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AGUS74 KWCO 051512

HMDNWC



National Hydrologic Discussion - EXPERIMENTAL

NWS National Water Center - Tuscaloosa AL

1012 AM CDT FRI APR 5 2024



.Synopsis...

Potentially widespread flooding impacts possible early-to-mid next week  

across the ArkLaTex through the Lower Mississippi River Valley and  

Southeast... Ongoing river flooding from the Midwest through the  

Mid-Atlantic... Snow melt induced river rises across portions of the  

Intermountain West... Potential river rises late next week for the  

Northeast...



.Discussion...



.ArkLaTex through the Lower Mississippi River Valley and Southeast...

All flooding hazards are possible beginning day 4 (Mon) and continuing  

through day 7 (Thu) from eastern OK/TX through AR, LA, MS, and western AL  

in response to a multi-day heavy rainfall event. Considerable flooding  

impacts are not out of the question as event total rainfall amounts of 5 -  

7" could fall across the already wet region (2 - 8" positive departures  

from normal precipitation over the past 30 days, RFC QPE) where deep layer  

soils are moist (0 - 100 cm relative soil moisture of 50 - 70%, NASA SPoRT)  

and many rivers are still elevated from previous flooding (USGS, normal to  

above normal streamflows). There is still some inherent uncertainty with  

the extent and magnitude of potential flooding given this event is still 4  

- 5+ days out, but at minimum scattered flash flooding and minor river  

flooding can be expected by mid next week. Isolated moderate and major  

river flooding cannot be completely ruled out.



The NBM and GFS forced National Water Model (NWM) Medium Range Forecasts  

(MRF) are showing widespread river rises across the aforementioned region  

beginning on days 5 - 6 (Tue - Wed), with potential for rapid onset  

flooding and impactful flows (flows with annual exceedance probabilities  

below 20%). Ensemble guidance (PQPF river ensembles, HEFS) shows potential  

for minor-to-moderate river flooding as well.



Further east across eastern AL and the remainder of the Southeast, flooding  

potential is a bit more uncertain for mid-to-late next week, but there  

looks to be at least a minimal threat for isolated flash and river flooding.



.Midwest through the Mid-Atlantic...

Ongoing river flooding and unimpeded recessions will continue through the  

weekend. The Upper Ohio River will fall out of moderate flood by the end of  

this weekend. Other larger rivers such as the lower Wabash RIver (IN/IL),  

Illinois River (IL), and the Rock River (WI) will not crest until  

mid-to-late next week. Widespread rainfall will return to the region  

mid-to-late next week, which could bring new and renewed river rises across  

the Ohio River Basin by the end of next week (MMEFS/NWM). Trends will need  

to be monitored closely as this region is very vulnerable to new flooding  

impacts given the recent rainfall and ongoing river flooding.



.Intermountain West...

Snow melt will slow through the weekend (SNODAS) as a relatively cold low  

pressure system  brings snow to the region through this weekend.  

Nonetheless, snow melt from earlier this week will make its way into the  

rivers across the middle-to-upper Snake River Basin in southeast  

OR/southern ID/northern NV and produce rises on many mountain streams with  

very cold water. The NWM is suggesting high probabilities for high water  

flows (1.6 year ARI flow) on many of the headwaters draining the terrain  

into the Snake River Plain through the weekend.



.Northeast...

Above freezing temperatures and dew points will return late next week as a  

low pressure system brings precipitation to the region. Given the snowpack  

in place, and potential ripening of the snowpack throughout next week,  

there could be concern for flooding impacts if there is a combination of  

rainfall and snow melt. Soils are wet (NASA SPoRT/NWM) and streamflows are  

elevated (USGS) across much of the region, plus the NWM and MMEFS/HEFS are  

showing potential for new river rises across much of PA, NY, and New  

England by the end of next week, so there is a signal for flooding  

concerns, but confidence remains low on what the exact hydrologic response  

will be given this event is a week away.



//JEC



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