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Area Forecast Discussion (AFD)

Issued by Chicago, IL (LOT)

FXUS63 KLOT 180834

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

931 PM CDT

Going forecast on track and no big changes needed this evening.
Will be expanding the higher coverage (areas of) fog to the
remainder of the CWA. While confidence isn't terribly high in fog
development, the low dewpoint depressions, light and forecast to 
get lighter winds, and large breaks in the stratus would seem to 
suggest a somewhat greater potential for more coverage of fog 
overnight. Certainly don't have the confidence for a headline, but
potential is there for dense fog to develop tonight and will be 
something for the oncoming overnight shift to keep an eye on. 

- Izzi


156 PM CDT

Through Tuesday night...

High pressure expected to be situated across the region for much 
of the period, with mainly dry conditions persisting. Any 
showers/thunderstorm developing along surface trough/front to the 
south this afternoon and evening will likely remain to the south. 
Additional showers/thunderstorms expected to develop along 
approaching surface trough over IA and southwest WI on Tuesday. 
Once again, this precip will likely stay just to the 
west/northwest of the CWA. Additional development expected to 
remain focused on this feature, but expect this axis to begin 
drifting into parts of northern IL Tuesday night. Thunder will be 
possible, however, instability is not looking overly high at this 
time. Some potential for some clearing skies this afternoon, but 
think cloudy skies will continue to be the trend into tonight and 
Tuesday. With the cloudy skies likely lingering on Tuesday, did 
lower high temps. If cloud cover holds on longer, temps will need 
to be lowered even more. Did add fog for tonight into Tuesday 
morning, as the pattern doesn't really look to change too much. 
Similar coverage is possible, and will once again need to monitor 
for the possibility of dense fog.



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.