Graphic for MPD #0009

Mesoscale Precipitation Discussion 0009
NWS Weather Prediction Center College Park MD
1105 PM EST Tue Jan 09 2024

Areas affected...portions of southern
NY...CT...RI...MA...NH...southern VT...far southern ME

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

Valid 100400Z - 101000Z

Summary...Scattered to numerous instances of flash flooding are
likely with localized rainfall totals of 1-3" atop potentially
rapid snow water equivalent snowmelt of 1-2".

Discussion...Strong low to mid-level (925-700 mb) warm air
advection (WAA) is ongoing across much of New England as a surface
warm front begins to traverse southern portions of the area. This
warm front is in association with an anomalously strong low
pressure system (surface pressure ~975 mb) and accompanying
upper-level low over the Great Lakes region. As precipitable water
values continue to increase to 1.1-1.4 inches (near record
territory, per ALB/GYX/CHH/OKX sounding climatology), upper-level
divergence is also abundant via the right-exit region of a 140+ kt
jet streak (at 250 mb). While instability is certainly the primary
factor that is lacking for heavy rainfall, the strong dynamics
(WAA, frontogenesis, UL divergence) will largely make up for this
lack of instability.

The new 00z hi-res guidance is in excellent agreement regarding
expected rainfall rates and totals in the near term, suggesting
that rates of 0.25-0.75"/hr will largely prevail (though may
locally approach as high as 1"/hr) with localized totals of 1-3"
(through 10z). While totals of 1-2" are most likely for the vast
majority of the area, these amounts should mostly fall a bit short
of the associated 6-hr flash flood guidance (which largely ranges
from 2-3"). Antecedent conditions are also near normal, per NASA
SPoRT 0-100 cm soil moisture generally near the 50th percentile
(with the exception of southern VT, northern NH, and adjacent
portions of ME which are near the 90th percentile).

While an event of this magnitude and relatively short duration
would typically not be of great concern (despite the impressive
dynamics and highly anomalous tropospheric moisture), the factor
that tips the scales in this case is the still unaccounted for
unmelted snowpack. According to NOHRSC, unmelted snow water
equivalent ranges from 1.0-2.0" across the bulk of the region.
This is rather significant, given that dew points are already
rising into the 40s and will reach the 50s in southern portions of
the region. This quickly warming air mass (and falling rainfall)
should efficiently melt at least half (if not more) of the
snowpack. This will effectively add at least another 0.5-1.0" of
melt on top of the expected 1-3". For this reason, at least
scattered instances of flash flooding appear to be likely, and
should snowpack melt sufficiently instances of flash flooding may
become numerous across the region.

Churchill

ATTN...WFO...ALY...BGM...BOX...BTV...GYX...OKX...

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

LAT...LON   44807106 44737076 44577054 44297037 43917018
            43607033 43187064 42227068 41997092 41637076
            41417121 41307178 41247281 41107336 40957387
            41147426 41347453 41607465 42157461 42367417
            42707380 42777322 43247301 43587269 43617234
            43977211 44237191 44467152 44717135
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Last Updated: 1107 PM EST Tue Jan 09 2024