575
AGUS74 KWCO 291518
HMDNWC

National Hydrologic Discussion - EXPERIMENTAL
NWS National Water Center - Tuscaloosa AL
1015 AM CDT SUN SEP 29 2024

.Synopsis...
Widespread river flooding and lingering flooding impacts across the
Southeast into the Southern Appalachians... Isolated urban and small stream
flooding possible in the Central Appalachians...

.Discussion...

.Southeast into the Southern Appalachians...
Residual considerable flooding impacts are expected to persist for the
foreseeable future as the Southern Appalachians recover from Helene. Rivers
draining the Blue Ridge Escarpment such as the Broad, French Broad, and
Catawba have crested, and should continue to slowly recede. A much needed
dry spell over the next week should allow for adequate recessions.

Further south into the coastal plains, widespread minor and moderate river
flooding, as well as isolated major river flooding, will persist well into
next week, as runoff from Helene continues to route downstream. Crests are
expected along these rivers by mid-week, and dry weather through day 7
(Sat) should allow for unimpeded recessions. Looking ahead, both the GFS
and NBM-forced National Water Model (NWM) are indicating scattered stream
responses across the northern Gulf Coast as potentially heavy rainfall
impacts the region, although there is high uncertainty in the exact
location and magnitude due to the extended nature of the forecast. However,
given how saturated this region is from Helene, this region will need to be
closely monitored by potential new flooding impacts.

.Central Appalachians...
Periods of convective rainfall through day 2 (Mon) may generate isolated
urban and small stream flooding. Antecedent conditions across MD and
southern PA are generally on the dry side, which may preclude most if any
hydrologic responses. However, further south into VA and WV, recent
rainfall has wetted soils, thus elevating soil moisture and streamflow
conditions. This factor, combined with the complex terrain of the region,
will enhance the runoff potential, especially in areas where more robust
rainfall rates are to materialize. The HRRR-forced NWM is indicating
isolated small stream responses across the region for later today. Isolated
significant stream rises are possible, as corresponding annual exceedance
probabilities from the High Flow Magnitude are as low as 10%.

//Pritchard



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