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Area forecast discussion 
National Weather Service Chicago/Romeoville, Illinois
542 PM CDT Wednesday Jul 27 2016

Short term...
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
WI...to 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 Illinois 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 Illinois later this afternoon. As a
result, I have focused some chance probability of precipitation 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 Kentucky 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 Illinois 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
evening.

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 Illinois. These increasing northeasterly winds
over Lake Michigan could set up dangerous swimming conditions
with large waves over NE Illinois late Thursday into Friday.

Kjb

&&

Long term...
340 PM CDT

Thursday night through Wednesday...

The cold front positioned over the area Thursday afternoon will
continue to drop south and stall out somewhere across central Illinois by
Thursday night and Friday. Shower and thunderstorm chances decrease
as the boundary moves south out of the local area and cooler, drier
northerly flow becomes established. However, continued mention of
chance to slight chance pops is warranted Thursday evening and again
Friday through the weekend, especially in southern portions of the
area, given the ongoing proximity of this boundary and pockets of
weak shortwave energy aloft. Interestingly, the GFS has the 582dm
line at 500mb positioned over or very near the local County Warning Area from
Thursday through late Saturday, which is just one basic indication
of the status quo pattern through this period.

Continuing to follow the 582dm line as a guide to our evolving
weather scenario, this height line begins to lift north by Saturday
night and Sunday as an upper level ridge builds across the plains.
Warmer Gulf air also begins to reestablish itself at low levels
during the early part of next week, but forcing for anything more
than isolated convection is meager under the upper level ridge at
least through midweek. Thus the pattern appears favorable for
another period of hot and humid conditions, though likely not to
the degree seen late last week.

Lenning

&&

Aviation...
for the 00z tafs...

Broad high pressure remains over the Great Lakes through the
Mississippi Valley. A well defined lake breeze boundary has been
very slowly inland and finally reached Ord at about 2230z turning
winds to enely at 7-10kt. Expect the generally Ely wind direction
with speeds less than 10 kt to persist through sunset at which
time winds will becm lgt/vrbl. The lake breeze should reach mdw by
arnd 2300z shifting wind there from swly to Ely. Dpa/rfd should
remain unaffected by the lake breeze as it should dissipate before
reaching as far west as dpa. Prevailing winds over nrn IL, away
from the lake should be generally swly 5-10kt through sunset
before becmg lgt/vrbl overnight. Some guidance is suggesting that
vis could drop to 3-4sm in br durg the early morning hours as sfc
dewpoints inch up overnight and winds becm lgt/vrb, so have added
3-4sm br at 08-09z for the 00z update. There is a chance that some
isold, sheltered area could drop a little lower than 3sm if cloud
cover would decrease more than is currently anticipated.

The other concern for the period will be thunder chances tomorrow.
Latest guidance suggests that the most favored timing will be
durg the afternoon hours as a mid level impulse crosses the
region. Exact timing and duration remains a bit uncertain, but
given the timing of the mid level impulse crossing the region durg
the afternoon hours, will continue to carry afternoon timing in a
prob30 group.

&&

Marine...
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.

Lenning

&&

Lot watches/warnings/advisories...
Illinois...none.
In...none.
Lm...none.
&&

$$

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