640
AGUS74 KWCO 212008
HMDNWC

National Hydrologic Discussion - EXPERIMENTAL
NWS National Water Center - Tuscaloosa AL
300 PM CDT TUE JUL 21 2026

...Isolated flash and urban flooding along the Gulf Coast this week...Flash
and urban flooding impacts are possible across the Desert Southwest into
the Northern Rockies through Wednesday...Flash, urban, and small stream
flooding possible over the Ohio and Tennessee Valley and Mid Atlantic into
the Northeast into tomorrow...Flooding impacts possible in the Carolinas
and southern Virginia from multiple rounds of heavy rainfall through the
week...Moderate to major river flooding will continue across portions of
South-Central Texas through the week...Atmospheric river impacts portions
of Alaska through Friday...

.Central Gulf Coast...
Tropical Storm Bertha continues to exhibit a flash flood threat along the
Gulf coast through the week. Current forecasts suggest that the heaviest
precipitation is likely to occur primarily along the I-10 corridor from the
Florida Panhandle into the central Louisiana coast. The heaviest rainfall
is expected to remain off the coast, and top soils and streamflows are
generally near normal across much of this area, which will help to mitigate
widespread flooding from the rain bands that make it onshore. The initial
concerns for this event are likely flash and urban flood impacts wherever
repeated rounds of rainfall materialize. The storm is forecast to proceed
westward along the Gulf Coast and potentially reach Texas by Friday,
although as of now the flood threat in Texas appears to be minimal.

.Desert Southwest into the Northern Rockies...
The threat of isolated flash and urban flooding continues through Wednesday
for much of the Desert Southwest and central/northern Rockies. Antecedent
conditions in Arizona and western New Mexico are more hydrologically
sensitive due to the persistent, almost daily rounds of rain that have
fallen over the last week. The rest of the region is not as primed for
runoff to form, but nonetheless heavy rain in complex terrain could lead to
flash and urban flooding, along with flooding slot canyons, arroyos, low
water crossings, and low-lying areas. Anywhere heavy rainfall sets up in
the vicinity of a burn scar is at an elevated risk of flash flooding and
debris flows.

.Carolinas through the Delmarva Peninsula...
Widespread rainfall totals of 3 - 7" are forecast to fall across the region
beginning Wednesday and carrying into the weekend. As a result, flash,
urban, small stream flooding is possible, along with rises on rivers.
Although recent rainfall has moistened topsoils in eastern North Carolina,
deeper layer soils remain very dry and streamflows are mostly below normal,
with the exception of the Raleigh area where streams are running much above
normal from recent heavy rain. The dry deep layer soils will help mitigate
widespread river flooding, although river ensembles are beginning to show
potential for isolated rises to minor flooding or higher. The primary
threat from this event will be flash/urban at first, but as the repeated
rounds of rain begin overwhelming the dry antecedent conditions,
out-of-bank rises on small streams and rivers will become a threat as well.
The flood threat, especially for rivers, will be closely watched and
updated as the event plays out. The NBM-forced National Water model
continues to signal fairly widespread exceedance of the 50% annual
exceedance probability (AEP) threshold across parts of eastern North
Carolina, while the GFS is struggling to capture the flood potential in the
region at all.

.Tennessee and Ohio Valleys...
Heavy rainfall is ongoing across the region, and additional rounds are
possible through tonight and into the early morning tomorrow, which will
bring the potential for isolated flash, urban, and small stream flooding.
Soils are near to slightly below normal leading into the event, which will
allow some infiltration capacity for rainfall before it converts to runoff,
but streamflows are near normal to above normal across the region and
out-of-bank rises on flashier small streams are possible. The areas of
higher concern are of course any urban environment where impermeable
surfaces enhance the runoff potential, and in the complex terrain of the
Appalachian Mountains. Signals from the HRRR-forced NWM are suggesting
there is some potential for locally significant flows, with pockets of AEPs
in the 20 - 2% range and rapid-onset flooding probabilities in the 25 - 75%
range, suggesting recent runs are at least somewhat consistent in placement
of potential impacts. Additionally, the SRF unit discharge service shows
isolated low-order streams with flows in the 300 - 800 cfs/sqmi range
mostly across West Virginia, which adds confidence in the occurrence of
locally significant flooding.

.Mid-Atlantic into the Northeast...
Multiple rounds of heavy rainfall will move through the region today into
late tomorrow morning bringing an increased potential of flooding to the
region. Though this area has generally been dry, urban basins within the
region will quickly convert rainfall into runoff, resulting in flash and
river flooding. Given the isolated to widely scattered nature of forecast
convection, the HRRR-forced NWM has been slow in indicating the potential
of possible impacts. However, recent runs have started indicating the
potential of Rapid Onset Flooding, especially along the coast near New
York, with NWM Highflow Magnitude Forecast also indicating the potential
for isolated flows of 4 - 10% within the region increasing concern for
locally significant streamflows within the urban corridor.

.South-Central Texas...
Moderate to major river flooding is observed or forecast across
southcentral TX following recent heavy rainfall. The only river currently
in major flood status is the Nueces, with the Frio expected to reach the
major flood threshold late this evening into the overnight hours. Very
minimal, if any, rainfall is expected over the next 7 days which should
allow recessions to continue unimpeded.

.Alaska...
Small stream and minor to moderate river flooding, along with isolated
flash flooding,  is possible along the Kenai Peninsula and into the
Anchorage area through Friday as an atmospheric rivers impacts the region.

//Bliss/GKendrick

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



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