Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 270207

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
907 PM CDT TUE JUL 26 2016

906 PM CDT

Had earlier issued an SPS for funnel cloud sightings in LaSalle
County, with a lone brief shower in an environment with high 0-3km
CAPE and some low level vorticity conducive for these brief
funnels. The threat for them has ended with the loss of daytime
heating. Main concern the rest of the night is with patchy fog
development given light to calm winds under the surface high,
clearing skies, the fairly humid air mass and recent rains. Have
stuck with a mention of patchy fog in the grids overnight into
early Wednesday, but will need to monitor trends overnight for the
development of any locally dense fog. Outside of any fog issues,
low temperatures in the low-mid 60s outside Chicago and lower 70s
in the city appear to be on track.



222 PM CDT

Through Wednesday...

Surface high pressure will dominate across the region through
Wednesday. This will result in light and variable winds and
seasonably cool temperatures tonight. Some fog may develop later
tonight...especially outside of the Chicago metro area as
temperatures look to cool a bit below this afternoon dew points.
Given the high will be overhead with light winds, and because some
patchy fog was reported this morning to our west under this high,
I have opted to include the mention for such in the forecast later
tonight. Any fog will be short lived and quickly dissipate with
sunrise. Another mild day is likely Wednesday with light winds and
another afternoon lake breeze. Temperatures inland could end up
another degree or two warmer than todays readings, which would
mean the potential for upper 80s highs. Closer to the lake,
however, temperatures will remain in the low 80s.

Given the lack of any organized focus, it appears the threat of
thunderstorms will be very low through early Wednesday evening.
With this in mind, I have mentioned a dry forecast through the day
on Wednesday. The better chances for storms looks to arrive by
Thursday, see more on this in the long term discussion.



303 PM CDT

Wednesday night through Tuesday...

A shift to northwesterly flow aloft will occur during the end of
this week with a couple short wave disturbances during Thursday-
Friday. For Wednesday Night, continue to not be very excited for
precipitation chances for most of the CWA given it lies between
forcing. Afternoon convective initiation on Wednesday, or re-
enhancement of convection, looks to occur over western/central
Wisconsin and some of this may drift to near the IL/WI state line
by later in the night. Limited forcing should allow for the
Wisconsin convection to gradually weaken after dark.

The chances for showers and some storms look to be the highest on
Thursday and Thursday Night when one short wave drifts over and
convergence improves, though chances are still not that high. An
upper low over the Gulf States this afternoon will drift up the
Ohio River Valley on Thursday and looks to rob moisture and low-
level jet support for our area during that time. So intensity and
possibly coverage looks to be kept in check. So only maintain
chance of storms. A second short wave may pass on Friday before
quieter weather under high pressure for the weekend.

Low-level northeast flow on Thursday and Friday will probably keep
some areas in the upper 70s, especially with the cloud cover
presently forecast for Thursday in the northern CWA. In addition,
these northeast winds may bring waves up to 3 to 5 ft along the
Illinois and possibly Indiana shores, which would lead to a risk
of rip currents mainly on unprotected beaches.



For the 00Z TAFs...

No real concerns through much of the forecast period as high
pressure continues to dominate the region. Generally VFR
conditions expected through the period, while winds vary tonight
and then turn easterly early Wednesday afternoon with lake
influence. However, speeds are expected to remain below 10 KT.
Patchy fog will likely develop late tonight and impact the
RFD/DPA area, with MVFR vis expected at this time.



232 PM CDT

High pressure centered over the southern Great Lakes will remain
in control through the day Wednesday. A northerly wind shift will
basically develop over mid-lake on Wednesday evening and shift
southward as a cool front through Wednesday night. This front will
turn winds to the northeast which will remain as such through
Friday night. Some gusts above 20 kt are possible during that
northeasterly time but do not appear to be the mean. Otherwise no
high impact weather forecast with waves expected to be 3 to
4...maybe 5 the Illinois nearshore Thursday Night into
early Friday.






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