210
AGUS74 KWCO 171527 CCA
HMDNWC

National Hydrologic Discussion - EXPERIMENTAL
NWS National Water Center - Tuscaloosa AL
1015 AM CDT WED JUL 17 2024

.Synopsis...
Locally significant flash and small stream flooding ongoing in Ozarks and
Lower Mississippi Valley... River flooding is ongoing in the Midwest and
Great Lakes... Monsoon activity increases this week in the Southwest...
Flooding possible in Puerto Rico, Guam, and in the Eastern Carolinas
through the National Capital Region... Isolated small stream and river
flooding in portions of Alaska...

.Discussion...

.Ozarks and Lower Mississippi Valley ...
Slow moving to stationary showers and storms ongoing across the Ozarks will
persist through the remainder of the morning, bringing intense rainfall
rates and additional flash and small stream flooding impacts. Significant
flooding has already occurred near Yellville and Summit AR, where 4 - 6"+
of rain has fallen over the past 12 hours (MRMS QPE), and given the slow
moving nature of these storms, new and worsening flooding impacts are
likely here and other places in northern AR and southern MO. This area is
also hydrologically sensitive given the steep and hilly terrain of the
Ozarks, which will serve to further enhance runoff conditions. These
factors will likely lead to impacts of rapid stream rises and pluvial
flooding of typical flood prone or low-lying areas. At this time, the
HRRR-forced National Water Model is indicating a concentrated area of small
stream responses across much of northern AR. Corresponding annual
exceedance probabilities (AEPs), per the High Flow Magnitude Forecast, are
as low as 2% from near Springfield MO, to Batesville AR. This includes
areas near the Mountain Home, AR area, which coincides with the locations
of where the most significant impacts have occurred thus far. After a brief
lull early this afternoon, additional showers and storms are expected over
this same region, as well as further south into central AR, western MS,
western TN, and as far south as the ArkLaTex. These storms have the
potential to produce heavy rainfall as well. Impacts will primarily be
driven by rainfall intensity and slow storm motions.

.Midwest and Great Lakes...
Scattered minor to isolated moderate and major river flooding is ongoing
and forecast, primarily in portions of northern IL, northern IN, southern
WI, and southern MI. The majority of these rivers are expected to crest
through late week, and fall out of flood stage shortly thereafter. A few
rivers in the slower drainage basins are forecast to linger into the
weekend. Scattered minor river flooding is ongoing or forecast throughout
the region. Generally dry conditions settle in for the remainder of the
week allowing unimpeded recessions to begin.

The broad flood wave on the Mississippi River, where moderate to major
flooding is ongoing, appears to be located south of Burlington, IA. Minor
and moderate flooding is forecast ahead of the wave which appears to
suggest that strong receding trends have reduced the overall flooding
impacts. The crest will work its way downstream just north of St. Louis, MO
by midweek next week.

.Southwest...
Increasing coverage and intensity of monsoon rains could lead to flash
flooding along arroyos and complex terrain, along with urban flooding,
particularly in NM, western AZ, and southern CO where the heaviest rainfall
is forecast. Slow moving convection may cause flash flooding issues,
especially over the usual vulnerable areas like burn scars, dry washes, and
slot canyons.  Localized rainfall maxima of 2 - 4" remain possible into
next week for this region. This will likely increase the threat for locally
significant impacts in these areas, as intense rainfall on hydrophobic
soils will enhance runoff conditions.

.Eastern Carolinas through the National Capital Region...
Isolated flash and urban flooding is possible for the D.C. metropolitan
area, and surrounding cities, through tonight as moderate to locally heavy
rainfall (1 - 2"+) is forecast. While urban areas are the immediate concern
for this event, antecedent conditions support a reduced risk of flooding
concerns moving forward today. Dry soils and low streamflows (NASA SPoRT
and USGS) will help to mitigate much of the small stream flooding and
runoff concerns for today. The HRRR-forced NWM is only highlighting lower
order reaches reaching high water threshold, further adding confidence in
this event primarily being a flash and urban flood threat. Short bursts of
intense rainfall could elicit some flooding responses in culverts and
ponding in roadways.

Further south into the Carolinas, flash and urban flooding will be possible
in response to several rounds ofMid-South moderate to locally heavy rainfall that
will impact the region starting on day 2 (Thu) and through early next week.
Soils are generally dry (<50% 0 - 10 cm RSM, NASA SPoRT) and much of the
region is in D1 - D3 drought (US Drought Monitor), particularly near the
coastal state lines of the Carolinas. While this rainfall would be
beneficial for much of the area, repeated rounds of heavy rainfall could
still overwhelm soil conditions and enhance runoff. Urban areas and areas
of poor drainage are the primary concern for this event with small stream
response being less likely unless training occurs. This is further backed
by low flows on streams and absent NWM MRF high flow signals providing
additional confidence of ample in-channel storage availability.

.Alaska...
Another round of heavy rainfall (1.5 - 3") in southeastern AK will continue
through this evening, and could still cause isolated instances of small
stream and river flooding. Prior rounds of rainfall have wetted soils and
raised streamflow levels in both rivers and streams. These vulnerable
antecedent conditions increase confidence in hydrologic responses
occurring, particularly in the Chilkat River Basin and from Yukatat to
Sitka, where significant river rises and high water are expected.
Additionally, temperature rises today will lead to increased snow and ice
melt into glacier-fed rivers situated at the foothills of steep terrain,
heightening the threat of small stream and river responses in the area.

.Puerto Rico...
Isolated urban and small stream flooding is possible today for western and
northeastern portions of the island. Previous rainfall has wetted the soils
in the northwestern and extreme eastern area of the island (NASA SPoRT)
however, there is available storage capacity to handle forecast rainfall,
and streamflows are generally near normal for this time of the year (USGS).
A return of heavy rainfall is also possible on day 4 (Sat) for the interior
portions of the island. Flooding impacts are largely going to be the same
with a similar setup and antecedent conditions.

.Guam...
Heavy rainfall (3 - 5", with locally higher amounts) is expected tonight
(local) through day 4 (Sat) due to a surface troughing pattern developing
over the Mariana Islands, likely causing flash, urban, and small stream
flooding. Low-lying and poor drainage areas, including roadways, are most
vulnerable.

//Capp/Pritchard



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