208
AGUS74 KWCO 121546
HMDNWC

National Hydrologic Discussion - EXPERIMENTAL
NWS National Water Center - Tuscaloosa AL
1015 AM CDT WED MAR 12 2025

.Synopsis...
Rainfall and potential flooding across the Pacific Northwest and
California...Flooding impacts possible this weekend across the Lower
Mississippi Valley into the Ohio Valley...Snowmelt resulting in rises on
rivers and streams in Eastern Oregon into Idaho... Widespread river
flooding possible early next week in the Northeast...

.Discussion...

.Pacific Northwest and California...
Multiple rounds of rainfall will begin to impact portions of coastal CA and
the foothills of the Sierra Nevada through day 5, with the heaviest
rainfall expected through day 2 (Thu) and again on day 5 across northern
CA. Ahead of this unsettled weather, antecedent conditions are variable,
with wetter soils (55 - 70% RSM, 0 - 10 cm NASA SPoRT) across northern CA,
along the Sierra Nevada foothills, and portions of SoCal (San Diego Metro)
to generally drier soils (30 - 50% RSM, 0 - 10 cm NASA SPoRT) along the
coast from the Bay Area to LA. Streamflows are generally at or just above
climatological normals for this time of year (USGS). These factors suggest
that some infiltration capacity is available, but may be at least locally
overwhelmed in areas of more robust rainfall totals/hourly rates, leading
to rapid rises on streams and creeks, and isolated flooding of urban and
low-lying areas. Flash flooding and debris flows are possible near recently
burned areas, particularly near the LA metro. River flooding isnt of much
concern, as HEFS guidance indicates generally action stage rises to
isolated minor flooding along the coast (10% exceedance). Guidance from
both the GFS and NBM-forced National Water Model (NWM) continues to
indicate isolated small stream responses across the region, generally
within the next 48 hours and again on day 6 - 7 (Mon - Tue). Corresponding
annual exceedance probabilities (AEPs) are generally near 50%, suggesting
modest stream rises on a regional scale. However, isolated AEPs as low as
2% are noted near the Bay Area and along the SoCal Coastline, further
giving confidence that significant stream responses should be highly
localized.

Further north into the Pacific Northwest, the heaviest rainfall is expected
on days 3 - 5 (Fri - Sun), where urban and small stream flooding is
possible. Soils are significantly wetter than areas further south in CA (60
- 85% RSM, 0 - 10 cm NASA SPoRT). Streamflows are generally near normal for
this time of year (USGS). As such, it will not take much rainfall to at
least locally overwhelm current infiltration capacity, leading to runoff
into urban areas and area creeks/rivers. However, given the channel
availability in streams, widespread river flooding is not expected. HEFS
guidance (10% exceedance) increases confidence in this, as the guidance
indicates isolated river rises along rivers draining the western Cascades
and Olympics. Isolated minor river flooding cannot be ruled out, as HEFS is
indicating minor river flooding across western OR however, these rivers
are generally quick responding to moderate to heavy rainfall. Guidance from
the GFS-forced NWM is indicating isolated small stream responses across
portions of southwest OR on days 4 - 5 (Sat - Sun). Corresponding AEPs do
indicate locally significant stream rises as AEPs as low as 4% on smaller
streams southwest of Eugene, OR. However, impacts are expected to remain
rather localized in nature.

.Lower Mississippi Valley into the Ohio Valley...
Periods of heavy rainfall may elicit flooding impacts beginning on day 4
(Fri) and continuing through the weekend. While dry weather over the past
few days has allowed for some soil moisture recovery (55 - 70% RSM, 0 - 10
cm NASA SPoRT), streamflows remain above climatological normals (USGS) and
given the potential for intense rainfall rates, current infiltration
capacity may be overwhelmed, leading to runoff into already swollen
rivers/streams, and leading to subsequent flooding. River flooding is of
concern as well, as both HEFS and MMEFS guidance continues to indicate the
potential for minor river flooding, particularly across the Tombigbee
(MS/AL), Middle Tennessee (AL), Cumberland (TN), and Green (KY) river
basins. Guidance from both the GFS and NBM-forced NWM has been consistently
indicating the potential for small stream responses across the region, with
the highest probabilities (greater than 50%) from northeast MS to central
KY on days 4 - 5 (Sat - Sun). Corresponding AEPs suggest the potential for
significant stream rises, as AEPs as low as 2% are noted from northern AL
through middle TN and southern KY. However, it is possible that the GFS is
overestimating the amount of precipitation in this region in comparison to
other models, so these AEPs are likely overdone. Nevertheless, all modes of
flooding are possible across the region, with locally significant small
stream flooding possible, especially across flashier creeks in northern AL
and Middle TN.

.Northeast...
Confidence continues to increase in the likelihood of widespread,
potentially significant flooding across much of the region this weekend and
early next week as a storm system brings high dewpoints, warm temperatures
and heavy rain, thus leading to rapid snowmelt. Warmer temperatures. Warmer
temperatures well ahead of this storm system is already causing some snow
melt and thermal ice rot on frozen rivers, thus increasing the snowmelt
runoff and ice jam potential. As the system gets closer, warm dewpoints (40
- 50F) will further accelerate the ongoing snowmelt and thermal ice rot.
As far as available snowmelt, guidance from SNODAS indicates 2 - 6" of snow
water equivalent available for melt across the region, with at least 2 - 4"
of melt possible. This melt, combined with 1 - 2" of rainfall, will likely
cause river flooding, some of which may be significant. MMEFS and HEFS
guidance supports this, as the ensembles have been suggesting the potential
for scattered moderate to major flooding across much of NY state, VT, and
western MA. Guidance from both the GFS and NBM-forced NWM are likely
underdoing the magnitude and coverage of small stream responses across NY
state as the NWM is underestimating the amount of available SWE in the
snowpack. As such, the sparse coverage is not representative of the
anticipated responses as river flooding is expected to be widespread across
the region.

.Eastern Oregon into Idaho...
Periods of moderate rainfall and high elevation snowmelt will continue to
cause elevated flows and isolated minor river flooding across portions of
the Lower Malheur, Lower Owyhee, Silvies, and Weiser river basins through
the week.

//JDP





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