FOUS30 KWBC 071236
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
836 AM EDT Wed Aug 7 2024

Day 1
Valid 12Z 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...

...Southeast...

Heavy rain from the very slow-moving Debby is expected to persist
across portions of Northeastern South Carolina and Southeast North
Carolina; likely resulting in continued significant rainfall and
flooding over the High risk area. Bands of heavy rain are forecast
to pivot all the way north into Southern Virginia and as far west
as Central North Carolina, but the coverage and magnitude of the
heavy rain threat is much less compared to near Debby's
circulation. Forecast QPF still references between 4-8+" through
the period. HREF neighborhood probability for >5" is very
aggressive with a large footprint of 80-90+% probs located across
the entire High Risk area with the primary location situated just
inland of Wilmington, NC. This area has the opportunity for a 48hr
total exceeding 15" leading to the threat for a prolonged life-
threatening flash flood scenario.

...Mid Atlantic...

A weak front will bisect the Mid Atlantic with multiple surface
waves riding along the boundary, especially early on in the period.
Area PWATs will remain elevated as moisture on the northern
periphery of Debby will intersect a stationary front and align
west to east along said surface front. The primary period for
impact is likely over the course of the morning as a stronger
mid-level perturbation traverses over the Central Mid Atlantic
with convection situated along and within the confines of the
front. There is a signal for a narrow corridor of heavy rainfall
followed by a broader footprint from the Piedmont region of
Maryland to the DelMarVa.

Neighborhood probabilities have increased since the previous HREF
issuance with decent areal extent of 50-70% probs for >3" with a
20-30% area for >5" located over much of DE into the eastern shore
of MD. 2"/hr rates are also forecast within the heavier convective
cores as depicted within the HREF probability fields, running a
good 25-45% outline between 12-18z across the Delmarva,
correlating with the higher neighborhood probabilities mentioned
before. A Slight Risk area remains in effect for eastern Maryland,
Delaware, southern New Jersey and southeast Pennsylvania.

...Southwest...

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 RISKFOR 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
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