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Excessive Rainfall Discussion
(Caution: Version displayed is not the latest version. - Issued 1959Z Aug 12, 2022)
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Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
359 PM EDT Fri Aug 12 2022

Day 1
Valid 16Z Fri Aug 12 2022 - 12Z Sat Aug 13 2022


--16Z Update--

The daytime update includes primarily minor adjustments based on
latest QPF trends from CAM and global guidance. In coordination
with PSR, have expanded the Slight Risk farther west as a
700-500mb inverted trough traverses the Lower Colorado River
Basin. 12Z PSR sounding measured a PW value of 1.86", which is
well above the 90th climatological percentile and in line with the
climatological max moving average. Storm motions will also be
quite slow with forward motions ranging between 5-10 knots. The
latest 12Z HREF 6-hr QPF > 10-year ARI probabilities between
18-00Z this afternoon were as high as 60-70% in some cases along
the Mogollon Rim, highlighting the potential for excessive
rainfall totals in parts of the region. In addition to the endless
supply of anomalous moisture mentioned from overnight, soil
moisture percentiles remain remarkably high throughout the Lower
Colorado River Basin on north into the Great Basin and up to the
Great Salt Lake. NASA SPoRT-LIS 0-40cm soil moisture percentiles
were >98%, indicating soils will struggle to absorb much, if any,
rainfall and lead to instances of rapid runoff. Areas most at risk
for flash flooding are within these overly saturated soils, as
well as within burn scars, slot canyons, dry washes, and urbanized
communities. Elsewhere, backed off the Marginal Risk in southeast
Georgia as CAMs and global guidance have come in drier than
previous forecast cycles.


Previous Discussion...

...Southwest, eastern Great Basin, and northern Rockies...
Little change is expected in the upper pattern, with a strong high
remaining centered over the central High Plains as a upper
low/trough lingers along the Northwest coast.  Deep southerly flow
between the two features will support an axis of anomalous
moisture extending from portions of the Southwest through the
eastern Great Basin, into parts of the northern Rockies, with PWs
2-4 standard deviations above normal across the region.  This will
continue to fuel daily shower and thunderstorm development and the
potential for heavy amounts.  Shortwave energy lifting north along
the western periphery of the ridge is expected to interact with
this moisture, supporting areas of organized convection and
heavier precipitation.  This includes areas that have already seen
repeating days of active monsoon convection.  Overall, the Slight
and Marginal Risk areas remain largely unchanged from the previous
Day 2 Outlook, with only relatively minor adjustments made to
their footprints based on trends shown by the overnight guidance.

...Gulf Coast to the eastern Carolinas...
Deep moisture ahead of cold front dropping across the Southeast
will continue to support diurnal convection and the potential for
locally heavy amounts today.  Across the eastern Carolinas, an
amplifying upper trough and right-entrance region upper jet
forcing is expected to raise the threat for organized
precipitation and heavy amounts.  While this area has been quite
dry recently, overnight heavy rains across portions of the area,
along with the potential for additional heavy amounts may warrant
an upgrade to Slight Risk later today.  Along the Gulf Coast, deep
moisture (PWs 1-3 standard deviations above normal) and weak
steering flow will continue to support slow-moving storms, capable
of producing heavy rains.  Given the wet antecedent soil
conditions that exist from southeastern Louisiana to the Florida
Panhandle, flash flooding will remain a concern across this area.

...Upper Midwest...
Shortwave energy moving off of the top of the ridge is expected to
initiate elevated showers and storms that are forecast to spread
southeast across portions of the region.  An initial round
currently developing over eastern North Dakota is forecast to move
across central into southeastern Minnesota later this morning,
with some potential for training and locally heavy amounts.  A
second round is forecast to develop tonight into Saturday morning,
impacting areas farther to the east across Wisconsin, with the
models once again showing some signal for training and the threat
for locally heavy amounts.


Day 2
Valid 12Z Sat Aug 13 2022 - 12Z Sun Aug 14 2022


--20Z Update--

Minor adjustments were made to the Marginal and Slight Risk areas
to account for latest 12Z CAMs and global guidance. The Southwest
and Intermountain West look to remain active as 500mb impulses on
the western periphery of an upper level ridge atop the Central
Plains act as a catalyst for widespread thudnerstorm development.
AHPS 14-day precipitation totals have ranged as much as 400-600%
of normal from the Lower Colorado River basin on north through he
eastern Great Basin and into northern Utah and western Wyoming.
Farther south, one area of interest is an 850mb low taking shape
in the northwest Gulf of Mexico that should lead to increased
areal extent of thunderstorms in South Texas. There does remain
some spread in guidance regarding timing and rainfall totals, so
have maintained the Marginal Risk inherited from overnight.


Previous Discussion...

...Southwest, eastern Great Basin, and northern Rockies...
Relatively little change is expected in the overall upper pattern
with deep southerly flow continuing to channel both ample moisture
and energy from the Southwest through the eastern Great Basin into
the portions of the northern Rockies.  PW anomalies are likely to
remain high across this region (2-4 standardized anomalies above
normal), fueling another day of diurnal convection across the
region as mid-level energy supports areas of enhanced organization
and heavy amounts.  With little change in the pattern, the
Marginal and Slight Risk footprints are quite similar to those on
Day 1.

...Gulf Coast to the Carolina Coast...
Boundary dropping in from the north is expected to stall along the
central and eastern Gulf Coast, providing some additional focus
for shower and storm development.  With ample moisture remaining
in place and the steering flow weak, the threat for slow-moving
heavy rainfall producers will persist.  A few of the overnight
guidance members are quite bullish in the development of an
easterly wave moving across the northwest Gulf, with some models
bring heavy rains onto the lower Texas coast during the period.

...Upper Great Lakes into the Ohio Valley...
Elevated convection developing late in the Day 1 period is
expected to continue into the early part of Day 2, with training
convection moving from northern Wisconsin into northwestern lower
Michigan Saturday morning.  While differing on the axis of heavier
amounts, most of the guidance is presenting some signal for
training/backbuilding convection across lower Michigan, raising
the threat for at least locally heavy amounts the potential for
flash flooding across the region on Saturday.


Day 3

The Day 3 outlook will be updated by 2030Z.

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