711
AGUS74 KWCO 281525
HMDNWC

National Hydrologic Discussion - EXPERIMENTAL
NWS National Water Center - Tuscaloosa AL
1015 AM CDT FRI MAR 28 2025

.Synopsis...
Flash, urban, and isolated minor river flooding possible in Texas and
Louisiana... Combination of rainfall and snowmelt may elicit river and
stream responses across the Pacific Northwest and Northern Rockies...
Multiple rounds of rainfall forecast across the Upper Great Lakes and Ohio
Valley...

.Discussion...

.Texas and Louisiana...
The system that has been moving over southern TX over the past day has now
moved mostly off the coast and is expected to push into LA and far eastern
TX later today. While the extent of significant rainfall for this event has
been limited to far southern Texas, impressive rainfall totals up to 16"
have fallen (MRMS, 48hr QPE), resulting in localized flash flooding impacts
in the Harlingen/McAllen area. The highly concentrated nature of the
rainfall has limited the riverine responses, with only isolated rises to
action or at worst minor flood stage.

Flash and urban flooding are possible in LA and eastern TX through today in
response to the heavy rainfall.

.Pacific Northwest and Northern Rockies...
Periods of rainfall and snowmelt through today will continue the potential
for isolated minor river flooding across OR, ID, western MT, northern CA,
and northern NV. In WA, minor river flooding is forecast on the Skokomish
River near Potlatch. Elevated temperatures and dewpoints in the mid-30s to
mid 40s are driving accelerated snowmelt at low-to-mid elevations (SNODAS,
NWM). This snowmelt will coincide with additional rainfall on day 1 (Fri),
which could trigger small stream and river responses as the soil column (65
- 90%, 0-100 cm RSM, NASA SPoRT) reaches its capacity to handle enhanced
runoff. The NBM-forced NWM MRF peak flow arrival time forecast and river
ensemble guidance (30% HEFS) continue to indicate isolated minor river
flooding, as well as small stream responses. Additionally, rainfall along
the Pacific Coast and northwest CA through day 7 (Thu) may result in rapid
rises on small creeks and tributaries. This is a typical scenario for the
region, as March often marks a period of high streamflows on the West
Coast, with significant runoff due to combined rain and snowmelt.

.Upper Great Lakes, Ohio Valley, into the Lower Mississippi Valley...
Multiple rounds of widespread rainfall are forecast across the region over
the next week (2 - 4", WPC 7-day QPF) beginning day 3 (Sun). Antecedent
conditions are generally near normal, and the initial rounds of rainfall
this weekend and early next week are not likely to produce flooding
impacts, but as subsequent events begin compounding, isolated flooding
impacts cannot be ruled out. River ensembles (HEFS/GEFS 10-day 30%)
indicate the potential for widespread rises to minor flooding in the White
and Wabash basins in Indiana, and flows from the GFS-forced NWM are
exceeding the 10 - 2% AEP threshold in the Ohio Valley in the days 6 - 7
timeframe (Wed - Thu), suggesting significant flooding cannot be ruled out
in the extended period.

//Bliss



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