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Area Forecast Discussion (AFD)

Issued by Chicago, IL (LOT)

FXUS63 KLOT 172350

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
650 PM CDT Sat Aug 17 2019

228 PM CDT

Through Sunday...

Several forecast challenges/concerns this afternoon through Sunday 
morning, initially with respect to any additional scattered 
shower/storm development this afternoon and then more widespread 
shower and storm potential overnight into early Sunday morning.

In the near term, focus revolves around another incoming mid-level 
wave across eastern Iowa this afternoon, which is expected to 
gradually shift eastward across northern IL through the remainder of 
the daylight hours. Despite the atmosphere being considerably worked 
over this morning, a cu field is already evident across parts of 
eastern IA and northwest IL out ahead of the wave in an increasingly 
unstable environment, along with some isolated to scattered radar 
returns (no lightning as of yet). Given the anticipated arrival of 
greater support aloft and some recovery across northeast 
IL/northwest IN, will continue the mention of scattered showers and 
thunderstorms across the forecast area this afternoon...most likely 
between 20z and 00z before support strips off to the east and a lull 
in activity is expected to become the rule this evening. There 
remains a non-zero chance through this afternoon for a stronger 
storm or two, mainly capable of producing gusty winds and perhaps 
some hail.

However, focus rather quickly shifts to the tonight into early 
Sunday time frame as an active pattern is expected to remain in 
place. Additional, potentially widespread, showers and storms appear 
likely during this time frame as no real change to the overall 
pattern and environment is anticipated, with highly active mid-
levels and strong warm air advection supported by a stout LLJ 
expected to be in place across the region. Latest trends continue to 
suggest a complex of thunderstorms developing across southern 
MN/Iowa late this evening into the early overnight hours before 
tracking eastward toward the CWA toward sunrise Sunday. While 
confidence isn't stellar, feel the most likely timing for storms to 
track through the region is somewhere between 09-14z, with most 
guidance in agreement with this timing. The exception is recent HRRR 
runs, which have significantly slowed shower/storm chances to after 
12z/Sunday. there will remain a severe weather threat with these 
storms, primarily in the form of strong/damaging winds, and to a 
lesser extent, large hail. Given lots of deep layer moisture (PWATs 
progged AOA 1.75 inches), heavy downpours with at least an isolated 
flooding risk will also be a concern.

Beyond Sunday morning, would expect some drier conditions to return 
for part of the day, although with lingering surface trough/boundary 
across the region, wouldn't be surprised to see an additional 
isolated shower and/or storm develop late in the day/early evening. 
High temperatures Sunday expected to range from the low-upper 80s 
across the forecast area with afternoon heat indices near 90 
degrees for many areas.



226 PM CDT

Sunday Evening through Saturday...

A surface trough will cross the area Sunday evening into Sunday
night. This will lead to lingering chances for isolated to
scattered showers and thunderstorms during the evening, with the
threat decreasing from northwest to southeast into the overnight.
Surface high pressure will move in from the west Monday and depart
Tuesday. The high will bring modestly drier air with it and
somewhat cooler temps for Monday with highs in the lower 80s north
to mid 80s south. Winds shift southerly Tuesday bringing warmer
and more humid back into the area. An upper trough will cross the
area with a weak surface reflection accompanying it. Will need to
see what kind of thunderstorm chances will be present with this,
but guidance seems to be more focused on something tracking more
to the south of the area. Will keep chance pops going Tuesday
night and Wednesday morning. A deeper upper low will move across
Ontario Wednesday pushing cooler and drier air into the local area
as another surface high builds in, this time from the north.
Guidance is at odds with some of the details with the GFS favoring
slightly active upper flow which suggests subtle chances for
shower/thunder development late this week. Will continue with a
dry forecast from Wednesday afternoon into the weekend but
continue to keep an eye on how the pattern evolves. 



For the 00Z TAFs...

650 PM...Forecast concerns include...

Possible mvfr cigs this evening.
Isolated showers/thunderstorms this evening.
Thunderstorms Sunday morning.
Strong/gusty southwest winds Sunday afternoon.

There is still a chance for an isolated thunderstorm this evening
with a cell near dbq to the west and another near beh to the east.
Not currently expecting anything more than isolated coverage so
have left out mention from the tafs except for vicinity shower 
mention at rfd this evening. Focus then shifts to the predawn
hours Sunday morning when a broken line of thunderstorms is
expected to move across northwest IL and then across the rest of
the terminals through mid morning Sunday. These storms will 
likely have some gusty wind potential as well as possible ifr 
cigs/vis. Confidence is low for additional convection Sunday 
afternoon into Sunday evening. It appears any new thunderstorms 
Sunday afternoon would be east or south of the terminals with 
perhaps a better chance for thunder later Sunday evening.

There is still patchy mvfr cigs across the area early this evening
and these are expected to move northeast and scatter within the 
next few hours.

Wind will remain south/southwest under 10kts this evening...likely
shifting a bit more southwest ahead of the expected thunderstorms
Sunday morning. Wind directions behind the storms will be 
dependent on how widespread the activity becomes but a 
south/southeast direction for a short time is likely before winds 
shift to the southwest. Forecast soundings show quite a bit of 
wind in the low levels through mid afternoon. The sooner clouds 
clear and mixing begins the more likely gusts could be stronger 
than currently indicated. Wind speeds/gusts will diminish with 
sunset Sunday evening. cms