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AGUS74 KWCO 081514

HMDNWC



National Hydrologic Discussion - EXPERIMENTAL

NWS National Water Center - Tuscaloosa AL

1015 AM CDT MON APR 8 2024



.Synopsis...

Considerable flooding impacts expected across portions of Northeast Texas,  

Lower Mississippi River Valley and Southeast... Rain and snow melt  

potentially leading to river rises late this week in the Northeast...  

Localized flooding and potential renewed rises across the Lower Ohio River  

Basin through the Mid-Atlantic...



.Discussion...



.Northeast Texas, Lower Mississippi River Valley and Southeast...

Considerable flooding impacts are likely beginning this afternoon from  

northeast TX through the MS Delta (southern AR/northern LA/western MS) and  

will continue through day 3 (Wed) from the MS Delta into portions of the TN  

Valley and much of AL as multiple days of heavy rainfall (3 - 7"+) are  

expected.



Across the northeast TX, the ArkLaTex, and the MS Delta, soils are somewhat  

conducive for efficient runoff processes as rainfall over the past month  

(+2 - 8" departures from normal, RFC QPE) has left soils down to 100 cm wet  

(60 - 80% RSM, NASA SPoRT). Despite top soils recovering from recent  

rainfall and increased vegetation growth over the past month, the increased  

deep layer soil moisture suggests that infiltration capacity will be  

quickly reached with the amount and intensity of rainfall expected.  

Streamflows are generally near annual median flows for this time of year,  

with the exception of northeast TX, where elevated flows persist on the  

upper Sabine, Upper Trinity, and Upper Neches basins. The latest guidance  

from the HRRR-forced National Water Model (NWM) is showing isolated stream  

responses across the region, with the more robust signals across central MS  

for later today. These responses will likely expand in coverage and  

intensity with time. Guidance from the GFS and NBM-forced NWM continue to  

show widespread small stream responses across this region through day 3,  

with corresponding annual exceedance probabilities (AEPs) as low as 2%  

along small streams in southeast AR, north LA, and west-central MS,  

increasing confidence in the potential for considerable flooding impacts  

across this region. Minor to moderate river flooding is also possible as  

well, as suggested by PQPF and HEFS ensemble guidance. Additionally, major  

river flooding can not be ruled out in areas of heavier rainfall (RFC  

ensemble guidance).



While periods of rainfall are possible today and tomorrow, the bulk of the  

activity will occur across the remainder of MS and much of AL on day 3  

(Wed), with considerable flooding impacts possible over the same area as  

well. Antecedent conditions there are similar to areas further west, as top  

soils have largely recovered from recent rainfall, but deep layer soils  

remain wet (60 - 70%), suggesting that infiltration capacity will be  

limited. The expected amounts and intensity of the rainfall will be the  

primary drivers for flooding impacts. Confidence in the exact placement of  

the greatest flooding impacts is somewhat unclear. The GFS-forced NWM  

places the more robust stream responses over east MS and portions of west  

and central AL, while the NBM-forced NWM places the more robust stream  

responses over northern MS. Nevertheless, considerable flooding impacts are  

possible in areas of heavier rainfall, especially urban centers along the  

I-20 corridor. River ensemble guidance (MMEFS/HEFS) as well as PQPF  

guidance suggests areas of minor to moderate river flooding across portions  

of the Pearl (MS), Big Black (MS), and Tombigbee (MS/AL) basins.



.Northeast...

Confidence is increasing in the potential for new and renewed river  

flooding impacts on days 4 - 5 (Thu - Fri) across the region as a storm  

system brings periods of moderate to locally heavy rainfall, with  

increasing temperatures and dew points (40 - 50F). On a regional scale,  

rainfall totals of 1 - 3" (WPC) are expected, with amounts up to 4" across  

portions of the Catskills, Adirondacks, and White Mountains, and this will  

all occur over a ripened snowpack, adding a snowmelt component as well.  

Antecedent conditions ahead of this event are saturated due repeated rounds  

of rainfall over the past month (+3 - 8" departures from normal,  

RFC QPE) and this additional rainfall and snowmelt will likely eliminate  

any remaining infiltration capacity, causing immediate runoff directly into  

streams and rivers and generating small stream and river flooding.  

Additionally, this system is expected to bring strong winds as well, as  

both the GFS and NBM are suggesting windy conditions, which combined with  

the warm dew points, will accelerate the snowmelt. In regards to the  

snowmelt, NWM guidance is indicating up to 2" of SWE loss over the next 48  

hours, which will immediately runoff into already elevated streams and  

rivers. During the event, there remains some degree of uncertainty with the  

amount of snowmelt, as this event remains out of the SNODAS forecast range,  

however, given the vulnerable antecedent conditions and atmospheric set up,  

it is likely that whatever is left of the snowpack, combined with the  

runoff, will further enhance the subsequent river flooding potential. Both  

the GFS and NBM-forced NWM guidance continue to indicate scattered to  

widespread small stream responses across the region, with corresponding  

AEPs generally near 50%. However, AEPs down to 2% are noted on some of the  

smaller reaches in higher terrain, indicating the potential for more  

significant flooding in those areas. MMEFS guidance continues to show the  

potential for minor river flooding across much of PA, NY, NJ, and New  

England later this week.



.Lower Ohio River Basin through the Mid-Atlantic...

Localized urban and small stream flooding impacts, as well as new and  

renewed river rises, are possible on days 2 - 4 (Tue - Thu) as widespread  

rainfall returns to these regions. The area at greatest risk will be across  

the Lower Ohio River Basin, as both the White (IN) and Wabash (IL/IN) river  

basins remain in flood and this coincides with the area of heaviest  

expected rainfall (2 - 4", WPC). Ensemble guidance (MMEFS/HEFS) as well as  

guidance from both the GFS and NBM-forced NWM continue to show scattered to  

widespread small stream and river responses across the region later this  

week.



Along the Mid-Atlantic coast, minor to isolated moderate coastal flooding  

impacts are possible through mid week from Chesapeake Bay to Long Island  

Sound. Check with your local NWS office for more information on related  

coastal flooding impacts.



//JDP











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