Area Forecast Discussion
Issued by NWS Chicago, IL

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926
FXUS63 KLOT 211749
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1249 PM CDT Wed Jun 21 2017

.SHORT TERM...
1039 AM CDT

Through Tonight...

Main forecast concern this update remains rain and thunderstorm
chances this afternoon and tonight.

Clusters of showers and storms continue to propagate east across IA
and into western IL this morning in an area of good warm air
advection/isentropic lift.  With low level jet helping to feed showers
and storms weakening and remnants moving into a much less
favorable and dryer airmass further east over the local area...
expect convection to weaken considerably as it approaches the
western counties (Lee/Ogle/LaSalle, etc.) of the forecast area in
the next few hours. Still with that... clouds will certainly
increase along with some sprinkles and showers still holding
together... and even an isolated storm. Initial cluster of
rain/storms tracking more ESE and should impact LaSalle-Streator-
Pontiac corridor while second cluster tracks further north
impacting Dixon-Oregon and eventually DeKalb corridor later this
afternoon.

Expect coverage to be scattered at best by the time the remnants
reach the local area through the afternoon.  Focus then shifts to
development of additional showers/thunderstorms tonight with area of
WAA/isentropic lift shifting further east ahead of returning warm
front over central IL and low pressure over the central plains.
Placement of this convection remains challenging as better chances
appear associated with re-intensifying low level jet later this
evening and north of our area. However can`t rule out an isolated-
scattered shower or storm... mainly north sections of the forecast
area ... although CAMs trending dryer this time period with focus
shifting farther north later.

Ed F

&&

.LONG TERM...
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

&&

.AVIATION...
For the 18Z TAFs...

Main concern for terminals in the next 24 hours are timing and
risk of thunderstorms tonight. Light winds will continue this
afternoon and evening with SE flow increasing slightly. High and
mid level cLouds will thicken through the afternoon in advance of
rain to the west moving into a dryer/unfavorable environment
overhead and decaying. May see a few sprinkles survive all the
way east to the airports toward late afternoon. Better chances
for rain come tonight however uncertainty remains high as latest
model trends suggesting rain and storms staying mainly north of
the area in region of better forcing. Best chances albeit minimal
for rain/storms looks to be during the 04z-06z timeframe
overnight.

Thursday shaping up to be a breezy gusty day (southwest winds G22) with
high clouds. Rain and storm chances gradually increase but expect
better chances to hold off till evening or later.

Ed F

&&

.MARINE...
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.

JEE

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...None.
&&

$$

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