560
AGUS74 KWCO 301211
HMDNWC

National Hydrologic Discussion - EXPERIMENTAL
NWS National Water Center - Tuscaloosa AL
700 AM CDT WED SEP 30 2026

.Update...

The Northern Mariana Islands were introduced on this mornings FHO
issuance, with Saipan and Tinian included in a Considerable flood threat
area, where significant flash flooding impacts may occur Thursday and into
the weekend as a developing tropical system approaches the islands. Guam
and Rota were included in a Limited flood threat area, where monsoonal
moisture may support locally heavy rainfall and flash flooding, but with a
lower overall threat than across Saipan and Tinian.

Additionally, the Considerable area across Arizona, New Mexico, and the
Texas High Plains was downgraded to Limited as the broader flood threat
continues to diminish, though locally considerable flooding impacts remain
possible. Adjustments were also made to the broader Limited to align with
current rainfall forecasts.

No changes were made to the remaining regions.

//RMF

Previous Discussion as Follows

...Considerable flash flooding impacts possible in the Southwest into the
Central High Plains... Locally considerable flash and river flooding impacts
possible for the Central Plains into the Middle Mississippi Valley... Locally
considerable flash, urban, and river flooding possible across the Southern
Plains...

.Southwest into the Central High Plains...
Considerable flooding impacts remain possible for portions of eastern AZ
and much of NM into the TX Panhandle through tonight as deep tropical
moisture continues to surge through the region. Due to heavy rainfall over
the past 24 hours, most of the topsoils across central AZ and NM are at 60
- 70% saturation (RSM NASA SPoRT) and increasing, making these basins much
more sensitive to additional rainfall. Streamflows are still increasing,
and many are already at an all time high for this time of year (USGS),
which suggests little to no available in-channel storage capacity. The
HRRR-forced National Water Model (NWM) Short Range Forecast (SRF) is
highlighting the strongest responses across eastern AZ through much of NM
and into the TX Panhandle, suggesting the most potential for rapid onset
flooding across this corridor. The NWM 18-hour Maximum High Flow Magnitude
Forecast is also highlighting a similar area across eastern AZ into central
NM of annual exceedance probabilities lower than 10% (higher magnitude
flows) suggesting increasing potential for out of banks flows.
Additionally, isolated minor river flooding is ongoing or forecast across
parts of eastern AZ. Given the continued widespread possibility for heavy
rainfall, the remainder of the region is highlighted in a Limited flood
threat indicating the potential for flash, urban and small stream flooding
impacts, some of which may be locally considerable, particularly near burn
scars, low-water crossings, and mountain drainages or areas of complex
terrain.

.Central Plains into the Middle Mississippi Valley...
Flash, urban, and river flooding impacts are possible through Thursday as a
storm system moves across the region. Widespread totals of 2 - 4" and
locally up to 6" of rainfall are forecasted (WPC). Areas that are more
saturated, such as parts of northern NE, are more vulnerable to flooding
impacts, some of which may be locally considerable depending on where the
highest rainfall totals occur. NWM SRF signals have consistently been
showing the potential for rapid onset flooding, particularly across
southeastern NE, northeastern Kansas, and into central and eastern IA.
Additionally, river ensemble forecasts (HEFS) are showing a robust river
response with potential widespread minor, scattered moderate, and isolated
major flooding at the 50% chance exceedance. With these strong responses,
confidence has increased in the potential for locally considerable flooding
impacts across the region.

.Southern Plains...
Locally considerable flash, urban and river flooding is possible due to
heavy rainfall beginning Wednesday and continuing into early Friday. The
heaviest rainfall is forecast to be around 3 - 5" with locally higher
amounts near and around the I-35 corridor through central TX. Very dry
antecedent conditions across central TX should help mitigate runoff
initially, but potentially repeated rainfall and higher rates could quickly
overwhelm soils and lead to rapid response runoff. Both the GFS- and
NBM-forced NWM are showing strong responses across central TX suggesting
some increased confidence with rapid stream responses along with flash and
urban flooding impacts, some of which may be locally considerable. However,
there is still some uncertainty in the exact placement of greatest flooding
impacts. River flooding will also be a concern for this event, but there is
still some uncertainty with the magnitude of river flooding impacts at this
point in the forecast although river ensembles are suggesting potential for
river rises across in central TX. Larger rivers will see delayed responses
later this week as water routes downstream.

//EAS/Cordak

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