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AGUS74 KWCO 261504

HMDNWC



National Hydrologic Discussion - EXPERIMENTAL

NWS National Water Center - Tuscaloosa AL

1015 AM CDT TUE MAR 26 2024



.Synopsis...

Isolated flooding impacts and riverine flooding is ongoing and forecast in  

the Southeast... Coastal and small stream flooding impacts through mid week  

for the Coastal Mid-Atlantic and Northeast... Isolated hydrologic responses  

possible late in the week for Southern California... Unimpeded river and  

small streams recessions begin for the Lower Mississippi Valley... and  

Northeast Texas...



.Discussion...



.Southeast...

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

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

low-pressure system impacts the area. Soils remain dry (0 - 10 cm RSM NASA  

SPoRT) eastward to the coastal ranges of GA and the Carolinas. Moderate  

rainfall totals of 1 - 3" (WPC QPF) are expected through day 2 (Wed), with  

locally higher amounts possible, particularly in the Apalachee Bay (FL)  

area and along the I-95 corridor of GA through the Carolinas. The National  

Water Model (NWM) Short Range Forecast (SRF) continues to signal low  

probabilities for rapid-onset flooding in AL and GA for day 1 (Tue) which  

suggests that a localized flood threat may occur where the heaviest  

precipitation materializes.



Minor flooding is forecast on the Tombigbee River (MS/AL) while minor to  

isolated moderate flooding is forecast along the coastal ranges of GA and  

into the Carolinas. 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. Stalled recessions and new and renewed rises  

are expected to continue through day 2 (Wed) before dry conditions settle  

in for the remainder of the week allowing unimpeded recessions to begin.



.Coastal Mid-Atlantic and Northeast...

Widespread minor to moderate coastal flooding is ongoing and forecast  

across much of the region. By day 2 (Wed) and day 3 (Thu), moderate to  

locally heavy rainfall (1 - 3" WPC) returns to the region bringing with  

renewed potential for small stream and urban flooding impacts, and  

potentially a riverine flood threat (MMEFS). Soils get progressively wetter  

(0 - 100 cm NASA SPoRT) as you go north into ME where snowmelt may play a  

role in the flooding potential late this week. The NWM Snow Water  

Equivalent (SWE) change has been consistent with SNODAS; the NWM SWE change  

for day 3 (Thu) is indicating a melt off of up to 0.2" - 0.4" of SWE each  

day in portions of coastal New England into ME, increasing the total runoff.



The NWM Medium Range Forecast (MRF) continues to show some consistency in  

highwater arrival times beginning later this week corresponding to the  

forecast rainfall. Furthermore, the extent of rapid-onset flooding (ROF)  

signals has increased, along with a slight increase in ROF probabilities  

along much of the Eastern Seaboard. However, uncertainty still remains if  

the combination of snowmelt and rainfall will cause more widespread impacts  

in the Northeast, or not. Ensemble guidance (MMEFS) indicates that new and  

renewed rises are possible beginning late in day 2 (Wed) for the  

Mid-Atlantic and day 3 (Thu) for the Northeast as runoff from rain and  

snowmelt begins to route into the tributaries and mainstems.



.Southern California...

Rainfall beginning on day (4) and persisting through the weekend may bring  

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

locally higher amounts possible (WPC QPF) are forecast in the Los Angeles  

area across the Ventura, San Gabriel Coastal, Santa Ana, Laguna, and San  

Diego Coastal river basins. Isolated flash and urban flooding impacts may  

be possible. However, some uncertainty remains in the exact placement and  

magnitude of potential impacts. The NWM MRF is beginning to signal  

scattered high water arrival times over the next 4 - 5 days (Fri - Sat)  

correlating well with the expected rainfall. Additionally, due to the  

heavier rainfall earlier in the season, it is possible that rock and  

mudslides may also impact residents in areas of steeper terrain.



.Lower Mississippi Valley...

As rainfall comes to an end by this afternoon, isolated small stream  

flooding, and minor river flooding are the remaining hydrologic impacts for  

the basin. The NWM MRF is signaling peak flow arrival times will generally  

begin to take place over the next 24 hours. However, some of the basins  

adjacent to the Mississippi River will take longer to peak. Dry conditions  

over the next week will allow for unimpeded recession to begin for streams  

and rivers.



.Northeast Texas...

Scattered minor flooding is ongoing and forecast through the weekend as  

flood water continues to route downstream from repeated rounds of rainfall  

over the past week (1 - 3" RFC QPE). While high river flows are expected to  

continue, particularly in the Big Cypress, Sulfur, Sabine, Neches, and  

Upper Trinity river basins, unimpeded recessions are expected throughout  

the remainder of the week due to dry conditions settling across the region.



//Capp









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