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AGUS74 KWCO 071514

HMDNWC



National Hydrologic Discussion - EXPERIMENTAL

NWS National Water Center - Tuscaloosa AL

1014 AM CDT SUN APR 7 2024



.Synopsis...

Considerable flooding impacts expected across portions of the ArkLaTex  

through the Lower Mississippi River Valley and Southeast... Ongoing river  

flooding and potential for renewed rises across the Midwest through the  

Mid-Atlantic... Rain and snow melt potentially leading to river rises late  

this week in the Northeast...



.Discussion...



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

Considerable flooding impacts are likely beginning day 2 (Mon) (east  

TX/OK/lower MS River Basin) and continuing through day 5 (Thu) across the  

Southeast as multiple days of heavy rainfall are expected. Rainfall totals  

of 3 - 7" are expected (WPC), and the area from northeast TX through  

southern AR/northern LA into central MS has already received above-average  

rainfall over the past month (2 - 8" positive departures from normal  

precipitation over the past 30 days, RFC QPE). As such the antecedent  

conditions are favorable for efficient runoff processes. Top soils have had  

time to recover from past events, but remain near normal levels (40 - 65%  

RSM, NASA SPoRT). Deeper layer soils are more saturated (50 - 75% RSM),  

suggesting infiltration capacity will quickly be reached and lead to  

runoff/flooding impacts. Streamflows are generally near normal except in  

east TX, where above to much above average flows persist in the Sabine,  

Upper Trinity, and Upper Neches basins (USGS mean annual streamflow). While  

some uncertainty still exists for where the most significant flooding will  

be, confidence is increasing that flash/urban flooding is possible  

beginning day 2 (Mon) and small stream flooding is possible beginning day 3  

(Tue). Minor riverine flooding is expected, and rises to moderate or even  

major flood stage are becoming increasingly likely.



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

(MRF) differ somewhat in the placement of potential flooding impacts, but  

both forcings are showing scattered annual exceedance probability (AEP)  

signals in the 10 - 2% range in AR/MS/northeast TX (NBM) and LA/MS (GFS),  

supporting the potential for considerable flooding impacts. The GFS forced  

5-day high water probability and rapid onset flooding time-lagged ensembles  

continue to show increasing probabilities over AR/LA/MS, increasing  

confidence in the location of where the more widespread river responses may  

be. 7-day PQPF and HEFS guidance supports the potential for fairly  

widespread minor to isolated moderate-to-major river flooding from east TX  

through central MS.



Further east in AL/GA and the remainder of the Southeast, flooding  

potential is a bit more uncertain for mid-to-late week and does not look to  

be as significant as further west, but isolated flooding impacts are  

likely, especially from central AL through north-central GA where signals  

for river rises have been most consistent from the NWM MRF.



.Midwest through the Mid-Atlantic...

Ongoing river flooding and unimpeded recessions will continue through day 2  

(Mon). Widespread rainfall will return to the region on days 3 - 5 (Tue -  

Thu), which could bring new and renewed river rises across the Ohio River  

Basin by the end of the week (MMEFS/NWM). The heaviest QPF totals (2 - 4",  

WPC) will be across lower portions of the Ohio River Basin where there is  

ongoing flooding still in recession across the White and Wabash basins  

(IN/IL). This area of southern IN/IL and perhaps into western KY may see  

the best chance for river flooding impacts, with this idea being supported  

by MMEFS and NWM forecasts showing fairly widespread riverine response  

across the region.



.Northeast...

Above freezing temperatures and dew points will return as a low pressure  

system brings rainfall to the region on days 5 - 6 (Thu - Fri). Rainfall  

totals of 1 - 3" (locally 4" in the Catskill, Adirondack, and White  

mountains) are expected (WPC) on top of a ripening snowpack, and with well  

above average rainfall over the past month (4 - 8" positive departures from  

normal precipitation over the past 30 days, RFC QPE), the combined  

rain-snowmelt threat could lead to impactful flooding. The NBM suggests  

potential for windy conditions as well, which in combination with dew  

points near 50 F could help with the energy transfer to and  

ripening/melting of the snowpack. Antecedent conditions makes the region  

quite hydrologically sensitive, with wet soils (60 - 75% RSM, 0 - 10cm NASA  

SPoRT) and much above average annual streamflows (USGS). With the event out  

of the forecast range of SNODAS, uncertainty in the amount of snowmelt that  

will occur remains high, but both the GFS and NBM forced NWM MRFs are  

signaling widespread flows with 50% or lower AEPs, and MMEFS/HEFS continues  

to suggest there is potential for scattered minor riverine flooding across  

much of PA, NY, NJ, and New England by the end of the week.



//JEC



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