FOUS30 KWBC 071554
QPFERD
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1154 AM EDT Wed Aug 7 2024
Day 1
Valid 16Z Wed Aug 07 2024 - 12Z Thu Aug 08 2024
...THERE IS A HIGH RISK FOR EXCESSIVE RAINFALL FOR PORTIONS OF
SOUTH AND NORTH CAROLINA...
...WIDESPREAD AND NUMEROUS INSTANCES OF FLASH FLOODING WITH
SIGNIFICANT AND CONSIDERABLE FLOODING LIKELY...
...16Z Update Summary...
Minor adjustments to the High Risk across the Carolinas was made to
reflect the latest QPF footprint based off CAMs and relative
ensemble output. SLGT risk across the Mid Atlantic was expanded
west through parts of Central MD and Northern VA. Only modest
changes were necessary for the MRGL out west with a small expansion
eastward into northern KS for the eventual complex moving out of
the Front Range into the area overnight. More details of each
located in the respective sub-headings below....
Kleebauer
...Southeast...
16Z Update:
Debby continues to meander off the South Carolina=20
coast with a disheveled appearance based on satellite=20
presentation. A lot of drier air has wrapped into the core with the
system struggling to shake off the drier environment leading to=20
less vigor within any organized convective patterns around the=20
storm itself. There is still a solid background of tropical=20
moisture associated with the storm as indicated within the latest=20
12z soundings out of both KCHS and KMHX with PWATs hovering between
2.2-2.6" across Eastern SC and Southeastern NC. Diurnal=20
instability will pick up inland with a redevelopment of heavy=20
convection focused on the western periphery of the low center. 12z=20
HREF and associated CAMs were all bullish on an arc of heavy=20
convection from Charleston to points north and northeast, about=20
25-60 miles inland from the coast. A general 3-6" with upwards of=20
8" will be plausible during the afternoon and evening from the band
of heavy rainfall with rates hovering between 1-2"/hr at peak=20
intensity. Considering the recent rainfall overhead, the soil=20
moisture percentiles have plummeted with the top-layer soils=20
trending very wet, running through the 80-90th percentile according
to NASA SPoRT output. With the a prevalent signal for at least 3"=20
of rainfall based within the latest EAS probability fields (>90%)=20
around the above area, the High Risk was maintained, but some=20
adjustment on the northern and southern fringes where probabilities
drop off significantly away from the prospectus area of heavy=20
rainfall.=20
Further away from the main area of interest, scattered heavy
thunderstorms will be plausible as they rotate outward from the
storms center with embedded heavy rain rates exceeding 1"/hr in the
stronger cores. The airmass further away from Debby is drier, but
marginal instability and sufficient deep layer moisture could still
cause some concern away from the primary focal point. Later this
evening, Debby will move back inland with a west-northwest
trajectory through the SC Low Country leading to more organized
convection spreading inland until it reaches the Carolina Piedmont.
This is when probabilities for the heaviest rain focus away from
the coast and out towards the I-77 and I-95 corridors with areas
like Columbia and Raleigh to Charlotte becoming the areas of focus
as we enter the end of the period. This allowed for a small
westward nudge in the SLGT and MRGL risk areas with a minor
adjustment on the MDT risk based on the timing of Debby's westward
progression and some heavier QPF being placed west of the previous
forecast.
Kleebauer
...Mid Atlantic...
16Z Update:
A quasi-stationary front is currently analyzed across the Central
Mid Atlantic with a corridor of higher theta-E's and relevant
instability located over the Delmarva and adjacent Piedmont across
Central MD and Northern VA. This will be the primary focus for
scattered convection this afternoon and evening with the initiation
occurring during peak diurnal heating along with the passage of
modest mid-level perturbation moving east-southeastward out of PA.
Majority of the region will see only light totals, but the moist
unstable environment in place will allow for multiple cells to
produce rainfall rates of 1-2"/hr, easily able to cause issues
within the DMV due to higher runoff potential from urbanization,
and antecedent wet grounds from last periods rainfall. Neighborhood
probabilities are very bullish thanks to the 12z CAMs outputs this
morning relaying >3" probs between 50-80% between US50 and the=20
Mason Dixon. This same area is also carrying some elevated=20
probabilities for >5" with a 20-45% footprint running west to east=20
along I-70 to the northern Delmarva. This is the proxy for the=20
frontal location during the period with the CAMs depicting the=20
focal point for convection the next 12-18 hrs. First Guess Fields=20
were also a little more robust in the depiction with a SLGT risk=20
extending out towards the Blue Ridge on the latest update. There=20
was an expansion further west, but only out to US15 with a=20
southern extension encompassing the District, Southern MD, and=20
portions of the Northern Neck of VA through the Southern Delmarva.=20
The best risk will likely be north of US50, but some CAMs were=20
privy to a smattering of convection down across the region which is
in the better deep layer moisture extending to the north of Debby.
=20
Kleebauer
...Southwest...
16Z Update: Little has changed from the previous forecast across
the Western and Central US. HREF probabilities are virtually
identical in the depiction with a >3" neighborhood probability
still lying within the 20-30% range with the best chance occurring
over the Front Range of WY near Cheyenne down into the Northwest
corner of KS. There was some CAMs support for the complex of
storms to migrate a bit further southeast into Northern KS before
dissipating by the morning a cold front associated with the
convective pattern pushes eastward and the LLJ ahead of the front
gets squashed leading to less of a convergence signal. Still
enough to warrant a small expansion across the aforementioned area,
but certainly not enough to raise the risk away from the MRGL
forecast. The Monsoon convection across AZ will flare up once again
later this afternoon and evening with probability fields targeting
Southeast AZ near the Huachucas up into the I-10 corridor near
Tuscon. More convection will fire along the Mogollon Rim as well,
but the QPF footprint was not amped enough to signal anything
higher than a standard MRGL. This allowed for a general continuity
over the Southwest overall.
