930
AGUS74 KWCO 071957
HMDNWC

National Hydrologic Discussion - EXPERIMENTAL
NWS National Water Center - Tuscaloosa AL
300 PM CDT MON SEP 7 2026

...Considerable flash flooding and mudslides are possible for the Hawaiian
Islands due to Hurricane Lowell through Wednesday...Flash and small stream
flooding is possible through Tuesday morning for Northern Montana...Isolated
flash and urban flooding is possible in the Upper Ohio Valley Thursday...

.Hawaii...
Considerable flash flooding is likely for Kauai County and along the
southeastern slopes of the Big Island due to torrential rainfall from
Hurricane Lowell. Across the remainder of the island chain locally
considerable flooding impacts are possible. All categories of impacts are
expected to continue through Wednesday with mudslides possible in areas of
steep or complex terrain. These areas remain hydrologically vulnerable from
previous rainfall events and have been primed from some rainfall ahead of
this storm. As such, the continued heavy rainfall rates will initiate
quick-response runoff and sharp rises on streams and creeks. Urban flooding
is very likely to occur in the Considerable selections which may generate
significant property damage due to the rapid runoff.

.Northern Montana...
Several rounds of thunderstorms 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 will 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 continues to suggest some potential for
isolated 20 - 10% annual exceedance probabilities, with peaks flowing
generally over the next 48 hours.

.Upper Ohio Valley...
A series of thunderstorms are possible Thursday across the area, which may
bring isolated flash and urban flooding concerns. While there has been some
recovery of the ground conditions from previous rainfall events,
streamflows are still much above normal to high for this time of year. This
combined with the possibility of enhanced runoff over steep terrain would
suggest hydrologic conditions in the basins would not need much rainfall to
generate isolated flooding impacts. Training rainfall is also a concern,
which could generate some additional flooding impacts to typically flood
prone and low-lying areas. The GFS-forced NWM is suggesting some isolated
chances of rapid onset flooding, with annual exceedance probabilities
generally between 50 - 20%. The highest probabilities of rapid onset
flooding are currently in eastern Ohio, western Pennsylvania, and
northwestern West Virginia. While there is still some uncertainty to the
overall placement and magnitude of responses due to the extended timing of
this event, this area will be included in the FHO this afternoon as a
Limited with the expectation that there may be some minor adjustments in
the coming days as model guidance continues to coalesce into a solution.

.Iowa...
Training thunderstorms are possible Tuesday across the Middle Mississippi
with the highest chances in Iowa. However, dry antecedent conditions and
normal streamflows would suggest there is an initial buffer to hydrologic
responses. Model guidance is quite spread for this rainfall, significantly
influencing the overall confidence of any placement or extent of potential
impacts. While convective rainfall and training thunderstorms can cause
localized flooding, there is not enough model consensus to provide enough
confidence to include a Limited selection for this afternoons FHO issuance.

//JAC

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