FOUS30 KWBC 102007
QPFERD
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
407 PM EDT Sat Aug 10 2024
Day 1
Valid 16Z Sat Aug 10 2024 - 12Z Sun Aug 11 2024
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE
CAROLINAS INTO SOUTHEASTERN VIRGINIA...
...16Z Update Summary...
Only minor changes were implemented across the risk areas forecast
with no changes to the SLGT risk in the Carolinas. Full updates can
be found in each respective sub-heading below....
Kleebauer
...Carolinas & Virginia...
16Z Update: Not seeing much of an agitated cumulus field across
Eastern NC as of recently, but all CAMs show a general initiation
of convection across the Carolinas after 18z with scattered to
widespread showers and storms anticipated through the first half of
the evening. HREF blended mean QPF depicts a corridor of 1-2"
totals with some locally elevated signatures in the mean across
Southeast NC, just inland of Wilmington. HREF neighborhood
probabilities are relatively high within SC (20-50%) for at least
3" with a much more robust signal in-of Eastern NC with an
extensive areal coverage of 50-80% for the >3" threshold, as well
as some low-end 15-30% signals for >5" northwest of Morehead City.
This area is a prime target for flood concerns as the region
continues to deal with the aftermath of Debby's rainfall. Moderate
to major river flooding is ongoing for much of the area within the
Eastern Carolinas, so additional rainfall will be a great concern
to exacerbate the flood risks when convection arises. The SLGT risk
forecast was maintained given the persistent signals and
antecedent conditions that could escalate flash flood concerns.
Kleebauer
..Previous Discussion..
A stalled out front over the western Carolinas and central Virginia
will act as a focus for storms to develop this afternoon on the
warm/moist southeastern side of the front. The atmosphere will be
characterized with plentiful deep moisture with PWATs over 2 inches
across much of the region. HRRR guidance shows several clusters of
storms forming across southeast Virginia and the central Carolinas
late this afternoon. The storms will then track southeastward
towards the coast into an area where instability could exceed 4,000
J/kg along the coast. This level of instability and moisture will
support potential for storms to produce rainfall rates to 3 inches
per hour. Fortunately the guidance suggests the storms will keep
moving into the evening, but those heavy rainfall rates would have
the potential to cause flash flooding even if antecedent conditions
weren't so favorable.
Debby's remnants tracking across this region over the last few days
has led to very saturated soils all across Virginia and the
Carolinas. With numerous streams, creeks, and rivers already well
into flood stage, additional rainfall from storms Saturday
afternoon will struggle to drain, resulting in additional flooding
and prolonging ongoing flooding. In coordination with all of the
impacted offices, a Slight Risk area was introduced with this
update.
00Z HREF guidance peaked at over 50% chance of exceeding 6 hour
FFGs in the Slight Risk area, with an over 70% chance of exceeding
3 inches of rain across much of eastern North Carolina. These
values added to the confidence to upgrade the ERO risk.
Wegman
...Rockies into the Plains...
16Z Update: Across the Plains, an elongated complex of thunderstorms is
beginning to fade across the Northeast NM through the TX Panhandle
into OK, but a few flash flood warnings were issued due to training
convection along a convergent zone that aligned with the I-40
corridor. Outflow generated from the complex on the eastern
periphery of the convection is migrating slowly eastward along the
Red River into the neighboring areas of OK and TX. Models are not
handling that area well with much of the CAMs void of precip in the
region despite the radar saying otherwise. This was a trickier
portion of the setup given the lack of consistency with ongoing
convection, so decided to hedge a little further south with the
MRGL risk into TX as it follows the Red River to as far east as the
DFW metro. Kept the risk area out of the metroplex given the
southern extent of the outflow lying north of the I-20 corridor,
but wanted to cover for an isolated threat of heavy rain as
boundary layer destabilization along the Red River this afternoon
could offer the threat of some overachieving cells that may not be
well forecast within the current CAMs suite.
