535
AGUS74 KWCO 141523
HMDNWC

National Hydrologic Discussion - EXPERIMENTAL
NWS National Water Center - Tuscaloosa AL
1015 AM CDT FRI JUN 14 2024

.Synopsis...
Flooding threat to continue in Florida...Potential flooding impacts through
next week across portions of the Central Plains and the Upper
Midwest...Heavy rainfall potential along the Western and Central Gulf
Coast...River flooding continues in East Texas...Isolated flooding possible
today in the Northeast...

.Discussion...

.Florida...
Heavy showers and storms with efficient rainfall rates will develop and
overspread the region again today into tonight, likely generating new and
renewed instances of flash, urban, and areal flooding, some of which may be
locally significant. Extremely heavy rainfall over the past 3 days (5 -
10", locally 15 - 25", 72-hour MRMS QPE) has completely exhausted any
remaining infiltration capacity for this additional rainfall today and has
generated considerable and locally catastrophic impacts, especially in
areas from Naples and further east to West Palm Beach and points south to
the Miami metro and Alligator Alley. While the FL Keys have seen bouts of
heavy rainfall, they have not received extreme amounts as areas further
north and while also saturated, significant responses are not as likely in
this area. Nevertheless, this rainfall will immediately runoff into nearby
small streams, canals, drainage systems, and urban areas. There remains
some uncertainty, however, on where exactly the heaviest shower and storm
activity develops, as the HRRR, which forces the National Water Model Short
Range Forecast (NWM SRF), places the heaviest rainfall over the FL Keys,
which is the furthest south of all model guidance. Additionally, recent
runs of the HRRR model have been slow to initialize the developing
activity, which also decreases confidence in the exact location of impacts.
The latest NWM SRF is indicating generally 50% probabilities of rapid
onset flooding, with corresponding annual exceedance probabilities (AEPs)
as low as 2% in some areas along the coast, indicating there is still some
likelihood in locally considerable impacts given the extremely vulnerable
antecedent conditions. The greatest risk of flooding impacts will be across
the Miami metro and along and south of Alligator Alley. An overall
decreasing trend in rainfall begins on day 2 (Sat), which should allow for
flooded areas to recover from this week`s heavy rainfall.

.Central Plains and the Upper Midwest...
Persistent rounds of storms through the Central Plains and the Upper
Midwest beginning today and continuing through day 7 (Thu), 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 (60
- 70% RSM, NASA SPoRT) and normal to much above-normal streamflows (USGS).
Additionally, the area is experiencing ongoing minor flooding, particularly
on the Rock and Minnesota rivers. In contrast, the Central Plains have
drier soils and normal to below-normal streamflows for this time of year.
Cumulative rainfall totals for days 1 - 3 (Fri - Sun) are expected to be
between 1 - 2". This will likely further prime both regions and possibly
contribute to new and renewed rises in MN and WI. The greater threat for
hydrologic responses lies in the later forecast period, as repeated rounds
of rainfall affect the already primed region. Due to it being the
convective season, later forecast periods are subject to much uncertainty
in magnitude and placement. If the QPF for days 4 - 7 (Sun - Wed)
materializes, subsequent urban, flash, and small stream flooding is likely
to occur.

The NWM Medium Range Forecasts (MRF), are signaling the potential for
rapid-onset flooding (ROF). However, the GFS-forced NWM indicates more
widespread responses, spanning from central NE to much of southern MN and
into parts of extreme eastern WI. The NBM-forced NWM is more muted in its
signaling, but shares a similar linear track to the GFS-forced NWM.
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 portions of eastern NE,
while the NBM-forced MRF HFMF has generally higher AEPs (50% or greater),
suggesting only modest rises. Discrepancies between model guidance can be
attributed to differences in the QPF totals, contributing to lower
confidence in magnitude of the prolonged rain event.

.Western and Central Gulf Coast...
Periods of heavy rainfall are forecast for days 3 - 7 (Sun - Thu), bringing
the potential for flooding impacts. There is uncertainty about the
intensity and spread of the rainfall, as well as its potential
flood-related impacts, due to model variability and timing of the rainfall.
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) due
to ongoing drought conditions. These conditions will help mitigate flood
responses, at least initially. The soils in this region are primarily
composed of clay, which have particularly slow infiltration rates. This
increases the likelihood of these dry soils behaving hydrophobically in
response to the anticipated heavy rainfall, potentially worsening flooding
impacts. The GFS-forced MRF has been consistently signaling for potential
rapid-onset flooding, primarily in southern LA. However, for portions of
southern TX, signals have been inconsistent in placement, lowering
confidence in where potential flood responses may occur in this region.

.East Texas...
Moderate flooding will continue across the Sabine, Trinity, and Neches
river basins into the weekend. Generally light rainfall amounts over the
next week should allow for continued recessions.

.Northeast...
Isolated instances of urban and small stream flooding are possible from
northern NJ to central MA as scattered showers and storms bring locally
heavy rainfall this afternoon. The greatest likelihood of impacts would be
in urban areas along the I-95 corridor as antecedent conditions are
generally near normal for this time of year outside of the urban areas. The
NWM SRF shows little to no stream responses across this region, which
supports the idea of the main threat being urban flooding. Impacts will be
heavily dependent on rainfall rates and training.



//Pritchard/Wood



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