255
AGUS74 KWCO 201529
HMDNWC

National Hydrologic Discussion - EXPERIMENTAL
NWS National Water Center - Tuscaloosa AL
1015 AM CDT THU JUN 20 2024

.Synopsis...
Lingering flooding expected in Texas... Locally considerable flooding is
possible in the Northern Plains, Upper Midwest, and Upper Great
Lakes...Flash flooding possible in the Four Corners region...Localized
urban flooding possible in the Northeast... Snow-melt induced flooding in
Alaska...

.Discussion...

.Texas...
Lingering flooding from Tropical Depression Alberto will continue across
southern TX 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. Antecedent soil
conditions have moistened due to the rain across south TX with a relative
soil moisture of 65 - 80%. While not saturated, there is still some
infiltration and in-channel storage capacity available but with the
continuation of rainfall and given antecedent conditions, additional
flooding may be possible.. The HRRR-forced National Water Model (NWM) Short
Range Forecast (SRF) rapid-onset flooding probabilities continue to be
fairly low-end (less than 50%) with scattered annual exceedance
probabilities (AEPs) below 20% from the High Flow Magnitude Forecast
(HFMF). These signals align with current high water flows likely through
normally dry streams and arroyos across south TX.

.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 across the
region. The first round of heavy rainfall is forecast starting today and
will continue through day 3 (Sat), peaking on day 2 (Fri). A brief lull is
then expected on day 4 (Sun) before additional rounds on days 5 - 7 (Mon -
Wed), 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 flooding 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.
.
Four Corners...
Locally heavy rainfall, bringing the potential for flash flooding in states
surrounding the Four Corners region, will be of concern today into day 2
(Fri). Areas most at risk include flooding on recent burn scars, in slot
canyons and dry washes.

.Northeast...
The potential exists for localized flooding impacts in primarily urban
areas on days 2 - 4 (Fri - Sun) as showers and thunderstorms develop across
the region. Outside of sensitive urban areas, near normal relative soil
moisture values (50 - 60%, 0 - 10 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.

//Kirkpatrick



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