744
AGUS74 KWCO 131523
HMDNWC

National Hydrologic Discussion - EXPERIMENTAL
NWS National Water Center - Tuscaloosa AL
1015 AM CDT THU JUN 13 2024

.Synopsis...
Considerable to locally catastrophic flash and urban flooding in Florida...
River flooding continues in East Texas... Potential for isolated flooding
impacts in the Central Plains and the Upper Midwest... Heavy rainfall next
week along the Western and Central Gulf Coast... Snowmelt-induced rises
continue across California, Intermountain West, and Rockies...

.Discussion...

.Florida...
Ongoing heavy showers and storms are expected to persist across the
southern half of the peninsula into the weekend. Considerable flash and
urban flooding impacts have already occurred and are likely to continue
into tomorrow. Outside of urban areas, the top soils are at or near
saturation (80 - 90% RSM, 0 - 10 cm NASA SPoRT) following prolonged heavy
rainfall of 4 - 8", widespread, with localized amounts over 20" (48 hrs,
MRMS). With the expectation of prolonged and slow moving heavy showers and
storms again today and tapering off into the weekend, remaining soil
infiltration capacity will likely be overwhelmed and rainfall will
immediately runoff into nearby canals, drainage systems, and urban areas,
generating large areas of standing water on roadways, overflowing of
canals, storm drains and culverts, and flooding of poorly drained and
low-lying areas in urban centers, with Naples and West Palm Beach down to
Miami being most at risk. Locally catastrophic impacts are possible in
these urban areas.

Small stream flooding is possible, though not as likely, given the
underlying karst topography and streamflows on a regional scale being
normal to below normal for this time of year (USGS). However, rapid rises
on small streams and canals are possible where heavy rainfall persists. The
HRRR-forced National Water Model (NWM) supports this idea, as it depicts
low to medium probabilities (26 - 50%) of rapid stream rises in urban areas
along the I-75 and I-95 corridors. Corresponding annual exceedance
probabilities (AEPs) supports a high likelihood of considerable impacts, as
most AEPs are below 10%, with isolated responses as low as 2%. The greatest
impacts are expected in urban areas and these impacts will largely be
dependent on rainfall intensity.

.East Texas...
Moderate to major river flooding will continue across the Sabine, Trinity,
and Neches river basins into the weekend. Crests are expected late this
week as the flood wave routes downstream. Little to no precipitation
through day 4 (Sun) should allow for unimpeded recessions across the area.

.Central Plains and the Upper Midwest...
Persistent rounds of storms through the Central Plains and the Upper
Midwest starting day 2 (Fri) and continuing through day 7 (Wed), may cause
isolated instances of urban, flash, and small stream flooding. Much of
eastern MN and WI are vulnerable to hydrologic impacts due to wet soils (65
- 80% RSM, NASA SPoRT) and normal to much above-normal streamflows (USGS).
Additionally the area has ongoing minor flooding, particularly on the Rock,
Little Sioux, and Minnesota rivers. In contrast, the Central Plains has
drier soils and generally normal streamflows for this time of year.
Cumulative rainfall totals for days 1 - 3 (Thu - Sat) are between 1 - 2",
with locally higher amounts, (WPC) and will likely generate instant runoff
in eastern MN and WI regions, flowing into already swollen streams and
possibly contributing to new and renewed rises. However, for the Central
Plains, early rainfall will prime the area for later rounds of anticipated
rainfall. Due to it being a convective season, later forecast periods are
subject to much uncertainty in magnitude and placement. If days 4 - 7 (Sun
- Wed) QPF materializes, subsequent urban, flash, and small stream flooding
are likely to occur for both regions.

The NWM Medium Range Forecasts (MRF), GFS, and NBM are signaling the
potential for rapid-onset flooding. However, the GFS-forced NWM indicates
more widespread responses, particularly in eastern NE and much of IA. These
are more southeastern than the NBM signals due to differences in QPF
placement. Regarding the overall magnitude of the streamflow responses, the
GFS-forced MRF High Flow Magnitude Forecast (HFMF) is signaling widespread
low annual exceedance probabilities (AEPs, 10% or lower) for much of
central and southern NE into extreme south western IA, while the NBM-forced
MRF HFMF has generally higher AEPs (50% or more), suggesting only modest
rises. Discrepancies between model guidance adds to a lower confidence in
magnitude and placement of the prolonged rain event.

.Western and Central Gulf Coast...
Heavy rainfall is forecast days 4 - 7 (Mon - Thu), bringing with it
potential for flooding impacts.
There is much uncertainty about the intensity and westward spread of the
rainfall, as well as its potential flood-related impacts, due to the
convective season and the timing of these events being later in the
forecast period. Antecedent conditions are generally not conducive to
hydrologic impacts, except for southeast TX. Portions of southeast TX are
especially vulnerable to additional rainfall, with many reservoirs greater
than 90% full and ongoing moderate flooding. Elsewhere in the region, soils
are on the drier side (20 - 50% RSM, 0 - 10 cm NASA SPoRT) and streamflows
are normal to much below normal (USGS). The NWM MRF has been signaling
potential for rapid-onset flooding, primarily in southern LA and TX, and
further east across southern MS and AL. However, due to significant model
variability, the location and amplitude of impacts still remain uncertain.

.California, Intermountain West, and Rockies...
Periods of scattered rainfall will continue to promote increased runoff
across much of the Intermountain West, north/central Rockies, and the
Sierra Nevada this week. Isolated minor river flooding in northern NV,
extreme eastern ID, and western CO is ongoing or expected to crest this
weekend into early next week. Area streams, creeks, and rivers will
continue to see rises as these conditions persist.

//Bliss/Wood



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