Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
409 PM EDT Sat Sep 23 2023

Day 1
Valid 2008Z Sat Sep 23 2023 - 12Z Sun Sep 24 2023

...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF SOUTHEAST VIRGINIA, AND EASTERN OKLAHOMA INTO SOUTHWEST
MISSOURI AND WESTERN ARKANSAS...

20z Update: Just did some trimming of the risk areas on the south
side of Ophelia where heavy rainfall is ending. The MDT risk still
looks good over southeast VA near the center of the system, where
efficient warm rain processes are resulting in hourly rainfall
locally near or just over 1". This MDT risk extends from near the
NC/VA border north through the Richmond metro area where flash
flooding remains most likely into the evening hours.

Chenard

16z Update:
Not much change to the ERO areas associated with Ophelia. The MDT
risk area still looks in good shape over northern NC into
southeast VA. An additional 2-4" of rain remains possible within
that area..with rain rates periodically peaking towards 1" in an
hour. This should be enough to continue to pose a more
concentrated flash flood risk over this corridor...with locally
considerable impacts possible.

Over the central U.S. we did make some tweaks to the MDT risk area
over OK/MO. There has been an eastward trend in the 12z HREF
guidance. Still think the biggest training/backbuilding risk is
over eastern OK (so some of the newer guidance may be undergoing
this potential to some degree), but there is certainly also an
increasing risk a bit further north and east into northwest AR and
southwest MO. Areas of 3"+ amounts appear likely within the MDT
risk area...with localized totals of 5-6" possible as well. Hourly
rainfall rates look pretty impressive with this setup..with 2-4"
in an hour appearing probable. Thus a more organized flash flood
risk still looks on track...with locally considerable impacts
possible.

Considered expanding this MDT a bit further north over western MO
where guidance is trending up with QPF and FFG exceedance
probabilities. However opted against this for now as soil
conditions are a bit less sensitive with northward extent, and the
better chance of more prolonged training/repeat activity is
probably south within the current MDT risk area. So will consider
this portion of western MO a higher end Slight for now and
continue to monitor trends.

Chenard

...Previous Discussion...

...Mid-Atlantic...
Tropical Storm Ophelia is close to making landfall somewhere near
the Crystal Coast of North Carolina and will continue to track
north through eastern North Carolina today. Ophelia will contain
an exceptional easterly Integrated Vapor Transport (IVT) ranging
between 750-1,000 kg/m/s, which is above the 99th climatological
percentile according to NAEFS for 12Z Saturday. As Ophelia weakens
throughout the day, so will the IVT, but it will still routinely
be above the 90th climatological percentile to go along with a
large areal extent of 1.5-1.75" PWs. In fact, 1.5" PWs will likely
be observed as far north as southern New England. Between 12Z
Saturday and 00Z Sunday, PWs will hover around or slightly above
2" from eastern NC on north along the I-95 corridor into the
Richmond metro area. As Ophelia continues undergoing extratropical
transition, it will in effect develop an occluded front and a warm
sector that will lift north through eastern NC and the VA
Tidewater. Most CAMs agree as much as 500-1,000 J/kg of MLCAPE
will be available while elevated freezing levels from the NC
Piedmont to the DelMarVa Peninsula suggest war cloud layers could
be up to 13,000ft deep. This is a recipe for highly efficient
rainfall rates within thunderstorm activity. This is especially
the case along the northern and western flanks of Ophelia's
circulation where the aforementioned strong easterly moisture
transport intersects the surface occluded front and ascends
rapidly as a warm conveyor belt, leading to vigorous convection to
the north and west of Ophelia's core. The 00Z HREF did depict
30-35% probabilities of rainfall totals >5" from south-central VA
to central NC, but localized amounts could approach 8". In
coordination with the Wakefield, Raleigh, and Morehead City WFOs,
have expanded the Moderate Risk into central NC and southern VA.

Farther north, hourly rainfall rates will struggle to top 1"/hr,
but the relatively deep warm cloud layers will still support
efficient rainfall processes. Instability (MLCAPE of 250-500 J/kg)
will be highest over from the Eastern Shore and northern DelMarVa
to southern NJ Saturday afternoon and into Saturday evening. The
northern flanks of the Marginal and Slight Risks were pulled south
slightly given the lack of sufficient instability into southern
New England through Saturday evening.

