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Area Forecast Discussion (AFD)

Issued by Chicago, IL (LOT)

FXUS63 KLOT 180849

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
349 AM CDT Tue Jun 18 2019

349 AM CDT

Through Tonight...

For the remainder of the overnight hours and through Monday, 
conditions will be relatively quiet though as high cloud cover 
pushes off to the east, areas of fog, and some patchy dense fog, is 
developing across the region.  Views from the coastal web cams show 
that the stratus has lowered to the surface.  While a dense fog 
advisory remains in effect for the nearshore waters of Lake 
Michigan, adjacent to nern Illinois and nwrn Indiana, conditions 
will need to be monitored for the potential for the dense fog to 
spread inland.

For the today, conditions will be relatively quiet.  The main 
forecast challenges will be sky cover and the development of a lake 
breeze and the impact on temperatures along the lake front.  Early 
this morning, as the cirrus canopy moves off to the east, some areas 
are seeing low stratus and fog develop and there is some uncertainty 
as to the areal extend and duration of the stratus.  So, will carry 
partly to mostly cloudy skies in the general forecast.  The sfc 
pressure field across the region will be very slack, so winds should 
be lgt/vrbl to light nely.  Weak upper ridging and height rises 
aloft, along with partial clearing should allow temperatures to 
reach close to seasonal normals with much of the area seeing highs 
in the upper 70s to around 80 F.  The weak nely flow and then lake 
breeze development will limit temperatures along the lake front to 
highs only in the upper 50s. A cold front is expected to reach far 
nwrn Illinois by late this afternoon and then slowly sag sewd across 
the region overnight tonight.  Latest guidance suggests that there 
will be weak forcing aloft even weaker sfc forcing with the 
boundary.  There will be a chance for some scattered showers and 
isolated thunderstorms associated with the frontal boundary, but 
instability should be very limited so any convective activity should 
be relatively weak.


330 AM CDT

Wednesday through Monday...

An active pattern returns for this slice of the forecast with 
trends indicating such a pattern may continue through the end of 
the month. First for a cool Wednesday, further refinement in 
details have been made to the rain forecast, with a south of I-80
emphasis in the heaviest rain and thunderstorm coverage 
continued. The peak CWA rain coverage timing looks evening-ish 
Wednesday. A more classic active summer pattern establishes itself
by later Friday through early next week with episodic 
thunderstorms within the region. Some severe weather and flooding
threat is there given pattern recognition and the forecast 
parameter space for instability, shear, and moisture at this 

The primary mid-level short wave for Wednesday is located over
Colorado this morning. As this shifts east in a quasi- 
zonal yet still split jet flow, it will encounter better upper 
jet support from both the northern stream and subtropics. A 
consistently forecast amplification of the mid to lower-level 
features of this wave is shown by Wednesday night as the system
center moves eastward across central Illinois into 
central/northern Indiana. The trend in the guidance mean solution 
the past 24 hours has been slightly south, highlighted by the 
18.00 GFS especially far south. The NAM on the other hand remains 
north, deeper, and slower and more concerning for a pronounced 
heavy rain threat and even a severe threat in the south. Feel a 
mean solution, close to the 18.00 EC captures the more likely 
outcome with what is a fairly progressive wave within a more cool 
season type track.

Scattered showers and possibly some storms will likely be
occurring during the first half of Wednesday. Our area will be 
under the northeast quadrant of this system, with forcing for 
ascent more tied to frontogenesis then deep isentropic ascent. As 
700-850mb winds turn more southerly with stronger moisture return 
occurring into the mid-later afternoon, there should be a fairly 
quick increase in rain coverage that would peak during the 
evening and potentially overnight. The forcing for ascent will be
assisted by a semi-coupled upper jet structure over the region, 
as noted earlier with system amplification during this time. A 
quick surge of moisture transport is predicted into the southern 
CWA, with precipitable waters forecast to around 1.6 inches. How 
far north the heaviest rain shield reaches will be dependent on 
the exact track of the system, but do feel at least showers are 
likely across the northern half of the area. The instability 
wedge aloft will at least make it into the area, but with a moist 
column and marginal lapse rates, the MUCAPE profiles are tall and 
narrow. So feel thunder coverage will struggle, especially north 
of I-80. If a further north solution, such as the NAM were to pan 
out, then thunder coverage would be a higher, and a more effective
warm sector may inch into our far southern CWA for a severe 
concern. If further south toward the GFS, even the southern CWA 
may miss a heavy enough rain for a flood threat.

