Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 230632

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
132 AM CDT SAT JUL 23 2016


For evening update:

Have removed low-chance pops across the western cwa for tonight.

Small area of isolated convection occurring over northeast Iowa
earlier this afternoon/evening have dissipated. These storms
appear to have been associated with weak moisture transport and
ascent aided by a weak mid-level speed max propagating across
Minnesota. AMDAR aircraft soundings from MDW/ORD depict fairly
substantial cap in the 800-700 mb layer however, and guidance
depicts weakening of 925-850 mb flow across eastern Iowa and
northwest IL tonight with low level jet focusing well to the west
across the Missouri Valley region.

Otherwise, a very moist summer air mass remains in place across
the region overnight. Surface dew points in the mid-upper 70s will
result in high RH/low T-Td depressions overnight as temperatures
cool diurnally. This will likely result in development of fog and
low clouds across portions of the forecast area. Some uncertainty
as to how low visibility will become, though some areas may see
patchy dense fog develop before morning. Will have to monitor for
development and thickening overnight.



304 PM CDT

Through Saturday...

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-
resolution guidance.

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 06Z TAFs...

A few isolated showers may pop up from time to time overnight and
then dive southeastward. Otherwise the concern initially is for
occasional mvfr cigs and possiblty some lower visibility. MVFR
cigs have been bouncing back and forth between broken and
scattered and is likely to be the case overnight. Regarding
visibility, low dewpoint depressions are in place away from the
urban terminals and winds are little to none, but there is a weak
wind component that may keep visbys from dropping too much
tonight especially towards the Chicago terminals. Favored areas
would be under the surface high from KVYS/KRPJ southeast to KRZL,
but even these may remain these areas may see stratus coupled with
cirrus. Will continue the mvfr vsby mention away from KORD/KMDW
with infrequent ifr possible.

Next concern is on thunder chances. There are a few subtle
features that may contribute to shower and t-storm chances. The
first is aforementioned, with KGYY favored before daybreak as
forcing is subtle but there is a weak cap in place over the area
and more so over east central IL and northwest IN where some quick
developing pop-up showers may form. Expect quite a bit of
convective development to occur across Iowa tonight where the LLJ
is focused. Convective debris may limit destabilization over our
area where subtle shortwaves in the coupled possibly with the lake
breeze may be enough to break the cap whic Confidence is low on
occurrence even though a few short term guidance sources depict
some isolated development to fester at times through the day, so
kept tafs dry at this point for uncertainty in coverage.

Best chances of thunderstorms are tied to the slow moving compact
showrtwave across the central plains that will approach the
Mississippi River late this afternon/early evening. With best
forcing headed north initially, it may just graze our area (KRFD)
this afternoon and evening and may be more of an issue on late
tonight into Sunday.



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.

     Heat Advisory...ILZ020-ILZ022 UNTIL 7 PM Saturday.

IN...Heat Advisory...INZ010-INZ011-INZ019 UNTIL 7 PM Saturday.




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