Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 130918 CCA
AFDLOT

Area Forecast Discussion...CORRECTED
National Weather Service Chicago/Romeoville, IL
418 AM CDT Wed Mar 13 2024

.KEY MESSAGES...

- Isolated to scattered showers and storms expected this
  afternoon into tonight.

- Showers and thunderstorms likely Thursday morning, with at
  least some potential for damaging winds in the area.

- Another round of thunderstorms remains possible Thursday
  afternoon into the evening. The remains a potential for severe
  weather and flash flooding during this time frame, but the
  likelihood of severe weather occurring then appears to be
  decreasing.

- Colder with below average temperatures later this weekend into
  early next week. A couple of periods of scattered snow showers
  possible.

&&

.DISCUSSION...
Issued at 358 AM CDT Wed Mar 13 2024

Through Thursday night:

An active weather pattern across the central portion of the CONUS
will lead to multiple rounds of showers and storms in northern
Illinois and northwest Indiana through Thursday night.

Early this morning, an area of warm air advection-driven showers is
evident on radar in the WFO Quad Cities CWA. This activity is
expected to slowly drift eastward, potentially entering our CWA
before daybreak, before fading away as the low-level jet in the
region wanes and forcing for ascent becomes too weak to sustain this
precipitation any longer. It appears that a couple of spotty showers
are trying to develop in our southern counties early this morning as
well, but am expecting these to meet a similar fate as the
precipitation to our west.

A quasi-stationary front demarcated by a fairly notable dew point
discontinuity is expected to settle in over our northern CWA this
morning. Easterly/northeasterly flow on the north side of this
boundary is expected to push a cooler marine layer inland today,
reinforcing the front and eventually shoving it farther inland later
this afternoon/evening. As a result, temperatures in the immediate
vicinity of the Lake Michigan shore (especially along the Illinois
lakeshore) will remain mired at or below 60 degrees today while
areas farther inland see high temperatures reach the mid 60s to low
70s. Convergence along the front coupled with modest warm air
advection/isentropic ascent over the top of it could spurt the
development of isolated to scattered showers and perhaps an stray
thunderstorm or two this afternoon in the vicinity of the front.
Have maintained slight chance and low-end chance PoPs this
afternoon into this evening to cover this possibility.

Things could start to get a little more interesting later tonight as
convection erupts well to our southwest in the vicinity of a surface
low and an ejecting shortwave embedded within longer wavelength
troughing from the southwestern to central CONUS. While the main
show tomorrow night looks to remain between the central Missouri and
central Mississippi river valleys, a burgeoning low-level jet could
foster an uptick of elevated convection here locally as early as the
late evening as the nose of this jet impinges upon our forecast
area. Steep mid-level lapse rates and decent shear profiles aloft
mean that we`ll need to keep our eye on the potential for hail
production with any beefier updrafts that sprout, but most of the
latest CAM runs don`t appear to get too feisty with this lead wave
of convection.

Thursday may be Pi Day, but the forecast for the day unfortunately
does not appear to be as easy as pie for our area due to multiple
reasons. The biggest question mark for the day revolves around how
Wednesday night`s convection to our southwest evolves and how it
looks come dawn on Thursday. The bulk of the 00Z/06Z CAM runs show
some variant of the upstream convection growing upscale into an MCS
that would track anywhere from central Illinois to as far north as
the I-80 or I-88 corridors early Thursday morning, based on Corfidi
vectors and progged CAPE gradients. If this solution were to indeed
verify, then that will have two significant implications on our
forecast for Thursday.

First, this would likely mean than some threat for damaging wind-
producing severe thunderstorms could materialize in or just south of
our forecast area Thursday morning. Forecast soundings show that
thermodynamic profiles should be stable near the surface as the MCS
moves into the area, which may make it difficult for damaging winds
to actually descend to ground level. However, the near-surface
stability doesn`t look overly strong, and either a later arrival
time of the MCS and/or some assistance from gravity waves could be
enough to make the damaging wind threat more likely to be realized.
The highest chances for this would be in our southern CWA, but it`s
quite possible that the core of the MCS misses us to the south.
Either way, widespread rain and isolated lightning strikes look
like a good bet across the entirety of our forecast area Thursday
morning.

Secondly, this morning MCS scenario could very well curtail a more
robust all hazards severe weather threat from materializing during
the afternoon and evening. A stronger MCS in the morning would
likely shunt the effective warm frontal zone well to our south with
not-so-great prospects for it to return far enough north to serve
as a focus for severe convection in our neck of the woods later in
the day. In this scenario, thunderstorms could still develop later
in the afternoon or evening along the incoming cold front, provided
that enough destabilization occurs. However, with lapse rates
getting chewed up by the morning convection, what looks to be a
pretty sizable amount of cloud cover hindering solar insolation
from contributing much towards destabilization, and with warm air
advection potentially being impeded to some extent by the convection
to our south, there are plenty of reasons to be skeptical that
we`ll be able to recover enough to support severe convection
during the afternoon or evening. That said, suppose that such a
scenario could not be ruled out if the morning MCS were weaker
and/or departed earlier.