With the moist advection nose followed by a developing deformation
area, there is increasing likelihood of a 9-15 hour window of 
moderate to heavy rain with potential embedded convection in the 
southern CWA. Considered a Flash Flood Watch for the far south
given the antecedent regional high soil moisture. For now the 
collaborative decision was a Hydrologic Outlook (ESF) for this 
area to highlight awareness on top of other messaging.

As for temperatures Wednesday, increasing northeast winds under a
thickening cloud canopy will probably support cooling in the 
afternoon. Given the current forecast regime, temperatures by mid 
to late afternoon in lake adjacent areas would be falling 
into the upper 50s. Similar to this past Sunday, the wind push 
coming down the lake may be a little stronger and lead to a 
higher rip current risk by late in the day into Thursday morning.

This system will depart the area Thursday morning with northerly
winds in place that likely will be breezy in the morning.
Depending on timing of the clearing, highs in lake adjacent
counties are forecast to struggle to climb out of the 60s.

A longwave trough is then forecast over the western CONUS,
comprised of a slow-moving northern Rockies closed low. The 
forcing ahead of this will support multiple surface lows Friday 
through at least Sunday, tracking northeast across the 
Plains/Midwest. The low-level mass response ahead of this 
persisting for several days should draw ample moisture northward 
too, with near 70 dew points forecast by a guidance blend by 
Saturday. The pattern on Friday afternoon into Friday night does 
indicate some potential for robust convection upstream that 
could track into the area depending on how quickly progressing the
instability and moisture return is. Saturday looks potentially 
quite active in the Corn Belt to Upper Midwest region, but as is 
typical with an active pattern, much will be modulated on previous
rounds of convection. Also it still is five days out so plenty of
specifics will be ironed out over time and major shifts in timing
of convection certainly may occur. Will continue noting the active
weather pattern in Hazardous Weather Outlook and start in Weather
Story graphics. As for temperatures, there will be some influence
from convection, but more summer-like nighttime lows appear 
likely if not too rain-cooled, and sustained highs into the 80s 
are more favored.



For the 06Z TAFs...

Primary forecast for the overnight hours will be lowering stratus
and fog development. Main concern for the daytime hours will be
timing of improvement of cigs/vis as well as lake breeze
development and timing of a wind shift at the Chicago area

Cigs have quickly trended down to ifr at ORD/MDW and LIFR at GYY.
Latest satellite imagery shows the low stratus gradually spreading
inland and it may reach DPA as well, though timing there is still
uncertain and the latest high res guidance is not handling the
status as well as the past couple nights. Based more on
persistence, have trended cigs a little lower than the previous
update and it is possible that cigs at ORD/MDW could drop to 300
ft before daybreak. Fog trends are also uncertain, but based on
observations from lake front web cams indicating only low stratus
and little visibility restriction, and the extent of high cloud 
over the area, it's possible that the dense fog may be confined to
GYY as far as the terminals are concerned and may be more patchy
elsewhere, favoring low-lying, sheltered locations.

With a very weak pressure gradient over the region, expect that
winds should be mainly light and variable overnight and well into
the daytime hours. This weak gradient should help set up a lake
breeze and expect it to push inland through ORD/MDW by late
afternoon, though with a synoptically northeast wind direction,
the lake breeze may be more of a surge to 8-9 kt and a slight
direction shift from nely to enely-ely.


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