence moisture convergence across the Southeast and how
convection will propagate.  Based on the available model guidance
and ensembles, it appears there is enough signal to suggest
localized flash flooding is possible.  Therefore, a Marginal Risk
for flash flooding was introduced at this update. 


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