Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 201727

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1227 PM CDT Fri Apr 20 2018

1059 AM CDT

Morning Update...

Only minor tweaks to going forecast, primarily to remove morning
fog over north central IL, and to nudge afternoon max temps up a
degree or two in most areas away from Lake Michigan. Otherwise,
forecast remains right on track.



225 AM CDT

Through Tonight...

An expansive area of high pressure is located across the mid and
upper Mississippi Valley this morning and will provide fair weather
conditions across northern Illinois and northwest Indiana today.
Yesterday`s high temperatures out-performed guidance and could be a
similar situation today under full mid-April sunshine and with
snowpack having already melted. 925MB temps are progged to be
about 3C warmer today, and with similar mixing expected today,
should result in surface temps 5-6F warmer than yesterday. The
exception would be areas away from Lake Michigan in Lake County
(IL) and McHenry County which held onto snow much longer
yesterday, and will likely be closer to 10 degrees warmer today
compared to yesterday. Light flow will allow an afternoon lake
breeze to form, and suppress temperatures in the low to mid 40s
near the lake front. Light easterly gradient will develop behind
the ridge axis this evening and overnight and cloud cover is
expected to increase overnight ahead of an approaching wave over
the Great Plains. Temperatures are expected to bottom out in the
low to mid 30s.



225 AM CDT

Saturday through Friday...

Quiet weather will continue throughout much of the extended time
frame with only slightly below normal temperatures expected
through the end of next week.

Fair weather and mild conditions relative to the cold streak we`ve
experienced of late will persist over the weekend. Main change
from today will be increased cloud cover on the northern periphery
of an upper low that undercuts the region moving into the Deep
South over the weekend. GFS develops a few showers in portions of
the CWA Saturday afternoon as a weak shortwave moves across the
area, but forecast soundings show very dry conditions below 750mb.
With the other models dry, opted to maintain a dry forecast at
this point. Even if the GFS verifies and precip occurs, it`d
likely only be a few high based sprinkles. Temps look to top out
in the upper 50s/around 60 both Saturday and Sunday, but onshore
flow will suppress high temps in the 40s near Lake Michigan.
Should see a few more peeks of sunshine on Monday as the upper low
continues to move away from the local area, and widespread 60s
away from the lake seem a better likelihood.

Much of the same on Tuesday, but there is some chance for showers
Tuesday night into Wednesday as a cold front sags south towards
the region. Main areas of vorticity are progged to bifurcate the
local area and surface cold front only gradually sags into the
area at a diurnally unfavorable time, so precip chances aren`t
terribly high but something to keep an eye on in the coming days.
Another wave is expected late in the week and will provide
additional chances for some showers.



For the 18Z TAFs...

No significant aviation concerns next 24-30 hours.

Surface high pressure drifting east across the area early this
afternoon, with calm or light variable winds predominant. Lake
breeze has developed and begun to move inland off Lake Michigan
per ORD TDWR data, and extrapolation of the boundary would suggest
it will arrive at MDW by 19Z and ORD by 1930Z. Some airport
sensors already showing some light NE or E component per CWSU ZAU,
prior to lake breeze boundary passage.

With surface high off to the east of the terminals tonight and
Saturday, modest east flow is expected to persist. VFR conditions
will prevail, though with a thickening and gradual lowering of
high and mid cloud late tonight into the day Saturday.



159 AM CDT

Little in the way of concerns over the lake with strong high
pressure moving overhead into this weekend. There will be variable
light winds at times over the open water with this, along with
afternoon lake breezes in the nearshore. Low pressure is expected
to develop south of the lake in the Tuesday-Wednesday time frame,
but there is uncertainty as to how strong. Winds look to turn
northerly during midweek in response to this.






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