Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 250750

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
250 AM CDT Fri May 25 2018

249 AM CDT

Through tonight...

The upper level ridge will shift east today allowing an upper
level trough to pass over the region this afternoon and evening.
Despite increasing cloud cover this afternoon, I think
temperatures will be warmer than guidance indicates. Upper 80s and
a few 90s are expected. A weak lake breeze is expected in the
mid-afternoon, but I think locations along the lake will get into
the low 80s before winds turn on shore.

Convection is the next concern. Convection currently over
Missouri and MN/WI will dissipate this morning. If the convection
over Missouri produces a MCV, I think the MCV will pass south of
the forecast area. There`s a small chance that storms could clip
the far southern portion of the forecast area, but I think
stronger convection in central IL will be the main show.

Most CAMs are focusing convection along the upper level wave and
cold front in MN, WI, and IA this afternoon. Upper level flow
becomes west to west-northwest late this aftn/early eve directing
bowing segments of storms toward northern IL. Forecast soundings
feature pretty dry low levels and a rapidly forming stable layer,
so I agree with SPC`s outlook that storms will be weakening as
they approach northern IL. I still kept a chance of storms in the
forecast through tonight, but my confidence is low in coverage
and how far storms will make it across the forecast area. As
such, I kept precip chances in the chance range or lower.



249 AM CDT

Saturday through Thursday...

Overview...Continued warmth with a couple chances of storms. 90s
are still forecast for Sunday with heat indices in the low to mid
90s away from the lake.

I kept a slight chance of storms Saturday, but I have low
confidence in storms actually occurring and coverage. Soundings
feature a good cap, rather dry, and there isn`t a clear source of
forcing outside of daytime heating. High temps will once again be
in the upper 80s to around 90 away from the lake.

Low 90s are expected Sunday, but a stronger lake breeze is
possible leading to lake side cooling. Monday and Tuesday look
similar to Sunday with lake cooling and highs in the 80s to 90s
away from the lake.

Temps remain in the 80s for the remainder of the forecast with
periodic chances of thunderstorms mid to late in the week.



For the 06Z TAFs...

The forecast for the Chicago area TAFs through today and tonight:

- Chance for an early evening lake breeze wind shift to easterly

- Chance for thunderstorms tonight (as early as 23Z), but low
  confidence in this

Near due south winds will turn southwest shortly after daybreak
and begin to increase in speed. Occasional gusts in the mid teens
are possible today. A lake breeze is likely to develop along the
Illinois and Indiana shore and may reach GYY during the afternoon,
but otherwise should remain ease of ORD and MDW prior to 23Z.
During the early evening, as the winds over land diminish, there
is potential for the lake breeze to push westward and turn winds
easterly, at least temporarily. Confidence in this is low (about
30 percent chance), so have not included in the TAFs at this time.
If it occurs, winds are likely to become somewhat light and
variable shortly after and then back to light south-southwest by
late evening.

Multiple small areas of convection across the Plains and Upper
Midwest early this morning are indicative of several small scale
upper level disturbances in the region. These will have influence
into today`s convection. There is likely to be scattered storms by
mid-afternoon into western Illinois on a consensus of guidance as
one such disturbance slowly moves east-northeast. How much of
this activity can build toward the Chicago area into the early-mid
evening is uncertain at this time, and have leaned more toward it
staying south / not as widespread. Another upper level
disturbance is expected to be moving southeast from the Upper
Midwest tonight and will also bring a chance of storms. None of
these have high enough confidence in solution yet for mention in
the 06Z TAFs.



203 AM CDT

South-southwest winds will be on the increase today over the open
waters of Lake Michigan, especially across the northern half,
where gusts of 20-25 kt should be frequent. Winds will be less on
Saturday but still south-southwesterly over the open waters.
Throughout the holiday weekend, the Illinois and Indiana
nearshores will experience afternoon lake breezes, and these
should have a little more gusto Sunday and Monday, but no small
craft advisory conditions are expected.

Thunderstorms are possible by late day across the north, and
across the entire lake tonight. Severe weather is not anticipated,
but a few storms could have gusty winds. Chances for storms are
less for the rest of the weekend, though cannot rule out isolated
storms on Saturday and Saturday evening.






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