Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 210808

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
308 AM CDT Wed Jun 21 2017

308 AM CDT

Through tonight...

Primary short term concern this morning is precip chances through

GOES-16 IR imagery early this morning shows blossoming deck of mid
level cloudiness across southern MN and northern IA in response to
strengthening WAA regime in advance of shortwave trough noted on
GOES-16 mid and high level water vapor imagery over Nebraska. Many
hi-res models suggest that convection will break out over N IA
this morning, with remnants of that activity spilling east into N
IL this afternoon. Low levels remain quite dry and we will be
entering the typical diurnal min in WAA, so activity should be on
the decline, but it is looking increasingly possible that some
sprinkles or light showers could make into into especially western
portions of the CWA this afternoon before dissipating completely.
Have introduced some low pops to cover this possibility.

WAA will ramp up again tonight as 50kt+ 850mb low level jet
develops. Sfc warm front is forecast to lift north tonight with
strong ascent to the north of the front on the nose of the 50kt+
LLJ which looks to take aim mostly on WI. Have maintained some
pops tonight, especially far northern CWA, but if convection does
impact N IL it wouldn`t be for long before focus shifts north into

It looks like Thursday will generally be precipitation free for
most of the CWA, with the bigger weather headline Thurs being the
increasing heat and humidity. Local 925mb temp climo supports
highs climbing into the low/mid 90s with 925mb temps in the mid
20s Celsius. Will need to keep an eye on Tropical Storm Cindy`s
moisture which could make it far enough north to provide our SE
CWA with more cloudiness and perhaps some afternoon convection,
which could keep SE areas cooler than points north and west.

- Izzi


308 AM CDT

Thursday through Tuesday...

Cold front will move southeast across the area Thursday night,
with timing a bit slower than previous model runs. That could mean
a delayed start time for precip until later in the night, which is
typically a less favorable time for cold frontal convection.
However, the associated upper trough is fairly strong by late June
standards, so felt comfortable maintaining likely pops Thursday
night. Latest guidance suggest the the trough and associated cold
front will be more progressive than earlier runs advertised, so
while heavy rainfall is still possible, it looks to be somewhat
lesser of a concern than last night.

Long wave trough is then forecast to become more established over
NE North America heading in the weekend with northwest flow at the
sfc and aloft bringing seasonably cool (mild) temps and
comfortable humidity. ECMWF and GFS do hint at a shortwave
embedded within the NW flow potentially bringing some showers to
the area Sat afternoon/evening, but are much less amplified with
weekend shortwave compared to earlier runs, suggesting a lesser
chance of significant/washout type rainfall this weekend.

The very comfortable temperatures (70s during the day, 50s away
from the city at night) will continue into Monday before trough is
forecast to move eastward and somewhat more seasonable temps
return next week.

- Izzi


For the 06Z TAFs...

Light winds are expected tonight. GYY continues to report fog, and
may see a few other areas report shallow ground fog. Thinking DPA
also has a chance of fog developing, but do not have enough
confidence in fog to put it in the TAF at this time.

East winds continue today, and winds veer to southeast through the
late afternoon and evening. I have low confidence in precip timing
and coverage tonight into Thursday morning. Decided to leave the
PROB30 as is and not include a PROB30 for GYY as precip should
stay to the north. Winds then become south to SSW Wednesday night.



214 AM CDT

High pressure over the lake will shift east this afternoon. Light
winds become southerly this evening. Meanwhile, low pressure shifts
east across southern Canada. May see a brief period of 15-25 kt
especially over the southern half of the lake late tonight into
Thursday morning as the pressure gradient tightens between the
departing high and Canadian low.  Will have borderline small craft
advisory conditions Thursday, but not confident enough to put a
small craft advisory out at this time. Will have 15-25 kt winds in
the nearshore forecast though.

Guidance suggests a secondary surface low may form over Lake
Michigan along the Canadian low`s cold front Thursday evening. One
model features 30+ kt winds in response to the low while other models
are not as aggressive. Decided to cap wind speeds at 15-25 kt.
Southwest winds become northwest immediately after the front, but
then back to westerly Friday.  A prolonged period of west winds is
then forecast as the remnants of Tropical Storm Cindy travel east
over the Tennessee River Valley through the end of the week.






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