Mesoscale Precipitation Discussion 0766
NWS Weather Prediction Center College Park MD
1223 PM EDT Wed Jul 31 2024

Areas affected...portions of Upstate New York into central and
northern New England

Concerning...Heavy rainfall...Flash flooding possible

Valid 311630Z - 312230Z

Summary...Showers and thunderstorms will expand in coverage and
lift northward through the evening. Rainfall rates of 1-2+"/hr are
likely, which through repeated rounds could produce 2-3" of rain
with locally higher amounts. Flash flooding is possible.

Discussion...The regional radar mosaic this afternoon shows a
rapid expansion of convective development blossoming across New
England and Upstate New York. These thunderstorms are developing
within favorable thermodynamics characterized by PWs measured by
GPS and morning U/A soundings of 1.75 to 2 inches, nearing the
daily record according to the SPC sounding climatology, and MLCAPE
that has climbed to around 1000 J/kg. Into this environment,
forcing for ascent is impinging from the south and west through
weak vorticity maxima embedded within the flow and the broad
mid-level trough axis approaching from PA to drive height falls.
Additionally, a jet streak positioned to the northeast is leaving
its favorable RRQ over New England to additionally enhance deep
layer lift. Rainfall rates within this fresh convection have
generally remained around 1"/hr, but increasing glaciation noted
in the GOES-E day cloud phase imagery collocated with increasing
lightning cast probabilities indicates updrafts are strengthening
and rainfall rates should quickly follow.

The CAMs are in good agreement that convection will expand,
although the exact placement of what will be primarily scattered
cells or multi-cells through 20-25 kts of bulk shear is uncertain.
This is reflected by modest 1"/6hrs EAS probabilities, but
high(>80%) and moderate(>30%) neighborhood probabilities for 1"
and 3", respectively, in 6 hours. There is high confidence that
convection will expand as thermodynamic advection continues from
the south pushing PWs to broadly over 2" and SBCAPE to nearly 2000
J/kg in much of New England and Upstate NY. These cells will
generally lift northward progressively on 0-6km mean winds of
15-20 kts, but the widespread coverage should result in multiple
rounds in many areas. Additionally, nearly unidirectional flow
noted in morning soundings suggests brief training is possible
along narrow N-S corridors. Rain rates will likely reach 1-2"/hr
in most of these cells, and the HRRR 15-min accumulation product
suggests locally 0.75"+ of rain is possible this evening which
indicates brief intense rates to 3"/hr are possible. These axes of
enhanced training are where the heaviest rainfall is expected, and
this is most likely from far eastern Upstate NY into much of VT
where 2-3" of rain is possible.

FFG across the region has been compromised from recent heavy
rainfall, and there are some local minimums of less than 1.5"/3hrs
in the Green Mountains and Northeast Kingdom of Vermont. This area
is generally sensitive anyway due the terrain features supporting
rapid runoff when rates become intense, but the antecedent rain
has made the area even more vulnerable. The generally progressive
nature will be somewhat inhibiting to impacts, but where any
short-duration training or repeated rounds can occur, instances of
flash flooding will be possible.


Weiss

ATTN...WFO...ALY...BOX...BTV...GYX...

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

LAT...LON   45357189 45257096 45107019 44617028 44087068
            43527152 43257207 42827239 42437277 42207338
            42577393 43267425 44157421 45147397