570
AGUS74 KWCO 152008
HMDNWC

National Hydrologic Discussion - EXPERIMENTAL
NWS National Water Center - Tuscaloosa AL
300 PM CDT TUE SEP 15 2026

...Considerable flooding impacts possible across the Central Plains into
the Upper Great Lakes and Ohio Valley...Locally considerable flash and small
stream flooding impacts are possible throughout the week across the
Southwest into the Great Basin...Flash and river flooding is possible
through mid-week for portions of Alaska...Flash flooding is possible
Wednesday (local time) for American Samoa...

.Central Plains into the Upper Great Lakes and Ohio Valley...
Considerable flooding impacts remain possible through late this week in
portions of Iowa, northern Missouri, extreme southeast Nebraska, and
extreme northeastern Kansas as additional rounds of rainfall are forecast
across the region. Previous model guidance had suggested more of an
overlapping threat than the current signals may suggest. The Considerable
category will be maintained through this evening, but if confidence in
model guidance continues to diminish through the evening, this area may be
downgraded for the morning issuance. Through tonight, the current National
Water Model (NWM) Short Range Forecast (SRF) guidance is suggesting that
the greatest magnitude responses of 10 - 4% are possible in the Upper Grand
(MO), Platte (MO), and Delaware (KS) river basins. Elevated unit discharge
signals have also provided some additional confidence in rapid responses on
rivers and streams in these basins. River ensemble guidance continues to
suggest some potential for significant river responses in Iowa and northern
Missouri as a reasonable worst-case scenario, but as mentioned above, this
is largely influenced by the overall placement and magnitude of rainfall
over the next 48 hours.

By Thursday and continuing into the weekend, the system lifts northward and
increases in extent more into the Upper Great Lakes and Ohio River Valley.
Large swaths of 1 - 3"+ of rainfall are forecast, which will likely
overwhelm soils and streams in the area, increasing the vulnerability for
hydrologic impacts. Locally considerable impacts should not be ruled out,
particularly in the flashier basins or in typically flood-prone or
low-lying areas. The GFS and NBM-forced NWM are signaling some isolated
chances of significant flows in the Lake Winnebago as well as the Upper and
Middle Rock river basins in Wisconsin with peak flows forecast this
weekend. Scattered NWM responses are beginning to show across the Ohio
Valley and are likely to increase in extent if current trends continue.

.Southwest into the Great Basin...
Locally considerable flash and small stream flooding impacts are possible
throughout the week, especially across Arizona and New Mexico. Areas of
steep terrain, slot canyons, and recent burn scars present the highest
flood impact potential through Friday. In the short-term, the HRRR
indicates scattered showers with .5" - 2" of rainfall totals across
Arizona, New Mexico, southern Utah and southwest Colorado this through this
evening which may lead to isolated flooding. The NASA SPoRT 0-10cm Relative
Soil Moisture(RSM) index indicates localized RSM up to 65% across the
southwest portion of Arizona, western counties of New Mexico, and
southwestern counties of Colorado where soils have moderate to high runoff
potential and will receive repetitive events of rainfall throughout the
week.

The region is expected to remain in an active rainfall pattern through the
week which is reflected by the NWM MRF. The NWM MRF (both GFS and NBM
forced versions) continues to indicate potential for fairly widespread
small stream responses in headwater basins draining the terrain of the
eastern slopes of the Black Range Mountains, the San Andres Mountains, and
the Sangre de Cristo Mountains in New Mexico through Friday. This is a
fairly robust and uncommon signal from the NWM for this part of the
country, and supports this regions continued inclusion in the FHO, with
the Limited selection also expanded into the Great Basin to capture the
increasing flood threat to the north.

.Alaska...
Small stream and localized river flooding is possible as heavy rain
continues through Wednesday across portions of west-central to
south-central Alaska. With already saturated ground conditions across the
region (Variable locations 85% 0-10cm Relative Soil Moisture SPoRT) and
already elevated streamflows, highwater flows and bankfull conditions are
likely through the week as area rivers respond. Localized flooding on
smaller creeks and streams are likely. However, significant mainstem river
flooding is not expected.

.American Samoa...
Locally heavy rainfall is forecast through Wednesday (local time) which may
cause isolated flash and urban flooding, particularly in typically flood
prone or low-lying areas. As the system moves across the island, saturated
soils will enhance the flood threat and may cause landslides, particularly
in areas of steep or complex terrain. As such, this area will be included
in the Limited selection for this issuance.

.Hawaii...
Remnant moisture from Norbert may bring scattered heavy rainfall totals to
the island chain late Wednesday through Friday which may elicit isolated
flash, urban, and small stream flooding responses. An active tropical
season has kept soils moist and streams vulnerable to enhanced runoff with
little in-channel storage. While there are favorable hydrologic conditions
for responses, confidence is not quite high enough in the placement or
magnitude of rainfall to introduce a Limited selection for the Hawaiian
Islands, but as the event draws closer this area will be reevaluated for a
possible inclusion.

//RK/JAC

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