...Upper Midwest to the Southern Plains...
An approaching 500mb shortwave trough over the Four Corners region
is set to eject into the Southern Plains Saturday afternoon. At
the surface, a cold front racing south through the Central Plains
will encounter another frontal boundary located north of the Red
River and stretching south and east through the ArkLaTex. In
response to the falling heights from both the upper level
disturbance from the Four Corners and a more robust upper trough
over the Northern Plains, an 850mb LLJ will introduce added
moisture flux into the region. One the primary concerns is
850-300mb mean winds become oriented quasi-parallel to the front
over the ArkLaTex, and with a prolonged LLJ present, backbuilding
and training storms are becoming increasingly likely. PWs will
approach 2" and surpass it as storms develop, plus there is likely
to be no shortage of instability thanks to 2,000-3,000 J/kg of
MLCAPE. The 00Z HREF probabilities for FFG exceedance have
continued to rise, especially across eastern OK. HREF
probabilities for 3-hr QPF > 3-hr FFGs are as high as 60-80%
between 00-03Z and 03-06Z across eastern OK. Meanwhile, the longer
duration into southwest MO and northwest AR is a concern with 6-hr
QPF > 6-hr FFG probabilities up to 40-50%. Chose to expand the
Moderate Risk a little farther east as a result. The last QPF
forecast calls for 3-4" of rainfall, but localized amounts
surpassing 6" is well within the realm of possibility as 24-hr QPF
> 5" probabilities on the HREF are 40-45% in east central OK.

Farther north, an upper trough taking on an increasingly negative
tilt will spawn a strengthening low pressure system that looks to
track into the Red River of the North by Saturday evening. An
excellent fetch of 850mb moisture flux out ahead of the forming
colluded front will wrap around the 850mb low and lead to a well
developed comma head of storms on the 850mb low's northwest flank.
00Z HREF showed 3-hr QPF > 3-hr FFGs probabilities north of Pierre
that were as high as 30-40% between 18-21Z Saturday. For the day
as a whole, probabilistic HREF guidance showed up to a 60-80%
chance for 24-hr rainfall totals surpassing 3". The 750 kg/m/s IVT
over central MN will play a big role in supplying the anomalous
moisture aloft and MLCAPE between 500-1,000 J/kg will provide
sufficient instability to generate Excessive Rainfall Rates. The
inherited Slight Risk remains in good standing, as does the
Marginal Risk in southern MT and northern WY near the Big Horns.
Adequate upslope flow and a complimentary tandem of available
moisture/instability over saturated soils makes this area still
prone to flash flooding on Saturday.

Mullinax


Day 2
Valid 12Z Sun Sep 24 2023 - 12Z Mon Sep 25 2023

...THERE ARE SLIGHT RISKS OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE NORTHEAST AND ARKLATEX...

20z Update:

...Northeast...
Only minor changes made to the Slight risk area...with it now
stretching from far northeast MD, into eastern PA, much of NJ, far
southern NY and most of CT. The flood threat does appear to have
increased some across this corridor compared to earlier
forecasts...but still solidly a Slight risk. As what is left of
Ophelia slows down we'll have increasing and persistent
convergence focused across this corridor. Instability continues to
be a limiting factor, however this setup does have embedded low
topped warm rain convective potential. So even some very weak
instability could be enough to get hourly rainfall into the
0.5"-0.75" range on a localized basis (with the 12z HREF
supporting rates of this magnitude as well)...with event total
rain locally pushing into the 2-4" range. Do think this will be
enough to produce some localized area of flooding, especially if
higher totals end up falling over a more sensitive urban location.
The HREF supports the greatest threat from northeast MD into
eastern PA and NJ...however it's worth noting that the 12z GEM REG
and ECMWF...along with both HAFS hurricane models are a bit
quicker with the system...thus spreading the higher rainfall more
into CT (and possibly RI) as well.

...Southern Plains to lower MS Valley...
Not much change to to the inherited Slight risk here. Still looks
like a higher end Slight with the potential for a targeted MDT
still on the table with future updates. Some risk should be
ongoing at 12z over the ArkLaTex area...with redevelopment of
convection later Sunday near the TX/OK border which will then
gradually spread east and/or southeast. For the risk to end up
more at the MDT level we would either need overlap of convection
and saturated soil conditions (which remains unclear) and/or a
more pronounced training signal later Sunday. Thus for now
sticking with the Slight risk seems to make the most sense, and we
can continue to monitor trends.

Chenard

..Previous Discussion...

...Northeast...
Ophelia's remnant circulation by Sunday morning is forecast to
cross the Potomac River and inch its way toward the northern
DelMarVa Peninsula by Sunday evening. There remains no shortage of
moisture with PWs of 1.75" likely to be measured from Northern VA
and Northern MD on east across the DelMarVa and into southern NJ.
Ophelia's 850mb low will continue to introduce added 850mb
moisture flux directly into the pivoting frontal boundary tracking
north through the Delaware bay and into southern NJ. From far
southeast PA to southern NJ and along the southern New England
coast, the 00Z HREF shows as much as 100-250 J/kg of MUCAPE
available. While this amount of instability will limit high end
rainfall rates, sampled model soundings from the Philly metro area
around midday show warm cloud layers up to 13,000ft deep and
highly saturated profiles. This is a recipe for highly efficient
rainfall processes that could still result in 1.5"/hr rainfall
rates within the strongest of storms. The Slight and Marginal Risk
areas on the far northern and western flanks continue to be
trimmed off to an extent given the lack of instability in the more
interior portions of the Northeast. Urbanized areas that contain a
greater concentration of hydrophobic surfaces will be more
susceptible to possible flash flooding.

