395
AGUS74 KWCO 182047
HMDNWC

National Hydrologic Discussion - EXPERIMENTAL
NWS National Water Center - Tuscaloosa AL
300 PM CDT FRI SEP 18 2026

...Considerable flooding impacts possible across portions of the
Upper
Midwest into the Upper Ohio Valley Flooding impacts possible for
the
Central Appalachians into the Mid-Atlantic Isolated flash and
small
stream flooding impacts are possible across New Mexico Flash and
small stream flooding possible for the Northern Rockies into the
Northern High Plains

.Upper Midwest into the Upper Ohio Valley
A Considerable flood threat area was introduced today across portions
of northern Illinois into northern Indiana, where multiple rounds of
heavy rainfall through the weekend will increase the potential for
significant flash, urban, and river flooding impacts. Additionally a
Limited flood threat area remains in place across the broader Upper
Midwest into the Upper Ohio Valley region where flooding impacts will
be possible through Monday.

In the short-term, the primary focus will be across portions of
southeast Minnesota, northeast Iowa, and southern Wisconsin through
early Saturday morning where isolated flash and urban flooding will
be
possible as the first round of heavy rainfall moves through.
Antecedent conditions leading into this event are variable, with near
surface layer soils across Minnesota and Iowa wet and primed
following
recent rainfall  ...65 - 80 percent RSM, 0 - 10 cm, NASA SPoRT ...
and
streamflows more locally elevated. Wisconsin on the other hand
remains
on the drier side, which will help to limit hydrologic responses
initially. While relatively dry deeper soils across portions of the
region provide some additional infiltration capacity, high rainfall
rates can quickly exceed infiltration rates and produce sharp
responses on smaller streams and urban drainage systems. This
corridor
is also where the HRRR-forced National Water Model  ...NWM ... Short
Range Forecast...SRF ... is beginning to highlight rapid-onset
flood
potential, though probabilities are generally less than 50 percent as
the bulk of the QPF is not fully captured in the 12-hr window yet.
Regardless, rain rates will be the primary driver of any likely
flooding with this first round.

The focus then shifts a little further south on Saturday, where
considerable flash and urban flooding is possible from northeast
Illinois into northern Indiana. River flooding will remain an
additional concern across portions of this area, particularly where
already elevated flows coincide with additional heavy rainfall.
Ongoing moderate river flooding in northeast Indiana, along with
elevated flows farther west, will limit the capacity of rivers and
streams to absorb additional rainfall. Additional rises and prolonged
recession times will therefore remain possible on already elevated
rivers and streams, especially where heavier rainfall repeatedly
affects the same basins. HEFS ensemble guidance also supports the
potential for additional rises across portions of the area, with the
30 percent worst-case scenarios highlighting higher-end possibilities
where additional heavy rainfall occurs. This is also supported by
both
the GFS- and NBM-forced NWM Medium Range Forecasts which continue to
highlight this corridor for hydrologic responses over the next few
days.

.Central Appalachians into the Mid-Atlantic
Flooding impacts are possible beginning as early as Sunday and into
early next week due to multiple rounds of rainfall for West Virginia
and Virginia into southern New York and Connecticut. Antecedent
conditions are variable across the region which may initially
mitigate
runoff, but more saturated areas may become overwhelmed with repeated
rainfall and initiate efficient runoff. Flooding in heavily urbanized
areas, particularly along the I-90 corridor,  is also possible where
high rain rates or repeated rainfall set up. The NBM- and GFS-Forced
NWM are beginning to show some agreement for impacts beginning on
Sunday for the northern portions of the area of concern, but exact
placement of greatest impacts are still uncertain.

.New Mexico
Isolated flash and small stream flooding is possible through tonight
as a surge of monsoonal moisture continues to fuel multiple rounds of
thunderstorms across the state. From previous rainfall over the past
week along with an overall active season has caused basins in the
area
to be more hydrologically sensitive and thus more prone to excessive
runoff where additional rainfall occurs. Excessive runoff into
rivers,
streams, creeks, and dry washes are possible with several locations
already experiencing flooding impacts. While mainstem river flooding
is unlikely, sharp rises from rapid runoff can cause additional
flooding impacts as it continues downstream into drier areas. Also of
concern are areas of steep terrain, slot canyons and burn scars that
could experience rapid runoff responses where heavier rainfall rates
materialize.

After a brief break in thunderstorm potential over the weekend, a
renewed flood potential returns beginning next Tuesday as another
surge of monsoonal moisture is forecast for the area. A break in
heavy
rainfall may allow for soils to recover and thus mitigate flooding
threats during the initial return of thunderstorms. The NBM- and
GFS-Forced NWM both currently support an increased potential for
flooding impacts within the 10-day timeframe; however, there is still
uncertainty on exact placement and magnitude of impacts this far out
in the forecast.

.Northern Rockies into the Northern High Plains
Flash and small stream flooding is possible for Idaho into late
tonight and through Sunday afternoon for southern Montana and
northern
Wyoming as ongoing and forecast rainfall continues. Antecedent
conditions remain fairly dry, but areas that receive repeated
rainfall
may eventually become overwhelmed and initiate runoff along with
quick
rises on streams and creeks. Burned areas are also of great concern
for excessive runoff and debris flows should heavier rainfall rates
set up.

.Alaska
Additional rounds of rainfall are expected across Interior Alaska
through the weekend, with already wet antecedent conditions allowing
for efficient runoff and may result in sharp rises on smaller streams
and tributaries. The primary areas of interest remain the Upper
Kuskokwim Valley and the Chena River Basin, though the overall
flooding threat is expected to remain isolated and mostly low-end.
Given the limited nature of the expected impacts, no FHO area is
warranted.

.Hawaii
Hawaii was removed from this afternoons FHO issuance as the flood
threat has diminished.

//EAS/RMF

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