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Area Forecast Discussion (AFD)

Issued by Chicago, IL (LOT)

FXUS63 KLOT 231140

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
640 AM CDT Sun Jun 23 2019

146 AM CDT

Through Tonight...

A bit more active weather will return to the region the next few
days. Just quickly looking at satellite early this morning reveals
active convection across the plains. Deep layer southwest flow is
in place ahead of a decent upper low across the Rockies. Multiple
smaller scale impulses will continue to advect toward the lower
Great Lakes which will increase concerns for storms at times, but
confidence on the timing of these individual waves is on the lower
side. At the surface, a warm front extends from low pressure here
Omaha to across western and central Illinois.

Evening soundings reveal a fairly unstable airmass off to our 
southwest across the central and southern Plains behind the warm
front, with some 3500-4000 J/Kg of CAPE per SGF, where a warm and
very humid airmass exits (mid 70s dewpoints). Locally instability
is still present aloft (1000 J/KG per ILX and 7.5-8 C/km lapse 
rates from 700-500 mb) with a sharp gradient toward Lake Michigan 
where a cooler and drier airmass is in place thanks to east flow 
on the western flanks of high pressure over the northern Great 

We have an organized cluster of thunderstorms along the 
Mississippi Valley with a defined MCV on radar mosaic noted. Arcs 
of showers ahead of this activity continue in the warm advection
ahead of the MCS, with thunder noted across central IL, while the
activity is devoid of lightning close to the lake. Fairly good 
agreement exists in the first wave early this morning moving 
through getting a push from a modest low level jet, with a batch 
of showers and some storms, and also decent agreement in these 
weakening near Lake Michigan and fading out for time this morning.
There could still be some activity around in the morning given 
the multitude of waves in the flow, but this does appear to still 
be a lull in coverage.

The pattern remains messy into the afternoon with a stronger
shortwave emanating from the southern plans expected to push
across central IL, while additional energy will swing by across
our area and points northward, but good signal for forcing for
showers and thunderstorms given a shortwave trough, likely not 
too far behind the morning wave and possibly leading to 
development as early as midday. With increasing southwest flow, 
this will push the warm front and associated warm and humid air 
across a decent portion of the area, likely only the near 
lakefront areas in IL hanging onto cooler onshore flow and holding
up the front. Mid- level lapse rates are still forecast to be 
pretty weak today and there should be quite a bit of convective 
debris, so even with the influx of higher theta-e air at the 
surface, instability should be on the moderate end of the 
spectrum, but could still be appreciable given the moist low 
levels and should lead to some robust convection. Also, shear 
will be weaker especially across the northern half of the area.
Thus SPC has the area in a marginal/isolated severe risk. 
However, will need to keep an eye on wind fields as any convective
enhancement to the shortwave could result in stronger shear and a
bit more appreciable of a severe threat, especially across 
central IL into northwest Indiana closer to the southern stream 
wave. Also, with PWATs surging to nearly 2", heavy rainfall is 
also likely in spots in spite of residence time for storms not 
expected to be fairly long.

Showers and storms should wrap up this evening as trough passes 
with just seasonably warm and humid conditions left in the wake of
this wave. Isentropic lift is not great Sunday night, but with 
the approach of the main upper low and continued southwest flow, 
some showers may linger tonight. 



146 AM CDT

Monday through Saturday...

The main trough axis that had been slowly moving east through the
Great Plains will finally accelerate NE into the Upper 
Mississippi Valley through Monday night. Guidance is in fairly 
good agreement that the trough axis will clear the CWA by Monday 
evening, with an associated cold front/occlusion tracking ENE 
across the CWA during the afternoon. A decent amount of mid- level
cloud cover will likely limit instability to some degree Monday 
afternoon. However, if pockets of modest instability do develop, 
sufficient deep-layer shear will support some organized convection
with an isolated severe risk. And with a negatively tilted trough
shifting through, this is a favorable pattern for increased 
precip chances.

It appears Monday's activity will come in several waves, one in
the morning and the scattered in the later afternoon. Precip 
activity will quickly end from SW to NE in the evening. Behind 
the front, SW/WSW winds should gust 25 mph to briefly 30 mph late 
afternoon through early evening. Slightly lighter winds but still 
breezy conditions continue through Tuesday.

Most of Tuesday appears dry given zonal flow aloft and low
pressure well to our north, though there are some hints of faster
moving flow which could lead to increased shower/storm chances
sometime later Tuesday/Tuesday evening.

Upper level heights will be on the increase mid to last week, with
continued moist/warm southwest flow in place. This suggests mostly
dry conditions and more of a continued summer-like feel with temps
in the 80s and generally more humid conditions with dewpoints
lingering in the mid 60s. But we remain in a precarious position
with waves rotating around the ridge axis, and some uncertainty
with how far west the core of the upper ridge will get, so there
is still some chances for precip at times and a possible return to
NE flow off the lake this weekend per the EC/CMC. 



For the 12Z TAFs...

640 AM...Forecast concerns include...

Showers/isolated thunder ending this morning.
Scattered thunderstorms this afternoon/evening.
Mvfr cigs this morning.
Wind directions this afternoon.
Ifr cigs tonight/Monday morning.

Showers with embedded isolated thunder will move east of the
terminals in the next hour or so. A dry period is expected this 
morning into early this afternoon.

Scattered thunderstorms are expected to redevelop this afternoon 
but there is increasing uncertainty for coverage. Timing also 
appears to have shifted a little later. There is currently 
widespread convection across the central Plains and how this may
affect thunder potential this afternoon is uncertain. Its possible
that the best chance of redevelopment is from central/northeast IL
into IN with perhaps nothing across northwest IL/rfd. Have kept
the tempo for now and tweaked timing just a little later.

Mvfr cigs are currently patchy across the area and appear to
scatter immediately behind the showers but addition lower cigs
have formed across west/central IL and short term guidance shows
at least some form of mvfr cigs possible mid/late morning and
trends will need to be monitored.

East/southeast winds around 10kts to start will turn southeast
this morning and remain in the 10-12kt range. There remains some
potential for winds to turn south/southwest this afternoon but
confidence remains low. Any thunderstorms that develop may also
affect the winds and so maintained south/southeast wind directions
this afternoon. Winds should favor light southerly tonight and
then turn southwest Monday morning.

Trends still support ifr cigs overnight into Monday morning but
guidance appears to be backing off of the lifr cig potential.
Confidence for cigs tonight is very low and opted to make no
changes to this forecast. Some patchy fog will also be possible.