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AGUS74 KWCO 121527

HMDNWC



National Hydrologic Discussion - EXPERIMENTAL

NWS National Water Center - Tuscaloosa AL

1015 AM CDT FRI APR 12 2024



.Synopsis...

Small stream and river flooding impacts likely across the Northeast...River  

flooding will persist across portions of East Texas, Lower Mississippi  

Valley, and Southeast, and the Ohio Valley...Elevated flows from recent  

rainfall in the Mid-Atlantic...Flash flooding possible through this  

afternoon in Hawaii...



.Discussion...



.Northeast...

Persistent moderate rainfall (1 - 3") is expected across portions of the  

Hudson Valley (NY) and portions of New England (CT and RI through western  

ME) through tonight and may elicit small stream flooding impacts, as well  

as areas of minor river flooding. This rainfall is occurring over an  

isothermal snowpack, and the combined overflow from rainfall and subsequent  

snowmelt (up to 4", SNODAS) will likely immediately runoff into already  

elevated streams (USGS), as there is little to no infiltration capacity  

available in the soils given their current degree of saturation (60 - 100%  

soil saturation, 0 - 40 cm, NWM). Another component to this snowpack is  

that the rainfall yesterday (up to 1.75") compressed it, and paired with  

the warmer rainfall expected today, this is expected to increase the  

degree/intensity of the melt. A complete and rapid loss of the snowmelt  

below 2000 is likely, especially along the the southern and southeastern  

slopes of the Green (VT) and White (NH) mountains, thus making high  

elevation streams in this area the greatest concern for experiencing small  

stream flooding impacts. Rapid rises on area rivers are likely as well,  

especially in the Upper Merrimack (NH), Saco (ME), Androscoggin (ME),  

Kennebec (ME), and Penobscot (ME) river basins, where minor river flooding  

is forecast. Rainfall rates are not expected to be particularly robust (up  

to 0.75"/hr, HRRR), however, poor drainage flooding is likely in urban  

areas as runoff from overflowing streams creates areas of standing water on  

roadways.



The HRRR, GFS, and NBM-forced National Water Model (NWM) is indicating  

scattered small stream responses across the regions, generally within the  

next 24 hours from CT and RI to Downeast ME. In regards to the magnitude of  

small stream responses, the High Flow Magnitude Forecast is indicating  

isolated annual exceedance probabilities (AEPs) as low as 2% along high  

elevation small streams in portions of eastern NH and western ME,  

indicating that locally significant small stream rises are possible.



.East Texas, Lower Mississippi Valley, and Southeast...

Widespread minor to moderate river flooding, and isolated major river  

flooding (TX/LA) is ongoing and forecast throughout these regions in  

response to heavy rainfall earlier this week. The majority of these  

locations are still rising and will crest within the next several days. No  

significant rainfall is expected through day 4 (Mon), which should allow  

for crests and subsequent recessions to occur unimpeded, although flooding  

along larger mainstem rivers will persist well into next week.



.Ohio Valley...

Widespread minor to isolated moderate river flooding impacts are ongoing  

and forecast across much of IN, OH, western PA, and western WV in response  

to rainfall yesterday. Most areas are at or near crest, however, areas  

along the Lower Wabash (IL/IN) and Lower White (IN) will crest early next  

week. Dry weather through day 4 (Mon) should allow for unimpeded crests and  

recessions on smaller rivers.



.Mid-Atlantic...

River rises are ongoing across the region in response to rainfall  

yesterday. Flooding is not expected, however, these rivers will stay  

elevated through the weekend. Moderate to major coastal flooding impacts  

remain possible today along Chesapeake Bay. Check with your local NWS  

office for more information related to coastal flooding impacts.



.Hawaii...

Showers and thunderstorms with enhanced rainfall rates (up to 2"/hr) may  

elicit flash and urban flooding impacts across Kauai and Niihau through  

this afternoon. Flood prone roads and other low lying areas may close due  

to elevated runoff and overflowing streams.



//Pritchard







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