Mesoscale Precipitation Discussion 0744
NWS Weather Prediction Center College Park MD
400 AM EDT Sun Jul 16 2023

Areas affected...Portions of the northern Mid-Atlantic into the
Northeast and southern New England

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

Valid 160800Z - 161400Z

Summary...Scattered instances of flash flooding early this morning
will likely become widespread across portions of southern New
England by mid-morning. Localized 3-5"+ totals are possible, and
some significant flash flooding is expected to develop across more
vulnerable terrain.

Discussion...Showers and thunderstorms have been organizing over
the past couple of hours across portions of the northern
Mid-Atlantic, within a pre-frontal surface trough and broad
cyclonic flow aloft. The mesoscale environment in the vicinity and
downstream of this convection is characterized by ML CAPE of
500-2000 J/kg, precipitable water values of 1.6-2.1 inches
(between the 90th percentile and max moving average, per IAD/ALY
sounding climatology), and effective bulk shear of 30-50 kts. This
parameter space should continue to support organized convection,
which has been orienting itself linearly from SSW-NNE (parallel
with the mean flow) and allowing for localized training of 1-2"/hr
rainfall rates. This is very concerning, as most of the region has
already seen anywhere from 300-600% of normal rainfall (ranging
from 3-12 inches) over the past week (through Saturday morning).
This has resulted in rather low Flash Flood Guidance (FFGs),
generally ranging from 1-2" (with the exception of portions of the
Delmarva and southern NJ, where guidance is as high as 3-4" for a
3-hr period).

Flash flooding is considered likely across the region, given the
very favorable environment and very wet antecedent conditions.
Scattered coverage is generally expected through much of the
morning, and mainly occurring where the aforementioned training
bands ultimately track. Scattered convective activity is also
beginning to proliferate across portions of southern New England,
and this shallower activity may also be capable of 1-2" short-term
totals with highly efficient warm rain processes dominating. As
the low-level flow continues to back towards the south across this
area, the approaching deeper convection from the southwest will
start to merge with and overcome this convection, where the 00z
HREF indicates alarmingly high 40-km neighborhood exceedance
probabilities for 3" and 5" thresholds through 15z (30-60% and
10-30%, respectively). This suggests that flooding will become
more widespread across portions of southern NY/NH and western
CT/MA by mid-morning, and some of this flooding will likely become
significant (i.e. life threatening) as well (particularly where
3-5"+ totals coincide with more vulnerable terrain, including
flood-prone low-lying and urbanized areas).

Churchill

ATTN...WFO...AKQ...ALY...BGM...BOX...BTV...CTP...GYX...LWX...
OKX...PHI...

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

LAT...LON   45387068 44227065 43317075 42407152 41617203
            40957290 40117413 39357488 38577557 39067657
            39887617 40747585 41217603 41957546 42527509
            43237415 43687394 44537403 44957356 45197271
            45247188