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AGUS74 KWCO 021515

HMDNWC



National Hydrologic Discussion - EXPERIMENTAL

NWS National Water Center - Tuscaloosa AL

1015 AM CDT TUE APR 2 2024



.Synopsis...

Flooding impacts and river flooding expected in the Ohio and Tennessee  

Valleys into the Mid-Atlantic... River flooding ongoing and forecast in the  

Mid-Mississippi Valley and Upper Great Lakes...



.Discussion...



Ohio and Tennessee Valleys into the Mid-Atlantic...

Urban and small stream flooding impacts, as well as isolated flash  

flooding, will continue to be possible through this evening across the  

OH/TN Valleys, and through day 2 (Wed) for portions of the upper  

Mid-Atlantic in response to rounds of moderate to heavy rainfall.  

Widespread river rises, including minor to isolated moderate flooding is  

ongoing or forecast throughout much of the region, with MMEFS suggesting  

additional rises to flood stage could be possible depending where the  

heaviest rainfall sets up. Latest guidance from WPC indicates the axis of  

heaviest rainfall (2 - 4", 2-day totals) is expected in PA/NJ, while totals  

up to 2" are forecast elsewhere. While the top layer (0 - 10 cm) of soils  

across IN/OH is nearly saturated due to recent rainfall (85%+ RSM, NASA  

SPoRT), soil moisture quickly decreases with depth in the column due to  

previous dry conditions, with overall values between 50 - 70% (0 - 100 cm)  

throughout the region. These conditions should initially allow rainfall to  

be absorbed into the system, however, any areas that received training  

convection or prolonged intense rainfall rates will see the potential for  

significant runoff and associated flash, urban, or small stream flooding  

impacts. This is also manifested through the responses of streams and  

rivers across IN/OH and portions of PA/NJ, where real-time streamflows are  

elevated much above normal (USGS), diminishing available channel capacity  

for any excess runoff. However, streamflows in areas south of the Ohio  

River are generally near to below normal, which will help mitigate any  

widespread hydrologic responses, but isolated flooding impacts remain  

possible rapid rises in any areas where rainfall rates and duration are  

high enough to overcome these conditions, particularly in areas of complex  

terrain.



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

been consistently signaling the potential for rapid-onset flooding across  

portions of the OH/TN Valleys, with some of the more robust signals across  

OH, WV, and southwestern PA however, the highwater thresholds for the OH  

Valley have been considered very low relative to the rest of the U.S.  

suggesting signals may be overdone. Corresponding annual exceedance  

probabilities  (AEPs) are mostly near 50%, however, isolated pockets of  

AEPs below 20% are seen in north-central TN through central KY, and western  

PA, suggesting locally out-of-bank small stream rises are possible. The NWM  

SRF is also beginning to capture the QPF for the event as it moves into the  

Mid-Atlantic, and signals will likely start to expand across the region  

later this morning. Similarly, the GFS-forced NWM Medium Range Forecast  

continues to signal the potential for isolated rapid-onset flooding across  

the upper Mid-Atlantic, with the highest probabilities (greater than 50%)  

occurring across central PA. Corresponding AEPs remain generally near 50%,  

suggesting rises should stay mostly in-channel, but scattered AEPs below  

20% through central PA and northern NJ, combined with wet antecedent  

conditions, support the potential for at least isolated flooding impacts,  

especially at any locations where repeated heavy rainfall rates occur over  

smaller and more responsive basins.



.Mid-Mississippi Valley and Upper Great Lakes...

Minor to isolated moderate river flooding is ongoing or forecast across  

portions of eastern MO, northern IL, and southeast WI in response to recent  

heavy rainfall over the last 24 hrs. Lingering rainfall through this  

afternoon shouldnt be heavy enough to result in any new flooding impacts,  

other than contributing additional runoff to the already elevated river  

systems. This flooding is forecast through the remainder of the week for  

most locations as rivers continue to crest and elevated flows route  

downstream.



//Freeman







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