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Area Forecast Discussion (AFD)

Issued by Chicago, IL (LOT)

FXUS63 KLOT 180534

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1234 AM CDT Sun Aug 18 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 06Z TAFs...

Numerous aviation weather concerns through the upcoming TAF
period, including:

-Potential for splotchy MVFR-IFR cigs/vsbys tonight, mainly near
 DPA but possible at all sites
-Timing an anticipated line of arcing showers and thunderstorms
 late tonight and into Sunday morning
-Cessation of TS chances
-Gusty southwest winds this afternoon
-Potential for additional showers and storms this evening and
-Northwest wind shift with a cold front late tonight

Things are relatively quiet at this hour at our area terminals,
although things are expected to change pretty rapidly after about
10-11z. Mostly clear skies combined with generally light southerly
winds and dewpoint depressions in the 3-5 degree range may support
some BR/FG potential. Currently think the greatest chances exist
at DPA and GYY, but an outside chance exists at ORD, MDW, and RFD.
This should be relatively short-lived as incoming convection will
disrupt any fog formation. 

Timing out an area of strong to severe convection currently
rolling through NW Iowa and south-central Minnesota yields
approximate arrival times at RFD towards 10-11z, and into the
Chicago-area terminals around 12-13z or so. Pure extrapolation is
challenging at this juncture, however, as some additional forward
acceleration is possible, followed by a slowing as the MCS 
attempts to build into a slowly veering low-level jet. So minimal 
changes were made to the inherited TAF timing for TSRA as a 
result. Additionally, elevated convection may develop well ahead 
of this incoming complex, hence the buffer zone with VCTS (and a 
longer window at RFD). 

After this complex clears the region, breezy southwesterly winds
are expected this afternoon although the magnitude of any gusts
remains uncertain due to a stout inversion just aloft. Think
occasional gusts to 25 kts will be within reach, however. 

Think there is at least a low potential for additional showers and
storms to percolate Sunday evening and overnight, mainly INVOF 
the Chicago-area terminals. Confidence in this, however, remains 
too low to warrant a VCTS or VCSH mention but this will be 
monitored for potential inclusion in subsequent TAFs. Finally, a 
cold FROPA will bring a north to northwest wind shift the the area
terminals Sunday night.