In this case, we`re seeing several favorable signals in model
guidance, especially for western thirds of MA/CT. First, there is
sufficient 0-3km CAPE (over 200 J/kg) as warm front lifts through
during late afternoon and early evening. Local CSTAR research has
shown that we only need to see values of 25-75 J/kg in tropical
remnant environments so that should easily be attainable. Second,
environment itself favors high amounts of shear in lowest levels and
in this case we have over 200 m2s2 of 0-1km shear, much of which is
focused in 0-500m layer. Research has also shown the 0-500m layer
can be a good discriminator for tornadoes vs non tornado
environments (values of 95 or higher). Lastly, LCL heights are down
to 500m which is below the typical height of about 700m.
While these factors line up very well and have support from SREF
Tornado Ingredients, it`s not always a lock in these situations.
Certainly we`ll be on the eastern edge of more favorable area
(region closer to the actual remnant low itself). But we have to
watch its warm front extending eastward as well as possibility of
mesolow formation along boundary to help enhance spinup. It`s also
possible to have all of this occur without any lightning.
It`s important to stress that these types of circulations, while
potentially supportive of tornadoes, tend to be very shallow in
nature and do not last very long. Radar detection can be difficult
especially at farther distances from the radar. These situations are
usually handled by issuing Severe Thunderstorm Warnings with the
"Tornado Possible" tag since it is difficult to track these
transient couplets. However, any longer-lived couplets that do
manage to form can be handled with Tornado Warnings.