Area Forecast Discussion
Issued by NWS Chicago, IL

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698
FXUS63 KLOT 271624 AAB
AFDLOT

Area Forecast Discussion...UPDATED
National Weather Service Chicago/Romeoville, IL
1124 AM CDT Mon Mar 27 2017

.UPDATE...
1120 AM CDT

Continue to keep a close eye on webcams and observations near the
lake front, with present stratus within 50 ft above ground level
near the shore. There likely are a few fingers of dense fog onto
the northeast Illinois and northwest Indiana shore based on webcam
imagery, but likely only extending a couple hundred feet inland
or so before becoming elevated.

Otherwise, winds have turned onshore and could detect the weak
lake breeze move in on radar. Within the marine layer, there has
been some patchy drizzle and some lowering of visibility inland.
Do not see dense fog being a threat inland this afternoon, and
given wind speeds actually expected to pick up after dark, do not
foresee it being a problem.

Temperatures should go nowhere near the lake front with the
onshore flow and downtown has been steady at 41 all morning.
Elsewhere, readings are not climbing much either. Have made some
minor cooling adjustments.

The upper low centered in southwest Missouri late this morning
remains pretty compact on water vapor, and its forcing is as well.
The most persistent rain is likely only in the far south it
appears. Further north, light scattered showers on weak
isentropic ascent are favored, with likely little to none
along/north of I-88.

MTF

&&

.SHORT TERM...
243 AM CDT

Through tonight...

Early this morning, foggy conditions are in place as a weak
surface trough moves across the area resulting in weak flow. To
our southwest, water vapor imagery shows an upper low spinning
over eastern KS while a surface low is analyzed over far
southeastern KS. As the low moves east across Missouri today, a
col area will move over northern Illinois and northwest Indiana
resulting in light and variable or calm winds through much of the
morning over the CWA which will delay or slow improvement of
visibility possibly into the afternoon for some areas. This
afternoon, surface low is progged to move east across the mid
Mississippi Valley into far downstate Illinois with a
northeasterly gradient developing over northern Illinois. Onshore
flow and fog over lake Michigan will suppress temperatures near
the lake front. Expect high temperatures in the mid to upper 50s
with a few spots south of I-80 tagging 60. Near the lake,
temperatures are expected to top out in the mid to upper 40s.

Precipitation out ahead of the low is expected to begin
overspreading the CWA south of I-80 by late morning or early
afternoon, then overspread the I-88 to I-90 corridor mid
afternoon.  Models have trended slightly farther south, and it
now appears the northern tier and possibly two tiers of counties
in Illinois could stay dry with precipitation most likely across
the south half of the CWA. Very weak instability only clips the
far southern counties in the CWA so have kept thunder mention
along and south of the Kankakee River Valley.

Deubelbeiss

&&

.LONG TERM...
243 AM CDT

Tuesday through Sunday...

A sprawling area of high pressure will spread south from the
Canadian Prairies into the Upper Midwest and western Great Lakes
region Tuesday and Wednesday bringing dry conditions and some
peeks of sunshine. North to northeast flow and persistent onshore
flow will keep temperatures lower near the lake front. Local
climate data indicates that downtown Chicago struggles to reach
the mid 40s in lake August on days with persistent onshore flow.
Have undercut some of the guidance near the lake front keeping
temperatures in the low 40s immediately along the shore while
inland temperatures should be able to warm into the low to mid
50s.

Models continue to come into better agreement on area of low
pressure lifting across the region late in the week. While timing
differences remain moderately large, models are now honing in on
a low track from near St Louis to northern Indiana. Despite
falling on the cold side of the low track, forecast soundings from
the GFS indicate the column will be warm enough for all rain.
Models remain in reasonably good agreement through Saturday
building high pressure across the Midwest, but begin to diverge
thereafter on handling a cutoff low that develops over the western
states late this week into the weekend.

Deubelbeiss

&&

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

Main forecast concerns are with IFR/LIFR ceilings, reduced vis
this morning, wind shift to the northeast this morning.

Weak surface trough is pushing through northeast Illinois at this
time, and is allowing for some drizzle or light rain to develop.
This precip will remain light and likely last only for the next
couple of hours. However, do think this will assist with keeping
vis lower, in the 1-2sm range. IFR and LIFR ceilings in place this
morning and at this time, do think there will be some slight
improvement. This may not occur quite as soon though as am
monitoring really low ceilings and vis in dense fog right along
the nearshore waters of Lake Michigan. As this trough pushes
through over the next couple of hours, winds will turn to the
northeast. Although speeds will initially stay low, this may allow
for these lower conditions to move inland and help keep
conditions lower. Confidence is low with regards to the inland
extent of these lower conditions, but will need to monitor this
morning for possible lower conditions than currently forecast.
Still have a slow improvement in ceilings later today, but once
again, lower ceilings may still linger. Trends would suggest that
the bulk of the precip expected later today may stay just to the
south of the terminals. With these trends, have trended away from
this precip in the TAFs, but have not yet removed completely. At
this time, GYY has the highest chance to observe these showers.

Rodriguez

&&

.MARINE...
317 AM CDT

Low pressure over the central Great Lakes is departing to the
northeast this morning, while west to northwest winds are in
place. Expect a trend towards the northwest for much of the lake
this morning, and then more northerly throughout today. However,
speeds don`t appear to be too strong for much of today. Very moist
conditions still in place over much of the lake, with dense fog
likely being observed over the entire lake with the exception of
the Indiana nearshore. Have the dense fog advisory in place
through later this morning and do think this end time still seems
reasonable. Will need to monitor for fog to possibly linger longer
over portions of the lake today though, especially over the
southern end of the lake. High pressure to the northwest and low
pressure tracking through the region just south of the lake
tonight into Tuesday will allow northerly winds to increase.
Expect speeds of 15 to 25 KT with some gusts up to 30 KT over much
of the lake tonight, with these speeds and direction likely
continuing into Tuesday. These stronger winds and building waves
will likely provide conditions hazardous to small craft by late
tonight and especially into Tuesday. These stronger northerly
winds will persist through midweek.

Rodriguez

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...Dense Fog Advisory...nearshore waters
     until 4 PM Monday.

     Dense Fog Advisory...LMZ777-LMZ779 until 3 PM Monday.

&&

$$

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