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Excessive Rainfall Discussion
 
(Caution: Version displayed is not the latest version. - Issued 0100Z Aug 11, 2022)
 
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Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
900 PM EDT Wed Aug 10 2022

Day 1
Valid 01Z Thu Aug 11 2022 - 12Z Thu Aug 11 2022

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR MUCH OF THE
MID-ATLANTIC THROUGH THE TENNESSEE VALLEY, PORTIONS OF THE
SOUTHWEST AND GREAT BASIN, AND THE SOUTHERN MISSISSIPPI BASIN TO
LOUISIANA AND CENTRAL GULF COAST...

...01Z Update...
Began trimming area off the Slight and Marginal Risk in the
eastern United States.  Radar and satellite trends were for less
convection and weaker convection from the Tennessee Valley
southward. Maintained the Slight Risk area over the Mid-Atlantic
region...rates had come down but rain was still falling in areas
that received moderate to heavy rainfall earlier in the day. 
Farther south, the signal from the models was for much of the
rainfall to dissipate...and that any showers and thunderstorms
that linger overnight may pose a non-zero threat of excessive
rainfall but should be isolated.

No changes were made in the western U.S. other than to trim a
small portion of the Marginal Risk area on the east side where
late day convection had already diminished. Most of the overnight
activity should be located near the axis of highest precipitable
water...and the current forecast has that well handled.

Bann


...Mid-Atlantic through the Lower Mississippi Valley...

16Z Update...
Few changes needed here.  A front pushing into a very moist
atmosphere already in place will be the focus for another round of
afternoon and evening convection capable of producing 1.5 to 2.0
inch per hour amounts which would challenge...if not
exceed...flash flood guidance.  12Z upper air analysis showed
precipitable water value generally ranging from 1.8 to 2.1 inches
and thermal/moisture profiles support efficient rainfall processes
with MLCAPE values of 800 to 1500 J per kg already in place. 
Spaghetti plots of high-resolution guidance with a diverse set of
underlying physics cores fit nicely in the on-going Slight Risk at
the 1 inch per hour and 2 inch per hour thresholds through the
afternoon and evening.

Bann

...Previous Discussion...

The front responsible for the Excessive Rainfall threat from the
Middle Mississippi Valley to the central Appalachians is beginning
to push southward. Thus the area from the Tennessee Valley to the
Mid-Atlantic coast is in the SLGT for today. PWAT anomalies are up
to 2 standard deviations above normal along the front today, as
PWs throughout the region will be >2.0" this afternoon with some
values up to 2.25" possible. Ample instability is also in place
with MLCAPE values up to ~2,500 J/kg especially over eastern
Virginia, which will be the last area to see convection today.
Latest 00Z HREF probabilities were highest in parts of the central
Appalachians through much of MD and VA west of the Bay. Low level
mean flow oriented orthogonally to the mountain ranges could lead
to localized enhancement of rainfall rates for western areas.
Meanwhile, 1-hr FFGs are as low as 1.5-2.0" in parts of the
DC-Baltimore metro areas, and 1-hr FFGs in parts of eastern WV and
downtown DC are less than 1.0". There does remain some spread in
global and CAM guidance on totals and which locations get the
heaviest rainfall totals. However, the signals at the synoptic and
mesoscale levels are there for a flash flood threat encompassing a
rather large portion of the east-central U.S. Hourly rainfall
rates >2" are a good bet within the most vigorous convection.
Areas that would be most susceptible to flash flooding are
locations that receive heavy rainfall today, as well as in areas
where possible training cells take shape. Corfidi vectors over the
DMV will be opposite the flow earlier in the day, which could
allow for some training of convection at that point, but they flip
to the NW by the time the strongest convection moves through. In
addition, portions of VA could see multiple rounds of convection
today, and depending on when that convection moves through, could
cause localized flash flooding concerns where later rounds of
convection are strongest.

...Southwest and Great Basin...

16Z Update...
Expanded the western periphery of the Marginal and Slight Risk
areas very slightly based on latest trends seen in water vapor
satellite and position of the mid/upper level flow shown by 12Z
upper air. Overall, the changes were minor and did not reflect a
significant shift if forecast reasoning from the previous shift. 
Certainly, the deep monsoon moisture remains in place and the axis
of deepest moisture has shown a slight eastward shift in the past
24 hours.  Spaghetti plots from the 12Z hres_qpf suite shows the
better potential for one hour rainfall amounts or greater in an
hour in Arizona and southern Nevada closer to the highest
precipitable water values...but a Slight Risk is certainly
warranted northward where there is overlap between the deep
moisture and any mid-level forcing.

