I get if you're in the East Coast you probably like El Nino.
Definitely confusing if anyone on the West Coast likes it...
Soggy weather for @Anti Marine Layer, and warm dry torch all winter for the PNW with limited snow.
LOT certainly not downplaying things for tomorrow. I would say that I am kind of concerned about that higher tornado threat maintaining itself a bit farther east into Chicago metro as if anything, there is a bit of an uptick in low-level shear around/after 00z (it's already pretty strong prior to that though)
Thursday Afternoon and Evening:
Focus then turns to the threat for severe weather. During the
afternoon and evening hours, the core of the upper-level
shortwave will eject northeastward into the Upper Mississippi
River Valley as the surface low lifts from near Kansas City,
Missouri to La Crosse, Wisconsin. Steepening mid-level lapse
rates by virtue of differential cyclonic vorticity advection
atop the continued moistening boundary layer (surface dew points
rising toward the mid 60s) will support the development and
northeastward advection of an uncapped instability plume
characterized by MLCAPE >1000 J/kg into the Mississippi River
Valley by mid-aternoon. At the same time, a 500mb speed max will
arc northeastward toward the Great Lakes atop the already
strong low-level wind field. Conceptually speaking, the synoptic
pattern including the expected path of the surface low across
eastern Iowa during peak heating matches analogs for significant
severe weather events, including tornado outbreaks, across the
region.
At this point, there appears to be two windows for severe
weather tomorrow afternoon. The first window will be during the
early afternoon hours (call it noon to 4 PM) as any residual
elevated warm-air advection convection, or newly forced
convection along a prefrontal trough, attempts to become rooted
to the eastern edge of northeastward-moving instability axis.
Often, this can lead to supercell structures that are nearly but
not fully surface-based owing to relatively cool, or at least
not primely unstable, storm-relative inflow originating off the
main instability axis. Should a supercell manage to become
surface-based during the early afternoon window, a threat for
tornadoes could emerge. Though in most cases, these kinds of
early-day supercells do struggle to latch onto the surface and
largely present a threat for lightning and downpours. (The hail
threat should be mitigated by strong low-level storm relative
inflow beneath weak shear above 3 km).
The more concerning time window for severe weather will be
during the late afternoon and evening hours (4 PM to 9 PM) ahead
of the cold front where low-level moisture will be greater,
mid-level lapse rates will be steeper, and synoptic-scale
forcing will be strongest. With semi-orthoginal orientations
between deep-layer shear vectors and the cold front, initial
storm mode will favor discrete supercells with strong mean-flow
supporting east-northeasterly storm motions of 45 to 55 mph. The
impressively sheared low-level environment characterized by
0-1km shear/SRH of nearly 40kt/275 J/kg, moisture-laden
instability axis supporting LCLs beneath 1000m, and strong
southwesterly flow to support frictionally-generated turbulent
near-surface vorticity streams will lead to an environment
uniquely favorable for strong and long-track tornadoes. Based
on the current arrival time of the instability axis, the favored
area for such a threat for tornadoes appears highest from
eastern Iowa through northern Illinois extending toward I-39,
though perhaps as far east as I-55 in the most aggressive
destabilization scenarios. Some upscale growth into clusters may
transition the primary threat to damaging winds toward Lake
Michigan and northwestern Indiana with time. And, the strong
low-level storm relative inflow beneath weak shear above 3 km
should tend to limit the threat for damaging, or at least giant,
hail. So, the main threat tomorrow could very well be
tornadoes.
As is often the case with severe weather set-ups, a regional
tornado outbreak tomorrow is hardly set in stone. One very easy
to envision failure mode is for morning and early afternoon
shower and afternoon coverage to be expansive, limiting the
ability for meaningful destabilization in the northeastward-
moving instability axis by late afternoon. In such a scenario,
new thunderstorm development along/ahead of the cold front could
be kept to a minimum or lead to low-topped "stringy" convective
elements that struggle to deepen within the strongly sheared
low-level environment. Or, thunderstorms that develop upstream
in Iowa could fall apart while moving into northern Illinois if
the northeastward movement of the instability axis is impeded.
This is all to say, the occurrence of severe weather in our
area tomorrow is not a guarantee.
When put altogether, the Storm Prediction Center has upgraded
much of our area to a Level 3/5 threat level for severe weather
(driven by tornado probabilities). Tomorrow will be a day to
stay very weather aware, especially if west of I-39, between 4
and 9 PM.