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Area Forecast Discussion (AFD)

Issued by Chicago, IL (LOT)

FXUS63 KLOT 231756

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1256 PM CDT Sun Jun 23 2019

1111 AM CDT

MCV apparent on WSR-88D regional mosaic near St Louis late 
this morning will continue to lift northeastward and likely reach
northern IL later this afternoon. Large area of stratus is 
blanketing much of the CWA late this morning in advance of a 
northward moving warm front that's located over central IL. Band 
of convection arcing out ahead of the KSTL MCV should continue to 
move northeast toward our CWA. Will probably see at least some 
breaks in the stratus to allow for some modest boundary layer 
heating prior to this convection's arrival. Over our southeastern 
1/3 or so of the CWA, sunshine will be more prevalent, allowing 
for greater destabilization prior to convection arriving. 

Even in the southeastern most CWA instability is forecast to be
modest and weakening of mid-upper level winds this afternoon will
result in decreasing deep layer shear magnitudes. Overall, it 
would appear as the gradient in thunder chances will be aligned NE
to SW with the highest chances Benton County and lowest chances 
north central IL over our NW CWA. Given the fairly high freezing 
levels, weak mid- level rates, not seeing much of a severe threat 
this afternoon. 

- Izzi


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 18Z TAFs...

Upper level disturbance will move across the region this afternoon
and evening bringing increasing coverage of showers and perhaps a
couple TSRA this afternoon, with rain possibly lingering into the
evening hours. In the wake of the rain this evening, guidance
continues to hit the low CIGS pretty hard tonight into Monday
morning with IFR and possibly even nearly LIFR. At this point,
made no changes to previous TAFs which had conveyed this trend
nicely, though perhaps erring on the optimistic side with guidance
a somewhat lower. Low CIGS will likely lift and scattered out
Monday morning with potential for a band of scattered TSRA to move
across the terminals midday into the early afternoon Monday. Gusty
southwest winds will develop in the wake of the storms Monday

- Izzi