Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 191111

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
611 AM CDT Sat Aug 19 2017

314 AM CDT

Through tonight...

Main concerns for the short term forecast period will focus on
isold/sct showers and thunderstorms this morning and then
temperature trends along the lakefront this afternoon.

A relatively high amplitude, progressive pattern has set up with
long wave troughing over the Great Lakes and lower Ohio Valley and
broad ridging over the swrn CONUS through the northern Plains.  A
series of shortwaves dropping through nwly flow aloft on the swrn
periphery of the long wave trough have kept isold/sct shra/tsra over
the region overnight.  Latest radar trends suggest that some
convective activity associated with a short wave over sern Iowa will
keep some sct shra/isold tsra going through the morning hours.
latest radar trends and short range guidance suggest that any
additional activity this morning should be confined to locations
south of the I-80 corridor and will likely end by late morning or
close to noon.  The sfc pressure gradient over the region will be
very weak today with high pressure ridging extending from the
central GLFMEX through the Great Lakes.  The weak gradient and
differential heating between land and the relatively cooler Lake
Michigan will allow a lake breeze to develop today.  Winds should
turn onshore by early afternoon limiting warming over lakefront
locations.  High temps today should reach the lower 80s as morning
convective debris cloud burns off and gives way to mostly sunny
skies this afternoon.  Lakefront locations should see temperatures
top out in the middle 70s.  Conditions overnight tonight should be
quiet, with mostly clear skies, light winds and temperatures
dropping into the lower 60s for locations away from the Chicago
metro urban heat island, where lows will only drop to the upper 60s
or around 70 F.


314 AM CDT

Sunday through Friday...

For the longer term forecast period, the main focus is on the sky
cover forecast for Monday during the period of the eclipse.  But
before that, forecast concerns will be with pcpn chances associated
with a low pressure system tracking through Canada, dragging a cold
front through the Upper Mississippi Valley Sunday and another low
developing over the plains, associated with a southern stream
shortwave lifting out of the southern Rockies.  The longer term
guidance is in generally good agreement on the evolution of this
pattern, with shra/tsra developing invof the cold front as it tracks
through the upper Mississippi Valley.  Meanwhile, a quasi-stationary
frontal boundary is expected to develop, extending east of the low
over the western plains to the middle Mississippi Valley.  This
boundary may also be the focus for sct shra/tsra by late Sunday
night and into Monday.  While there should be some chances for pcpn
through the day on Monday, do not expect that pcpn and associated
cloud cover will be widespread through the day and that there should
be a least periods of sct-bkn cloud cover through the day.  Pcpn
chances will increase through the evening and overnight with likely
PoPs and more widespread showers and scattered thunderstorms.  The
Day 3 SPC Convective Outlook highlights the upper Midwest, including
northern IL for a Marginal Risk of severe weather, mainly for the
overnight hours Monday night.  The upper level pattern will become
more zonal by Monday night, with a broad belt of strong mid to upper
level winds and cooling aloft.  Combined with warm/moist conditions
at the sfc...temps in the upper 80s and dewpoints approaching 70 F,
there may be sufficient deep layer instability to support organized
convective activity, with upscale development into lines or clusters
of storms.  The primary severe threat would be for strong, damaging
wind gusts.  Showers and thunderstorms may linger into the day on
Tuesday as the srn stream shortwave pushes east of the region and a
northern stream shortwave drops southeast through the upper
Mississippi Valley and across the upper Great Lakes.

Tuesday night through the end of the week should be dry, with
temperatures trending toward below normal as high pressure drops out
of Canada and remains centered over the Great Lakes region.  By
Wednesday, highs will only be in the middle to upper 70s across much
of the area.  Northerly sfc flow will keep winds off of the lake and
lakefront locations should see temps only reaching the lower 70s.


For the 12Z TAFs...

Limited aviation weather concerns today. Winds will be variable at
times but light, and a lake breeze wind shift is expected for ORD
and MDW this afternoon.

Underneath a surface ridge, patches of MVFR fog at some outlying
airports will dissipate quickly early this morning. This ridge
will settle directly overhead today allowing for light and
occasionally variable wind direction. It may result that ORD and
MDW turn to a northeast wind of <7 kt as early as late morning.
If this occurs, it will not be a lake breeze and more of just a
flop in winds. Confidence in speeds remaining <7 kt through early
afternoon is medium- high.

The true lake breeze should slowly progress inland during the
afternoon hours, with a 1-2 hour window behind the lake breeze
where 7-9 kt winds, at least occasionally, will be likely. This
reaching GYY and MDW has high confidence, while for ORD is medium-
high confidence. As for timing in the TAFs of this lake breeze,
confidence is low.



105 AM CDT

Lake Michigan will see a mainly quiet weather weekend. For this
morning in the Illinois and Indiana nearshore, winds will at
times show variability in direction with light speeds. An onshore
lake breeze will develop by early afternoon. For the open waters,
southerly winds will prevail, especially Sunday. Gust speeds look
to remain below 20 kt for most of the lake, with the exception in
the far north on Sunday.

A weak trough will ease its way southward over the northern part
of the lake by Tuesday morning. Low pressure will develop across
Michigan into the eastern Lakes region Tuesday afternoon, and help
accelerate the trough southward as a cold front. While still
several days out, this certainly has potential for 25-30 kt
northerly gusts including into the Indiana and Illinois nearshores
by Tuesday afternoon into the night.






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