Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 272005

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
305 PM CDT MON JUN 27 2016

231 PM CDT

Through Tuesday...

Generally sunny skies with temps in the upper 80s still ongoing
this afternoon, while continued stream of drier air moves through
the area. With reinforcing pushes of drier and cooler air
occurring this afternoon into the evening, most locations have
reached the warmest temps of the day and will see temps fall over
the next several hours. Expect increasing cloud cover currently
being observed to the north spread south this evening and
overnight, but with any upstream showers to likely slide just to
the north and northeast of the area. Still anticipating a quick
transition to north and northeast winds late this afternoon into
the early evening, with most short term guidance showing this
solution. This seems reasonable given the substantial weakening
occurring in the surface gradient, and with upstream obs beginning
to show this trend. With temps in the 80s this afternoon,
locations near the lake will feel this wind shift the most with a
quick transition to cooler temps off the lake. Do think there will
be some possible stronger speeds with this wind shift, but with
the prevailing strong winds not anticipated until later tonight.
High pressure will help erode lingering cloud cover on Tuesday but
with a cooler air mass in place, temps will stay in the 70s inland
and 60s near the lake.



243 PM CDT

Tuesday night through Monday...

The weather pattern will be primarily quiet through the extended
period, with the minor exception of potential thunderstorms
Thursday and Thursday night. This is the main forecast concern.

Cool high pressure will result in clear skies, winds going calm
and fairly chilly overnight lows Tuesday night in the lower-mid
50s away from Chicago and upper 50s to around 60 in and near the
city. This will be followed by a beautiful day Wednesday, with
highs slightly below seasonable. Other than some afternoon cumulus
development due to a weak midlevel disturbance, there will be
plenty of sunshine, with highs warming to the upper 70s to around
80 inland. Early lake breeze formation will keep lakeshore areas
in the lower 70s.

After a quiet and slightly milder night Wednesday night,
attention turns to a cold frontal boundary approaching the mid and
upper MS valley. Ahead of the front, 925 mb temperatures in the
upper teens celsisus will support surface temperatures warming to
the upper 70s to lower 80s. Afternoon dewpoints will likely rise
to the upper 50s to lower 60s range. Though a shower cannot be
ruled out west of I-55 in the morning, main period of concern is
the afternoon and evening.

There is uncertainty on timing of the approaching front, with the
12z GFS/GGEM quicker than the ECMWF, as well as dewpoints and
associated instability by the late afternoon. Global models are in
decent agreement on a sheared wave trailing from main trough
across the northern Lakes swinging across northern Illinois during
the evening as surface cold front translates southeast. GFS
dewpoints are very likely overdone, but even forecast dewpoints in
grids could yield some minor instability. Should isolated or
widely scattered storms or clusters of storms develop in far
northern Illinois (mainly north of I-88) during the late afternoon
and early evening during more favored diurnal period, 30-35 kt NW
flow at 500 mb could support some storm organization. Coupled with
steep low level lapse rates, have some concern for an isolated
strong or marginally severe storm (mainly along/north of I-88 as
mentioned above), with gusty winds the main threat if this
occurs. The later frontal timing of the ECMWF and also lower
instability would result in little to no strong/severe threat.

The front will be through most if not all of the CWA by Friday
morning, keeping threat for any additional convection in at most
far southeast CWA. The rest of the area will be dry and cooler
with steady northerly winds keeping the shore coolest. Will need
to watch for strength of northerly winds and resulting wave
heights with regard to hazardous swim conditions on Friday.
Surface high pressure locking over the central Great Lakes will
result in a very pleasant Independence Day holiday weekend with
primarily dry conditions. Temperatures will start out a bit below
normal under influence of troughing south of Hudson Bay, and then
moderate close to normal by July 4th as mid and upper ridging
builds northward. Onshore flow through the weekend will result in
cooler temperatures along the lakeshore.



For the 18Z TAFs...

Main forecast concerns/challenges this TAF period are with a wind
shift to the northeast at/above 10 KT this evening, and then with
MVFR ceilings tonight.

VFR conditions ongoing across the terminals early this afternoon,
with these conditions expected to persist through this afternoon
and evening. Upstream front dropping down the lake this afternoon
will allow for an increase in cloud cover tonight as well as a
wind shift, but any precip to stay to the north and northeast. Did
push up arrival of north/northeast winds with a wind shift to the
northeast this evening ahead of the front, and then increased
speeds later in the night with the arrival of the front.
Confidence is higher with this wind shift but lower with the exact
timing as some guidance is indicating an earlier northeast wind
shift, as early as 22-23z for areas near the lake. Feel that the
current stronger westerly winds should hold this shift until later
in the evening when the winds diminish. However, will continue to
monitor what the winds at observation points in eastern Wisconsin
do over the next couple of hours to try and gain a better handle
on trends. Upstream MVFR ceilings will likely continue to move
down towards the terminals, and have maintained this in the TAFs.



305 PM CDT

Main concern is with gusty northerly winds behind a cold front
tonight and Tuesday. North winds of 15-25 kt are in store, which
will yield building waves and conditions hazardous for small craft
late tonight through late Tuesday night for the Illinois and
Indiana nearshore waters. As the winds quickly increase this
evening, a few 30 kt gusts are possible over the northern portion
of the lake, and a few 30 kt gusts may occur Tuesday afternoon
over the southern half of the lake.

High pressure building in will cause winds to quickly diminish
from north to south Tuesday night. After a period of west to
southwest winds Wednesday and Thursday, the next cold front will
cross the lake Thursday night, with brisk northerly winds on
Friday. For now, mentioned 15-20 kt winds in the GLF forecast, but
will have to monitor for gusts up to 25 kt and conditions
hazardous for small craft. Winds will again quickly diminish by
Saturday as high pressure builds across the Great Lakes region.



LM...Small Craft Advisory...LMZ740-LMZ741-LMZ742-LMZ743-LMZ744-
     LMZ745...1 AM Tuesday TO 4 AM Wednesday.




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