501
AGUS74 KWCO 301513
HMDNWC

National Hydrologic Discussion - EXPERIMENTAL
NWS National Water Center - Tuscaloosa AL
1013 AM CDT SUN JUN 30 2024

.Synopsis...
Flooding impacts possible across Southern New England this
afternoon/evening... Renewed flooding possible this week for the Upper and
Middle Mississippi Valley.... Daily convection across the Four Corners
Region... Heavy rainfall potential Monday and Tuesday across Puerto Rico as
TS Beryl tracks south of the region... Increased snowmelt and glacier
runoff in Alaska...

.Discussion...

.Southern New England...
Isolated flash, urban, and small stream flooding is possible this afternoon
and evening as multiple rounds of convection progress across the region.
Soils remain wet due to recent rainfall (50 - 65% relative soil moisture
(RSM) 0 - 10 / 0 - 100 cm NASA SPoRT), therefore successive rounds of
intense rainfall may be able to overcome soil infiltration capacity and
lead to excessive runoff and subsequent flooding impacts. The HRRR-forced
National Water Model (NWM) Short Range Forecast (SRF) is consistently
signaling potential for rapid onset flooding (ROF) across CT, southern MA,
and RI this afternoon into the evening hours, with basin averaged
probabilities for ROF increasing above 25%. Probabilities for high water
flows (1.3 ARI Flows) are above 50%, and rises on small headwater
tributaries could be locally significant as suggested by isolated flows
with AEPs 10% being forecast (NWM SRF). Antecedent normal to below normal
climatological streamflows (USGS) should help mitigate initial riverine
response, and flooding impacts will be contingent on basins receiving
repeated rounds of heavy rainfall.

.Upper and Middle Mississippi Valley...
Moderate to major river flooding is ongoing across far southeast SD,
southern MN, and northern IA and is forecast through this week along the
Missouri (moderate) and Mississippi (moderate to major) River mainstems.
Rain returns to the region beginning day 2 (Mon) and lingers through day 6
(Fri). Ample deep layer soil moisture remains in place across eastern SD,
southern MN, IA, and WI (60 - 80% RSM 0 - 100 cm NASA SPoRT), suggesting
that the already elevated/flooded rivers across the region will likely
respond fairly efficiently to the expected additional rainfall/runoff.
Generally 1 - 3" inches of rain is expected by the end of the week, which
is likely not enough to cause widespread new and renewed river flooding,
however some basins could see some new and renewed river rises/flooding
(NWM Medium Range forecast (MRF) High Water Probability) if heavier totals
(3 - 4"+) materialize. At the very minimum, delayed recessions of ongoing
flooding and isolated flash flooding can be expected.

New river flooding impacts will be possible, with some new and renewed
rises into minor and moderate flood being possible (HEFS, CR Ensemble QPF
hydrographs) if the heavier aforementioned rainfall totals come to
fruition. The NWM MRF (NBM and GFS forced) continue to indicate potential
for widespread new and renewed river rises by mid week in response to this
rainfall with peak flows mostly on days 3 - 6 (Tue - Fri) across southern
MN, western WI, and IA. The magnitude of response is still uncertain, but
the NWM MRF is indicating potential for 4 - 20% AEP flows on many smaller
tributary rivers across the aforementioned region, so trends will need to
be closely monitored heading through the week.

.Four Corners Region...
Daily convection will continue the potential for isolated significant flash
flooding in arroyos, slot canyons, dry washes, urban, and recently burned
areas through the end of the week. Locations that have received multiple
rounds of rainfall over the past week are most susceptible to new flooding
impacts.

.Puerto Rico...
Moderate to heavy rainfall (1 - 4") Monday through Tuesday across southeast
Puerto Rico will bring the potential for isolated flooding impacts.
Streamflows across this portion of the island are generally normal to above
normal in comparison to both the daily and annual climatological normals,
suggesting soil capacity is nearing saturation in response to recent
rainfall. This increases the potential for rapid runoff and flooding
concerns, however any flooding impacts are expected to remain isolated
given the small areal coverage of heavier rainfall and lack of significant
rainfall totals.

.Alaska...
Recently warm temperatures continue to lead to increased snowmelt and
glacier runoff, leading to rivers running high in areas across southern AK.
Minor flooding on the Chilkat River near Klukwan continues through at least
the weekend. Also, elevated flows and localized flooding is possible along
the Skwentna, Yentna, and Nuyakuk Rivers.

//JEC

$$