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901
FXUS63 KLOT 230829
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
329 AM CDT Mon Sep 23 2024

.KEY MESSAGES...

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- Locally heavy rainfall possible tonight into Tuesday with an
  axis of 1 to locally +2 inches of rain possible.

- An unusually high degree of uncertainty exists in the forecast
  from Friday onwards.
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&&

.DISCUSSION...
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Issued at 329 AM CDT Mon Sep 23 2024

Through Tuesday:

Challenging forecast this morning with a seemingly increasing
threat of a swath of heavy rainfall tonight into Tuesday across
portions of our CWA.

Early morning water vapor imagery shows a compact, but very well
defined closed mid-upper level low tracking east along the KS/NE
border. This closed low is embedded within the broader NE-SW
oriented upper trough that`s the remnants of the big cut off low
that had been parked over the southwestern U.S. To the southeast
of this trough, there is an plume of moisture extending from TX
northeast into the lower Ohio Valley associated with the
subtropical jet.

Meteorologically, a fascinating and complex evolution to the
pattern is progged to take place over the next 48 hours as a
140kt+ 250mb jet crashes ashore into western Canada today ahead of
a progressive and fairly high amplitude upper ridge that will
spread into western North America. The downstream effects of this
evolution is progged to cause the positively tilted trough and
embedded upper low over the central Plains to get shoved eastward
toward the Mississippi Valley tonight into Tuesday. As this
occurs, the trough is progged to take on an increasingly negative
tilt with the quickly approaching upper ridge and strong Pacific
jet expected to lead to this negatively tilted trough eventually
closing off into an upper low over the lower Missouri Valley
Tuesday.

As this is occurring aloft, a weak surface low currently over AR
is expected to track to northeast to near KSTL by later this
evening, then to southern Lake Michigan by 18z Tuesday. As this
low tracks northeast, it should begin to deepen tonight into
Tuesday morning in response to an increasingly diffluent flow
associated mid-upper trough becoming negatively tilted. In
addition, guidance suggests that there will be a coupling of a
pair of upper level jet streaks helping maximize upper level
divergence tonight into Tuesday morning over the region.

In response to this low level cyclogenesis, a low level jet will
develop tonight and result in strong northward moisture transport.
There is a pretty strong signal in GFS, ECMWF, and RAP that an
axis of heavy rainfall will develop near the elevated baroclinic
zone where low level moisture convergence will be maximized
tonight into Tuesday morning. PWATs are progged to increase to
175% to nearly 200% of normal along this axis of maximized
moisture convergence. In addition, the strong ascent and weakest
convective stability/instability is progged to be in the
low/middle troposphere, below the freezing level. This could
result in warm rain processes dominating in this band tonight,
potentially enhancing the heavy rainfall threat.

Important to briefly note that the NAM and some of the CAMS are
much weaker with cyclogenesis and subsequently are not depicting
this northern axis of heavy rainfall tonight/Tuesday. A complex
synoptic evolution like this seems more likely to be handled
better by the global models, so have largely discounted the
HREF/NAM solutions and weighted the forecast heavily toward the
GFS and ECMWF. This would suggest the axis of heavy rainfall would
end up from west central IL east-northeast into the Chicago metro
area. Certainly some wiggle room and the location of the highest
rainfall totals may need to be adjusted in subsequent forecasts.

One last "interesting" aspect to this system is the potential for
some lake enhancement to the rainfall. Forecast soundings over
southern Lake Michigan certainly would favor some convective lake
enhancement potentially resulting in heavier rainfall inland
front the southwestern portions of Lake Michigan. Where exactly
this lake enhancement sets up will hinge on the precise track of
the surface low and where the subsequent lake enhanced convergence
zone develops. Given water temps still in the 70s and potential
for a rather deep marine convective layer, there could easily be
some localized rainfall totals over 2" where lake enhancement
occurs.

Generally speaking, the antecedent moderate drought-like
conditions should limit the flooding threat. However, if
confidence increases in the potential for heavier rainfall
setting up in the highly urbanized areas of Chicago, then later
shifts may need to consider a targeted flood watch for Chicago
since moderate drought won`t increase the amount of water concrete
can soak up. This would be of particular concern if it looks like
lake enhancement will set up over the highly urbanized areas.

