Area Forecast Discussion
Issued by NWS Chicago, IL
FXUS63 KLOT 222343
Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
643 PM CDT FRI JUL 22 2016
304 PM CDT
Have made no changes to current heat headlines for Saturday and no
major changes to the low chances for storms tonight and Saturday
morning, mainly in north central Illinois and then slowly
increasing chances Saturday afternoon.
Influence of last night`s and this morning`s MCS can still be seen
on visible satellite this afternoon. The outflow from this arcs
northward across central/eastern Iowa with a zone of 20-25kt
850-925mb warm advection across that same area. Have seen isolated
storms develop in that area and expect an uptick through late
afternoon as a subtle short wave disturbance moves eastward near
the MN/IA border and the cap is eroded. The instability gradient
and thicknesses slope southeastward and would expect any organized
convection to propagate that way, possibly clipping north central
Illinois during the evening hours and east central Illinois in the
late evening/overnight hours. Confidence is low on this, but
somewhat higher confidence than in recent days that the Chicago
area and far northwest Indiana will remain dry.
Afternoon dew points have recovered into the lower to mid 70s
across the area and may inch up a couple more degrees into this
evening. The humid air mass will not allow the temperatures to
drop much, but they could reach the temperatures for a few hours
which under the surface ridge could set the stage for patchy
shallow fog or even some haze. So have added that into the
overnight forecast. There may even be stratus development based
on the lowest level profiles forecast by some of the high-
For Saturday outside of morning fog/stratus possibility and the
small chance of festering convection in the south/southwest, the
morning should be sunny. Given the 925mb temperatures forecast to
be 26-27C, highs in the mid 90s are possible, but at this time
have gone lower 90s in most locations. Light onshore flow is
expected to develop across northeast Illinois and far northwest
Indiana which should keep them cooler. Heat index values may be
close or reach advisory criteria for some counties not currently
under a headline for Saturday, but given the uncertainty on
clouds/afternoon storms have collaborated on waiting for any new
headline issuance or changes.
As for Saturday afternoon storms, as the higher theta-e returns
from the southwest there are increasing chances reflected. Also,
if convection can sustain itself from the Dakotas/Minnesota late
tonight and Saturday morning, this would possibly reach into or
near the CWA by mid-late afternoon Saturday. Some guidance
indicates this and being in the zone of ridge-riding complexes,
this is a possibility at some points late Saturday or into the
329 PM CDT
Saturday night through Friday...
Main forecast concerns/challenges are with thunderstorm trends
Saturday night through Sunday evening across the entire CWA w/
multiple rounds of thunderstorms possible, as well as the
possibility for hot and humid conditions to persist into Sunday.
Early Saturday evening thunderstorm chances will be based on what
occurs during Saturday afternoon. With either scattered
thunderstorm development to linger into the evening or with a
possible complex approaching the CWA during this time, chance pops
for thunderstorms seem reasonable. After a possible lull in the
activity around the midnight time frame, there is a fairly
consistent signal for redevelopment to occur over the Central
Plains and Upper Midwest Saturday night as LLJ is really focused
across this region and while strong instability remains in place.
If this does occur, MCS development seems probable with a likely
track into northern Illinois and southern Wisconsin late Saturday
night into Sunday morning following instability axis. A wind
threat would be possible with this thunderstorm complex and with
high PWATs still in place, heavy rainfall will accompany any
storm. Thunderstorms should then continue east of the CWA through
mid/late morning Sunday, with a dry period then anticipated.
In the wake of these thunderstorms, a clearing trend should occur
with at least partly cloudy skies then expected. With synoptic
boundary approaching the CWA Sunday but still to the west and with
thermal ridge moving overhead, temperatures in the low 90s should
occur CWA wide. Lingering cloud cover will control the extent of
the warming, but enough clearing were to occur, then warmer temps
in the mid 90s are possible. Dewpoints are anticipated to be well
into the 70s out ahead of this approaching boundary, which will
provide humid conditions once again. Sunday will need to be
monitored for this possible heat/humidity. The extent of clearing
and warming during the day Sunday will be key with additional
thunderstorm development in the afternoon and evening, with
guidance varying on coverage/placement of any additional
thunderstorms. However, with front pushing through the CWA during
this time, would think at least widely scattered development will
at least be possible ahead of it and have continued chance pops
into the evening. Confidence of intensity of these storms is low
at this time, but isolated stronger development will be possible.
High pressure then expected to build across the region into Monday
with drier and slightly cooler temps expected, while chances for
thunderstorms remain low/zero into the start of next week.
For the 00Z TAFs...
643 pm...Forecast concerns include a lake breeze this evening and
again Saturday afternoon...fog/low cloud potential early Saturday
morning...and small chance of thunderstorms Saturday afternoon or
Current lake breeze has been moving inland slowly and is near mdw
with a shift easterly expected there soon. Still another hour or
more from reaching ord and confidence regarding timing is low as
is wind direction and how long they may stay easterly. Weak
gradient across the area will allow winds to become light/variable
this evening and calm overnight. Winds will turn light southerly
mid/late Saturday morning and then another lake breeze is possible
Saturday afternoon...turning winds easterly at ord/mdw/gyy though
only medium confidence on timing.
Most guidance develops fog away from the metro area tonight with
some potential for at least patchy dense fog along with low mvfr
or ifr cigs...which could develop as fog begins to lift after
sunrise. Trended lower at rfd/dpa but confidence for ord/mdw and
gyy is lower. More likely a lower mvfr cig will develop rather
than low vis from fog...but trends will need to be monitored for
possible changes with the 03z updates.
Current convection developing across northeast IA is expected to
turn mainly south/southeast later this evening and remain west of
the terminals...though may approach rfd in a weakening phase later
this evening. Additional thunderstorms are possible across IA/MN
Saturday but how far southeast and how fast these move is quite
uncertain. There is a small chance for thunderstorms into
northwest IL and rfd Saturday afternoon and to ord Saturday
evening but confidence is low and opted to stay dry until better
trends/timing emerge. cms
322 PM CDT
Generally light winds will continue through Saturday with high
pressure building over the Western Great Lakes. A weak cold front
will turn winds from southwest over the north half of the lake
Friday afternoon to light northerly Saturday morning. The pressure
gradient will increase Saturday night as the trough of a low
pressure system moving across Ontario approaches. This will result
in south to southwest winds/gusts up to 25 kt on Sunday. A cold
front trailing from the low pressure system will move across the
lake Sunday night, with fairly light west and northwest winds on
Monday behind the front, though some potential for gusts up to 20
kt. High pressure spreading to the Eastern Great Lakes by mid week
will bring light winds Monday night through Wednesday.
Thunderstorms are possible across the lake over the weekend,
especially Saturday night into Sunday, with locally higher winds
and waves possible with any stronger thunderstorms.
IL...Excessive Heat Warning...ILZ008-ILZ010-ILZ019-ILZ021-ILZ023-
ILZ032-ILZ033-ILZ039 UNTIL 7 PM Saturday.
ILZ014 UNTIL 7 PM Friday.
Heat Advisory...ILZ020-ILZ022 UNTIL 7 PM Saturday.
IN...Heat Advisory...INZ001-INZ002 UNTIL 7 PM Friday.
Heat Advisory...INZ010-INZ011-INZ019 UNTIL 7 PM Saturday.
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