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746
FXUS63 KLOT 070517
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1217 AM CDT Fri Jun 7 2024

.KEY MESSAGES...

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- Gusty west to northwesterly winds will continue this morning
  and ease in the afternoon.

- Chances for showers will return on Saturday.

- Pleasant conditions are expected Sunday through early next
  week. A return to warm and humid conditions is poised to
  return toward the end of next week.
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&&

.DISCUSSION...
Issued at 307 PM CDT Thu Jun 6 2024

Through Friday night:

A recent hand surface analysis revealed a sprawling 992mb surface
low pressure system is centered in central Ontario well to the
northeast of a 1016mb high pressure system over the Plains. The
Great Lakes are positioned directly inbetween both features and
indeed within a seasonably tight low-level pressure gradient,
which continues to support gusty northwest winds of 35-45 mph.
Efficient mixing has also allowed for humidity levels to plummet
today, which when combined with temperatures in the mid to
upper 70s, is leading to a dry, summer-like day.

Tonight, a subtle mid-level shortwave anchored by a modest pocket of
locally cooler 850mb air diving southeast into Minnesota will reach
northern Illinois and northwestern Indiana allowing for at least
patches of partly cloudy skies and perhaps even a few rain showers
(mainly near the Wisconsin state line). Even as diurnal mixing
ceases, modest cold-air advection associated with the wave may allow
for breezy west winds (20-25 mph) to continue through the
overnight hours, partially offsetting nocturnal cooling and
limiting overnight lows to the low to mid 50s (warmer in the
urban heat island of Chicago).

Over the next 24 hours, the low pressure system responsible for our
gusty winds will only slowly move eastward. As a result, tomorrow
looks to start similarly to today with mostly sunny skies and a
"pop" of gusty west to northwest winds. In fact, if we don`t totally
decouple tonight, one can envision a scenario where winds quickly
gust over 45 mph by 7 or 8 AM, as hinted by forecast BUFKIT profiles
from the HRRR. Confidence in such a scenario is too low to include
in our formal forecast (will instead cap gusts at 35-40 mph), but
we`ll need to keep a close eye on observations overnight just in
case. Winds should begin to finally ease during the afternoon
as the surface low finally moves far enough away and the surface
high approaches to slacken the low-level pressure gradient. In
fact, a lake breeze may sneak into northwestern Indiana by
dinnertime. Highs tomorrow should be a tad cooler than today and
in the mid to upper 70s.

Tomorrow looks quiet with light winds and thickening upper-level
cloud cover. Chances for rain will increase toward daybreak (see the
next section of our AFD for more information below).

Borchardt

Saturday through Thursday:

Isolated to scattered showers and storms are expected Saturday. An
MCS will be moving off the Plains into the Mississippi River Valley
region Saturday morning. However, model guidance is in pretty good
agreement that this system will move south of the area. If it drifts
slightly north, then southern portions of the CWA could see some
showers and storms Saturday morning into the afternoon. Left slight
chance to chance PoPs in the forecast for the far south. However,
the system will be leaving the instability behind in the Plains, so
it will be weakening as it progresses east, therefore the severe
threat is very low.

Also on Saturday, a shortwave will move across the area bringing
shower and storm chances to the northern portions of the CWA as
well. Model guidance is split on whether the precipitation will stay
north of the area or if the northern portions of the CWA will get
clipped. Kept chance PoPs across the northern half of the CWA as
this system bringing rain to the area seems more plausible than the
remnant MCS across the southern CWA. Northwesterly flow aloft at 850
mb will prevent any Gulf moisture from reaching the area, so dew
points will be low and CAPE will be lacking. Therefore, any showers
or storms that form on Saturday are unlikely to be severe.

Although there is a chance for showers and storms across the entire
CWA from two different systems on Saturday, it is possible that the
decaying MCS will miss to the south and the showers and storms
associated with the shortwave will miss to the north. Therefore, it
is not out of the question that the whole CWA stays dry on Saturday.
Temperatures will be in the mid-to-upper 70s with winds around 10-15
mph.

Dry weather and sunny skies will return Sunday with an upper-level
ridge and high pressure beginning to build into the area. An upper-
level low is expected to eject from the main flow early next week
and become cutoff over the northern Plains. This low will not have a
surface low associated with it, but rather will become trapped under
a ridge with a broad surface high. It is possible that an inverted
trough will form ahead of the high pressure, bringing a slight
chance for showers on Monday afternoon, but confidence is very low
in shower development due to disagreement in the models in the
progression of the cutoff low.

The next chance for precipitation will come Wednesday as some models
show yet another shortwave and low-pressure system moving across the
area with a cold front crossing the area sometime Wednesday. Left
slight chance PoPs for now and will continue to monitor this as the
week progresses.

Temperatures through the first half of the week will be seasonable
to slightly cool with highs in the low-to-mid 70s, however, towards
the end of the forecast period there is a signal for an increase in
temperatures. High temperatures are expected to climb into the mid-
to-upper 80s by Thursday and persist into the weekend. It appears
that summer may truly be on the way late next week.

Carothers

&&

.AVIATION /06Z TAFS THROUGH 12Z SATURDAY/...
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Issued at 1215 AM CDT Fri Jun 7 2024

Key Messages:

- Gusty WNW winds to around 30kt develop after daybreak and
  ease mid to late afternoon

Sporadically gusty winds to 20-25kt may continue at times
overnight mainly at the Chicago terminals due to occasional
mixing into the low-level jet overhead. Elsewhere expect gusts
to be less frequent overnight. After daybreak winds will
steadily pick back up with gusts in the lower 30kt range
expected by 13-14Z. Gusts will be highest through early
afternoon then ease as the low-level jet weakens and high clouds
begin to expand in coverage across the area. Winds then become
light and variable overnight. VFR and dry conditions are
forecast through the period.

Petr
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&&

.LOT WATCHES/WARNINGS/ADVISORIES...
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IL...None.
IN...None.
LM...Small Craft Advisory until 7 PM CDT this evening for the IL
     nearshore waters.

     Small Craft Advisory until 3 PM CDT this afternoon for the IN
     nearshore waters.
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&&

$$

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