522
AGUS74 KWCO 281527
HMDNWC

National Hydrologic Discussion - EXPERIMENTAL
NWS National Water Center - Tuscaloosa AL
915 AM CST FRI FEB 28 2025

.Synopsis...
Flooding impacts possible next week from the Mid-South to Lower Ohio River
Valley...Small stream and river flooding possible next week across the
Northeast and New England...Snowmelt induced flooding continues in
Montana...

.Discussion...

.Mid-South to Lower Ohio River Valley...
Isolated flooding impacts, including river flooding, are possible on days 5
- 6 (Tue - Wed) across portions of the Mid-South (AR/TN/MS) through the
Lower OH Valley (western KY) as a line of showers and storms move through
the region. As mentioned in previous discussions, antecedent conditions
remain somewhat vulnerable to new and renewed flooding impacts as soils are
still wet and streamflows are still above climatological normals for this
time of year following significant flooding across portions of these
regions two weeks ago. Although there has been a dry period, it is possible
that antecedent conditions will not fully recover due to a lack of green up
across the region. River ensemble guidance (MMEFS/HEFS) continues to
indicate the potential for river rises and isolated river flooding across
this region on days 5 - 7 (Tue - Thu). One limiting factor is the
progressive nature of these storms, although there is potential for locally
enhanced rainfall rates which may quickly overwhelm current capacity,
leading to runoff into already swollen creeks/streams and impervious
surfaces in urban areas and subsequent flooding. Guidance from both the GFS
and NBM-forced NWM continue to indicate scattered small stream responses
across these regions on days 5 and 6 (Tue - Wed), with corresponding annual
exceedance probabilities (AEPs) generally at or greater than 50%. There is
still some uncertainty in the location of the impacts as there has been a
northward shift in the overall QPF footprint from previous days, but given
the vulnerability of the region, confidence is increasing in the potential
for flooding impacts with this system.

.Northeast and New England...
Confidence continues to increase in the potential for localized small
stream and river rises/flooding mid next week as a frontal system brings
periods of moderate to locally heavy rainfall. Ahead of this rainfall,
soils are frozen due to prolonged below freezing temperatures. There is a
snowmelt component as well, as there is generally 2 - 4" (with locally
higher amounts) of snow water equivalent (SWE) in the snowpack. This system
will bring 1 - 2 days of above normal temperatures and dew points exceeding
40F, which will help ripen the pack. These factors suggest an increased
likelihood of immediate runoff into streams and rivers leading to
river/stream rises and localized flooding. MMEFS (NAEFS/GEFS) guidance
continues to suggest scattered action stage rises and minor river flooding,
especially across much of NY state, VT, and southern New England (MA/CT)
where higher dewpoints are expected. Ice jams are of concern as well, as
the expected warmer temperatures and snowmelt may encourage ice movement.
Guidance from both the GFS and NBM-forced NWM continue to indicate small
stream responses across portions of western and southern NY state, as well
as northern PA on days 6 - 7 (Wed - Thu). Corresponding AEPs suggest that
mostly within bank rises are most likely, as the majority of the AEPs are
at or greater than 50%. However, there are some AEPs as low as 20 and 10%
along small streams draining the Upper Susquehanna (NY/PA) basins. This
also is in line with where we are the most robust responses from MMEFS,
which increases confidence in the potential for flooding in this area.
There is still some uncertainty in the magnitude of the responses given the
modeled QPF differences between various models, but overall, confidence is
increasing for the potential for hydrologic responses mid to late next week.

.Montana...
Snowmelt-induced flooding impacts will continue across north-central
portions of the state through today. Ice jams and subsequent flooding on
smaller rivers and streams remain possible as warm temperatures promote ice
breakup and movement.

//JDP





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