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AGUS74 KWCO 291518

HMDNWC



National Hydrologic Discussion - EXPERIMENTAL

NWS National Water Center - Tuscaloosa AL

1015 AM CDT FRI MAR 29 2024



.Synopsis...

Rainfall and snowmelt-induced flooding likely for the Northeast... Urban  

and small stream flooding impacts possible this weekend in Southern  

California... Recessions continue across Northeast Texas and the Southeast... 

Rainfall potential next week across the Mid-Mississippi and Ohio Valleys...



.Discussion...



.Northeast...

Periods of steady to moderate rainfall and snowmelt through the day will  

continue to produce areas of small stream rises and isolated minor river  

flooding from northern CT through coastal ME. Antecedent conditions are  

saturated, and warmer temperatures have left the current snowpack ripened  

in some areas. SNODAS and the NWM indicate up to 1" of snow water  

equivalent (SWE) loss, with locally higher amounts in higher elevations of  

NH and ME. Models appear to be melting slower than observations in portions  

of ME, so runoff may be more significant there. Guidance from the HRRR,  

GFS, and NBM-forced NWM are indicating scattered small stream responses  

from northern CT through ME generally within the next 24 hours. Given the  

lack of enhanced rainfall rates, the most likely impacts will be rapid  

rises on small streams and rivers draining from high terrain, as well as  

standing water on low-water crossings and poorly-drained areas in urban  

locations. This increase in flows may cause ice movement along the St. John  

River in ME.



Looking ahead, rainfall returns to the region mid-week next week. While the  

exact location and magnitude of impacts remain uncertain, this area will  

need to be closely monitored for additional impacts given the current  

saturated antecedent conditions.



.Southern California...

Urban and small stream flooding impacts are possible beginning today and  

continuing through the weekend as an atmospheric river brings periods of  

moderate to locally heavy rainfall (3-day totals of 2 - 4", WPC) to the  

region. The GFS and NBM-forced NWM continue to suggest the potential for  

scattered 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 on day 2 (Sat). There is potential for  

locally significant stream rises along the foothills of the Transverse  

Ranges, as supported by low annual exceedance probabilities (AEPs). The  

greatest risk of flooding will primarily be in urban areas and along small  

streams draining from steep terrain, especially if more robust rainfall  

rates are to materialize.



.Northeast Texas and the Southeast...

Minor river flooding will continue through at least early next week across  

these regions as runoff from rainfall earlier in the week routes  

downstream. Most locations are expected to crest this weekend and recede  

shortly thereafter. Light rainfall returns to the region early next week;  

however, confidence in the resultant hydrologic impacts remain uncertain at  

this time.



.Mid-Mississippi and Ohio Valleys...

Rainfall through the weekend will help moisten conditions prior to the  

arrival of moderate to locally heavy rainfall day 4 (Mon), leading to  

potential 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. Localized flooding does remain a  

concern especially with heavy rainfall and training.



//Ayala



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