Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 271954

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
254 PM CDT WED JUL 27 2016

254 PM CDT

Through Thursday...

The main forecast concerns are with the chances and timing of
thunderstorms tonight and on Thursday as a cold front moves over
the area.

Currently a cluster of thunderstorms is ongoing over east central the north of MKE. These thunderstorms are pushing an
outflow boundary southward towards the MKE area. It is possible
that if this outflow boundary continues to shift southward into
northeastern IL late this afternoon that a few storms could impact
far northeast Illinois around 6 to 8 pm this evening. A few
additional showers or storms could also try to develop near the
lake breeze boundary over northeast IL later this afternoon. As a
result, I have focused some chance POPS across the area late this
afternoon and early this evening. Thereafter there may be some
widely scattered showers or storms overnight, but most of the
area looks to stay dry.

A cold front is expected to shift over far northern Illinois early
Thursday, before setting up somewhere near the I80 corridor during
the day as a surface wave of low pressure develops and tracks
eastward along the front. This front looks to be the focus for
thunderstorm development by Thursday afternoon as a northern
stream short wave disturbance shift over the area. It appears that
a second upper level disturbance will shift east-northeastward
well to our south over KY on Thursday. This could focus the better
moisture and hence heavy rain producing thunderstorms over that
region. In spite of this, it does appear that at least widely
scattered storm development will be probable over portions of
northern IL and northwestern IN Thursday afternoon. Overall the
threat of severe weather looks low with this activity, due to weak
deep layer shear and mid-level lapse rates. Therefore, only
isolated strong wind gusts and heavy downpours look possible with
these storms. Some of this activity could linger into Thursday

Temperatures with the front look to cool off into the lower 80s
north, with 70s likely along the lake front due to the onshore
winds over northeastern IL. These increasing northeasterly winds
over Lake Michigan could set up dangerous swimming conditions
with large waves over NE IL late Thursday into Friday.



335 AM CDT

Friday through Tuesday...

Broad upper troughing remains in place Friday and Saturday, while
surface high pressure remains positioned north of the area with
drier northeast flow in place across WI and into IL. Have
maintained chances of precip Friday, still on the lower end.

Upper heights increase some Saturday in spite of a weak trough
lingering, and then increasing more significantly on Sunday.
Cannot rule out showers a a weak theta-e ridge will be focused
into NE IL/NW IN Saturday and less so Sunday, but no washouts
either day as dry northeast low- level flow will continue.

Warm and more humid conditions will arrive to start the work week
as southwest flow increases ahead an upper ridge/hot dome that
will expand eastward. CPC has the region pegged with fairly high
probabilities of above average temperatures during this time
frame. 850-925 Temps support readings well into the 90s, while a
max of blended guidance gives temperatures in the 90-95. Dewpoints
will again be elevated in this pattern. GFS Ensemble guidance has
the highest confidence on the Tuesday-Wednesday time frame, though
of note the ECMWF keeps us more on the edge of the heat dome an in
a more favorable position for impacts from both nearby and
possibly local convection. Blended guidance keeps slight chance to
eventually chance pops M-W.



For the 18Z TAFs...

Surface high pressure over the area continues to result in light
winds across the area early this afternoon. A lake breeze is
beginning to shift inland over the northeastern IL near shore
areas. This lake breeze should approach the eastern terminals later
this afternoon, with light (5 to 10 KT) easterly winds likely in
its wake. Prior to its passage, it appears the winds will remain
south-southwest up to 6 KT. Otherwise, similar to yesterday, VFR
CU with bases around 4-6,000 FT will continue to develop over the
area, and they could result in periods of a BKN CIG this

A cold front will drop southward over the area on Thursday
morning. This will result in more of a northeasterly wind on
Thursday. It also appears the threat of some scattered showers
and some thunderstorms will increase during the day. At this time,
though it appears the main focus for thunderstorms could end up
in the vicinity of the cold front late Thursday morning and
afternoon, which could end up near I-80 by early afternoon. As a
result, it is possible most of the thunderstorm active could miss
the terminals just to the south. However, given uncertainties this
far out, I have opted to hold onto the PROB30 for thunder, but
have pushed it off a couple hours.



228 PM CDT

Today`s somewhat weak and variable wind regime will evolve to a
stronger northeasterly fetch tonight and long northerly fetch by
Thursday evening. It is not yet clear whether wave heights at the
south end of this fetch will prompt a small craft advisory, but
they should at least be in the 3 to 5 ft range by late Thursday
into Thursday night. This north to northeasterly fetch will
persist into the weekend. Monday will be a time of transition to
southerly flow again, which then will persist into the middle of
next week.






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