...Southern Plains to the Lower Mississippi Valley...
The same setup that resulted in widespread thunderstorms on
Saturday will continue to make its way south into central TX and
as far east as the Lower MS Valley. Latest guidance is keying in
on the ArkLaTex with the best odds of receiving Excessive
Rainfall. An 850mb low bordering the Red River will direct a
continuous fetch of 850mb moisture flux at a stalled front
positioned somewhere near the ArkLaTex, resulting in PWs
approaching 2" and as much as 2,000-3,000 J/kg of MLCAPE. Similar
to Saturday, the steady stream of low level moisture becomes
oriented quasi-parallel to the stationary front, providing a
suitable setup for potential backbuilding and training storms.
Storms will be more than capable of producing 2-3"/hr rainfall
rates, and given the potential training of thunderstorms,
localized amounts could reach 4-5". Did increase the footprint of
the Slight Risk area from central TX to the northern AR Ozarks.
Should confidence grow in a swath of 5+" amounts in new guidance
over the next 12-24 hours, there is a chance a Moderate Risk may
be needed for parts of the ArkLaTex in subsequent forecast cycles.

...Southwest Oregon...
The leading edge of one of the upcoming cold season's first
atmospheric rivers will be knocking on the door of the Pacific
Northwest coast Sunday night. A narrow swath of heavy rainfall
will be directed at the Oregon coast Sunday night thanks to a
robust 750 kg/m/s IVT that is 6-8 standard deviations above normal
for late September. Rainfall totals could range between 1-2" with
locally higher amounts closer to 3", but the bulk of this is
falling within a 6 hour span. Given those possible rainfall rates,
have introduced a Marginal Risk with any nearby burn scars most
at-risk for possible flash flooding.

Mullinax

Day 3
Valid 12Z Mon Sep 25 2023 - 12Z Tue Sep 26 2023

...THERE ARE MARGINAL RISKS OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE PACIFIC NORTHWEST, SOUTHERN PLAINS, LOWER AND UPPER
MISSISSIPPI VALLEY...

20z Update:
No big changes to the risk areas with this update...just some
tweaking based on the latest guidance trends. Some chance we may
need to hang on to a Marginal risk into day 3 over portions of
southern New England...but too much model spread at this time, so
opted to hold off. Something to keep an eye on...but should be a
localized threat at best.

Chenard

...Previous Discussion...

...Pacific Northwest...
A powerful upper low approaching the Pacific Northwest will direct
an atmospheric river at the West Coast on Monday, delivering a
soaking rain to residents of northern California on north to
western Oregon and Washington. The upper low off the coast is very
impressive for late September with heights within the 500-850mb
layer are routinely 3-4 standard deviations below normal according
to NAEFS. This powerful upper trough will direct a robust 500-750
kg/ms/s IVT (peaking around 8 standard deviations above normal) at
northern CA and western OR where PWs will be as high as 2-3
standard deviations above normal. This is effectively the first
bonafide atmospheric river of the season for the Pacific
Northwest. Fortunately, parts of far northern California and
southern Oregon could use the rain as parts of the region are in
D0-D2 drought. The primary concerns are residual burn scars where
soils will be more susceptible to possible debris flows. The
Marginal Risk remains in place as rates do not appear to be too
excessive, but detrimental impacts could be felt in burn scars.

...Southern Plains & Lower Mississippi Valley...
A frontal boundary stretching from West Texas to the Ozarks is
forecast to undergo frontolysis throughout the day while at the
same time, southerly 850mb moisture flux is expected to wain
through the day. These features will still be present at a time
when a weak 500mb disturbance approaches from the north, providing
some modest vertical ascent atop the atmosphere. PWs are currently
forecast to reach as high as 1.75" (1-2 standard deviations above
normal according to NAEFS) and MLCAPE between 1,000-1,500 J/jk
from central TX to the Lower Mississippi Valley. The available PWs
and instability should support >2"/hr rainfall rates within the
more intense cells, and soils could be a little more saturated
after recent rainfall on Sunday. The Marginal Risk remains in
place with any consideration of a Slight Risk likely to be
discussed in future forecast cycles when more CAMs guidance
becomes available to key in on the more at-risk areas.

...Upper Mississippi Valley...

A meandering closed low over the Upper Midwest provides a suitable
environment for divergent flow atop the atmospheric column on
Monday. PWs will be as high as 1.25" and MLCAPE is currently shown
to range up to 250-500 J/kg. A steady funneling of 850mb moisture
flux ahead of a dissipating occluded front should help to supply
developing storms with enough moisture to generate storms
containing rainfall rates >1.5"/hr. This round of storms is also
coming on the heels of a 2-day stretch where as much as 2-3" or
rainfall is forecast through Sunday night, so soils will be a
little more saturated compared to earlier in the week. With that
said, opted to introduce a Marginal Risk for Excessive Rainfall as
flash flooding has a chance to occur in poor drainage areas, more
urbanized communities, and where soils are most sensitive.

Mullinax


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