Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 181855

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
155 PM CDT Sun Aug 18 2019

154 PM CDT

Through Monday night...

In the near term, focus revolves around the low potential for
additional isolated to widely scattered showers and/or storms this
afternoon and evening. Otherwise, focus transitions to perhaps a
lingering precipitation threat early Monday.

Fairly tranquil sensible weather conditions across the forecast area
early this afternoon in the wake of this morning`s showers and
storms. Guidance is in fairly decent agreement with this trend
continuing for much of the afternoon; however, the atmosphere
continues to gradually recover from morning convection and with high
dewpoint air remaining in place this afternoon, suppose it remains a
possibility to pop isolated to widely scattered showers and/or
thunderstorms. Still think this will largely be the exception rather
than the rule as the vast majority of the region stays dry, and any
additional deep convection remains well to our south tied to the
better instability gradient. Gusty southwest winds will continue
this afternoon with gusts as high as 25-35 mph, gradually curtailing
this evening.

By this evening into the overnight hours, the surface trough/front
situated to our west-northwest will gradually sag southward, when
combined with a strengthening LLJ, may prove to be enough for at
least isolated shower/storm develop across parts of the forecast
area. Don`t foresee any of this development getting too out of hand,
although may still pose a briefly heavy rain threat given the
moisture-rich air in place. Isolated - widely scattered shower
chances continue into the morning hours on Monday while gradually
sagging even further south with the majority of any early day
activity focused along and south of I-80 (deeper convection confined
to central IL and points south and east). High temperatures Monday
varying from the upper 70s to near 80 along the immediate lakeshore
given onshore the mid 80s inland.

High pressure centered to our north Monday evening/overnight should
be plenty close to aid in bringing tranquil conditions to the region
for much of Monday night. Partly to mainly clear skies should allow
temperatures to fall several degrees cooler than they have been the
previous several nights with lows ranging from the low 60s north and
west to the mid-upper 60s elsewhere.



328 AM CDT

Monday through Saturday...

Surface trough/front expected to sag south of the CWA Monday
morning, with any precip developing out ahead of it likely exiting
the area. Much of the area should remain dry for much of Monday,
but it does appear that this front will hang up right around east
central IL and northwest IN. In this setup, may see additional
thunderstorms Monday afternoon along and south of the Kankakee
river valley in IL and IN. Skies will be clearing on Monday and
despite some cooling noted on Monday, temps in the mid 80s
expected. However, onshore flow expected to keep locations near
the lake in the upper 70s to around 80. Should finally see dry
weather Monday night but will need to quickly turn attention to
Tuesday, as thunderstorm chances return to the region. Rapidly
increasing moisture and instability with the possibility of some
rather steep lapse rates expected on Tuesday. This could support
another period with organized storms, and will definitely need to
monitor this period. Should see a front swing through by midweek
and bring a change to the pattern, with cooler/drier conditions
expected at least through the end of the work week.



For the 18Z TAFs...

Main concerns through the upcoming TAF period include:

-Gusty southwest winds late morning/afternoon.
-Potential for additional showers and storms late tonight.
-Northwest wind shift with a cold front late tonight and then
 veering northeast by tomorrow morning.

At issuance time, all precipitation has pushed east of the region,
but some scattered showers are beginning to develop along a pre-
frontal trough currently located between RFD and the Chicago area
terminals. This trough axis is also the wind shift line between
the residual sely/vrbl winds left over by the earlier convection
and the strengthening swly winds. Expecting the trough axis to
pass through the Chicago area terminals by arnd 19z with winds
gusting to arnd 25 kt. The trough axis is moving very quickly, so
any showers that manage to develop enough to produce a wetting
rain would likely be very short duration and not long enough to
warrant a mention in the TAFs. The cold front is expected to push
through the region late tonight, latest guidance suggesting a
05-06z fropa timing, which will bring a wind shift to nwly. Some
guidance is suggesting some shra/tsra development along the front,
while other models are dry. Given the uncertainty, will only carry
a prob30 mention and refine the pcpn details as necessary.  As
high pressure spreads across the upper Mississippi Valley tomorrow
morning, winds should then trend to more nely by arnd 12z and
should remain nely through the remainder of the day.





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