709
AGUS74 KWCO 181510
HMDNWC

National Hydrologic Discussion - EXPERIMENTAL
NWS National Water Center - Tuscaloosa AL
1015 AM CDT TUE JUN 18 2024

.Synopsis...
Considerable flooding expected in Texas... Locally considerable flooding
possible in the Northern Plains and Upper Midwest... Isolated flooding
possible in the Central Plains...

.Discussion...

.Texas...
Considerable flash, urban, small stream, and riverine flooding is
anticipated for parts of TX tonight through day 3 (Thu) as heavy rainfall
from Potential Tropical Cyclone One impacts the region. Widespread QPF
totals for this event range from 5 - 10" along coastal TX, decreasing
further inland, with areas as far inland as San Antonio forecasted to
receive up to 3" (WPC). Isolated rainfall amounts up to 15" may be possible
in southern TX by the end of this event, potentially leading to moderate
and isolated major river flooding.

Antecedent conditions vary, with east TX being on the wetter side after an
abnormally wet April and May (5 - 8"+ above normal precipitation). The area
has been spared significant rainfall over the last week, allowing soils to
dry to near normal conditions throughout the soil column (40 - 50% RSM, 0 -
100 cm), but streamflows remain well above average climatological norms
(USGS). Reservoirs across east TX are at or near capacity, and river
flooding is ongoing in the lower reaches of the Trinity, Sabine, and Neches
basins. Recent forecasts have shifted the highest QPF totals to the
southwest, with current 7-day totals for the area near 3 - 4". Given the
current hydrologic sensitivity of eastern TX, persistent heavy rainfall
will likely generate flash, urban, and small stream flooding impacts, some
locally considerable. Ensemble guidance (HEFS 30%) continues to indicate
the potential for minor to moderate flooding on rivers north and east of
Houston. The NBM-forced version of the National Water Model (NWM) shows
muted small stream responses despite the high QPF totals, which is not
surprising due to the high annual recurrence interval (ARI) set for TX,
however, small stream flooding impacts are still likely.

South and west of Houston, soil conditions are dry and streamflows are
running at normal to much below normal mean annual flows, allowing for some
infiltration and in-channel storage capacity for the initial rounds of
expected rainfall. This will help buffer initial hydrologic responses, but
with high forecasted rainfall totals and high rainfall rates, considerable
flooding impacts are still likely. The TX Hill Country will be an area to
watch, as basins are flashier due to the high relief and the area is
currently forecast to get 3 - 5"+ over the duration of the event, with
isolated rainfall amounts up to 15" possible in southern TX by the end of
this event. Considering the current reservoir capacities are below 30%
across the region (except near Victoria where reservoirs are near
capacity), this rainfall will be highly beneficial. As for river flooding,
ensemble guidance has started to pick up on the southwest shift in QPF,
with HEFS 30% exceedance showing minor to moderate river flooding in south
TX. Regardless, rises on rivers in south TX to at least minor flood status
are likely. The NBM-forced version of the NWM is showing scant signals
considering the amount of rainfall forecast, which is due to the high ARI
value from the NWM in TX. The GFS-forced version of the NWM is much more
aggressive, with widespread annual exceedance probabilities (AEPs) less
than 20% being signaled across south TX. These signals are likely overdone
in their wide areal coverage, but isolated occurrences of these extreme
signals are possible should the rainfall exceed current forecasts,
supporting the potential for locally considerable flooding impacts. Flash
and urban flooding, regardless of antecedent conditions, are still likely
wherever repeated rounds of heavy rainfall occur.

.Northern Plains and Upper Midwest...
Flash, urban, small stream, and river flooding, some locally considerable,
as a result of persistent rounds of moderate rainfall this week, threatens
the Northern Plains and Upper Midwest. This region has already received
well above average rainfall over the past month (2 - 5"+ positive
departures from normal precipitation) and rainfall over the last 72 hours
(widespread 2 - 4", locally higher, MRMS) have wetted soils, particularly
across southern MN into WI where soils down to the 100 cm layer are nearing
saturation  (65 - 95% RSM, NASA SPoRT). Additionally, area streams are
above average annual median flows (USGS), with ongoing and forecast minor
to isolated major river flooding in southern MN and northern IA as a result
from preceding rainfall. Each additional round of rainfall will serve to
further compound the flood potential.

In the short range, the heaviest rainfall is forecast to affect northern MN
this evening, where the aforementioned antecedent conditions will
facilitate efficient runoff, leading to the potential for all modes of
flooding. For days 3 - 5 (Thu - Sat), another system will move through this
primed region. As this consecutive round of precipitation occurs later in
the forecast period, uncertainty arises regarding the placement of higher
rainfall totals. However, if storms persist or linger over the same
vulnerable area for extended periods, locally considerable flood impacts
are possible.

The HRRR-forced NWM suggests a chance of rapid-onset flooding (ROF) this
evening. The past few runs have been consistent, showing high ROF
probabilities (50 - 75%), mostly situated in northeastern MN. Regarding the
overall magnitude of the streamflow responses, AEPs, per the High Flow
Magnitude forecast, have continually indicated significant stream rises
(AEPs as low as 2%) southwest of Minneapolis as well as in northeast MN.
The consistency in the placement and magnitude of the signals increases
confidence in hydrologic responses expected later today. In the medium
range, the GFS and NBM are indicating the potential for ROF. Both models
are indicating widespread small stream responses across much of the region
with lower AEPs concentrated in southeastern MN and into northwestern WI.
This is the greatest area of concern for potential considerable flooding
impacts later this week, with peak flows not expected until day 4 (Fri) for
smaller streams, and day 8 (Tue) and beyond for larger mainstem streams.

.Central Plains...
Heavy rainfall is forecasted for today and tomorrow across portions of the
Central Plains, with the highest totals (2 - 5", WPC) expected in
southwestern KS, posing a threat of isolated urban and flash flooding.
Antecedent conditions are not conducive to flooding, except in northeastern
NE, where widespread rainfall totals of 1 - 2 inches, with locally higher
amounts, in the past 48 hours have primed the region for hydrologic
responses. Elsewhere, near surface soils are dry (15 - 35% RSM, 0 - 10 cm
NASA SPoRT) and streamflows are normal to much below normal due to ongoing
drought conditions. Additionally, sandy soils characterized by high
infiltration rates will further mitigate any widespread responses, allowing
rainfall to be absorbed into the system. However, where storms train and/or
persist in high intensities for prolonged periods, the threat of flash and
urban flooding cannot be ruled out. The NWM has been consistently signaling
the potential for ROF in eastern NE and across central KS, with a cluster
of ROF probabilities (25 - 75%) situated in northeastern NE. Corresponding
AEPs, per the High Flow Magnitude Forecast, indicate isolated small stream
rises with AEPs less than 20% scattered across the region, suggesting small
stream rises could be possible in flashier, more responsive basins.

//Freeman/Wood



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