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AGUS74 KWCO 251517

HMDNWC



National Hydrologic Discussion - EXPERIMENTAL

NWS National Water Center - Tuscaloosa AL

1015 AM CDT MON MAR 25 2024



.Synopsis...

Localized flooding potential for East Texas... Isolated river and small  

stream flooding impacts possible in the Lower Mississippi Valley...  

Isolated flooding impacts and riverine flooding is ongoing and forecast  

through midweek in the Southeast... Urban and small stream flooding impacts  

possible for Puerto Rico and USVI... Ongoing and forecast coastal flooding  

impacts early this week for the Coastal Mid-Atlantic and  

Northeast... Isolated hydrologic responses possible late in the week for  

Southern California...



.Discussion...



.East Texas...

Moderate rainfall from a progressive low-pressure system this morning  

across portions of the East TX may lead to localized flooding impacts for  

typical flood prone areas and streams. Soil conditions are wet (0 - 10 cm  

RSM, NASA SPoRT) from repeated rounds of rainfall this past week (1 - 3"  

RFC QPE) and streamflows have remained normal to above normal (USGS),  

suggesting that while some infiltration and storage capacity are still  

available, lower order streams and poorly drained areas are primed for  

localized flooding impacts. In addition, stalled recessions and new and  

renewed rises to minor flood stage are forecast in northeast TX as runoff  

is routed downstream. In short, the primary hydrologic concern this morning  

is brief small stream and urban flooding impacts where the heaviest  

rainfall materializes. Afterwards, minor river flooding and smaller order  

streams will be able to recede unimpeded for the remainder of the week.



.Lower Mississippi Valley...

Moderate to heavy rainfall across the region today through early day 2  

(Tue) will bring the threat of localized flash, urban, and small stream  

flooding impacts, along with minor river flooding. Soil moisture and  

streamflows are near normal (0 - 10 cm RSM, NASA SPoRT USGS respectively),  

suggesting that initial rainfall will have infiltration capacity to work  

with. New and renewed rises along with stalled recessions are forecast due  

to the aforementioned rainfall impacting the region.



The National Water Model (NWM) Short Range Forecast (SRF) is signaling  

rapid-onset flooding (ROF) across the region, particularly in the Upper  

White (AR/MO), Lower Ouachita (AR, LA), and the Yazoo (MS) river basins.  

Additionally, high flow magnitudes across the region are generally  

signaling annual exceedance probabilities (AEPs) above 50%, with only a few  

isolated smaller streams signaling anything more significant (NWM SRF).  

Furthermore, confidence in the signals is increasing due to run-to-run  

consistency. Considering the antecedent conditions, and consistency of NWM  

output, the main hydrologic concern for the area will likely be isolated in  

nature and contained to the slower draining basins.



.Southeast...

Isolated flash, urban, and small stream flooding impacts, along with minor  

to isolated moderate river flooding is possible through mid-week as yet  

another low-pressure system impacts the area. Soils are somewhat varied as  

the wetter soils (50 - 75%, 0 - 10 CM RSM, NASA SPoRT) are located in the  

Blackland Prairies (MS/AL) and get more dry (40 - 50%) as you get to the  

coastal ranges of GA and the Carolinas. Moderate rainfall totals of 1 - 3"  

(WPC QPF) with locally higher amounts possible, particularly in the  

southern Appalachians and along the I-95 corridor of GA through the  

Carolinas. Ensemble guidance (MMEFS) suggests that a reasonable scenario of  

minor to isolated moderate flooding could occur across the region as water  

is routed downstream from the headwaters. Additionally, hydrologic impacts  

may reach as far south as the Gulf Coast, however, due to the antecedent  

condition and sandy soils, these impacts will likely be limited to urban  

areas with poor drainage or low-lying areas.



.Coastal Mid-Atlantic and Northeast...

Widespread minor to moderate coastal flooding, along with minor flooding  

further inland is ongoing and forecast across much of the region. By day 3  

(Wed) and day 4 (Thu) moderate to locally heavy rainfall (1 - 3" WPC)  

returns to the regions bringing with it a renewed small stream and riverine  

flood threat. The NWM Medium Range Forecast (MRF) is beginning to show some  

consistency in highwater arrival times beginning later this week  

corresponding to the forecast rainfall, but uncertainty remains with  

respect to the extent of potential flooding impacts.



.Puerto Rico and USVI...

Urban and small stream flooding impacts remain possible through day 2 (Tue)  

for Puerto Rico due to a passing cold front producing a high chance of  

wetting rainfall. As this front continues east today, the U.S. Virgin  

Islands may also see isolated flooding impacts in flood prone areas.



.Southern California...

Rainfall beginning on day (5) and persisting through the weekend may bring  

the chance for isolated flooding impacts. Rainfall totals of 1 - 3" (WPC  

QPF) are forecast, however, due to the extended timeframe of the rainfall,  

confidence in the magnitude and placement of any potential hydrologic  

impacts remains low at this time.



//Capp



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