One final wildcard to keep an eye on tomorrow is the potential for
some low topped convection/supercell threat over mainly our
northwest Indiana counties. The most likely scenario is that cloud
cover and rain should prevent any meaningful destabilization.
However, southeastern most CWA is progged to get into the warm
sector with the sfc low progged to track northeast to southern
Lake Michigan, so if there is any diurnal heating of the boundary
layer in this warm sector, the threat for low topped supercells
with an attendant low end tornado risk would increase. Right now,
this threat appears quite low and am not planning on messaging it
in our products, but it is something worth monitoring.

- Izzi


Tuesday Night through Sunday:

After any lingering precipitation from Tuesday`s system clears
the area overnight into Wednesday, Wednesday is shaping up to be
a pleasant late September day with seasonable highs in the mid
70s as surface high pressure begins to build into the region
from the west. In all likelihood, we should remain dry on
Thursday as well with the continued influence of the surface
anticyclone in the region. Thursday`s highs look like they`ll be
a smidge higher than Wednesday`s, so as long as an encroaching
mid- to high-level cloud shield from the south holds off long
enough on its arrival as the majority of available guidance
suggests. Onshore flow on both days will likely result in
slightly cooler temperature readings closer to the lakeshore.

Aloft, the mid-week weather pattern will be a bit more complex
as a trough diving southeastward out of the northern Plains will
get pinched off from the jet stream and become a closed-off
upper-level low that should center itself somewhere to our south
or southwest Wednesday into Thursday. At the same time, a high-
amplitude upper- level ridge will be setting up shop over the
central third of the continent, effectively yielding a Rex
block. Medium-range guidance often struggles to accurately model
the evolution of these blocking patterns several days out, and
to add further complexity to the weather pattern and forecast, a
tropical disturbance (and likely soon-to-be Tropical
Storm/Hurricane Helene) will likely be making landfall somewhere
along the eastern half of the Gulf Coast on Thursday, setting
itself up for an encounter with the aforementioned cut-off low.
How these two systems end up interacting will ultimately dictate
what weather we will see Friday through early next week, and
unfortunately, there remains too large of a spread in outcomes
advertised in both deterministic and ensemble guidance to be
able to say much aside from that we have low confidence in the
forecast for this time period. Thus, have continued to ride with
the NBM-delivered output for the long range portion of our
forecast, which features model-averaged daily highs in the 70s
and very broad- brushed slight chance and chance PoPs from
Friday onwards. Refinements to this portion of the forecast will
almost certainly be needed as forecast trends become clearer.

Ogorek
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&&

.AVIATION /06Z TAFS THROUGH 12Z TUESDAY/...
Issued at 112 AM CDT Mon Sep 23 2024

Key Messages:

- MVFR ceilings ending overnight/later this morning

- Another period of rain showers likely beginning sometime
  Monday night

The northern terminus of the MVFR stratus deck that is present
at the beginning of this TAF period will creep southward over
the course of the overnight time period, allowing for VFR
conditions to prevail once again prior to daybreak. The one
possible exception to this could be GYY, where MVFR ceilings
streaming inland from over Lake Michigan could prolong MVFR
conditions until mid-morning or so. GYY may also continue to
see 20+ kt northerly gusts until around daybreak as well.

Towards the end of the current TAF period, our next low pressure
system will be moving into the region from the southwest, with
rain showers along its northern periphery likely to spread near
or over the terminals sometime on Monday night. There is still
a good bit of uncertainty regarding the onset time of
precipitation and how far north the shield of steadier rainfall
will get, which will, in turn, dictate when sub-VFR ceilings and
visibilities may arrive and just how low ceilings and
visibilities will get. For now, opted to introduce a prevailing
MVFR SHRA group into the ORD and MDW TAFs at 06Z, but
adjustments will likely need to be made as forecast trends
become clearer.

Ogorek

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...Beach Hazards Statement through this evening for ILZ006-ILZ103-
     ILZ104.

IN...Beach Hazards Statement through late tonight for INZ001-INZ002.

LM...Small Craft Advisory until 7 PM CDT this evening for the IL
     nearshore waters.

     Small Craft Advisory until 10 PM CDT this evening for the IN
     nearshore waters.

&&

$$

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