086
AGUS74 KWCO 191522
HMDNWC

National Hydrologic Discussion - EXPERIMENTAL
NWS National Water Center - Tuscaloosa AL
1015 AM CDT WED JUN 19 2024

.Synopsis...
Considerable flooding expected in Texas... Locally considerable flooding is
possible in the Northern Plains, Upper Midwest, and Upper Great Lakes...
Isolated flooding possible in the Central Plains... Localized urban
flooding possible in the Northeast... Snow-melt induced flooding in
Alaska...

.Discussion...

.Texas...
Tropical Storm Alberto will bring considerable flash, urban, and riverine
flooding to portions of southern TX through day 2 (Thu), with the bulk of
the rainfall expected today. Although conditions are expected to gradually
improve as we head into the weekend, there is the potential for the wet
weather pattern to continue through at least early next week. Widespread
QPF totals for this event range from 5 - 10", with locally higher possible.
Antecedent soil conditions are dry across south TX 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 2 - 3"+ over the duration of the event.
Considering the current reservoir capacities are below 30% across the
region (except near Victoria where reservoirs are near capacity), this
rainfall will be generally beneficial outside of areas where heavy tropical
rainfall may develop. As for river flooding, HEFS ensemble guidance
continues to show minor to moderate river flooding in south TX, with
isolated minor to moderate flooding already being forecast.

The HRRR-forced National Water Model (NWM) Short Range Forecast (SRF) is
beginning to pick up on the QPF for this event and is showing a similar
story as the NWM MRF over the past couple of days. Rapid-onset flooding
probabilities continue to be fairly low-end (less than 50%), likely due to
the dry antecedent conditions and the QPF not being fully captured.
Scattered annual exceedance probabilities (AEPs) below 20% from the High
Flow Magnitude Forecast (HFMF) suggest considerable responses through
normally dry streams and arroyos across south TX, which implies that
flooding impacts are expected in areas that do not typically flood. 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 AEPs less than 20% being signaled. 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,
also 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, Upper Midwest, and Upper Great Lakes...
Flash, urban, and river flooding, with the potential for locally
considerable impacts, are expected this week and through the weekend from a
multi-day storm system bringing repeat rounds of heavy rainfall sets up
across the region. The first round of heavy rainfall is forecast starting
on day 2 (Thu) and will continue through day 4 (Sat), peaking on day 3
(Fri). A brief lull is then expected on day 5 (Sun) before additional
rounds on days 6 - 7 (Mon - Tue), although this rainfall should be less
significant. Current guidance places the axis of heaviest rainfall (7-day
totals) across eastern SD, southern MN, and central WI (3 - 5", locally
higher possible). The region remains hydrologically active and vulnerable
having received 150 - 200% of its normal rainfall over the past 30 days
(CPC). USGS streamgages are reporting widespread above normal flows both
historically and for this time of year. Soil moisture down to the 100 cm
layer remains on the wet side as well (65 - 80% RSM, NASA SPoRT),
suggesting limited infiltration capacity. Given these conditions, it will
not take much rainfall to generate immediate runoff and subsequent flooding
of small streams and creeks, as well as low-lying and poorly drained
locations in urban areas. Each additional round of rainfall will serve to
further compound the flood potential. Additionally, isolated moderate to
major river is ongoing or forecast across the Minnesota, Upper Mississippi,
and Crow river basins in southern MN, with ensemble guidance (HEFS 30%)
indicating additional rises along the Mississippi and any tributaries
affected by this heavy rain. Many locations along the Mississippi River
will approach flood stage towards next weekend. Both the NBM- and
GFS-forced NWM are responding accordingly with widespread responses and the
potential for rapid-onset flooding. Both models show good agreement with
the placement of the rainfall, however, the GFS is much more aggressive
with the overall magnitude of the responses.

.Central Plains and Mid-Mississippi Valley...
Slow-moving convection with the potential for enhanced rainfall rates will
continue to impact portions of the OK Panhandle into southern IA through
late this evening, bringing a renewed threat of isolated flash, urban, and
minor river flooding. Antecedent conditions are variable, with recent
periods of drought leaving much of NE/KS/OK dry. However, recent rainfall
has helped to moisten soils and elevate streamflows across eastern portions
of NE/KS into IA (50 - 65% RSM, 0 - 100 cm, NASA SPoRT). With the ongoing
rainfall, there may be a few isolated areas of more saturated soils not yet
reflected by NASA SPoRT. While this suggests infiltration capacity remains
available, locally heavy rainfall rates may be able to overcome this,
pushing runoff into already elevated smaller streams and rivers and produce
sharp rises as well as flash flooding.

The NWM SRF continues to signal the potential for rapid-onset flooding
across the OK Panhandle/southeast KS with 12-hr probabilities between 50 -
75%, and more low-end probabilities through central KS into southwest IA
(mostly less than 25%), however, these signals will likely continue to
increase as the storm develops throughout the day. The NWM SRF HFMF is
indicating fairly widespread streamflow AEPs as low as 4% across the
Panhandle north to Dodge City, KS, where high water flows are ongoing.
While these signals are likely overdone to an extent, these AEPs align with
the region of highest probabilities for small stream responses, increasing
confidence in flooding impacts in this region. Any areas where intense
rainfall rates persist or repeat will see the greatest threat for flooding
impacts.

.Northeast...
The potential exists for localized flooding impacts in primarily urban
areas on days 3 - 5 (Fri - Sun) as showers and thunderstorms develop across
the region. Outside of sensitive urban areas, near normal relative soil
moisture values (50 - 60%, 0 - 100 cm, NASA SPoRT) and near to below mean
annual streamflows (USGS) suggest sufficient infiltration capacity overall
to capture runoff. These factors, combined with modest rainfall amounts
(less than 2"), will help to mitigate any widespread hydrologic responses
and limit the threat to urban, low-lying, and poor drainage areas.

.Alaska...
Warm daytime and overnight temperatures are causing significant snow and
ice melt at high elevations, leading to minor flooding on the Chilkat River
near Klukwan, which is expected to persist due to diurnal temperature
trends, and localized flooding in the Yetna and Skwetna river basins.
Similar flooding is anticipated along the Nuyakuk River. These warm
conditions are projected to continue through early next week in southwest,
southcentral, and southeast areas of the state, further increasing the rate
of snowmelt runoff into rivers.

//Freeman



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