Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 252000
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
300 PM CDT Sat Mar 25 2017

.UPDATE...
1113 AM CDT

Overall the forecast is in fairly good shape, especially with the
expected warm front placement and temperatures. Have just adjusted
hourly trends especially north of I-88 and near the lakefront to
keep steady for a few more hours. Have also added a little more
definitive break in shower activity into early afternoon. Do
expect scattered showers and some thunderstorms to develop across
central Illinois this afternoon and early evening and track
northward across the forecast area. While the general severe
weather threat is low, will have to keep an eye on activity
along/south of I-80 for a conditional low instability/modest
shear isolated severe threat given the strength of the warm
frontal boundary.

Observations and satellite indicate the warm front draped from
near Peoria, IL eastward across the far southern forecast area to
near Rensselaer, IN. While the closed and stacked low over
central/southern Missouri is not moving much north of east nor
fast, the warm front has moved slowly northward likely aided by
upper support. In addition, clearing/thinning of some of the
clouds has spread north of the boundary, allowing for some solar
heating. It remains low confidence to say how quickly this front
will move north, but am anticipating the boundary to be near I-80
mid-late afternoon. Temperatures south of the boundary should
climb well into the 60s to low 70s. North of the boundary, little
recovery is expected near the lake (39 in downtown Chicago right
now) with a tad more recovery inland. If any of the clearing
continues further north, that would allow for temperatures to jump
quite a few degrees.

GOES-16 visible channel indicates some agitation of clouds across
southern/central Illinois in the past hour, though overall not
the most cumuliform look for most of upstream at the moment. While
weak lapse rates in high PWAT air for March (1-1.25 in.) will not
support much deep CAPE (250-500 J/kg), upward motion will become
better favored for continued scattered development with emanating
subtle short waves. Any low topped storms that do develop and
near/cross the boundary (i.e. conditional on those two
occurrences) in the southern forecast area may exhibit brief low-
level rotation. In addition, wet bulb zero values of 7500-9500 ft
could allow for pea size hail with any activity today.

So overall continue the forecast theme of periods of showers with
isolated storms mid-afternoon through the evening.

MTF

&&

.SHORT TERM...
303 AM CDT

Through Sunday night...

Showers and thunderstorms will continue across the region at times
throughout the weekend with a large north to south temperature
contrast expected thanks to a frontal boundary draped over the
area.

Early morning water vapor imagery shows a closed upper low
centered over central Oklahoma with moisture from the Gulf of
Mexico streaming northward towards the Great Lakes region. Over
northern Illinois, a surface cold front stretches from near BMI
east-northeast to RZL. Temperatures north of the front have
fallen into the 30s and 40s, while to the south, temperatures have
stayed in the upper 50s and 60s overnight. The front has surged
much farther south than was originally anticipated, and
unfortunately models do not have good agreement on how much if any
the front will move through the day today. The GFS lifts the
front back to near the I-88 corridor by mid to late this
afternoon, while the NAM shows little northward progression
through the day. This results in lower than average confidence in
temperatures through the day especially across the Chicago Metro
area which will likely see a 15-20 degree temp difference from
north to south. For now, leaning towards the NAM which verified
better on frontal position yesterday and has decent agreement with
the ECMWF but has a better handle on temps near the lake.

Meanwhile, the upper low over OK is progged to lift to central MO
by mid afternoon and a corridor of more tightly packed pressure
contours on the 295-305K isentropic surfaces lifts across the
region. This should lead to an increase in shower and thunderstorm
coverage roughly 9-12Z across the south lifting towards the WI
state line by 18-21Z. Afterwards, there may be a lull in activity
or at least lower coverage in precipitation as the forcing becomes
more nebulous. Would expect to see at least some continued spotty
activity as broad ascent associated with a persistent diffluent
region aloft and continuing mid level height falls, along with low
amplitude vort maxima emanating from the upper low move into the
very moist albeit weakly unstable environment that will persist
throughout the day. Overall thunderstorm threat is not too
terribly impressive due to weak mid level lapse rates, but
guidance continues to hint at weak MUCAPE <100 J/kg that could
result in a couple isolated thunderstorms given the ample forcing.

On Sunday, upper low is expected to lift to Central Illinois by
midday then shift to northwest Indiana Sunday evening. Showers and
possibly a couple thunderstorms will continue as the upper low
traverses the area, but anticipate precipitation to begin winding
down from the west through the afternoon as the 500mb trough axis
pass through and eventually mid and upper level height
rises/modest ridging builds in during the evening. Still a little
uncertainty in surface front placement, but models in a little
better agreement with the front lifting to or close to the WI
state line through the day Monday which should allow the upper 50s
and low 60s to overspread the CWA.

Deubelbeiss

&&

.LONG TERM...
252 PM CDT

Sunday through Saturday...