Further north into OK, the expectation for another round of
nocturnal convection has remained persistent within the current
hi-res suite with general agreement within the global deterministic
and ensemble blends with some heavier convective cores likely cause
localized flood concerns across portions of Central and Northern OK
after 03z. The prospects for significant flooding are low, but
non-zero given the probabilistic output off the latest HREF with a
>3" neighborhood probability around 20-40% bisecting portions of
West-Central OK across areas north of I-40 and OKC. The progressive
nature of the storms combined with very dry antecedent conditions
will mitigate a greater threat of flash flooding leading to a
maintenance of the MRGL risk with a lower chance of a targeted
upgrade.
The Southwest remains active with the Monsoon with the elevated
moist pattern nestled across the entire Desert Southwest up through
the Inter Mountain West as far north as WY. The greatest threat
will occur within the terrain with the highest flash flood risks
likely within remnant burn scars, slot canyons, and any urbanized
corridors where slow-moving convection lingers. Signals for 1 and 3
hour FFG exceedance are low and scattered among the outlined areas,
so the threat remains isolated in nature. The MRGL was maintained
given the sporadic depiction with the probabilities and mean QPF.
Kleebauer
..Previous Discussion..
An approaching positively tilted shortwave trough moving into
California will increase lift to its east across the Four Corners
region. Storms are likely to be widely scattered across this area,
which would support an isolated flash flooding instance or 2,
currently covered with the large Marginal Risk. Additional moisture
in southern Arizona and lift into the Peninsula Ranges of southern
California may cause local flash flooding issues, but the guidance
is inconclusive at best as to where any potential Slight risk area
could be drawn. In coordination with the impacted offices, have
opted to hold at a Marginal for now, but will reevaluate each day's
chances for flooding based on the previous day's convective
behavior.
MCS development with potential repeating storms in the Plains will
be a concern across Kansas through Oklahoma late Saturday night.
Antecedent conditions are quite dry for soil moisture across this
area, and the CAMs are not in great agreement on the location and
behavior of the storms, despite ample moisture available. Here too
a Slight Risk is close, and may need to be considered with future
updates.
Wegman
Day 2
Valid 12Z Sun Aug 11 2024 - 12Z Mon Aug 12 2024
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR THE CAROLINAS,
SOUTHWEST UTAH, AND FOR PORTIONS OF THE OZARKS AND CENTRAL=20
PLAINS...
...20Z Update Summary...
General continuity across the Carolinas with the SLGT risk over the
region as another day of scattered to widespread convection capable
of heavy rainfall will develop in-of the stationary front
bisecting the region. The SLGT risk across the Central Plains into
the Ozarks was maintained with some adjustments on the northeastern
edge of the risk area based on the latest probabilistic trends and
with coordination from the local Kansas City WFO. A new SLGT risk
was added across Southwestern and Central UT for the threat of=20
heavy rainfall with a greater heavy rain footprint compared to
previous periods. More on each region in the respective sub-
headings below....
Kleebauer
...Carolinas...
20Z Update: Minor adjustments were made across the Carolinas with
the biggest change coming across the northwest side of the SLGT
risk where guidance has scaled back on the westward extent of heavy
rainfall between Raleigh to Morehead City. There is still a
formidable signature for locally >3" of rainfall within the
neighborhood probability fields off the 12z HREF (60-90%) across
far Eastern NC to the west of Cape Hatteras. This is in correlation
to the strongest instability located within the deep layer moisture
lingering along the NC coastal plain. The heavy rain threat extends
down the coast of SC as well with the best potential for flash
flooding occurring close to Charleston and areas adjacent. This is
almost a repeat of what is occurring in D1, and with the antecedent
well-above normal soil moisture forecast, there was no reason to
deviate from the previous forecast.=20
Kleebauer
..Previous Discussion..
