Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 061505

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1005 AM CDT Tue Jun 6 2023

Issued at 1005 AM CDT Tue Jun 6 2023

Current showers and isolated thunder across southeast WI is
expected to move across northeast IL and the southern end of Lake
Michigan through early afternoon, likely in a slowly weakening
phase. Increased pops for northeast IL and the metro area through
early afternoon and also added slight chance thunder. Though
confidence for thunder is low and there may not be much thunder
with this current activity, especially if the weakening trend
emerges. If this activity holds together, a further increase in
pops will be needed, especially into northwest IN for this
afternoon. Overall trends remain low confidence.

Other change was to add haze, for smoke, for today for most of the
area. Many sites in the 6-8sm range with ORD/PWK now at 6sm with
haze with the 15z obs. cms


Issued at 247 AM CDT Tue Jun 6 2023

Through Wednesday...

An expansive Omega-type blocking pattern will persist in the mid
and upper levels across the CONUS through the period. The
positioning of this large scale pattern has generally favored our
area to be positioned under the warm upper ridge the past several
days, thus resulting in rather warm and cloud free weather. This
will change today, however, as the large cut-off upper low over
New England in this pattern begins to retrograde the upper ridge
and the associated very warm airmass westward through the mid to
late week time period. While this is expected to result in the
transition to a period of cooler and primarily dry weather mid to
late week following a back door cold frontal passage today, we
will see more cloud cover today into tonight, along with a chance
for a few isolated to widely scattered showers/storms.

The threat of thunderstorms today is expected to largely be west-
northwest of the area in parts of northeastern IA into
northwestern IL. This is the general region that will see the better
low-level moisture (dewpoints into the low 60s) and instability
this afternoon ahead of the approaching front. Farther east,
across much of northeastern IL into northwestern IL, low-level
moisture will remain more muted (dewpoints in the low to mid 50s),
thus limiting instability and the threat for storms with eastward
extent beyond the I-39 corridor. Shower activity will be possible
across the area at times today into tonight, but a lot of dry
time is likely as the coverage of the showers look to remain more
isolated to widely scattered with eastward extent. In fact, some
areas may remain precipitation free altogether. Any remaining
shower and thunderstorm activity will begin to shift into central
parts of IL later this evening and overnight as the surface front
continues its southward trek. We will then trend to a return to
dry, but cooler weather for the mid to later part of the week.

Sunny skies will return for Wednesday as a much drier post
frontal airmass shifts into the area on breezy northeasterly
winds. Conditions on Wednesday should be rather pleasant, with
very low humidity (~25%) and inland temperatures generally topping
out in mid 70s to near 80. Closer to the lake, onshore flow is
expected to keep temperatures in the upper 60s! The only weather
note of interest for Wednesday is the heightened threat of fire
spread due to the very dry and breezy conditions.



Issued at 340 AM CDT Tue Jun 6 2023

Wednesday night through Monday night...

The latter half of the work week looks like another quiet period
here locally with an Omega block still in place across the CONUS and
surface high pressure residing over the region. Seasonable
temperatures can be expected with Friday`s inland highs likely to be
about five degrees higher than Thursday`s inland mid 70s to 80
degree highs as ample sunshine on both days allows the resident
dry post-frontal air mass to warm up. Daily lake breezes will
continue to keep lake-adjacent locales about 10 degrees cooler
than locations farther inland on both days. Diurnal mixing should
result in afternoon relative humidity values bottoming out in the
20-30% range away from the lakeshore on both days as well, though
winds should remain in check enough to preclude a greater fire
weather threat as vegetation continues to dry up amidst our
ongoing dry spell.

A bit of pattern change does appear to be in store for this
weekend as the Omega high is yanked westward, effectively forming
a transient Rex block over the western CONUS while establishing
northwest flow aloft over the northern Plains and Midwest. This
will ultimately yield a cold front at the surface that will drop
through our forecast area this weekend and likely bring us our
next chance to see showers and possibly thunderstorms as just
enough moisture return is realized ahead of the frontal boundary.
The NBM has continued to come in with primarily chance PoPs for
Saturday and Sunday, which still look fine percentage-wise at this
range, but are still likely spread out too much temporally as a
result of forecast guidance continuing to exhibit a notable spread
in the exact timing of the cold frontal passage. The timing of
these PoPs will thus likely need some refinement over the coming
days as forecast trends become clearer. High temperatures this
weekend will also largely depend on the timing of the frontal
passage with highs in the 80s likely to occur ahead of the front
and highs in the 70s favored behind it.

Beyond this weekend, there is an increasingly evident signal for a
new closed upper-level low to emerge somewhere over the northeastern
quadrant of the CONUS as the Omega block attempts to re-establish
itself. While it`s unclear at this range whether that will be over
the Great Lakes or farther to the east, enough NBM members have
this upper low setting up shop far enough west for the blended
PoPs to end up in the slight chance range for the beginning of the
upcoming work week. Have left these PoPs largely untouched for
now given the existing uncertainties in how the upper-level
pattern will look like 6-7 days from now.



For the 12Z TAFs...

Aviation forecast concerns for this TAF period include:
* Widely scattered showers increasingly likely near the terminals
  this afternoon.
* Timing of a mid-day northeasterly wind shift at ORD and MDW.

Scattered showers have developed across central Wisconsin early
this morning, and confidence has increased in these showers
affecting the Chicago metro terminals enough to justify the
introduction of a VCSH mention in the 12Z TAFs. Lingering
uncertainties over how this activity will evolve and the exact
coverage of showers as they approach the terminals precluded the
introduction of a prevailing or TEMPO SHRA group at TAF issuance,
though adding a formal SHRA mention may be warranted forthcoming
TAF amendments once trends have become clearer. The better
moisture and instability will remain in northwestern Illinois
today, which should limit the thunder threat at the Chicago metro
terminals. RFD stands a better chance at seeing an isolated storm
nearby, but confidence in that occurring is still low.

A lake breeze-reinforced cold front is also set to cause a
northeasterly wind shift at ORD and MDW around mid-day today.
Currently favor a 17Z timing for the wind shift at both locations,
though this may still end up varying by up to an hour or so.
Winds should peak at a little over 10 kts behind the boundary this
afternoon before gradually coming down this evening.





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