Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 270746

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

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.



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

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.



For the 06Z TAFs...

Main forecast concerns are with persistent IFR/LIFR ceilings
through much of the forecast period, fog and reduced vis this
morning, and then a wind shift to the northeast later this

Ceilings steadily lowering once again at this time, with IFR
ceilings quickly falling to LIFR. The only site which has not
quite observed this trend is GYY, but anticipate this trend to
impact this site as well over the next 1-2 hours. These lower
ceilings then expected to be in place for much of today. Guidance
does vary to the extent of any improvement later today into
tonight, and have maintained forecast improvement to MVFR.
However, will monitor for the potential for IFR ceilings once
again later tonight. Fog will likely lower through early this
morning with RFD and DPA likely getting into the more dense fog,
with vis below 1 mile. This may impact ORD and MDW, and will need
to continue to monitor for this potential. Westerly winds will
gradually turn to the northwest this morning, and then to the
northeast by mid morning. Speeds should initially stay below 10
KT, but do anticipate speeds at or above 10 KT by early this



327 PM CDT

Low pressure will pass northeastward over the central part of the
lake this evening. Webcam imagery from the Illinois and Wisconsin
shores has indicated undulations in visibility, with dense fog at
times, and given that low track believe that more dense is
probable. Some of this will likely linger into Monday morning. The
low path also means wind direction at times could be variable
across the central part of the lake as it turns from southeast
late today to southwest and then to northwest late tonight.

The next in a series of surface lows will pass up the Ohio River
Valley Monday evening. Behind this, northerly winds will sharpen
Monday evening through Tuesday morning. This will bring Small
Craft Advisory waves and possibly winds to the Illinois and
Indiana nearshores.

North, northeast, or east winds will prevail much of the week.
With the next low late in the week, these winds are presently
forecast to stiffen to 30 kt over the open water.



LM...Dense Fog Advisory...LMZ740-LMZ741-LMZ742-LMZ777-LMZ779 until 9
     AM Monday.




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