Main forecast concerns for the longer term forecast period will
continue to be timing of a series of systems bringing periodic
chances for pcpn to the region as well as temperature trends.

The latest guidance continues to indicate a series of southern
stream systems lifting out of the swrn CONUS and tracking across the
region.  The models remain relatively consistent on this trend,
though some differences in the track remain, with the GFS
trending a little south of the NAM/ECMWF, the general idea of the
associated sfc low lifting tracking from the middle Mississippi
Valley and across nrn IL/srn WI tomorrow remains consistent with
previous runs. As the low tracks slowly to the northeast, an
associated warm that is pushing nwd through IL/IN today will be
north of the CWA, the warmer air overspreading the region today
will remain in place through the day on Sunday, allowing
temperatures to rebound back into the lower 60s. Instability will
be on the increase in advance of the system and at least some
partial clearing along with forcing from the sfc low and mid-level
shortwave will keep some chance for thunderstorms over portions
of the area through the day tomorrow. Generally expect the greater
chances for TS to be near the greater forcing of the sfc low/mid-
level shortwave. For timing, expect that the most favorable window
of opportunity for TS will be through late morning or early
afternoon, with the system lifting to the northeast durg the
afternoon hours. There should be a grief lull in pcpn sunday
night as weak shortwave ridging crosses the region in advance of
the next system. The models are in a little better agreement on
the track and timing of the next system. With strong high pressure
building out of central Canada and into the nrn plains/upper
Mississippi Valley, the track of this system should be more to the
south, through srn IL/IN and the Ohio Valley. The most widespread
pcpn with this system Monday into Monday night should be over
portions of the area south of the I-80 corridor, but there should
still be some light rain over the nrn portions of the CWA as well.
Temperatures on Monday should still trend higher than normal,
with much of the area seeing temps in the upper 50s to lower 60s,
but winds on Monday should trend toward nely through the day,
which would keep the far nrn portions of the CWA and the lakefront
a bit cooler, as flow turns off of the cool Lake Michigan waters.
Lakefront locations should should see temperatures only in the
upper 40s to lower 50s. High pressure will then spread east across
the upper Mississippi Valley and the Great Lakes region through
midweek, keeping generally northerly winds in place through much
of next week. The temperature trend through next week should be
for warmer conditions inland and cooler near the lake. As high
dominates the region through midweek, conditions should generally
be dry. The next significant chance for pcpn will not come until
late next week as another srn stream system lifts out of the swrn
CONUS.

&&

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

The concerns with the Chicago area TAFs are:

- Northeast winds through tonight with sporadic gusts this
  afternoon.

- Isolated SHRA early this afternoon gradually upticking in
  coverage by late day into the evening, with potential for
  isolated TSRA.

- 400-700 ft cigs probable to continue through tonight, with
  potential overnight and Sunday morning to dip just below this,
  but confidence in that occurring and for how long is low.

- Potential for more showers on Sunday with winds veering to
  south, though low-medium confidence in when that shift occurs.

Low pressure across Missouri this afternoon will move to far
western Illinois by daybreak Sunday and to near/just north of
Chicago by mid-afternoon Sunday. The warm front with this remains
south of the TAF sites this afternoon, with northeast flow locking
in lower clouds. Webcams and observations indicate 400-600 ft
cigs from ORD and MDW back to the lake, so do not see these
changing much. Would think the better potential to dip a tad lower
is tonight, especially in-between any showers overnight. Cannot
rule out 200 ft given the synoptic scenario and bases already at
400 ft, but feel it is a low chance right now.

As for showers, isolated to scattered activity is expected to
develop this afternoon and pass over northeast Illinois, though
the highest coverage is not expected until late day into the
evening. Some of this may contain thunder, though the instability
is quite limited, so did not have confidence enough to include in
the TAFs. Confidence in shower coverage overnight into Sunday is
lower, though at least isolated should be present.

There is uncertainty on how far north the warm front will reach
tonight, which is key because south of this boundary conditions
are likely to be VFR. The front reaching GYY is a real
possibility, even by late afternoon. As for MDW and ORD, it may
get pretty close, but feel that winds should continue out of the
northeast/east and maintain the low clouds and occasionally MVFR
to IFR visibility.

MTF

&&

.MARINE...
300 PM CDT

Winds are trending to easterly today with speeds up to 30 KT over
much of the lake as low pressure over Missouri slowly moves
across the mid Mississippi Valley tonight. Hazardous conditions
for small craft will persist through tonight, mainly for the
Illinois nearshore waters in the persistent east flow. As the low
then tracks north through Illinois on Sunday and central Lake
Michigan Sunday night, anticipate that speeds will diminish to
the 10 to 20 KT range. High pressure is expected to build across
the upper Great Lakes region Tuesday through Thursday setting up
generally north to northeast winds over the lake with more
moderate wind speeds.

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...Small Craft Advisory...IL nearshore waters until 5 PM Sunday.

&&

$$

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