Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
904 PM EDT Mon Jul 26 2021
Day 1
Valid 01Z Tue Jul 27 2021 - 12Z Tue Jul 27 2021
...THERE A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
SOUTHWEST AND SOUTHEAST...
...01Z Update...
Trimmed the Marginal and Slight Risk areas over southern
California and Arizona based on latest radar and satellite trends.
The downward trend in convection was reflected in the latest HREF
probabilities and an ensemble of the high resolution guidance.
There may still be scattered showers overnight but the risk of
additional problems are waning. Also removed far southwest New
Mexico and adjacent TX. Slow moving showers and thunderstorms
persist south of the international border and cross into southern
NM but models do not appear favorable for this rainfall to present
more than isolated heavy rainfall. Elsewhere, rainfall rates
should continue to decrease from the Central Plains eastward into
the Mid-Atlantic Region and Southeast.
Bann
...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
regions.
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.
Pagano/Jackson
Day 1 threat area:
www.wpc.ncep.noaa.gov/qpf/94epoints.txt
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
355 AM EDT Tue Jul 27 2021
Day 1
Valid 12Z Tue Jul 27 2021 - 12Z Wed Jul 28 2021
...THERE A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
THE SOUTHEAST...
...Southeast...
As a surface low tracks along the surface boundary through the
region, expect to see periods of heavy rainfall. PW values sit
near 2.25 inches sitting near 2.5 std deviations above normal for
this area. Weak steering flow and possible training storms, could
lead to flash flooding. Latest model guidance places signals of 2
to 4 inches for portions of central to southern GA up along the
coast of of SC. The Slight Risk was left in play and adjusted
slightly to account for latest model guidance. Much of the
Southeast will see periods of heavy rain with this system, with
much of this area seeing 150-400% of normal precipitation over the
past two weeks, making flash flooding more probable for this
portion of the Country. The Marginal Risk area was also adjusted
slightly to account for latest Hi-Res guidance and forecast QPF.
...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. The Marginal
Risk area was maintained over terrain from central OR to southern
AZ by way of UT with only minor adjustments made to account for
latest guidance.
...Northeast...
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.
Chiari/Jackson
Day 1 threat area:
www.wpc.ncep.noaa.gov/qpf/94epoints.txt
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
356 AM EDT Tue Jul 27 2021
Day 2
Valid 12Z Tue Jul 27 2021 - 12Z Wed Jul 28 2021
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
THE UPPER GREAT LAKES INTO THE UPPER MIDWEST...
...Upper Midwest/Great Lakes...
The next shortwave trough makes its way across the area during the
period with sufficient instability and PW values reaching upwards
of 2.0 inches to provide heavy rounds of precipitation for the
area. Soils will be primed during the day 1 period as mesoscale
features make their way across the region. Soil moisture are
already sitting in upwards of the 98th percentile and with day 1
QPF, the day 2 periods will see much lower FFG than what is
currently seen, which is already low. Flash flooding concerns are
on the higher end confidence wise and thus, a Slight Risk area has
been hoisted for portions of WI into MI. The Marginal Risk area
already in play for this region was expanded slightly to account
for differences among the models and latest QPF forecasts.
...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 and
the Marginal Risk area has been maintained.
...Southeast...
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 and combined with day 1 QPF,
the Marginal Risk area was kept in play.
Chiari/Jackson
Day 2 threat area:
www.wpc.ncep.noaa.gov/qpf/98epoints.txt
