Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
855 PM EDT Sat Jun 21 2025
Day 1
Valid 01Z Sun Jun 22 2025 - 12Z Sun Jun 22 2025

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE NORTHERN GREAT LAKES AND THE INTERIOR NORTHEAST...

01Z Update...
With some minor adjustments, Marginal Risk areas were maintained
over portions of northern Michigan, as well as northern New York
and Vermont.

Across northern Michigan, the latest HRRR has shown an uptick in
amounts centered near the Tip of the Mitt and the eastern U.P.,
where storms may develop and lift north from a slow-moving
boundary. While the latest mesoanalysis shows plenty of moisture
(PWs 1.8+ inches) and instability (MUCAPE 3000+ J/kg), it also
shows a fair amount of CIN, which along with a capped environment,
is likely to hinder any widespread development.

Further to the east, the complex developing over southwestern
Ontario/southern Quebec is expected to move into northern New York
and New England tonight. The latest guidance is not indicating
widespread heavy amounts, however a period of training may
contribute to some localized heavier totals (1-2 inches) and an
isolated flash flood threat.

Pereira

Previous Discussion...
...Northern Great Lakes...
Convection will likely be ongoing at 12z this morning across
portions of the UP of MI. It still looks like the storm mode should
be a progressive squall line, which would limit the duration of
heavy rainfall and flash flood risk. However even the progressive
convection will be capable of heavy rates, with a high likelihood
of some 1"+ per hour totals, and a low chance of 2" per hour. Also
can not rule out some brief backbuilding/training on the southwest
flank of convection as it moves eastward. So while the flash flood
risk should be relatively low, localized hydrologic issues can not
be ruled out this morning. Convective and model trends suggest the
better threat is over the UP of MI, and so we were able to cut
back on the southern extent of the inherited risk area with this
update.

...Northeast...
Not expecting much convection during the day across the Northeast,
however an uptick in activity is possible overnight into early
Sunday. By tonight steeper lapse rates advect into the region,
resulting in what should be a pretty rapid increase in MUCAPE over
western NY into portions of VT and NH. The ongoing MCS over MN and
MI should weaken as it approaches the Northeast this evening as
instability will not have increased yet. However the parent mid
level shortwave will remain upstream and expect an additional area
of organized convection to develop with this over southern Canada.
It is this next round that has a better chance of surviving into
portions of NY/VT/NH by later tonight.

The details of this remain unclear, but there should be enough
instability and low level moisture transport/convergence to
sustain some level of activity into the Northeast. Most indications
from the 00z high res guidance is for a quick moving area of
convection, and thus rainfall totals are not overly impressive.
However given the ingredients in place there does seem to be some
potential for an over performing area of convection with some brief
backbuilding characteristics. The better chance of this would be
along the instability gradient and southwest flank of
activity...probably central/upstate NY and/or portions of VT/NH. A
Marginal risk continues to suffice given the 00z suite of
models...however will need to closely monitor observational and
model trends through the day. Depending on how things evolve later
tonight into early Sunday there is some chance for an embedded
more focused flash flood risk within the broader Marginal, and can
not rule out the eventual need for a Slight risk.


Chenard


Day 1 threat area: www.wpc.ncep.noaa.gov/qpf/94epoints.txt