The stalled out front over the Carolinas will continue to draw deep
tropical moisture northward up the Southeast coast. The front will
have made enough progress to likely keep the rainfall down into
southeastern Virginia, but there will likely be more storms into
South Carolina. Thus, the Slight risk is just a bit south of the
Day 1/Saturday Slight Risk. Nonetheless with ongoing flooding from
Debby, additional rainfall, even if more widely scattered, will
have an outsized potential for impacts from additional flooding.
00Z HREF guidance through 00Z Monday suggests additional elevated
potential for exceeding FFGs in NC, and plentiful moisture and
instability with a stalled out front providing the forcing will
likely help organize the storms, increasing their flooding
potential.
Wegman
...Central Plains/Ozarks...
20Z Update: The area across Northwest OK will see an initial round
of convection with heavy rainfall capable of 1-3" within the
morning before the threat dwindles with the loss of the nocturnal
LLJ. A break will ensue for most of the afternoon and early evening
before the forecasted MCS initiating over KS moves into the=20
targeted region after nightfall. The SLGT risk from previous=20
forecast was maintained with general continuity with some shift=20
southward based on a shift in the heaviest QPF placement, a trend=20
that may continue just based on the recent ML output. The setup is=20 contingent
on the axis of highest theta-E in conjunction with the=20 northeastern extension
of the ridge across the Central and Southern
Plains. The event remains on the lower end of SLGT due to the=20
drier soil moisture residing over the expected area of impact. The=20
environment is conducive for heavy rain however, and the=20
probability fields are relatively favorable for pockets of 2-4" of=20
rainfall within a few hours of impact as the complex shifts=20
southeast out of KS into the MO and Northwestern OK. Considering=20
the consistency in the placement of the forecasted complexes during
the period and the continued favorable probabilistic output from=20
the CAMs, the SLGT was justified, but certainly on the lower end of
the risk threshold.
Kleebauer
..Previous Discussion..
MCS development on the leading edge of a low level jet (LLJ) is
expected from southeast Kansas through northwest Arkansas. Despite
recent dry weather, training convection with PWATs increasing to
around 2 inches will support storms capable of rates to 3 inches
per hour at times. This very heavy rainfall combined with terrain
interactions in the Ozarks will favor widely scattered instances of
flash flooding. In coordination with the impacted offices, a
Slight Risk upgrade was introduced with this update. Given CAMs
frequent trouble with correctly simulating storms with MCSs, there
are likely to be both temporal and intensity adjustments with the
expected rainfall. Despite this potential, there was good enough
agreement in the guidance to draw in the Slight risk, which for now
remains low-end given the recent dry weather and dry soils.
Wegman
...Great Basin and Southwest...
A slightly stronger mid-level ridge is forecast across the
Southwestern US with a zone of elevated low to mid-level moisture
advecting northward into the Great Basin and portions of the Inter
Mountain West during the day on Monday. The combination of +1 to +2
deviation PWATs, ample diurnal destabilization, and a weak mid-
level perturbation ejecting out of NV into UT will allow for a bit=20
more robust convective development in-of portions of Southwest and=20
Central UT leading a low-end SLGT risk placement across the=20
aforementioned area. Ensemble based QPF output is respectably high=20
compared to recent periods with signals for 1-2" across the terrain
Southwest UT, extending eastward thanks to the progression of the=20 disturbance
moving overhead. 12z HREF neighborhood probabilities up
around 50-70% for at least 2" exist within the corridor from=20
Gunnison down towards the AZ border in Western UT. 10-year ARI=20
exceedance probs are also running very high in the same region with
signals of 50-70% showing up in the recent HREF output. Modest=20
100-year ARI probs are also located within West-Central UT, so the=20
threat is unlike the recent convective pattern when those signals=20
were either muted or extremely marginal (Likely driven by one CAM=20
member). In coordination with the Salt Lake City WFO, a SLGT risk=20
was introduced to cover for the threat with emphasis on areas=20
around the I-70/15 split where the highest probabilities and QPF=20
are positioned.
