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Area Forecast Discussion (AFD)

Issued by Chicago, IL (LOT)

                            
000
FXUS63 KLOT 221737
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1237 PM CDT Sat Jun 22 2019

.UPDATE...
1133 AM CDT

Going forecast appears to be in fine shape with no appreciable
changes planned. Have seen a couple showers percolate up out of
the ACCAS deck over our far southern CWA, expect this activity to
continue to wane. Upstream, regional radar and GOES-16 imagery 
show a well defined mesoscale convective moving into central IA 
with a arc of convection out ahead of this feature. Haven't found 
any guidance that captures this feature, lending to lower than avg
forecast confidence. Given how well defined this feature is, have
a hard time believing it won't last through the afternoon. As it 
moves east, it should gradually be progressing into a more stable 
environment, so think that the most likely impact from this 
feature beyond an increase in cloudiness would be some scattered 
showers or sprinkles moving into our western CWA. Can't completely
rule out sufficient destabilization taking place and allowing for
an isolated storm toward evening in advance of this MCV, so no 
change

- Izzi

&&

.SHORT TERM...

Through Monday...

300 AM...Primary forecast concern and challenge is thunderstorm 
timing...as well as associated severe/heavy rain risk.

A large area of thunderstorms is currently moving east/southeast
across central IL and should remain mainly south of the cwa this 
morning but isolated showers and perhaps an isolated storm will be
possible through mid morning across the southern cwa. Recent 
development near I-80 in eastern IA will need to be monitored for 
possibly pulling pops a bit further north as trends emerge. Once 
this activity ends later this morning...the bulk of the daylight 
hours appear dry with only isolated showers maybe a storm in the 
southwest cwa toward sunset as shown by some of the high-res 
guidance. Have maintained slight chance pops for this possibility 
late afternoon into the evening but confidence is low.

Northeast flow has currently advected upper 40 dewpoints into the
northern cwa while temps are holding steady near 60 with the thick
cloud cover overhead. Eventually winds will turn a bit more to the
southeast with dewpoints slowly rising back into the 60s. Guidance
is fairly consistent with highs back into the mid/upper 70s most
areas and with any peaks of sunshine some areas might tag 80. IL
lakeshore will remain cooler with the onshore winds.

Chances for showers/thunderstorms begin to increasing this evening
with a reasonable consensus for an upper wave moving across the
area overnight but there are still some timing differences...as
early as late evening or as late as daybreak Sunday. Have shown an
increase in pops through the evening and maintained likely pops
overnight. Main concern during this time period would likely be
heavy rain with precipitable water values reaching into the 1.7 to
perhaps 1.9 inch range. Timing however may play a role into how
widespread the rain becomes...a later timing may mean more of a
weakening trend. Severe potential appears limited to the western/
southwest cwa within the better instability axis with strong wind
gusts the main threat but potential looks low.

Convective trends on Sunday will be quite dependent on how
activity unfolds early Sunday morning. However...there has been 
some trend for thunderstorms to develop during the afternoon right
across the cwa as weakening low pressure moves across northwest 
IL/southern WI. With expected clearing skies from morning storms 
and temps rising into the lower/mid 80s...modest instability and 
shear should allow some thunderstorms to form but how widespread/
organized they become is uncertain. Deeper mixing by afternoon 
could also result in a breezy/windy afternoon/early evening.

Sunday night looks potentially unsettled as the main low pressure
lifts from IA Sunday evening to the northern Great Lakes Monday 
night. Some warm air advection precip seems possible Sunday night
but low confidence from this distance. As a trailing cold front 
from the low moves across the area on Monday...additional showers
and thunderstorms are expected. Timing of this front will be key
in how widespread/strong any activity will become with the best
chance across the eastern cwa and in northwest IN. Monday
continues to look potentially breezy/windy...especially behind the
front. cms

&&

.LONG TERM...
300 AM CDT

Monday night through Friday...

Big story this period will be the building ridge and with it 
transition to very much summer like weather for mid and late week. 
With this transition also comes a period of at least mostly dry 
weather... to the likes of which we've not seen in some time. 

Models continue trend of building broad upper ridge across the 
central CONUS through the Tues-Fri period... and with this shifts 
the jet stream north of our area. Guidance suggests a couple of 
weak upper level shortwaves will ride over the ridging pattern 
during the 4 day period... bringing a small chance of showers and 
thunderstorms each time. Differences in the guidance exist in the
timing of these features unfortunately means very low pops over a
good part of the period however most of the period will be dry, 
and perhaps even all of the period dry for some locations. 

Temperatures will climb above seasonal averages for the last week of 
June... with highs generally in the mid and in some cases upper 80s 
Wed-Fri.  Weak high pressure at the surface will mean a weak 
gradient and hence lake breezes Wed-Fri bringing modest cooling 
lakeside.   

Ed F

&&

.AVIATION...
For the 18Z TAFs...

VFR conditions with easterly winds expected through this evening.
Cannot rule out a light shower or sprinkle early this evening out
toward RFD, but chances look low enough to omit this from the TAF.
Band of SHRA and sctd TSRA could move across the terminals early
Sunday morning, followed by a break, then another chance of SHRA &
TSRA in the afternoon. Covering both chances with PROB30 for now,
but as confidence likely grows in time, these will probably be
transitioned into a TEMPO or prevailing group. 

- Izzi

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...None.
&&

$$

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