Previous Discussion...
The expansive corridor of monsoon moisture in the West will
continue to preside across a large portion of the region with PW
percentiles well above the 90th percentile from the Great Basin to
the northern Rockies. MLCAPE is expected to range between
1,000-1,500 J/kg from the Lower Colorado River Basin on north to
the northern Great Basin. The ongoing surge of monsoon moisture
combined with a destabilizing environment and quasi-uniform
southerly flow through the depth of the atmospheric column favors
yet another active day with numerous heavy thunderstorms. PWATS
remain an astounding 4 standard deviations above normal from the
Great Basin north through ID. This equates to PWATs of 1-1.25". In
the Southwest, higher PWs (1.25-1.5" on average) and daytime
surface based heating will be the primary trigger by
mid-afternoon. The Mogollon Rim remains one of the most at risk
areas for flash flooding given the higher elevation there and
abnormally saturated soils according to NASA SPoRT-LIS (up to 99%
soil saturation in spots). With continued rainfall potential into
SE AZ and nearly saturated soil saturation, the SLGT was expanded
south to the Mexican border. Further, the SLGT was expanded south
and west into far southeastern CA as the saturated terrain in this
area, over 98% saturated, continues to pose an elevated threat for
flash flooding since any rain will not be able to be absorbed into
the soils.

...Southern Mississippi River Basin to Louisiana and the Central
Gulf Coast...

16Z Discussion...
Grouped the area from Arkansas and Louisiana in with the Gulf
Coast region for this issuance.  Convection developed early in
portions of Arkansas early this morning in a region of light north
to northeast flow aloft.  Given CAPE values expected to reach into
the 1500 to 2500 J per kg range by afternoon in a region with
precipitable water values close to 2 inches already in place...not
expecting enough CIN to prevent scattered convection forming
anywhere from Arkansas into northern Louisiana throughout the day.
 No change farther south with a low level disturbance out ahead of
an inverted trough in the Gulf of Mexico expected to interact with
a stalling frontal boundary that sets up over the Lower
Mississippi Valley that focuses and supports additional heavy rain
and the threat of excessive rainfall.

Bann

Previous Discussion...
A low level disturbance out ahead of an inverted trough axis in
the Gulf of Mexico will approach the region at the same time as a
stalling frontal boundary sets up over the Lower Mississippi
Valley. Stuck in between is the central Gulf Coast where PWs will
range between 2.0-2.3". MLCAPE up to 2,500 J/kg will support
robust convection throughout the region while weak steering flow
(<5 knots for much of the region) supports slow moving
thunderstorms capable of producing 2-3"/hr rates. The central Gulf
Coast is most at risk for the heaviest rainfall rates. Much of
this region has seen heavy rain the past few days, so the soils
are already saturated, lowering the amount of rain needed for
flash flooding. Given the ample moisture and instability present,
along with slow storm motions, flash flooding is possible in low
lying/poor drainage areas, as well as more urbanized metro areas.
Much of the CAMS guidance suggests that significant convection
will move ashore into AL and the FL panhandles today. Since these
areas are well saturated, the threat has expanded eastward today.


Wegman/Mullinax


Day 2
Valid 12Z Thu Aug 11 2022 - 12Z Fri Aug 12 2022

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE SOUTHWEST AND EASTERN GREAT BASIN...

...2030Z Update...
Overall, the larger scale guidance from the 12Z model runs showed
decent consistency with their previous runs in showing a somewhat
muted risk of excessive rainfall compared with the risk in the Day
1 period...with the broad mid- and upper-level high easing
eastward towards Colorado while a trough over the eastern U.S.
amplifies.  Despite the fact that ridging in the mid-level will
begin to disrupt the moisture transport from Mexico...precipitable
water values of 1.0 to 1.5 inches will remain over the western
U.S. with a continued risk of heavy to excessive rainfall along
and near the axis of greatest precipitable water...with the area
from the Mogollon Rim into southern/eastern Nevada into Utah being
the most susceptible given the favored topography being oriented
quasi-parallel to the mean flow (with burn scars, slot canyons and
dry washes being the most worrisome).

Along the Gulf Coast...The focus for heavy rainfall should
continue to be shunted southward as a front encroaches.  Some
models still produce 1 to 2.5 inches of rainfall onland, but the
signal for the best organized heavy rainfall has been pushed
off-shore by the 12Z models.  Opted to keep the Marginal risk
without too many modifications given the weak steering level winds
over the area and the potential for heavy downpours...although
confidence is not all that great.  Later shifts will be able to
evaluate.

Bann

...Previous Discussion...

...Southwest & Eastern Great Basin to the Northern Rockies...

