LOT’s AFD goes crazy
Wednesday:
An unusually powerful shortwave trough is progged to race
east-southeastward into the upper Midwest and western Great
Lakes Wednesday into Wednesday evening. Model guidance is in
good agreement on the synoptic scale with 500 mb heights
progged to be 3-4 standard deviations below normal for mid June.
At the sfc, an associated low pressure that could threat all-
time monthly record for June is expected to develop across
Wisconsin and eventually move into lower Michigan.
Unsurprisingly, the kinematic fields with such an anomalously
strong system are also nearing the upper echelons of what we
see this time of year in the Midwest. All of this to say that
synoptically, the ingredients are coming together for a
potentially dangerous outbreak of severe thunderstorms and
potentially long-tracked, intense tornadoes.
On the mesoscale, there are naturally greater uncertainties
regarding precisely where (and to a lesser extent, if) the
overlap of exceptionally strong deep and low level shear and
moderately strong instability will take place. SPC`s latest
SWODY2 highlights areas south of I-80 in our CWA with a rare
day 2 moderate risk (level 4 of 5) for Wednesday. The potential
ceiling for bad this severe weather event could get is quite
high, but it is also important to note there are obviously still
failure modes that could materialize on the mesoscale and
prevent the reasonable worst case scenario from unfolding.
The general expectation as it looks now is that a strong low
level jet (increasing to 60kt+ at 850 mb by 12z Wed) will result
in very strong low level theta-e advection and eventually the
development of strong to severe thunderstorms late tonight.
Initially development is expected over eastern IA, but as the
low level jet translates eastward into IL, it should support
this convection developing/moving into northern IL near/after
sunrise Wednesday morning. While this convection will be
elevated, strong effective shear and a reservoir of 1000-2000
J/kg of MUCAPE should allow for elevated supercells. The
greatest severe risk Wednesday morning looks to be over our
western and eventually southern CWA. In addition to the threat
large hail, strong shearing instability near a sharpening
frontal inversion would point to at least some threat for the
development of gravity wave associated convection. Should this
occur, a damaging wind threat could also develop, despite little
or no sfc based instability. In fact, there could even be
pockets of locally significant severe wind gusts (>75 mph) near
or just west/south of our CWA Wednesday morning. This
convection will probably evolve into an MCS as it tracks east
across northern IL and into northwest Indiana Wednesday morning.
The morning convection will likely augment the warm frontal
position and at least initially slow the northward progression
of the composite warm front/outflow boundary, delaying
destabilization north into our CWA. This seems to be the most
obvious potential failure mode: morning convection retarding
the northward surge of the warm front and subsequently the
stronger instability, keeping the extreme wind shear profiles
and strong synoptic forcing somewhat divorced from the more
favorable instability. While that is one potential obvious
failure mode, at this point, it seems unlikely to fully succeed
in completely disrupting the otherwise exceptionally favorable
synoptic set-up from resulting in a high end severe threat.
Though the precise location of the most likely area(s) to see a
high end event, could change some on the mesoscale as the event
nears.
The reason that this morning convection is unlikely to
completely stunt the northward surging warm front is the
extremely strong mass response expected as a result of the near
record deep low pressure system. With a far less intense low
last Thursday, an impressively large footprint of a cold pool
left behind from an MCS that lingered well into the afternoon
was able to be completely overcome in just a matter of a couple
of hours late in the day. Something similar seems plausible
again tomorrow afternoon where a dissipating outflow boundary
from early convection could separate an extremely volatile air
mass from a still sufficiently unstable air mass north of the
boundary could support severe thunderstorms with damaging winds
and isolated tornadoes.
There is variance in guidance with just how far north the
effective boundary will get, but somewhere generally in the
vicinity of I-80 is where a majority of the models show it
reaching its peak latitude in our CWA. To the south of this
boundary, the environment looks similar to what`s often seen in
the cool season major tornado outbreak in the southeastern
United States. Forecast hodographs are literally off the chart,
with 0-1 environmental SRH >500 m^2/s^2. This sort of extreme
low level shear, coupled with low LCLs and resultant strong
0-3km CAPE, fast storm motion, and favorable downstream
environment is the classic type of set-up long tracked strong-
violent tornadoes.
It`s important stress that while the synoptic set-up is classic
for a tornado outbreak, we are dealing with convection that
often alters the mesoscale environment in ways that cannot be
anticipated 12-24+ hours in advance. As noted above, there are
certainly scenarios where an alteration of the mesoscale
environment could dramatically reduce or shift the area of the
greatest tornado risk. For this reason, it is important to
monitor later forecasts closely. Finally, in addition to the
supercell tornado threat, a fast moving line of severe
thunderstorms/possible derecho may also accompany the front
producing widespread, potentially significant, wind damage as
well as line embedded tornadoes, some possibly strong.
Not to be overlooked, precipitable water values are progged to
get to near or just above 2", meaning that convection will
likely be extremely efficient rain producers. The stronger
convection could easily produce 2"+ of rain in just an hour or
so. Given the antecedent very moist ground and generally above
average streamflow, there is a threat of flash flooding
Wednesday. The greatest threat may be with the first round of
convection, since it will likely be a bit slower moving and
offer a better chance of some training cells than the second
round. When the area most at risk can be better refined, we will
need a flood watch for portions of the area for Wednesday as
well.
