525
AGUS74 KWCO 261520
HMDNWC

National Hydrologic Discussion - EXPERIMENTAL
NWS National Water Center - Tuscaloosa AL
1015 AM CDT SUN MAY 26 2024

.Synopsis...
Flash, urban, and small stream flooding impacts likely across the
Mid-Mississippi, Tennessee, and Ohio River Valleys through Monday
morning...Isolated flooding impacts possible across the Midwest through
tonight...Rainfall returns to the Southern Plains early this week...Ice jam
and snowmelt flooding impacts to continue in Alaska...Flooding possible
across Puerto Rico and USVI through the week...Isolated flooding impacts
possible on Memorial Day in portions of the Mid-Atlantic and Northeast...

.Discussion...

.Mid-Mississippi, Tennessee and Ohio River Valleys...
Flash, urban, and small stream flooding impacts are likely through early
day 2 (Mon) across southeastern MO, southern IL, much of KY, and northern
TN as multiple rounds of strong thunderstorms, with intense rainfall rates,
move through the region. While antecedent conditions are not necessarily
saturated area wide, the expected intense rainfall rates with these storms
in a short period of time will overwhelm remaining infiltration capacity
and rainfall will immediately runoff into nearby urban areas. Areas most at
risk of flooding impacts are much of western and central KY and northern TN
(particularly areas along the KY/TN state line) where antecedent conditions
are most conducive for flooding impacts (NASA SPoRT, USGS). As such, rapid
rises of small creeks and streams, potentially large areas of standing
water on roadways, overflowing of ditches and culverts, and poor drainage
flooding in low-lying and urban areas are likely, with locally considerable
flooding impacts possible in areas of heavier rainfall and where antecedent
conditions are most conducive for flooding impacts.

The HRRR-forced National Water Model (NWM) has been consistently indicating
medium to high probabilities (above 50%) of widespread rapid-onset flooding
of small streams, with the most robust responses across southeastern MO,
southern IL, and much of KY, though the evening hours. The corresponding
High Flow Magnitude Forecast indicates widespread annual exceedance
probabilities (AEPs) at or below 20% across the aforementioned regions, and
includes urban areas such as Paducah, Bowling Green, Clarksville, and
Nashville, increasing confidence in the potential for locally significant
responses across these regions. Guidance from both the GFS and NBM-forced
NWM are also signaling small stream responses across these regions,
although the latest AEPs from the High Flow Magnitude are not as widespread
nor as significant in magnitude as what the HRRR-forced NWM is indicating.
In regards to river flooding, minor flooding is ongoing and forecast across
the Green River Basin (KY), and renewed rises to minor and moderate flood
are expected across this basin in response to this rainfall. Additional
areas of minor river flooding (and potentially higher) are also possible in
areas of heavier rainfall.

.Midwest...
Urban and small stream flooding impacts remain possible through tonight
across portions of northern IL, eastern IA, and much of WI as showers and
thunderstorms bring locally heavy rainfall to the region. This region
remains vulnerable to hydrologic impacts given ongoing river flooding, as
well as saturated soils following repeated periods of heavy rainfall over
the past week. Guidance from all three NWM forcings (HRRR, GFS, and NBM) is
indicating scattered small stream rises across these regions over the next
24 hours, with peak flows generally occurring this evening. The most robust
AEPs (20% or less), per the High Flow Magnitude forecast, are across small
streams in the Wisconsin and Fox river basins across central WI. Widespread
river flooding isnt expected with this latest rainfall, but it may
aggravate ongoing river flooding across the region.

.Southern Plains...
Confidence is increasing in the potential for flooding impacts across much
of northern and central TX and southern OK as heavy rainfall impacts the
region on days 3 and 4 (Tue - Wed), with periods of additional rainfall
through day 7 (Sat). At this time, antecedent conditions are conducive for
hydrologic responses, as near surface soils are generally wet due to
repeated rounds of rainfall over the past week. A drying trend today and
tomorrow may allow for some soil recovery, but given the potential for
enhanced rainfall rates, these conditions may be at least locally
overwhelmed. Both the GFS and NBM-forced NWM continue to indicate isolated
stream responses on days 4 - 7 (Wed - Sat). River ensemble guidance (HEFS)
continues to suggest only isolated river rises, with low probabilities of
reaching flood stage at this time.

.Puerto Rico and USVI...
A wet pattern will persist across the islands through the week, continuing
the potential for flooding impacts. Soils are wet and streamflows are above
annual means given repeated rounds of heavy rainfall over the past couple
of weeks, increasing vulnerability to rapid stream rises and subsequent
flooding impacts. Isolated flash floods are possible as well where more
robust rainfall rates occur.

.Alaska...
The breakup along the Yukon river continues to push across the Yukon Delta,
bringing the potential in ice jam flooding impacts through this afternoon.
The ice jam is currently near Kotlik and any shift in the jam may generate
flooding in Nunam Iqua and Koltik, with the greatest risk in low-lying
areas near the river. Further north and east across the Yukon Flats,
snowmelt induced flooding impacts remain possible along the Porcupine River
and near Ft. Yukon. Flooding in these areas will continue to rise through
day 2 (Mon) and slowly recede through mid-week.



.Mid-Atlantic and Northeast...
There continues to be some potential for at least isolated flooding impacts
across these regions on day 2 (Mon), as a frontal boundary provides a focus
for locally heavy rainfall. The areas greatest at risk are along the urban
corridor (along and west of I-95) in NJ and eastern PA, portions of the
Susquehanna Valley (PA/NY), and along the steep terrain of the southern
Adirondacks and Catskills (NY) as these are the areas to receive the
heaviest rainfall. On a regional scale (and outside of sensitive urban
areas), soil moisture has increased following recent rainfall over the past
several days (60 - 80% RSM, 0 - 10 cm NASA SPoRT) and streamflows are
locally elevated across portions of the Susquehanna Valley (PA/NY) and the
anticipated rainfall rates may overwhelm current capacity. There continues
to be little to no stream response by both the GFS and NBM-forced NWM,
which suggests that the HRRR-forced NWM may provide the best guidance as
the event comes within the forecast period. River flooding is not expected,
although river rises cannot be ruled out in areas of heavier rainfall
(HEFS/MMEFS).

//Pritchard



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