Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 240708

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
208 AM CDT Thu May 24 2018

207 AM CDT

Through tonight...

Surface ridge still across the region early this morning, while
quiet/dry conditions are in place. Expect this high to push
further to the east today, while upper level ridge pushes
overhead. While the focus for any precip development will remain
west of the area through tonight, will likely see associated high
clouds continue to move across the region, but with mainly sunny
skies still expected. Warm up continues today with temps rising
well into the 80s across the area. Locations near the lake in NE
IL will be the exception, as onshore winds are expected to develop
later today. In this location, still think temps in the 70s are
expected, but the extent of the warming will be tied to when the
winds turn. Do think there will be enough time this morning to
warm up before this occurs, so did raise temps near the lake a
couple of degrees.



207 AM CDT

Friday through Wednesday...

Dry conditions still anticipated at the start of the period with
shower/thunderstorm development and instability axis still just to
the northwest. Much of the day on Friday is appearing dry, with
temps once again rising into the upper 80s. Continue to monitor
the afternoon and evening time frame when thunderstorm development
increases. Model guidance still varying with overall solution
during this time frame, though several features will come together
that could support thunderstorms. Upstream mid/upper level trough
with embedded shortwave energy will approach during this time,
with some sort of surface focus/trough appearing probable. During
this time, moisture/instability axis shifts overhead as well. All
of this will support the possibility for convection to develop,
although, still not exactly quite sure where this will initiate.

At this time, a likely scenario would be for scattered storms to
develop to the west/northwest of the CWA and drift into northern
IL. The shear does not appear overly high, however, the extent of
the instability could support at least an isolated strong to
severe threat Friday afternoon into Friday evening with the
highest threat over northern IL. Once again, guidance still not
locked onto a solution with some hires guidance that gets into
this period showing this convection developing overhead. Also,
some guidance has been hinting at an MCS developing to the
northwest late in the day Friday and then quickly diving southeast
into the area Friday night. Given the pattern and where the
instability axis is forecast to setup, this does not appear
completely unreasonable.

Confidence lowers further on thunderstorm chances on Saturday,
although chances do remain. Cyclonic flow and instability still in
place could support at least some isolated development if there
is some sort of focus at the surface. Sunday into Monday, chances
for showers and thunderstorms appear to be low. Well above normal
temps in the upper 80s to around 90 still expected Saturday into
Monday. During this time, dewpoints are looking to increase across
the region, and will support heat indices in the low to mid 90s.
If temps/dewpoints are slightly higher, could see heat indices
rise more to around the mid 90s, and possibly slightly higher.
Confidence is lower for temp trends near the lake during this
time, as guidance is now hinting at the possibility of lake breeze
development each day. Have continued the generally warmer temps
near the lake, but this could possibly change with later forecast



For the 06Z TAFs...

High pressure centered over the Great Lakes will result in light
south to southeasterly flow and VFR conditions through the day
Thursday. Expect mostly clear skies with only some passing high
cirrus. Only area of uncertainty is with winds associated with a
lake breeze Thursday afternoon. Some models show a lake breeze
forming with winds backing to the SSW (around 200) ahead of the
boundary. Other models maintain a light S/SSE winds through the
afternoon. Winds will back to the SE behind the lake breeze during
the late afternoon/evening hours, then gradually veer to the SSW



222 PM CDT

Dense fog continues to be the main weather story over the lake at
this time as surface high pressure dominates across the lake.
Unlike yesterday, we can see the fog rather well on visible
satellite imagery. While it is not over the entire lake, there is
quite a bit over the open waters of the lake, including the
northern half of the lake. For this reason, we have including the
north half of the lake in the advisory and are running it through
Thursday morning. Forecasting the ending of this dense fog is
always difficult, since the water temps in the open waters is
still so cold. However, it does appear that southerly winds will
begin to increase on Thursday (10 to 20 kt) as the surface high
shifts east. As a result, this may end up pushing any lingering
fog to the far northern portion of the lake on Thursday.

Southerly winds are then expected to continue into the weekend,
albeit light. A cold front then looks to shift southward down the
lake sometime late Sunday through Sunday night. Northerly winds
are then expected early next week in the wake of this front. This
may result in a bit higher waves on the southern end of the lake
by Monday.



LM...Dense Fog Advisory...LMZ777-LMZ779 until 10 AM Thursday.




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