AWUS01 KWNH 291315
FFGMPD
NYZ000-291900-

Mesoscale Precipitation Discussion 0810
NWS Weather Prediction Center College Park MD
914 AM EDT Sat Jul 29 2023

Areas affected...Much of Upstate New York...

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

Valid 291315Z - 291900Z

SUMMARY...Slow moving potentially repeating thunderstorms
downstream of approaching shortwave pose localized possible flash
flooding incidents through late morning/early afternoon.

DISCUSSION...GOES-E WV suite depicts a well defined shortwave
crossing Lake Erie and the Ontario Peninsula with a well defined anticyclonically curved cirrus shield indicative of continued
favorable upper-level divergent pattern aloft for
convective/system maintenance. At the surface a well defined
surface low resides below the shortwave in Lake Erie with a sharp
well defined stationary front extending due east through the
Mohawk Valley into central VT/NH. This frontal zone/shear zone
extends though a fairly deep layer with enhanced moisture pooled
along it. CIRA LPW denotes .8-1" values across W NY that extends
to the 850-700mb layer, though overall total moisture remains
above average at 1.75 range or about 1.5-2 standard deviation from
normal. Lapse rates are modest given the deep layer moisture, but
early morning clearing downstream supports 1000-1500 J/kg of
MLCAPE and in a weakly capped or uncapped environment, convection
is breaking out along and just north of the frontal boundary
across north-central Upstate New York.

Given the vigor of inflow/flux is generally oblique to the
orientation of the deeper frontal zone, moisture flux convergence
is weak to moderate and so rainfall efficiency is a bit limited
but still capable of 1.5"/hr at least initially for the initial
central NY to E NY cells. This will change rapidly with the
approach of the shortwave and low level cyclone, backing and
increasing flow will allow for stronger directional convergence
with similar 15-20kts of mid-level flow too, so short-term rates
could perk up to 1.75-2"/hr nearer the surface cyclone as it
approaches. So, the concern is the mean steering flow is
parallel to the boundary and should support solid opportunities
for redevelopment and repeating thunderstorm tracks across much of
west-central to east-central Upstate NY, spotty 2-3" totals are
expected.

If there is some good news, this axis has been relatively dry over
the last 1-2 weeks and may be able to have some beneficial
infiltration, but given the potential for multiple rounds, the
first may saturate the upper soils and limit infiltration for the
upstream cells, that are likely to be a bit more intense and could
have a bit more runoff. As such, spots of flash flooding are
considered possible through late morning into early afternoon.

Gallina

ATTN...WFO...ALY...BGM...BTV...BUF...

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

LAT...LON 44477565 44447410 44077356 43357339 42637371
42447504 42397726 42447850 42487913 43007914
43387885 43427701 43777643 44137633

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