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AGUS74 KWCO 101513

HMDNWC



National Hydrologic Discussion - EXPERIMENTAL

NWS National Water Center - Tuscaloosa AL

1015 AM CDT WED APR 10 2024



.Synopsis...

Considerable flooding impacts expected across portions of Northeast Texas  

and Lower Mississippi River Valley and the Southeast... Rain and  

snowmelt-induced river and small stream responses in the Northeast...  

Localized flooding and potential renewed rises across the Ohio Valley and  

Central Appalachians into the Mid-Atlantic... Flash flooding possible in  

Hawaii...



.Discussion...



.Northeast Texas and Lower Mississippi River Valley...

Considerable flooding impacts are ongoing this morning in eastern TX,  

southern LA, and central MS, with additional impacts, including flash,  

urban, and river flooding, as heavy rainfall, with the potential for  

training convection, continues across this already sensitive region.  

Locally considerable flash and urban flooding impacts are possible  

elsewhere, along with widespread minor to moderate river flooding, and  

isolated major flooding in TX. This system has already shown to be capable  

of producing enough rainfall to constitute home evacuations and flooding of  

homes near the channels of smaller order streams. Over the last 24 hours,  

rainfall totals of 4 - 7" have fallen in areas north of Austin, TX to  

Shreveport, LA, with 10"+ totals seen across southwest LA. This rainfall  

has saturated soil conditions and elevated streamflows throughout the  

region, suggesting the additional forecast rainfall (2 - 5", locally  

higher) will elicit rapid runoff into area rivers and streams with little,  

if any, channel storage capacity remaining. As such, new and renewed small  

stream and river flooding impacts are expected (PQPF). High rain rates and  

periods of training will continue to exacerbate the risk of considerable  

flash flooding impacts, especially in urban areas, along smaller creeks and  

streams, and in low-water crossings.



The HRRR-forced National Water Model (NWM) Short Range Forecast (SRF) has  

been consistently signaling widespread 12-hour probabilities at or above  

50% for rapid-onset flooding (ROF) potential throughout the region, with  

the highest probabilities (greater than 75%) clustered in and around  

Jackson, MS, south to the LA state line. ROF probabilities are also  

beginning to increase across southern AR, primarily from Texarkana to  

Monticello, due to both ongoing rainfall and the redevelopment of training  

rainfall later today. The NWM SRF High Flow Magnitude Forecast, is  

signaling widespread streamflow annual exceedance probabilities below 10%  

(with some as low as 2%) along smaller order streams throughout much of the  

region, suggesting that locally significant flash and small stream flooding  

impacts are likely, especially in locations where repeated heavy rainfall  

rates occur over smaller and more responsive basins.



.Southeast ...

Considerable flooding impacts associated with multiple rounds of heavy  

rainfall and flash flooding is possible today into tomorrow. The region  

south of I-10, including New Orleans, Mobile, and the FL Panhandle, will  

likely be most vulnerable to impacts with impervious surfaces and higher  

populations. Heavy rainfall tonight south of I-10, including New Orleans,  

Mobile, and the FL Panhandle, will likely result in rapid runoff.  

Antecedent conditions are already primed for rapid runoff with wet top  

soils (60 - 80% RSM, 0 - 10 cm) and streamflows around normal (USGS),  

suggesting elimination of infiltration capacity and rapid conversion of  

rainfall to runoff. Multiple axes of heavy rainfall remain possible and  

high rainfall rates are expected where showers and thunderstorms develop.  

River ensemble guidance (MMEFS/HEFS) as well as PQPF guidance suggests  

areas of minor river flooding can be expected across the region, but the  

main flooding threat remains rapid rises on small creeks/streams and  

flash/urban flooding.



The NWM SRF is not fully representative of the timing and placement of  

potential responses given that the HRRR QPF is a bit behind in timing in  

comparison to current radar observations. The NWM SRF 18-hour deterministic  

ROF forecast is signaling 70% waterway length flooding in many small basins  

between Mobile and Montgomery (AL), and seems to be a reasonable scenario  

this morning. Associated Maximum High Flow Magnitudes signals indicate AEPs  

of 20 - 2% with peak flow arrival times over the next 7 - 10 hours. Creek  

and stream flooding impacts will linger into tonight well beyond the fast  

moving heavy rainfall. Confidence in these signals is high as this storm  

has already been shown to produce high rainfall rates required for these  

responses. One factor potentially helping to mitigate significant river  

flooding (closer to the coast) is the fact that the heavy rainfall is  

moving very fast west to east (cross basin orientation), and the majority  

of the river basins near the coast are oriented north to south, helping to  

mitigate total resonance time of the heavy rainfall in individual basins.



