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AWUS01 KWNH 121256
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MEZ000-MAZ000-NHZ000-121845-

Mesoscale Precipitation Discussion 0155
NWS Weather Prediction Center College Park MD
855 AM EDT Fri Apr 12 2024

Areas affected...Northern New England

Concerning...Heavy rainfall...Flash flooding likely

Valid 121245Z - 121845Z

SUMMARY...Heavy showers over the next several hours combined with
snowmelt over the higher terrain is likely to result in areas of
flash flooding going through early this afternoon.

DISCUSSION...A deep layer trough of low pressure continues to
advance into the Northeast U.S. this morning with a surge of
strong warm air advection and moisture transport lifting up across
much of New England. In fact, a warm front continues to surge
northward across the interior of New England which is yielding the
arrival of a notably warm/moist airmass in off the western
Atlantic Ocean.

The early morning CIRA-ALPW data shows the LPW in the SFC/700mb
layer exceeding the 95th percentile of climatology, and this will
play a key role in conjunction with a southerly low-level jet of
50 to 60+ kts in driving efficient rainfall processes for elevated
rainfall rates. The CIRA-LVT magnitudes below 700 mb are
particularly strong right now nosing up across areas of
central/eastern MA and southern NH, with magnitudes exceeding 300
kg/m/s.

Radar imagery shows a batch of heavy shower activity including a
few thunderstorms riding northward across eastern MA and RI which
will likely get up into areas of central/southern NH and
especially southwest ME over the next few hours. A nose of
instability is associated with this area of convection with MUCAPE
values of as much as 500 to 1000 J/kg, and this coupled with the
efficient moisture column is driving rainfall rates of as much as
1"/hour with the stronger convective cells, although much of this
rainfall is tending to fall in as little as 30 to 45 minutes in
some cases.

Overall, the latest hires model guidance including the HRRR and
overnight HREF members appear to be a bit underdone with the
ongoing areas of heavy rain. The expectation is that rainfall
rates and coverage of heavy rain may increase going through
midday, and especially for areas of western and southwest ME. An
additional 2 to 3 inches of rain may occur locally, and especially
if some of the convective bands of eastern MA begins to locally
train over the same area into portions of western ME.

Snowmelt is a major concern at least locally up across the White
Mountains this morning in central and northern NH, and adjacent
areas of western ME with very warm air and high dewpoints
overrunning the remaining snowpack. Aside from this, the region in
general is quite sensitive with very moist/wet soil conditions.
Therefore, collectively, the arrival of additional heavy rainfall
over northern New England will encourage notable concerns for
runoff problems and flash flooding.

Orrison

...Please see www.wpc.ncep.noaa.gov for graphic product...

ATTN...WFO...BOX...CAR...GYX...

ATTN...RFC...NERFC...NWC...

LAT...LON   45626963 45446879 44726860 44166908 43597018
            42807104 42857174 43387198 44007195 44767124
            45347058