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Area Forecast Discussion (AFD)

Issued by Chicago, IL (LOT)

FXUS63 KLOT 161151

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
651 AM CDT Sun Jun 16 2019

304 AM CDT

Through Monday Night...

This Father's Day will be on the cooler and cloudy side,
especially for north of I-80. Initially, there will be a 
continued fog concern, especially near Lake Michigan this 
morning. Spotty showers will be departing early, but isolated 
redevelopment might occur by early to mid afternoon, and a few 
thunderstorms are possible mainly south of I-80.

A well-defined but gradually weakening MCS that progressed 
through the area late last night has left a moist air mass in its 
wake. With light flow, some fog is starting to redevelop early 
this morning. Given an early sunrise and still ample cloud cover, 
any dense fog should be patchy for most inland locations. Near 
Lake Michigan where offshore water temperatures are in the 50s, an
air mass that is more readily able to saturate and did last 
evening may present dense fog again by daybreak. So have continued
Dense Fog Advisory for near the lakeshore at this time. Will see 
how conditions are at 6 a.m. and if conditions are vastly better, 
will let it go. 

Otherwise across the area early this morning, a low amplitude mid-
level short wave is moving across northern Illinois resulting in
some spotty showers. These should end by mid-morning with slight
height rises then drifting across the area and likely little to no
activity in the way of precipitation for at least several hours.
Low-level clouds are fairly thick across the area, with an
aircraft sounding near MDW early this morning indicating a depth
of around 3,000 ft. Annual peak solar heating will start to 
mix/erode this, however the northern CWA is under an area of 
light cloud-bearing flow. Slight convergence and moisture 
layering within the 850/925 mb low centers that are slowly 
drifting overhead should also keep condensate in place for clouds
longer than would typically be expected. So have trended cloudy 
all day north of I-80, which with northeast winds of 10 mph 
(higher near the lake) will keep highs about 10 degrees below 
normal if the going forecast of upper 60s to lower 70s is correct.
Near the lake the most likely high temperature outcome is the 
lower 60s. Along/south of I-80, observational trends would 
indicate some sun should break out and cannot rule out even going
mostly sunny for a period. If this pans out, 80-85 for highs 
should be reached.

In terms of showers, there should be little in the way of forcing
aloft this afternoon. The effective front/composite outflow 
boundary from last night's activity will be in the far southern 
CWA or just south of it this afternoon. Despite that, some low-
level convergence is indicated further north, where winds turn 
from west-northwest to north-northeast. This could result in 
isolated showers. Further south between that area and the front, 
isolated thunderstorms are possible as MLCAPE values look to build
to 1,000-1,500 J/kg. Deep layer shear values AOB 25 kt will keep 
the strong to severe threat low, and again with little in the way
of upper forcing, convective coverage should be isolated. Any 
convection should move southeast into/through the evening.

The cloud cover forecast lowers in confidence tonight into 
Monday. Much of the available guidance is pessimistic through 
a lot of that time despite climatology weighing heavily against 
that, especially with Monday being precipitation-free. If staying
broken to overcast in coverage, clouds will impact temperatures 
both tonight and on Monday. With northeast winds in place on 
Monday, highs should still be below normal even if conditions 
trend sunnier. Do think there will scattering of the clouds, and 
have mainly mid 70s away from the lake. If clouds were to hang in 
there until late in the day and then clear, with high pressure 
moving overhead Monday night there may be some risk for fog



328 AM CDT

Tuesday through Saturday...

A quasi-zonal and not overly fast-paced upper flow will be present
during midweek, before both slightly more amplified and stronger 
flow are indicated, which could make for an active end of week 
and/or next weekend.

Tuesday will find the remnant surface front to the south of the 
area with weakening high pressure over the CWA. While some weak
instability is forecast, there looks to be little in the way of
forcing aloft or low-level focus. Late Tuesday night through
Wednesday, a gradually amplifying/strengthening short wave is
forecast to move across the middle of the country in the GFS and
EC solutions. The present track of this would keep the moderate
instability values just south of the CWA for a severe 
thunderstorm threat. However, precipitable waters in excess of 
1.5 inches return northward and the 925-700 mb moisture transport 
nose and convergence along it could be centered over the CWA 
potentially bringing a heavy rain risk. Over half of the 00Z GFS 
ensemble members provide in excess of an inch of rain in 24 hours 
across the southern half of the CWA...a decent warm season signal 
for three-four days out. At some point Thursday, possibly even 
beforehand, the area looks to get on the backside of this system 
with diminished rain chances.

Temperatures should be able to bounce back to normal range on
Tuesday, and if Wednesday has prevalent clouds and showers then
highs would probably dip back into the 70-75 arena.

For Friday into next weekend there is a fair amount of spread 
even in the 16.00Z GFS and EC deterministic solutions, however 
both would provide a convectively active pattern in the middle of 
the country, especially the EC with a longer wave trough over the
western CONUS. For now have trended a little warmer and preserve
thunderstorm chances within that time. Mass fields of flow and 
moisture would support some severe weather threat, possibly multi-
day if the EC were to pan out.



For the 12Z TAFs...

Under persistent nnely flow off of Lake Michigan, IFR/LIFR CIGS 
along with IFR/MVFR vis and patchy drizzle will continue through 
the morning, with only gradual improvement to lower end MVFR 
conditions by early afternoon. Weak low pressure will slowly track
south of the terminals across cntrl IL/IN today, keeping winds 
generally under 10 kt with a nnely direction through the period. 
Following what little diurnal warming is expected today, 
conditions should settle back down to IFR/LIFR again overnight 


IL...Dense Fog Advisory...ILZ006-ILZ014 until 10 AM Sunday.

IN...Dense Fog Advisory...INZ001-INZ002 until 10 AM Sunday.

LM...Dense Fog Advisory...nearshore waters 
     until 10 AM Sunday.