####018005330####
AGUS74 KWCO 061550

HMDNWC



National Hydrologic Discussion - EXPERIMENTAL

NWS National Water Center - Tuscaloosa AL

1015 AM CDT SAT APR 6 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 rain-on-snow event late next week for the  

Northeast...



.Discussion...



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

Flooding impacts are likely beginning day 3 (Mon) and continuing through  

day 6 (Thu) across east TX/OK, through LA, AR, MS, AL, and into GA as  

multiple days of heavy rainfall are expected. Rainfall totals of 3 - 7" are  

expected (WPC), and the area 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 the formation of 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  

uncertainty still exists for the magnitude and placement of flooding  

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

beginning day 3 (Mon) and small stream flooding is possible beginning day 4  

(Tue). The potential remains for some of these impacts to be considerable  

given the expected QPF and high rainfall rates. Minor riverine flooding is  

also expected, and rises to moderate or even major flood stage cannot be  

entirely ruled out.



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

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

forcings are showing widespread annual exceedance probability (AEP) signals  

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

potential for considerable impacts. The GFS forced 5-day high water  

probability and rapid onset flooding time-lagged ensembles are already  

showing modest signals over AR/LA/MS despite the event just now being fully  

in range of the forecast, increasing confidence in the flood forecast.  

7-day PQPF and HEFS guidance supports the potential for minor to isolated  

major river flooding.



Further east in GA 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 from earlier this week will continue to 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 as a low pressure  

system brings precipitation to the region on days 6 - 7 (Thu - Fri).  

Rainfall totals of 1 - 3" 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. 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 the  

NWM MRF is signaling widespread 50% or lower AEPs and MMEFS is suggesting  

there is potential for minor riverine flooding across much of PA, NY, and  

New England by the end of the week. This area will continue to be monitored  

as the event draws closer.



//Bliss







$$