Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 241729

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1229 PM CDT Thu May 24 2018

948 AM CDT

Minor tweaks made to going forecast to raise max temps slightly
this afternoon.

Surface high pressure has drifted east of the area this morning,
and was analyzed over the southern Lakes/upper Ohio Valley. West
of the surface ridge, modest south winds and sunny/mostly sunny
skies will combine to provide summer-like warmth to the forecast
area. Temps already rising into the mid-upper 70`s approaching 10
am, outpacing current forecast slightly. Model 925 mb temperature
progs support +22/23C later this afternoon, which would support
surface temps in the mid-upper 80`s. Based on this, have raised
temps a bit, with highs expected to range from 84-ish over
northwest IN counties to the 86-88 degree range farther west into
IL. Weak southwesterly lake breeze is anticipated, though would
expect the greatest impact to be along the immediate shore and
perhaps a few miles inland of the IL north shore.

Otherwise, no additional changes needed.



207 AM CDT

Through tonight...

Surface ridge still across the region early this morning, while
quiet/dry conditions are in place. Expect this high to push
further to the east today, while upper level ridge pushes
overhead. While the focus for any precip development will remain
west of the area through tonight, will likely see associated high
clouds continue to move across the region, but with mainly sunny
skies still expected. Warm up continues today with temps rising
well into the 80s across the area. Locations near the lake in NE
IL will be the exception, as onshore winds are expected to develop
later today. In this location, still think temps in the 70s are
expected, but the extent of the warming will be tied to when the
winds turn. Do think there will be enough time this morning to
warm up before this occurs, so did raise temps near the lake a
couple of degrees.



207 AM CDT

Friday through Wednesday...

Dry conditions still anticipated at the start of the period with
shower/thunderstorm development and instability axis still just to
the northwest. Much of the day on Friday is appearing dry, with
temps once again rising into the upper 80s. Continue to monitor
the afternoon and evening time frame when thunderstorm development
increases. Model guidance still varying with overall solution
during this time frame, though several features will come together
that could support thunderstorms. Upstream mid/upper level trough
with embedded shortwave energy will approach during this time,
with some sort of surface focus/trough appearing probable. During
this time, moisture/instability axis shifts overhead as well. All
of this will support the possibility for convection to develop,
although, still not exactly quite sure where this will initiate.

At this time, a likely scenario would be for scattered storms to
develop to the west/northwest of the CWA and drift into northern
IL. The shear does not appear overly high, however, the extent of
the instability could support at least an isolated strong to
severe threat Friday afternoon into Friday evening with the
highest threat over northern IL. Once again, guidance still not
locked onto a solution with some hires guidance that gets into
this period showing this convection developing overhead. Also,
some guidance has been hinting at an MCS developing to the
northwest late in the day Friday and then quickly diving southeast
into the area Friday night. Given the pattern and where the
instability axis is forecast to setup, this does not appear
completely unreasonable.

Confidence lowers further on thunderstorm chances on Saturday,
although chances do remain. Cyclonic flow and instability still in
place could support at least some isolated development if there
is some sort of focus at the surface. Sunday into Monday, chances
for showers and thunderstorms appear to be low. Well above normal
temps in the upper 80s to around 90 still expected Saturday into
Monday. During this time, dewpoints are looking to increase across
the region, and will support heat indices in the low to mid 90s.
If temps/dewpoints are slightly higher, could see heat indices
rise more to around the mid 90s, and possibly slightly higher.
Confidence is lower for temp trends near the lake during this
time, as guidance is now hinting at the possibility of lake breeze
development each day. Have continued the generally warmer temps
near the lake, but this could possibly change with later forecast



For the 18Z TAFs...

Quiet VFR aviation weather will persist through the current TAF
period. Only minor concern is timing lake breeze passage at the
Chicago area terminals later this afternoon.

Surface high pressure has moved off across the upper Ohio Valley
this midday, with light south-southeast gradient winds around the
western periphery of the surface ridge. With weak gradient, wind
direction has been varying somewhat from 230-260 deg or so, though
most obs were generally southerly. ORD TDWR depicts the developing
lake breeze from right along the shore in downtown Chicago, to
west of UGN farther north. Given temp differential from lower 80`s
inland to upper 60`s at the lake shore, would expect the lake
breeze to continue to push inland through the afternoon hours.
Winds should turn back to the south-southwest overnight tonight,
with a little stronger gradient Friday likely keeping the lake
breeze east of the terminals.

Little in the way of cloud cover across the region this afternoon.
Patchy mid/high cloud may become more widespread/thicken Friday
morning as a weak mid-level disturbance tracks into the region.
Potential for scattered thunderstorm development increases west of
the area during the afternoon, with some guidance bringing storms
into northwest/north central IL beyond the end of the current
forecast period early Friday evening.



244 AM CDT

High pressure over over the central Great Lakes region early this
morning will provide light winds across Lake Michigan this morning
in addition to at least patchy foggy conditions, dense in spots.
The high will move to the upper Ohio valley today while low
pressure moves into Manitoba later today. Fresh southerly flow
will develop in response and eventually peak in the 20 to 25 kt
range Friday. The low will move over Lake Michigan Saturday and
weaken, then a weak high will build in its place early next week.
This should result in relatively light flow over the weekend and
into early next week.






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