Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 271720
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1220 PM CDT Thu Jul 27 2017

.UPDATE...

1106 AM CDT

Expect cloud cover to be a bit slower to erode, but do gradually
expect some clearing as a lower dewpoint airmass to the north
filters in. Some instability south generally along and south of
I-80, with a nice non-linear depiction on GOES-R visible
satellite of enhanced cumulus growth, will allow a few showers
and maybe a storm this through the afternoon. This axis will
continue to drift south through the day.

KMD

&&

.SHORT TERM...
319 AM CDT

Through tonight...

The day will begin with light and variable winds at most locations,
but during the morning a prolonged period of northeasterly flow will
become established and stay in place through the weekend.  Showers
related to the late evening and overnight convection have mostly
exited the area as of the predawn hours, but there are multiple
signs that rain chances may linger into the morning and early
afternoon. First is the presence of a low pressure circulation
along a frontal zone straddling the area. Next is the very moist
airmass still in place. Third is the general agreement among
models of some weak shortwave energy aloft this morning. Overall
am not expecting much if any activity, but will leave a mention of
at least slight chances for most of the area through the morning,
and of chance POPs in our southeast CWA through mid afternoon.
With the northeast flow in place, temperatures today look to be 5
to 10 degrees cooler than yesterday.

The strengthening north-northeast winds in the wake of the low
will align well with the fetch of Lake Michigan, and as a result
build waves along the Illinois and Indiana shores. For Illinois,
the tightening pressure gradient and gusty winds move in early to
mid afternoon and for Indiana mid to late afternoon. So have
issued a Beach Hazards Statement for a dangerous rip current
threat beginning at those times. With onshore flow anticipated
through at least early Saturday morning, have run the Beach
Hazards Statement through then, and it is possible it will need to
be extended through midday Saturday.

Lenning/MTF

&&

.LONG TERM...
1219 PM CDT

Friday night through Thursday...

High pressure expected to slowly build across the region through
the weekend. As this occurs, persistent onshore flow will continue
to support dangerous beach conditions through Saturday morning,
that may persist slightly longer into the day due to the continued
north/northeast winds. At this time, no significant weather
expected this weekend into early next week. Shower and
thunderstorm chances will gradually increase towards the end of
the forecast period.

Rodriguez

&&

.AVIATION...
For the 12Z TAFs...

The concerns for the Chicago area TAFs today are:

- Patchy IFR/MVFR clouds through mid-morning with a small chance
  after that

- North-northeast winds steadily increasing in speed, reaching 10
  kt around 16Z

Winds have turned northeast at most airports early this morning,
albeit still light. These speeds will slowly increase this
morning and then more quickly this afternoon, with gusts to become
common. Some gusts are probable even after dark, and then again
becoming common during the day Friday. Confidence in wind
direction being north-northeast for most of the TAF period is
high, and is medium-high in forecast wind speed.

The back edge of the IFR cloud threat should move south with RFD
the most favored TAF airport to have IFR through mid-morning.
Cloud bases should lift/scatter some through the day, trending
VFR. Some clouds may develop in the northeast flow off the lake,
although those are likely to be no lower than 2500 ft.

An upper wave will move south over Michigan late tonight into
early Friday morning. While any showers are likely to remain to
the northeast, there is a chance for some MVFR clouds associated
with this near daybreak Saturday.

MTF

&&

.MARINE...
1219 PM CDT

As surface trough and associated frontal boundary have moved
south to the lower Great lakes today, northerly winds have been
observed over much of the lake. Still anticipate increasing
northerly winds this afternoon into this evening with winds of 15
to 25 kt returning. Occasional gusts to 30 kt are still a
possibility, especially across the south half. These stronger
winds and higher waves will support hazardous conditions for small
craft this afternoon and tonight. Expect northerly winds to
continue Friday into the start of the weekend, with speeds
periodically staying around 15 to 25 kt during this time.
Hazardous conditions for small craft will continue as well, and
will need to monitor for these conditions to possibly last longer
into Saturday. High pressure will finally build overhead, allowing
the northerly winds to diminish through the remainder of the
weekend.

Rodriguez

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...Beach Hazards Statement...ILZ006-ILZ014 until 10 AM Saturday.

IN...Beach Hazards Statement...INZ001-INZ002...5 PM Thursday to 10 AM
     Saturday.

LM...Small Craft Advisory...IL nearshore waters until 10 AM Saturday.

     Small Craft Advisory...LMZ743-LMZ744-LMZ745...5 PM Thursday to
     10 AM Saturday.

&&

$$

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