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AGUS74 KWCO 061959
HMDNWC

National Hydrologic Discussion - EXPERIMENTAL
NWS National Water Center - Tuscaloosa AL
300 PM CDT SUN SEP 6 2026

...Considerable flash flooding and mudslides are possible for the Hawaiian
Islands due to Hurricane Lowell through Tuesday...Locally considerable flash
and small stream flooding is possible this evening for the Mid-Missouri
River Basin...Flash and small stream flooding is possible through Tuesday
morning for Northern Montanna... Isolated flash and arroyo flooding possible
for the Great Basin and Southwest for tonight...

.Hawaii...
Considerable flash flooding is possible for Kauai County. Locally
considerable flash flooding is expected for the remainder of the island
chain, particularly on the southeastern slopes of the Big Island, all of
which will begin this evening and continue through Tuesday due to
potentially torrential rainfall from Hurricane Lowell. These areas remain
hydrologically vulnerable from previous rainfall events, thus the continued
heavy rainfall rates may initiate quick-response runoff and rises on
streams and creeks. As such, out of bank responses and significant urban
flooding impacts should not be ruled out. Mudslides will also be possible
in areas of steep terrain. As the event continues to unfold this area will
be closely monitored for any additional upgrades to "Considerable", notably
to the Big Island where terrain enhancement and moisture from the eastern
flank of the hurricane interact.

.Mid-Missouri River Basin...
Locally considerable flooding impacts are possible through late tonight
across the area with the greatest flooding concern in northwestern Iowa.
Current Hi-res guidance suggests that 2 - 4" of rainfall with isolated
higher amounts possible. Sharp out of bank rises along with flash and urban
flooding impacts are possible in locations that experience repeated rounds
of convective rainfall with embedded heavy rainfall rates. Regardless of
current antecedent conditions in the basins, if these amounts were to occur
it would quickly overwhelm any mitigating factors that are in place. The
HRRR-forced NWM is signaling some potential for locally significant
responses and rapid onset flooding on small order streams primarily in the
Rock (MN/IA), Maple (IA), East Soldier (IA), and Boyer (IA) river basins.
Fortunately, river ensemble guidance is not suggesting much in terms of
river flooding even in the 10% chance of exceedance. This area will be
placed in a "Limited" flood impact category for this evening with locally
considerable wording.

.Northern Montanna...
Several rounds of thunderstorms tonight through Tuesday morning will bring
locally heavy rainfall to the area which may cause flash and small stream
flooding impacts. Rock and mudslides are also possible in areas of steep or
complex terrain. With multiple days of rainfall antecedent conditions are
likely to be primed for hydrologic responses and efficient runoff.
Typically flood prone and low-lying areas are of concern along with rapid
rises on small order streams. The GFS-forced NWM is suggesting some
potential for 20 - 10% annual exceedance probabilities with peaks flowing
generally over the next 72 hours with the larger order rivers responding by
mid-week providing some additional confidence in "Limited" flooding impacts
selection.

.Great Basin and Southwest...
Isolated flash and arroyo flooding will be possible over portions of
Arizona, Utah, and Nevada starting early Sunday morning through late
tonight. Flash flooding and debris flows will also be possible near
recently burned areas.

.Georgia and South Carolina...
While there are some concerns for the potential for heavy rainfall in the
area, overall confidence is low for flooding impacts primarily due to
overall dry soils and near normal to much below normal streamflows. Broadly
these basins appear to be capable of handling moderate to locally heavy
rainfall without much impact. However, with the convective nature of the
rainfall and widely scattered placement localized flooding and/or nuisance
flooding impacts are possible. Due to the overall lack of NWM responses,
forecast scattered rainfall, and dry conditions this area will not be
included in a selection.

//JAC

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