377
FXUS63 KLOT 300842
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
342 AM CDT Tue Apr 30 2024

.KEY MESSAGES...

- Decaying showers and thunderstorms may move into the area
  late tonight (30-50% chance). Severe weather is not expected.

- Unseasonably warm temperatures continue through Thursday.

- Additional periods of showers and storms expected Wednesday
  night through Friday.

&&

.DISCUSSION...
Issued at 342 AM CDT Tue Apr 30 2024

Through Wednesday:

All is quiet early this morning with clear skies across the
area and surface high pressure settles across the region in the
wake of yesterday`s weather system. Fog has developed across
portions of east central Illinois and west central Indiana. So
far this has not expanded further north into Benton and Iroquois
Counties but felt prudent to include a patchy fog mention
through daybreak just in case. Can`t rule out shallow fog in a
few other spots (typical low- lying fog-prone areas) given
residual low-level moisture from rains the past couple of days.

Temperatures this morning start out notably cooler than what we
have seen the past few days, in the 40s to lower 50s. Ample
sunshine and height rises lifting into the area ahead of yet
another system developing across the Central Plains should allow
temperatures to readily warm into the 70s today, with the
exception being along the immediate lakeshore. The lake breeze
looks to hold off on pushing further inland until late
afternoon/early evening.

The low-level jet begins to ramp up heading into this evening
as showers and storms from our west approach the area. This will
allow winds to remain breezy overnight out of the southwest and
help limit overnight cooling to the upper 50s to near 60. The
narrow axis of instability associated with these storms is
expected to weaken with eastward extent. Accordingly, these
storms are expected to decrease in coverage and intensity as
they approach. A few rumbles of thunder can`t be fully ruled
out, mainly west of the Fox River valley and I- 55 as scattered
showers move through. Severe weather is not expected. Northwest
Indiana could very well end up dry with showers dissipating
prior to reaching the stateline.

With such a warm start to the day, temperatures on Wednesday
look to easily warm into the mid 70s with generally light
westerly flow. Strong subsidence in the low-levels and mixing
will allow for dew points to dip into the 40s resulting in RH
values in the lower 30% range. Given how much rain has fallen
the past couple of days, not expecting any fire spread concerns
even if wind gusts overperform. A lake breeze/backdoor front
will cool temperatures late afternoon/early evening along the
lakeshore and further inland across northwest Indiana. Any
precipitation with our next system looks to hold off until after
sunset.

Petr


Wednesday Night through Monday:

During the latter half of the week, a deepening upper-level trough
will swing into the Upper Midwest from the northern Rockies and help
induce a somewhat disjointed episode of cyclogenesis to our west. As
a surface low spawns in the central Plains, broad warm air
advection/isentropic ascent on its eastern flank will likely
encourage scattered showers and thunderstorms to develop and track
northward into our area late Wednesday night into Thursday morning
as an associated warm front lifts northward. Shower coverage should
be greater closer to the center of the incipient surface low, where
forcing will be maximized, so currently have chance PoPs going
across the western 2/3rds of our forecast area with slight chance
PoPs farther to the east during this time frame.

This initial wave of precipitation is likely to lift north of the
area by Thursday afternoon, and should afford at least part of our
forecast area some dry time on Thursday before the surface low and
its cold front approach from the west and bring another band of rain
that should feature at least isolated storms into the area sometime
Thursday afternoon or Thursday night. This secondary wave of rain is
then expected to linger into Friday before departing eastward along
with the cold front. Given the recent rainfall, the potential for
training convective elements ahead of the cold front, and that
precipitable water values near or in excess of 1.5" (near-record
values for this time of year here, per sounding climatology for DVN
and ILX) are progged to be drawn northward out ahead of the cold
front, we will need to monitor the potential for hydrologic concerns
during this late Thursday through early Friday time frame. However,
a good deal of spread still remains in forecast guidance regarding
the overall evolution of the low pressure system, and some solutions
(such as the operational 00Z ECMWF) advertise a low track/occlusion
that would favor the heavy rainfall threat remaining west of our
forecast area, and could even result in little to no rainfall being
observed here Thursday night into Friday. Thus, we`ll continue to
hold off on more formally messaging the potential hydro concerns
until there is some more agreement amongst forecast guidance that
such a threat should actually materialize.

In other news, breezy southerly flow should help temperatures rise
into the upper 70s to mid 80s south of the warm front on Thursday,
though onshore flow should keep lake-adjacent locales in Illinois
several degrees cooler. High temperatures in our western and
northern counties may also end up being lower than currently
advertised in our gridded forecast database if the warm front
doesn`t lift as far north as presently expected, or if the second
wave of rain arrives earlier in the afternoon. In the wake of
Friday`s cold frontal passage, temperatures are favored to return
closer to normal going into the weekend, when another incoming
disturbance looks to bring another chance for precipitation to the
region. Slightly warmer temperatures are then favored into early
next week with the overall weather pattern continuing to look rather
active across the central CONUS.

Ogorek

&&

.AVIATION /06Z TAFS THROUGH 12Z WEDNESDAY/...
Issued at 106 AM CDT Tue Apr 30 2024

Key Messages:

- Southeasterly wind shift expected late this afternoon/early
  this evening at the Chicago metro TAF sites

- Chance of showers late Tuesday night

Mostly clear skies and westerly winds to around 10 kts are
expected through this afternoon. A lake breeze will develop
this afternoon, but will likely remain tethered to the Lake
Michigan shoreline for most of its lifespan. It may begin to
progress inland late this afternoon/early this evening, but will
probably start to wash out before reaching ORD and MDW. However,
even if this happens, a synoptically-driven southeasterly wind
shift appears to be likely for a few hours this evening before
winds gradually start to trend back towards a southwesterly
direction.

A decaying convective complex coming out of Iowa is likely to
survive into northern Illinois late Tuesday night, but the
extent to which it may affect the terminals is still in question.
For now, have added a PROB30 mention for -SHRA to account for
this and will reassess again later. A few lightning strikes can`t
entirely be ruled out if this complex doesn`t weaken as quickly
as presently expected, but the probability of this occurring is
currently too low to warrant including a TSRA or VCTS mention
in the TAFs. The best chances to see any rain or lightning
should be at RFD. Some lower (potentially sub-VFR) cloud cover
may also accompany this activity and linger around into
Wednesday morning, but confidence in this currently low.

Lastly, there is also some potential for low-level wind shear
criteria to be breached Tuesday night into Wednesday morning.
However, winds at the surface look to remain elevated enough
during this time frame to preclude the need for a formal LLWS
mention in the TAFs.

Ogorek

&&

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

&&

$$

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