####018006028####
AGUS74 KWCO 081518

HMDNWC



National Hydrologic Discussion - EXPERIMENTAL

NWS National Water Center - Tuscaloosa AL

1015 AM CDT WED MAY 8 2024



.Synopsis...

Considerable flooding impacts continue in portions of East Texas...Locally  

considerable flash and urban impacts are possible for Middle and Lower  

Mississippi Valleys into the Ohio Valley... and Puerto Rico and the  

USVI...Ice jam breakups and snowmelt driven rises continue in  

Alaska...Localized flooding impacts are possible next week for portions of  

the Gulf Coast and Southeast...River and small stream rises ongoing in  

Central and Eastern Montana...



.Discussion...



.East Texas...

Considerable river impacts are ongoing and will continue through much of  

this week across portions of southeast Texas. Major river flooding will  

continue this week along the Neches and Trinity rivers, with scattered  

moderate river flooding impacts throughout the region. While drier  

conditions have provided some relief to these basins, additional rounds of  

rainfall forecast late this week into next week may renew river rises and  

delay the much needed recessions from these historic flows. The extent of  

river flooding impacts is not anticipated to be at the level of what was  

experienced last week. However, the nearly saturated soils (0 - 100 cm,  

NASA SPoRT) have not had an appreciable amount of time to recover and  

streamflows are still much above normal to high (USGS). Any additional  

rainfall will likely result in immediate runoff into very sensitive rivers.



.Middle and Lower Mississippi Valleys into the Ohio Valley...

Multiple rounds of rainfall forecast through day 2 (Thu) may cause locally  

considerable flooding impacts, particularly for portions of TN and KY where  

2 - 4" (locally higher possible) of rainfall totals are forecast. The  

primary flooding concern for this upcoming event is flash flooding and  

rapid rises on streams that may impact low-lying areas. However, MMEFS and  

PQPF guidance suggests that river responses are also possible with rises to  

minor and isolated moderate flooding not out of the question. Soils are  

generally dry, and streamflows are near normal where the heaviest rainfall  

is forecast. This setup would suggest that while runoff may not be the  

initial threat, heavy training rainfall can quickly overcome these  

antecedent conditions resulting in rapid rises on streams, as well as flash  

flooding.



The National Water Model Medium Range Forecast is signaling some isolated  

probabilities of high water with the best chances of high water generally  

around north-central TN and south-central KY at roughly 50%. Additionally,  

the NBM and GFS-forced Maximum High Flow Magnitude Forecast is signaling  

that annual exceedance probabilities are generally at high water to 50%,  

with some of the smaller reaches as low as 10% which would suggest some of  

the potential flooding responses from rises could be significant. While the  

magnitude of both forcings are similar, the extent varies somewhat with the  

NBM signals slightly further south than the GFS.



.Puerto Rico and the USVI...

A wet pattern continues to elevate the flood threat through the evening of  

day 2 (Thu) across the islands. Urban and small stream flooding impacts,  

along with the chance for isolated flash flooding and mudslides, remain  

possible as streamflows are elevated and soils are completely saturated.  

Any additional rainfall will immediately enhance the risk of further  

flooding impacts, some of which may be significant.



.Alaska...

Snowmelt-driven flooding continues to impact flood prone areas in the  

Cooper River Basin, particularly Moose and Trooper Creeks at Glennallen. As  

warmer conditions influence the area, continued rises on streams and rivers  

will last through the week. Further to the west, the communities of Akiak  

and Tuluksak will be impacted by ice jam breakups moving through the area.  

Additionally, river rises to minor and moderate flooding are possible in  

Kwethluk and Akiachak. Looking north to the Yukon River, an ice jam front  

is also expected to impact the community of Circle today. Ice jam breakups  

and formations are expected to continue along this breakup front as it  

makes its way down the Yukon River. Ice jams and breakup fronts can evolve  

rapidly, so refer to the local offices for up-to-date information.



.Gulf Coast and Southeast...

Rounds of potentially heavy rainfall totals of 1 - 3", locally higher  

amounts possible, may cause some isolated flooding impacts along portions  

of the Gulf Coast and inland into parts of LA, MS, AL, and FL late this  

week and again early next week. Antecedent soil conditions are generally  

dry along the coast. Streamflows are a bit of a different story with higher  

flows in the western and eastern portions of the Gulf Coast and much lower  

flows in the central portion. While day 2 into day 3 (Thu - Fri) may cause  

some localized flooding impacts, it will also serve as a potential  

hydrologic priming event. By day 6 (Mon) another slug of moisture enters  

the region renewing the localized flood threat along the coast. Model  

guidance may still need some time to better resolve the magnitude,  

location, and timing of expected rainfall. However, localized urban and  

pluvial flooding can not be ruled out in typical flood prone areas or areas  

that may have been elevated from rainfall this week.



.Central and Eastern Montana...

Rain continues this morning over Montana. Rainfall totals 1 - 3" over the  

past 24 hours have caused some out-of-bank responses on the smaller  

streams, as well as moderate river flooding on Clear Creek near Chinook. By  

day 2 (Thu), dry conditions should settle in across the state allowing for  

unimpeded recessions to begin.



//Capp



$$