.Northeast...

There is potential for urban and small stream flooding, as well as new and  

renewed river flooding from eastern NY through ME late week as a storm  

system brings periods of moderate rainfall to a ripened snowpack. The  

combination of gusty winds (30 - 40 kts at high elevations) and high dew  

points (mid 50s) on an isothermal snowpack is expected to bring 3 - 4" of  

melt in 48 hours. Early rainfall on day 2 (Thu) will likely compress the  

snowpack and with the arrival of warmer rain day 3 (Fri) there is potential  

for rapid melt. Full melt-out below 2,000 ft is likely, particularly on  

S/SE facing slopes. This melt will route downstream, causing potential  

flooding impacts on low elevation rivers as well. Melt combined with  

saturated antecedent conditions sets up ideal conditions for potential  

flash flooding and moderate river flooding in the Upper Merrimack, Saco,  

Androscoggin, Kennebec, and Penobscot basins. Outside of the snow covered  

areas, locally heavy rainfall may elicit poor drainage and nuisance  

flooding in urban areas, especially if heavier rainfall totals materialize.



Both the GFS and NBM-forced NWM Medium Range Forecast (MRF) guidance  

continue to indicate small stream responses across the region.  

Corresponding AEPs as low as 2% are noted on small streams draining the  

steep terrain of the White (NH) and Adirondack (NY) mountains, indicating  

confidence in the likelihood of small stream flooding impacts. MMEFS  

guidance continues to show the potential for minor river flooding across  

the region day 3 (Fri).



.Ohio Valley...

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

renewed river rises, remain possible as showers and thunderstorms impact  

the region today through day 2 (Thu), with the greatest potential for  

impacts across portions of western KY and southern IN. Rainfall totals of 1  

- 2" are expected (locally up to 3" possible). While top soils have mostly  

recovered from last week's rainfall, streamflows remain elevated,  

especially across the Lower Wabash (IL/IN) and Green (KY) river basins, as  

rivers in these areas remain in action stage and minor flood. Renewed river  

rises and prolonged river flooding remains the biggest concern, as this  

activity should be relatively progressive, though small stream and poor  

drainage flooding impacts in urban areas are possible dependent on  

training. Ensemble guidance (MMEFS/HEFS), as well as guidance from both the  

GFS and NBM-forced NWM MRF continue to support the idea of river and small  

stream responses later this week.



.Central Appalachians into the Mid-Atlantic...

Localized urban and small stream flooding impacts are possible day 2 (Thu)  

from portions of the Blue Ridge Mountains (WV/VA) to central PA (where  

flash flood guidance is low) as the same storm system bringing impacts to  

the Southern U.S. moves northeast, bringing periods of moderate to locally  

heavy rainfall (1 - 1.5+")  to the region. Soil moisture and streamflows  

are elevated from rainfall last week, which suggests lowered infiltration  

capacity for runoff. Guidance from both the GFS and NBM-forced NWM are  

indicating scattered small stream responses across southern WV through  

central PA on days 3 - 4 (Thu - Fri), with corresponding AEPs near the high  

water threshold (50% AEP), suggesting that most rises will remain in-bank,  

although some out-of-bank responses are possible on small streams draining  

steep terrain, as well as creeks draining into urban areas. River rises are  

possible as well, as MMEFS and HEFS guidance suggests some action stage  

rises across VA and PA later this week.



Coastal flooding impacts are possible through day 2 (Thu) from Chesapeake  

Bay to Long Island Sound. Check with your local NWS office for more  

information related to coastal flooding impacts.



.Hawaii...

Flash flooding due to heavy rainfall is possible days 2 - 3 (Thu - Fri) on  

Niihau and Kauai. Impermeable urban areas are especially vulnerable and  

significant flooding and property damage is possible. The slow moving  

nature of the rainfall will contribute to elevated runoff and flood prone  

roads and other low lying areas may be closed.



//Ayala/Freeman



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