336
AGUS74 KWCO 241521
HMDNWC

National Hydrologic Discussion - EXPERIMENTAL
NWS National Water Center - Tuscaloosa AL
1015 AM CDT TUE SEP 24 2024

.Synopsis...
Tropical Storm Helene to bring flooding impacts to portions of the
Southeast into the Mid-Atlantic... Isolated flash flooding possible across
the Middle Mississippi Valley... Coastal flooding along the Eastern
Seaboard... Potential for flooding in Puerto Rico...

.Discussion...

.Southeast into the Mid-Atlantic...
Considerable flooding impacts are possible with Tropical Storm Helene
poised to bring heavy rainfall to portions of these regions on days 2 - 4
(Wed - Fri). Looking at the Southeast, antecedent soil conditions remain
mostly unchanged much of FL and the coastal plain of GA/SC remain the
wettest throughout the soil column with much drier soils further inland in
GA, AL, SC, and TN (NASA SPoRT). Streamflows are reflective of the soil
conditions, with normal to above normal flows in FL and southern GA and
below to much below normal flows elsewhere in the Southeast. Drought
conditions are also present across much of the Southeast. While the dry
antecedent conditions should mitigate widespread significant impacts,
forecast rainfall amounts of 3 - 7" in northern FL, GA, AL, eastern TN, and
western SC are likely to produce flash, urban, and small stream flooding,
with considerable impacts possible in the complex terrain of the southern
Appalachians, on days 2 - 3 (Wed - Thu).

In the Mid-Atlantic, particularly in western portions of NC and VA,
antecedent soil conditions are wettest in western NC and much drier in
western VA (NASA SPoRT), and streamflows are below to much below normal
across the area. Much like in the Southeast, these generally dry antecedent
conditions should mitigate widespread significant impacts. However, the
complex terrain of these areas will allow for the possibility of
considerable impacts from the 3 - 7" forecast rainfall amounts, primarily
on days 3 - 4 (Thu - Fri).

River flooding is also possible across the aforementioned areas, though the
exact location and magnitude remain somewhat uncertain. Current river
ensemble forecasts (MMEFS) indicate varying location of river flooding,
with one solution indicating river flooding primarily across the Southeast
and the Carolinas (NAEFS) and other solutions limiting the most significant
flooding to the Carolinas (GEFS, HEFS). As such, confidence remains low in
determining the exact location and magnitude of river flooding from Helene,
though the chance of widespread minor to moderate and isolated major river
flooding in these regions appears to be increasing.

.Middle Mississippi Valley...
Moderate to heavy rainfall will impact the Ozarks and Middle Mississippi
Valley days 3 -  5 (Fri - Sun) and with it the threat of flash flooding is
possible. Even with recent rainfall, this region still remains very dry
with streamflows ranging from normal to much below normal (USGS). There is
potential for river flooding with southern region QPF ensembles signaling
potential for isolated minor flooding throughout the region. Soils still
remain on the drier side, likely providing infiltration capacity, at least
initially. Even though antecedent conditions are drier, forecast rainfall
may still produce flooding impacts, especially in flashier basins, and in
low lying flood prone areas. The complex terrain of the Ozarks will enhance
runoff and increase likelihood of flooding.

Though there is still uncertainty with regard to where the heaviest
rainfall will set up, both the NBM and GFS forced National Water Model
Medium Range Forecast are signaling potential for small stream responses
throughout this region in the days 4 - 5 time frame.

.Eastern Seaboard...
Widespread minor-to-moderate coastal flooding associated with high
astronomical tides and northeast flow continues to be possible from the
Carolinas to Long Island through today. Check with your local NWS office
for more information related to coastal flooding impacts.

.Puerto Rico...
Continued showers today may lead to potential for flooding and mudslides in
areas of steep terrain, mainly along Cordillera Central.

//Ayala/Watson

$$