The upper level ridge becomes situated more over the central High
Plains by Thursday with the axis of most anomalous PWs oriented
from the Lower CO River Valley to the northern High Plains and
Rockies. 90th percentile PW will span across a large portion of
the Intermountain West with > 3.0 standard deviation PWs values in
parts of southern NV, northern UT, eastern ID, northwest WY, and
into southern MT. Actual PWs in these areas will be 1.0-1.5" with
the higher end of those values in the northern High Plains and
Lower CO River Valley. Similar to Wednesday, traversing remnant
MCVs will be of interest along with any embedded 500mb
disturbances rotating around the southwest and northwest flanks of
the upper ridge. The area from the Mogollon Rim on north up the
Wasatch and into northern UT remain most susceptible to possible
flash flooding due to the favored topography oriented
quasi-parallel to the mean flow along the highest terrain. Rugged
terrain, slot canyons, and sensitive soils from either over
saturation or within burn scars are most at risk. This goes for
areas as far north as Yellowstone and the Tetons and as far south
and east the Sacramento mountains of southern New Mexico.

Changes with this forecast package include expanding the SLGT into
far southeastern CA, and westward to include more of eastern NV in
coordination with the VEF/ Las Vegas, NV and LKN/ Elko, NV
forecast offices.


...Central Gulf Coast...
Persistent onshore flow converges along a weak surface trough
extending from east TX through southeast LA with PWs still ranging
between 2 to 2.25", which is ~2.0 standard deviations above
normal. This moisture and convergence along with ample instability
and low steering flow warrants a risk of excessive rainfall,
particularly since much of the area east of Galveston Bay to the
FL Panhandle has had above normal rainfall over the past week.
Sufficient instability and weak storm motions (as weak as < 5
knots) would also allow torrential downpours potentially enough
residency time to cause localized flash flooding. A Marginal Risk
remains in place as confidence is on the lower side regarding
placement of the most intense thunderstorm activity. This also
goes hand-in-hand with where the heaviest rainfall both Tuesday
and Wednesday transpired, which if the heaviest rainfall for
Thursday lines up in these areas, would make those soils or
communities most susceptible to flash flooding. Once confidence in
rainfall totals increases and the heaviest concentration of storms
comes into focus, a Slight Risk upgrade could be issued in future
forecast cycles.

The MRGL risk area was expanded eastward to cover southern AL, the
FL panhandle and far southwestern GA in coordination with the
MOB/Mobile, AL and TAE/Tallahassee, FL forecast offices.

Wegman/Mullinax

Day 3
Valid 12Z Fri Aug 12 2022 - 12Z Sat Aug 13 2022

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE SOUTHWEST...

2030Z Update...
The overall pattern for Day 3 has changed little since the
previous outlook was issued.  Only minor adjustments were made to
the boundaries of the Slight Risk area out west given a subtle
southward shift in the latest mode QPF and which was reflected in
the WPC deterministic QPF.  These changes were minor and did not
reflect a significant change in the previous forecast reasoning
either out west or with the moisture along/near a quasi-stationary
front draped from the Gulf Coast northeastward to coastal
Georgia/South Carolina.

Bann


Previous Discussion...
...Southwest & Eastern Great Basin to the Northern Rockies...

The upper ridge continues its eastward drift with the center
reaching western KS Friday night. The monsoonal moisture plume
edges east as well with the 90th percentile PW anomaly spanning
from AZ to central MT. Similar to recent days, upper level
impulses will allow organized activity to lift north. Terrain such
as the Mogollon Rim through the slot canyons of southern UT remain
most susceptible to possible flash flooding due to orientation
quasi-parallel to the mean flow and a Slight Risk is in effect for
these areas. There is uncertainty in the best corridors as seen in
differences between the 00Z ECMWF and GFS with the GFS generally
with a narrower plume. However, higher afternoon PW over southeast
AZ Friday vs Thursday warrants a Slight Risk for southeast AZ to
the southern border. The Marginal Risk hedges this area between
central NV and southern NM. Very anomalous moisture (PWs 3 sigma
over normal) over western WY may warrant a Slight Risk extension
north to the Yellowstone area, so stay tuned to subsequent updates.


...Central Gulf Coast to the Carolina Coast...
Continued elevated Gulf moisture persists along the Gulf coast
through the Carolina Coast (south of the Outer Banks) into the
diurnal peak with a cold front approaching the Gulf Coast late in
the day. A surface trough along the coast will help focus the
heavy rain threat to along the entirety of the northern Gulf Coast
and Southeast Coast. Given the heavy rain threats from prior days
there is likely to be a need for a Slight Risk at some point, but
the more prone areas (besides urban areas) may have to make
themselves more evident after rain today/Thursday before the
confidence is great enough. Also worth noting is the westward
shift in the anomalous moisture plume which may allow enough heavy
rain the lower Rio Grande Valley to warrant an eventual excessive
rain risk - this is currently noted to be greater in the 00Z GFS
which features a faster cold front than the 00Z ECMWF.

Jackson


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