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AGUS74 KWCO 121158

HMDNWC



Area Hydrological Discussion #079 - EXPERIMENTAL

NWS National Water Center - Tuscaloosa, AL

651 AM CDT Fri Apr 12 2024



WHAT: Small stream and river flooding

WHERE: Northern Connecticut through western Maine

WHEN: Through early afternoon



FORECAST RAINFALL AND ANTECEDENT CONDITIONS...

QPF: 1 - 3" , locally higher (HRRR)

QPE: up to 1.5" (MRMS QPE)

Rates: up to 0.75"/hr (HRRR)

Soils: 60 - 100% saturation (0 - 40 cm, NWM Soil Moisture Analysis)

Streamflows: Much above normal (USGS)

Snowmelt: Up to 4" (SNODAS)



DISCUSSION...

Persistent moderate rainfall is expected across the area of concern

though the early afternoon hours, potentially eliciting widespread

small stream flooding impacts and pockets of minor river flooding.

This rainfall is occurring over an isothermal snowpack, and the

combined output from both sources will likely immediately runoff into

already elevated streams as there is little to no infiltration

capacity available in soils given their current degree of saturation.

Another component to this is rainfall yesterday compressed the

snowpack, and paired with the warmer rainfall today, this will

increase the degree/intensity of melt of the snowpack. A complete and

rapid loss of the snowmelt below 2000 is expected, especially along

the south and southeastern slopes of the Green (VT) and White

Mountains (NH), thus making high elevation small streams the greatest

concern for 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.

While rainfall rates are not expected to be particularly high, poor

drainage flooding is likely in urban areas as runoff from overflowing

streams create areas of standing water on roadways.



The latest guidance from the HRRR-forced NWM is indicating scattered

small stream responses from northern CT through western ME, with

increased probabilities (50 - 100%) of rapid onset flooding across

much of NH and western ME. Corresponding annual exceedance

probabilities, per the High Flow Magnitude Magnitude forecast, of 10%

or less (as low as 2%) are noted along small streams draining the

White Mountains and western ME mountains, indicating that locally

significant small stream rises are possible. A subsequent AHD may be

needed across portions of NH and ME later today as additional

rainfall impacts the region.



GRAPHICAL REPRESENTATION weather.gov/owp/operations-ahd



Additional National Water Center products are available at

weather.gov/owp/operations



//JDP



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