There is still some uncertainty in the overall evolution of the two
different "rounds" on Wednesday. The higher resolution guidance is
indicating that the first round will clip the southeast counties and
move east while allowing afternoon destabilization to occur before
the surface front and deep upper lift move in towards the afternoon
with another round of thunderstorms. GFS/ECMWF are understandably
having a more difficult time differentiating between the two
systems. Thinking the first round of storms won`t be that strong
given the timing and the dew points in the lower 50s thanks to the
southeasterly flow at the surface. Point soundings in Dekalb county
also indicate a strong low level inversion through the morning hours.
The afternoon is really when things could start to get interesting.
Given this is still over a day away, things are in better agreement
that "something" will develop in the afternoon hours. The difference
with Wed compared to previous events is the progressive nature of
the coastal convection and how much earlier than the secondary line
the coastal convection is occurring. Guidance is consistent on dew
points in the lower to mid 60s advecting into the NW AL areas with
the southwesterly flow through the column and into the 60s in the NE
AL area in the wake of the earlier precip. I do not see any kind of
expansive cirrus deck to keep us stable so in theory, if the earlier
convection can exit quick enough in the morning we should be able to
see some peaks of sun and warm up enough (mid to upper 70s) to break
a mid level cap and realize decent instability, aided by steep mid
level lapse rates. There will be no lack of shear as low level winds
increase after 18-21z areawide with 0-1km shear around 30 kts and
0-6km shear as high as 60- 70kts. Low level helicity won`t be too
impressive but given the instability and shear values, may not need
too much to get rotating updrafts. Soundings by the afternoon, if the
pieces align, are impressive, and would be indicative of large hail,
damaging winds, thanks to strong mid level winds and dry air in the
mid levels, and a tornado or two cannot be ruled out. Do agree with
the SPC enhanced risk in the east where we could have better moisture
return behind the morning round and more time to destabilize. It is
still too early for much more specific or enhanced wording though.
Next, are the uncertainties. First issue is the moisture recovery
and related to that, the afternoon mixing that could lower the dew
points, and if the earlier convection will disrupt the wind fields
and therefore, any moisture return. The last point was a big player
last Thursday when convection did not redevelop well in the
afternoon. If the earlier convection lingers in the east and actually
expands farther west than we currently see, the afternoon will not
warm and destabilize very much. This would leave a line of
thunderstorms moving through around 21z in the west which will likely
be strong and not too severe. There is also a pretty significant mid
level dry slot pushing in from the west around 18z which seems to be
well analyzed in soundings. Moisture is really confined to below
800mb which begs the question of how deep the convection will be able
to get. Also, with the strong low level jet, the dew points will
likely mix out a few deg lower than models are predicting as they
always underdo daytime mixing. The models are handling the mixing
out of the cap well but are not showing the lowering of dew points.
