Severe Threat...
Many of the criteria we look for with regard to severe weather
development in the region is forecast to be in place tomorrow,
though as typical, some of the signals are not as clear. So I
don`t believe it will be an over the top block buster event, but
it will be busy for a few hours late afternoon into the evening.
On the plus side, we`ll definitely have enough surface
instability with CAPE values expected to be 1000-1500 J/kg
across the interior by early afternoon. Thank you temperatures
around 90F and dewpoints in the mid 60s F. Conflicting guidance
from the GFS and NAM as the NAM suggests a weak EML tomorrow
with decent mid-level lapse rates. GFS is much weaker. 0-6km
shear is progged to be around 35kt, so it`s enough to help get
convection organized, but on the lower end of it being in a
sweet spot. Model soundings show weak capping across the area
which will help to keep convection squashed until the upper
level trough gets closer and helps to erode that cap. After
which time we should see rapid convective development. Looking
at various SPC parameters from SREF and HREF, I see signals in
the Supercell Composite and even the SigTor parameter. Noted the
12z HREF shows decent updraft helicity signals, especially in
the 20z-02z time range, suggestive of the potential for
rotating storms. SPC has a 5% risk for a tornado across the
western third of Massachusetts, and I have no reason to argue
with that. As typical this time of year, the southwest winds
will advect a slightly cooler and definitely more stable airmass
across the southeast portion of the forecast area. So it looks
like the primary severe threat area is from LWM to ORH to HFD
and points west. Some question as to the timing, as the various
hi-res models to differ by several hours as to when the
convection will develop and move into our region. Best estimate
at this point is that 4-8pm will be the peak time of activity
for us, though some convection may start as early as 2pm in the
Berkshires. Given the stable airmass in southeastern areas, and
the fact that it will be after sunset once it reaches far
eastern areas, thinking that convection will go elevated and
quickly weaken, but showers and a few rumbles of thunder may
still continue across SE MA, Cape and Islands until after
midnight. Took a blend of models for the hourly timing of the
PoPs. Kept the mention of gusty winds with t-storms during the
evening. Can`t rule out some hail with stronger storms, but that
seems to be a lower chance.