Kleebauer
...Previous Discussion...
The increased moisture flux from the Gulf of California will spread
to the north and east across the Four Corners region, helping
maintain enhanced rainfall during the afternoon and evening. There
is a growing consensus for upstream convective growth around the
northern periphery of the ridge with sights downstream into the
High Plains off the Colorado/Wyoming Front Range. The nature of
this convection is expected to be progressive which should limit the
flash flood potential to a low-end MRGL threat, however a narrow
corridor of 20-30% probability for >3" exists within the latest
HREF output with hourly rates generally between 1-2"/hr at peak
intensity. A Marginal Risk area covers much of the Four Corners
region and extends into southeast Wyoming, western Nebraska and
Kansas.
Campbell/Kleebauer
Day 2
Valid 12Z Thu Aug 08 2024 - 12Z Fri Aug 09 2024
...THERE IS A HIGH RISK FOR EXCESSIVE RAINFALL FOR PORTIONS OF
SOUTH AND NORTH CAROLINA...
...WIDESPREAD AND NUMEROUS INSTANCES OF FLASH FLOODING WITH
SIGNIFICANT AND CONSIDERABLE FLOODING LIKELY...
...Southeast and Mid Atlantic...
The latest trend with Debby's track is for it to press back inland
of the Carolinas with the primary circulation a bit further west
compared to the latest forecast. Ensemble trends within the QPF
field followed suit with further push towards the Carolina Piedmont
leading to a higher risk of heavier rain focused inland from I-77
and points east to around I-95. Current forecast is for a
widespread 3-6" with local to 8" of rainfall encompassing the
Midland's of South Carolina across into East-Central North Carolina
and South Carolina.
During this period the highly anomalous airmass will range from +2
to +4 standard deviations above normal for early August for this
part of the country and will be drawn northward ahead of an
approaching trough/cold front to the east. This will be conducive
for heavy precipitation to make it into parts of Southern and
Central Virginia with heavy rains extending back near the Southern Appalachians.
The setup remains tricky considering the weak
steering pattern around Debby as the low gets absorbed into the
eventual front.
A High Risk remains in effect for central/southern North Carolina
and northeast South Carolina with minimal adjustments from the
previous forecast. However, the southern bounds of the Moderate
and Slight Risk areas were trimmed northward given the threat for
excessive rainfall has reduced across southern South Carolina and
southeast Georgia.
...Four Corners, Central Rockies and High Plains...
The monsoonal pattern should yield a targeted focus for heavy rain
across portions of the Central and Southern Rockies. Shortwave
energy passing through the Rockies will help provide additional
forcing for convection to focus along the higher terrain of
Colorado and New Mexico. Model guidance suggests a concentration of
cells capable of producing 1 to 2 inches with very isolated maxes
greater than 3 inches. A Slight Risk remains in effect for
portions of central Colorado and northern New Mexico given the
increased risk for isolated to scattered flooding concerns. A
Marginal Risk covers much of the Four Corners region, western
Kansas and Nebraska, southwest South Dakota, eastern Wyoming and
extreme southeast Montana.
Campbell/Kleebauer
Day 3
Valid 12Z Fri Aug 09 2024 - 12Z Sat Aug 10 2024
...THERE IS A MODERATE RISK FOR EXCESSIVE RAINFALL FOR PORTIONS OF
THE NORTHEAST...
...Mid-Atlantic to the Northeast...
During this period Debby is anticipated to transition to
extratropical and absorb into the system in tracking through the
Northeast. The deep moisture from Debby will feed into the
advancing cold front and yield moderate to heavy rain from the
Piedmont region of the Maryland northward to Upstate New York,
Vermont and New Hampshire. A Moderate Risk area was inherited for
this period and was maintained with a small westward shift from the
previous area. As such, there was a modest westward adjustment of
the Slight Risk area over southern New England. While the new
rainfall over the Mid-Atlantic and Southeast region will be greatly
reduced there will still be a lingering Marginal/Slight Risk for
continued flooding concerns over the extremely sensitive soils.
...Four Corners, Central Rockies and High Plains...
Shortwave energy exiting from the Rockies into the Plains will
continue to support more organized convection across Colorado and
New Mexico and eastward into western Kansas and the panhandles of
Oklahoma and Texas. Moderate to heavy rainfall will focus in the
vicinity of the higher terrain. A Slight Risk was already in effect
for portions of central Colorado and north/northeast New Mexico but
with the latest guidance and QPF trends, the Slight was shifted a
little to the south. A broad Marginal Risk covers much of the Four
Corners region, east into western Kansas and northward into south-
central Montana.
Campbell
Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=
v/qpf/94epoints.txt__;!!DZ3fjg!_04uR-lckFpHqP2sfoTzDz7m9heOUB4f1dfZrJl9SZk0=
JaCz4c60sliuf1UkqRV1IoEmJJsYRuVY6JoR6A8-niNklsQ$=20
Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=
v/qpf/98epoints.txt__;!!DZ3fjg!_04uR-lckFpHqP2sfoTzDz7m9heOUB4f1dfZrJl9SZk0=
JaCz4c60sliuf1UkqRV1IoEmJJsYRuVY6JoR6A8-ufhMvXc$=20
Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=
v/qpf/99epoints.txt__;!!DZ3fjg!_04uR-lckFpHqP2sfoTzDz7m9heOUB4f1dfZrJl9SZk0=
JaCz4c60sliuf1UkqRV1IoEmJJsYRuVY6JoR6A8-OaAjZgM$=20
=3D =3D =3D
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