The threat may also extend further south into Northwest AZ, but the
setup is not as aggressive comparatively and lies a bit closer to
the MRGL risk based on the recent precip forecast. This will be an
area to monitor in future updates.=20
Kleebauer
Day 3
Valid 12Z Mon Aug 12 2024 - 12Z Tue Aug 13 2024
...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE FOUR CORNERS REGION, CENTRAL PLAINS, OZARKS, AND THE CAROLINAS...
...20Z Update...
The premise behind each MRGL risk area remains valid with a general
run-to-run consistency on the placement of the heaviest QPF axis'
over the course of the period. The area with the best opportunity
for a SLGT risk upgrade remains over the Ozarks thanks to the
opportunity for back-to-back days of complexes that could move
through the area. There's still some uncertainty on how the overall
mesoscale pattern plays out as remnant outflow boundaries from
convection the previous period could shift the threat north or
south pending how it evolves. The ensemble bias corrected QPF and
NBM 75th percentile QPF signatures align, more or less across that
span of Southern MO and Northern AR with some overlap from the
previous D2 SLGT risk located across Southwestern MO. Decided to
maintain continuity in the area to account for some shift in the
coming model cycles, but the prospects are still relevant for an
upgrade in the near future.=20
Elsewhere, the threat across The Colorado Front Range into
Northwest KS took a slight step forward in the prospects for any
upgrade with some global deterministic output yielding formidable
QPF maxima located near and along I-70 out by Goodland and points
west. Shortwave ejecting out of the Central Rockies could be
sufficient for primed large scale forcing within an environment
that favors above normal precip with PWAT deviations running
between +1 to +2 across the Central High Plains. Close proximity of
a stationary boundary will also allow for an axis of convergence
that would solidify a suitable initiation and storm motion
inflection point for the setup. For now, a MRGL risk remains in
effect with a point of interest for a future SLGT if the precip
signature maintains itself or increases in intensity.=20
Carolinas will be within one last day of potential flash flood
concerns as the stationary front lingers over the far eastern
sections of the area. The signal for excessive rainfall is less
than prior days, but still warrants some attention given the
antecedent conditions expected through the D3 period. Only minor
adjustments were made to reflect the mean QPF footprint within the ensembles.=20
Kleebauer
...Previous Discussion...
Ongoing rainfall from Sunday night may continue into Monday morning
across southern Missouri. With only a few hours of rain in the
morning, the Marginal across Missouri was maintained. However,
should rainfall from Day 2/Sunday overperform, then a future Slight
may be needed. Once again another round of storms Monday afternoon
across the Four Corners region is likely to result in isolated
flash flooding. However, there is greater uncertainty with
coverage by Monday in this region with lesser forcing than on
Sunday.
For the Carolinas, lingering moisture with the same front may cause
yet another round of storms Monday afternoon into the evening. With
greater potential that the bulk of the activity will be offshore by
Monday afternoon, the previously inherited Marginal risk remains
largely unchanged.
Wegman
Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=
v/qpf/94epoints.txt__;!!DZ3fjg!4TestN1HkrkVefuZ2Tb_IoGFE5ngXwcEEW9Aj-gfS5S0=
b-detPLiH0YYikMoy5CXjGGDS0p6xa0DAVhVUPw05_5cUFQ$=20
Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=
v/qpf/98epoints.txt__;!!DZ3fjg!4TestN1HkrkVefuZ2Tb_IoGFE5ngXwcEEW9Aj-gfS5S0=
b-detPLiH0YYikMoy5CXjGGDS0p6xa0DAVhVUPw02awsVzQ$=20
Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=
v/qpf/99epoints.txt__;!!DZ3fjg!4TestN1HkrkVefuZ2Tb_IoGFE5ngXwcEEW9Aj-gfS5S0=
b-detPLiH0YYikMoy5CXjGGDS0p6xa0DAVhVUPw0WZ7DAOo$=20
=3D =3D =3D
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