In the event that no morning MCS were to materialize, or that it
ends up being weak enough for the effective warm frontal zone to be
able to return northward into our CWA by the afternoon, then an
all hazards severe weather threat would be in play for at least
the southern half of our forecast area Thursday afternoon or
evening. This would include the potential for flash flooding,
given that storm motions are expected to be somewhat parallel
relative to the warm front, which would introduce the potential
for training convection. However, if the recent changes in hi-res
guidance hold true, then this scenario does not appear likely to
verify. At any rate, we`ll get a better idea of what is most
likely to occur as additional model guidance comes in and
observational trends emerge, so stay tuned for forecast updates.

Ogorek


Friday through Tuesday:

Blustery north winds will start the day early Friday, in the
wake of a cold front which exits the area late Thursday/Thursday
night. Winds will gradually diminish during the day however,
with dry and cooler (but more typically seasonable) weather
expected. Afternoon highs will vary from the mid 40s to lower
50s, with the coolest readings anticipated over the eastern
parts of the forecast area especially along and downwind of Lake
Michigan. Lingering cloud cover should decrease by Friday
night, as weak surface high pressure builds briefly across the
area.

Deterministic and ensemble guidance is in good agreement in
digging the first in a series of mid-level short waves southeast
across the Great Lakes region Saturday, with the upper pattern
evolving into a deep upper trough across much of eastern North
America from Sunday into early next week. Temperatures initially
warm Saturday as breezy (gusts 30-35 mph by afternoon)
southwest warm advection flow develops in association with low
pressure over the northern Lakes in association with the first
short wave. Highs in the mid-upper 50s (10-12 degrees above
average) appear likely ahead of a cold front which pushes
southeast across the area later in the afternoon and evening.
Dry low-levels look to make this mainly a precip-free frontal
passage, though some guidance does generate a few spotty
scattered showers into northwest IN by evening. Cold advection
behind the front continues to be reinforced by a couple of other
fairly strong mid-level short waves digging across the region
Sunday through Monday, helping to amplify and deepen the large
scale upper trough. Forecast soundings depict fairly dry
profiles throughout, though increasingly colder air aloft (-10
to -13C at H8 by late Monday) may support a couple of periods of
convective snow showers with the second and third vorts Sunday
afternoon into Monday.

Temps Monday may be hard pressed to get out of the mid-30s
across the area, a good 10 degrees cooler than average. No
changes made from blended NBM temps during the period, though
would not be surprised to see the NBM eventually trend down a
few degrees for highs both Sunday and Monday. Temps then look to
moderate a bit Tuesday as upper flow trends briefly more zonal
by mid-week.

Ratzer

&&

.AVIATION /06Z TAFS THROUGH 12Z THURSDAY/...
Issued at 1210 AM CDT Wed Mar 13 2024

Aviation Forecast Concerns:

- Weak cold frontal passage shifts winds ENE this morning.

- Scattered showers develop this afternoon, and persist with
  varying degrees of coverage into tonight. Thunder potential
  non-zero, but fairly low across terminals through midnight.

- Thunder potential gradually increases after midnight tonight,
  peaking pre-dawn.

Weak low pressure was over northern Lake Huron early this
morning, with a weak cold front trailing through eastern WI,
northwest IL and eastern IA as of 05Z/Midnight CDT. This front
is expected to push southeast this morning, shifting winds from
light southwest to an east-northeasterly direction around 10
kts. The wind shift may occur as early as 10Z for KRFD (though
with winds 5 kts or less prior to daytime mixing after sunrise),
and in the 14Z-15Z timeframe for KORD/KMDW. Winds will then
maintain an easterly component for the duration of the TAF
period, as the front continues to settle south of the terminals.


Another weak low pressure area was noted farther south along the
cold front across north/northwest MO at this hour, associated
with a mid-level disturbance tracking east across the lower MO
Valley. Mid-level moisture advection associated with this
disturbance will likely result in scattered shower development
across northern IL this afternoon, though stable low-levels
behind the aforementioned cold front and somewhat shallow nature
of moisture aloft is expected to limit coverage somewhat, as
well as thunder potential. While non-zero, the thunder potential
across the terminals this afternoon/evening is low enough (<20%)
to not carry in TAFs. Stronger mid-level moisture advection will
develop tonight however, in association with low pressure
deepening across the Plains. This will spread elevated
instability into the region after midnight tonight, ramping up
thunder chances across the terminals through the pre-dawn hours
of Thursday morning. Exact timing of best TS potential into
KORD/KMDW remains a bit uncertain, possibly arriving just
beyond the end of the current TAF, but have included a prob30
mention after 08Z for these increasing chances/coverage.

Ratzer

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...None.

&&

$$

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