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AGUS74 KWCO 301515

HMDNWC



National Hydrologic Discussion - EXPERIMENTAL

NWS National Water Center - Tuscaloosa AL

1015 AM CDT SAT MAR 30 2024



.Synopsis...

Urban and small stream flooding impacts possible this weekend in Southern  

California... Flooding potential next week across the Mid-Mississippi and  

Ohio Valleys...Ongoing highwater and flooding across New England and  

Northeast Texas and the Southeast...



.Discussion...



.Southern California...

Moderate to heavy rainfall is ongoing and will continue through the weekend  

along with the potential for isolated flooding impacts primarily in urban  

areas and along small streams draining from steep terrain. Rainfall over  

the past 24 hours (2 - 4" QPE, MRMS, gage) has wetted top-layer soils, and  

combined with non-impervious urban areas and the steep/complex terrain  

elsewhere, rapid runoff is possible especially if more robust rates  

materialize.



The National Water Model (NWM) Short Range Forecast (SRF) has consistently  

shown the potential for rapid-onset flooding (ROF) across the region with  

many ROF probabilities greater than 50%. The SRF Maximum High Flow  

Magnitude Annual Exceedance Probabilities (AEPs) suggest potential for  

out-of-bank flows, with AEPs ranging from 20 - 50%. Given these signals  

from the NWM and the antecedent conditions, there is potential for urban  

and small stream responses along the I-5 corridor from Santa Barbara to the  

International Border and along the foothills of both the Transverse and  

Peninsular ranges, primarily today.



.Mid-Mississippi and Ohio Valleys...

Rainfall through the weekend will moisten soils and increase streamflows  

prior to the arrival of moderate to locally heavy rainfall day 3 (Mon),  

bringing potential for localized hydrologic impacts. Current soil moisture  

indicates availability for initial infiltration (45 - 60% RSM, 0 - 10 cm  

NASA SPoRT), and streamflows are generally below normal for this time of  

year (USGS), with the exception of small portions of MO, OH, and IL, where  

streamflows are elevated from recent rainfall.



While guidance from both the NBM and GFS-forced NWM are indicating  

scattered small stream responses across these regions, the exact location  

and magnitude of impacts still remain uncertain given the extended nature  

of the forecast and are dependent on where heaviest rates materialize. WPC  

QPF is further south than the GFS and NBM QPF, thus further reducing  

confidence in the location of potential flood impacts. MMEFS supports the  

probability of flooding, signaling potential for isolated minor to moderate  

flooding, though QPF has backed off in more recent runs. Localized flooding  

does remain a concern especially with heavy rainfall and training.



.New England...

A combination of rainfall and snowmelt has left streams throughout the  

region above climatological normals. There are still rivers in minor  

flooding, though many are in decline or cresting this weekend. Little to no  

precipitation until mid-week next week should allow for unimpeded  

recessions of elevated rivers elsewhere across the region. Looking ahead,  

with the return of rainfall and saturated antecedent conditions this region  

will need to be closely monitored for additional flooding impacts.



.Northeast Texas and the Southeast...

Minor river flooding is expected to persist into early next week across the  

region due to runoff routing downstream. Most locations are anticipated to  

crest this weekend before gradually receding. There's a possibility of  

light rainfall returning to the region at the beginning of next week, but  

the specific hydrological impacts are currently uncertain.



//Ayala



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