Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 272337

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
637 PM CDT FRI MAY 27 2016

300 PM CDT

Through Tonight...

MCV from earlier morning activity in central and southern Illinois
is moving northeast through the area this afternoon. This activity
is moving into a weakly unstable environment, but is struggling to
grow into deeper convection given this weaker instability, deep
layer shear under 30 kt, and mid level lapse rates that are poor
as well. Expect shower coverage to hold at its current levels or
increase some in the coming hours, with some isolated to scattered
thunder activity as well. The severe threat today is low and
reflected in the marginal risk for severe storms focused farther
west. The concern for scattered storms this afternoon/evening will
be localized heavy downpours in the moisture laden and weak storm
motion environment...along with some gusty winds to 40 mph.
Slightly better instability is found in east central Illinois and
northwest Indiana.

There is another wave moving through the bootheel of Missouri and
southern Illinois that will rotate northeastward tonight ahead of
the negatively tilted trough in the plains, that will keep the
chance for showers and isolated thunderstorms through the evening
and we may see another uptick in activity during this time, but
likely less so overnight as the wave appears to weaken some and
the focus shifts towards the stronger forcing associated with the
upper trough itself. Confidence on coverage during this time is



300 PM CDT

Tomorrow through Friday...

The pattern remains active through late next week. Confidence in day
to day convection is not overly high because forcing is weak in most
cases. The atmosphere looks relatively primed for convection,
especially in the near term, but have less confidence in forcing.
Confidence decreases rapidly Thursday night through next Friday due
to model differences.  Highs will remain in the 80s through most of
next week. Lake breezes are possible next week leading to highs in
the upper 60s along the lake.

Saturday is looking very similar to today as far as precip is
concerned. A weak vort streamer lifts north through the day and
expecting convection to increase in coverage to scattered in the
afternoon. Kept PoPs in the generic chance range since the pop up
convection will be hard to catch deterministically in the grids.
Main threat will be heavy rain and perhaps a few will produce gusty
winds. CAPE values look to be less than 1000 J/kg with weak shear.
Pwat values remain high though around 1.75 inches. Storm motion
looks a little better with southwest to northeast movement at around
30-40 kt.

Convection dies off diurnally Saturday night...but left over
convection from the west shifts east through the night. Expecting
the storms to diminish as they do so. Could see some gusty winds as
the storms first breach the western CWA, as depicted by SPC`s day 2
marginal risk.  Areas along and west of I-39 are included in the
marginal risk.

Guidance differs more than I would like with the main upper level
trough Sunday. The GFS is further south than the other models and
thinking precip will be closely tied to the trough. The better storm
coverage will be over Wisconsin...but storms are possible mainly
north and along I-88. Kept a slight chance along and north of I-80
in case the GFS is right. Lifted Pops even further north Sunday
afternoon as the trough lifts northeast with a slight chance along
the IL/WI state line and east of I-55.  CAPE values are forecast to
be below 1000 J/kg again with even weaker shear so not expecting any
severe storms.  Forecast Pwat values are around an inch...but
thinking storms still may produce heavy rain.

We get a break in convection Sunday night through Monday morning!
The GFS and ECMWF bring convection associated with a shortwave into
north central IL Monday afternoon. Have medium-low confidence in the
convection making it into the CWA so capped PoPs in the low chance
range. The GFS is more aggressive than the ECMWF so also limited how
far east the precip gets.  Monday should be mainly dry for most if
not all locations.

Tuesday may be dry, but the GFS features some upper level vort
streamers that may help convection get going. Kept a chance of
thunderstorms in the forecast but have low confidence in PoPs
through the afternoon. Guidance is consistent with additional
convection proceeding an approaching cold front passing through
Wednesday night and Thursday. The GFS is still the most aggressive
with the precip since it has a stronger second upper level low
pressure trough over the plains.  Have low confidence in PoPs
Thursday night into Friday given the drastic differences in
guidance`s upper level patterns. The GFS has a cutoff low over
Kansas while the ECMWF has a closed low over Ontario and a trough
axis moving through the region late it in the week.  Moral of the
story is the pattern remains busy but exact details mid to late next
week are very fuzzy.  Temps in the 80s look solid...with a psbl cool
down into the 70s late next week.



For the 00Z TAFs...

Unsettled weather expected to continue through the TAF cycle.
Isold SHRA could continue to pop up early this evening, but more
organized area of SHRA/Isold TSRA over central IL should lift
north and affect the terminals mid-late this evening. TS will
probably dissipate before this convection arrives. While
additional SHRA cannot be ruled out overnight through Sunday
morning, chances appear low with dry conditions probable. Some
potential exists for sct`d SHRA/TSRA Sunday afternoon and evening
but confidence is low, so kept TAFs dry for now.



304 PM CDT

We have expanded the dense fog advisory for the northern two
thirds of the lake into Saturday as warm moist air runs over the
colder lake. It is not until midday before guidance begins to
erode the fog over the lake. Have kept a single advisory through
mid afternoon for the area for now, though it could erode a bit
sooner in the south and make take a bit longer for the north
portion of the lake. Otherwise, expect some showers and
thunderstorms through this evening, with lower coverage in the
coming days.

Otherwise low pressure will continue to slowly move toward Lake
Superior through the weekend before a weak cold front shifts winds
to light northerly with high pressure.






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