510
AGUS74 KWCO 191151
HMDNWC

National Hydrologic Discussion - EXPERIMENTAL
NWS National Water Center - Tuscaloosa AL
700 AM CDT SAT SEP 19 2026

.Update...

This morning`s update included a slight expansion of the through Day 3
Considerable selection for the Upper Midwest into the Ohio Valley further
north into Wisconsin, and an overall trimming of the western/northern edge
of the surrounding Limited selection to better match the QPF and recent
QPE. The Through Day 3 Limited selection for the Northern Rockies into the
Northern High Plains was trimmed out of Idaho as the threat has diminished,
and locally considerable" wording was added for the flash and small stream
flood threat, especially for over portions of southern Montana. The New
Mexico and Central Appalachians into the Mid-Atlantic selections were left
unchanged, but a close is being kept on New Mexico given the multiple days
of rainfall in the extended forecast. A Through Day 7 Limited selection was
also added for Guam and Rota through Thursday.

//Bliss

Previous discussion as follows

...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% 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% 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% 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

Additional National Water Center products are available at
www.weather.gov/owp/operations