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Area Forecast Discussion (AFD)

Issued by Chicago, IL (LOT)

FXUS63 KLOT 150119

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
819 PM CDT Sun Jul 14 2019

811 PM CDT

Thunderstorm activity is wrapping up this evening as remaining
cores in DeKalb County continue to fade. There is at least one
outflow boundary pushing south across eastern Lee and SW DeKalb
that has triggered at least some cumulus towers with a couple
lightning strikes occurring as well. Cannot rule out an additional
isolated small and short-lived thunderstorm in this area prior to
about 9 pm. Thunderstorm activity to our southwest towards the
Peoria area continues to slowly move northeast but will likely
fade not long after sunset. An outflow boundary is also noted
moving northward from Livingston County east-southeast into 
Benton Co. Think it'll be pretty tough to see any development 
along this and it may end up stalling out. With this boundary and 
the boundary that triggered the central IL storms in the vicinity,
will maintain slight chances for shower/thunder in the southern 
part of the forecast area into the overnight. 



151 PM CDT

Through tonight...

For the remainder of the afternoon and into the evening hours, main 
concern will be development and coverage of showers and 

Latest satellite imagery indicates an area of developing towering cu 
and isolated showers along a line from Dixon to Mazon to Rensselaer. 
This line is coincident with an axis of MUCAPE in excess of 3000 
J/kg and weak sfc convergence associated with a weak cold front that 
passed through the region last night.  This boundary is showing 
signs of slowly returning back north, bringing a chance for at least 
scattered showers and thunderstorms into the Rockford and southern 
portions of the Chicago Metro areas.  Convective activity should 
have a strong diurnal trend and expect that showers and 
thunderstorms should increase in coverage through the late afternoon 
hours and then diminish through the late evening hours, with the 
greatest coverage invof the sfc boundary. While severe thunderstorms 
are not expected at this time, high pwats in excess of 1.7" pooling 
along the front could allow for some stronger storms to produce 
locally heavy downpours and strong/gusty winds as well as frequent 


151 PM CDT

Monday through Saturday...

The upper level flow across the northern tier of the county will 
remain quite active, but as the remnants of the tropical system 
currently moving through the southern Gulf states approaches the 
region by late Monday night into Tuesday, this will keep the more 
active and stronger convection to our north on Monday.  We will 
still need to contend with lead energy ahead of this system 
interacting with an increasingly unstable airmass Monday afternoon 
that could trigger some showers and storms. 

The slow forward/northward momentum will an added boost once it 
catches up with the more progressive flow aloft at our latitude on 
Tuesday. So while there is a chance for precipitation late Monday 
night across the southern CWA, it is appearing that system would 
support returning shower/thunderstorm chances Tuesday into Wednesday 
morning. Highest chances would be south of I-80. Will need to 
monitor the potential for heavy rainfall, but with the core of this 
system likely staying just to the southeast, the higher threat would 
be just outside of the CWA. Another day of highs around 90 expected 
Monday, but with precip and cloud cover expected on Tuesday, 
slightly cooler temps in the 80s expected.

The upper ridge will expand over the area Wednesday, and this 
will a very warm and humid low level airmass to stream into the 
Great Lakes Wednesday and through the weekend. The ridge axis will
quickly flatten, and we will remain on the southern edge of the 
active flow across the upper Midwest. This would suggest possibly 
a drier day Thursday, but the theme for the extended will be for 
hot/humid conditions and periodic thunderstorm chances. A weakness
in the low level flow Wednesday could bring some brief lake 
cooling, but with the increasing southwest flow this should bring 
the warming all the way to the shore, conceivably our 
warmest/muggiest conditions thus far this season. The main concern
days are Thursday and Friday, when temps are expected to rise 
well into the 90s with dewpoints in the 70s. Heat indices well 
above 100 would be expected and are appearing increasingly likely.
This heat/humidity could linger into the weekend, but there is a 
possibility for a front to still move through on Friday and bring 
some relief. Either way, most guidance depicts another system 
moving through the northern tier of states this week which should 
make for a continuation of the heat and humidity at least into 
Saturday before we appear to possibly break things down later in 
the weekend. 



For the 00Z TAFs...

647 PM...Forecast concerns include...

Thunderstorms early this evening.
Gusty southwest winds Monday.
Chance of thunderstorms Monday afternoon.

Current thunderstorm activity is expected to dissipate with sunset
this evening. Still a few storms near ord with an outflow that may
allow additional storms to develop in the next hour or so. Another
outflow/enhanced lake breeze is moving west across northwest IL.
Additional storms may also fire on this boundary including
possibly at rfd. As storms collapse they will have the potential 
to produce locally strong winds.

Winds are expected to settle back to light east/southeast this
evening and then turn southwesterly overnight. Southwest winds
will then increase into the 10-15kt range with gusts into the 20kt
range by early afternoon which will continue through sunset Monday

There is a chance of isolated thunderstorms Monday afternoon
across the entire area including the terminals but confidence is
too low for mention with this forecast. cms