Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 211957

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
257 PM CDT WED SEP 21 2016

1134 AM CDT

Second round of thunderstorms continuing to drop south through
northern Illinois this morning with the more intense storms
currently over the far western CWA, along and west of I39. This
stronger development is following higher moisture/instability axis
situated from the west and southwest CWA back west into Iowa and
Missouri. Storms are staying sub-severe once again with only gusty
winds, small hail, and heavy downpours being observed. Although
this trend will likely continue as the storms to drop further south
over the next couple of hours, still a possibility for a stronger
storm that could produce larger hail or winds. Will continue to
monitor this possibility mainly along and west of areas from
Rochelle to Dwight in Illinois. However, would think this would be
limited given well capped environment still in place.



333 AM CDT

Through tonight...

Primary challenge in the short term is with convective chances.

A well anticipated MCS has blossomed over MN and central WI early
this morning. Mean flow would suggest a mainly easterly movement
across central and southern WI, however, orientation of the
instability axis and forward propagating Corfidi vectors suggest
that a more southeasterly motion will occur. This would
potentially place northern IL in the line of fire later this
morning. SPC objective mesoanalysis data suggests that fairly
robust inhibition exists between us and this line, though given
the mesoscale organization that this system is exhibiting am
thinking that there is a pretty good shot that some of the showers
and storms will make it into far northern IL. Have beefed up pops
in anticipation of this occurring this morning, especially north
of I-88 and I-290, but fully expect additional revisions to pops
will be needed.

Questions then become how far south into the area does this
activity make it and how quickly cloudiness clears out in its
wake. Have introduced more of a temp gradient from north to south
for highs today, given potential for more clouds lingering north.
Suspect that this activity should dissipate by mid to late morning
at latest, possibly sooner, but could leave outflow boundary which
would have at least a chance of sparking additional isolated
storms this afternoon. Confidence is low, but did maintain some
low pops north into the afternoon due to this threat.

Tonight, it appears the focus for convection will be north of the
WI border and we should be dry. The activity tonight appears less
likely to propagate south into our area, so Thursday stands a
better chance at being dry and very warm CWA-wide.



253 PM CDT

Thursday night through Wednesday...

Above normal warmth will continue across the region late in the week
and through the weekend with a pattern change set to bring more
seasonable conditions sometime early to middle next week.

Upper level ridge is progged to remain anchored from the lower
Mississippi Valley into the upper Midwest Thursday night through
Saturday night which should provide primarily dry conditions along
with continued warm temperatures with highs around or into the 80s
across northern Illinois and northwest Indiana. Will include slight
chances for showers and thunderstorms across the far northern tier
of counties in Illinois Thursday night through Friday associated
with the surface warm front as it lifts across Wisconsin, but the
best chances will be north of the state line. Meanwhile, deep upper
low currently just off the west coast is progged to slowly make its
way across the inter-mountain west through early Friday, then
will lift across the central and northern Plains over the weekend.
Attendant surface front is expected to occlude and stall over the
region late in the weekend and early next week with the greatest
chance for showers and thunderstorms for now appearing along the
Mississippi Valley through Sunday. There are some pretty large
timing differences on when the front will finally push across the
local area so confidence in the temperatures through the first
half of next week isn`t particularly high, and PoPs are broad-
brushed through that time frame. In general expect 80s for highs
out ahead of the front with highs in the 60s or near 70 behind the
front. If the slower ECMWF pans out, will likely have to bump up
temperatures considerably Monday and Tuesday, and however it pans
out, will have to tighten the temp/precip gradient as we get a
better handle on timing.




Record highs and warm lows:

              Chicago            Rockford
           High      Low      High      Low
Wed 9/21  92(1970) 76(1931)  94(1920) 70(1895)
Thu 9/22  92(1956) 73(1895)  93(1937) 65(1959)
Fri 9/23  91(1937) 71(1891)  92(1937) 66(1920)


For the 18Z TAFs...

Challenging forecast continues over the next couple of hours,
with regards to thunderstorm potential and wind direction/speed.
Latest radar imagery showing a slow moving area of showers and
thunderstorms still dropping south through northern Illinois. At
this time, the strongest thunderstorms are currently away from all
of the terminals, to the west/southwest. This will likely continue
to be the trend but with showers and embedded lightning still
possible for DPA/ORD/MDW over the next 1-2 hours. Isolated showers
may persist for a couple more hours beyond then, but think the
impacts would be limited. Focus for anymore precip will likely
remain to the northwest/north of all the terminals through the
rest of the period, with a very low chance for a possible brief
storm to move across the RFD area tonight. Winds continue to be
highly variable across the area due to persistent precip this
morning. However, think winds will likely hold onto a north to
northeast wind direction for most of the afternoon before turning
more southeast and the south tonight.



253 PM CDT

Very messy wind field across the south end of Lake Michigan this
afternoon behind thunderstorms that occurred earlier this morning. A
meso-high has formed over northern Illinois in the wake of the
thunderstorms and has resulted in a tight pressure gradient across
southern Lake Michigan. There is considerable variability in the
wind speeds and direction with a few gales being reported by coastal
obs and the south buoy. Expect that synoptic southerly flow
should return by mid evening as a warm front lifts back north
across the lake and the stronger winds should diminish over the
next few hours as the meso-high weakens. The front will stall and
push back south Thursday night into Friday as a strong Canadian
high builds over Ontario. A strong northeast breeze will develop
behind the front, then winds will gradually veer to the southeast
over the weekend as another low lifts across the Plains.



LM...Small Craft Advisory...ILLINOIS NEARSHORE WATERS UNTIL 4 PM Wednesday.




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