Area Forecast Discussion
Issued by NWS Chicago, IL

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000
FXUS63 KLOT 251745
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1245 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...
303 AM CDT

Monday through Saturday...

Active weather is expected to continue through the upcoming week
and into next weekend as a series of lows push across the
Midwest and Great Lakes region. Early part of the day Monday
should be dry and mild, but precip chances increase from the
south later in the day as low pressure lifts from southern
Missouri into Indiana late Monday. Sprawling area of high pressure
will build across the Canadian Prairies into the Upper Midwest
Tuesday and Wednesday resulting in a couple days of dry weather.
Persistent north to northeast flow this time frame will result in
onshore flow and cooler temperatures near Lake Michigan. Local
climatology indicates that temperatures struggle to reach the mid
40s for downtown Chicago on late March days with persistent
onshore flow. Bumped down temperatures several degrees below
guidance near the lake...into the mid 40s but may still be too
warm. Farther inland, still anticipate temperatures well into the
50s. Another low will traverse the region Thursday into Friday but
there is considerable spread among the various models and their
ensembles. GFS races a low to the mid Atlantic Coast by Friday
morning while the GEM and ECMWF deepen the low and lift it across
Iowa and Missouri, respectively, Friday morning. Will maintain
chance PoPs late in the week, but confidence in any details is
very low at this point.

Deubelbeiss

&&

.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...
335 AM CDT

Northerly winds in place early this morning, with speeds up to 30
KT over much of the lake. Speeds will likely remain the same with
even the north half observing these 30 KT speeds today, as low
pressure over the Plains slowly moves towards the mid Mississippi
Valley. This will provide continued hazardous conditions for small
craft today into tonight, and Sunday. As this low then tracks
north through Illinois on Sunday, do anticipate speeds to diminish
to the 15 to 25 KT range. This trend will continues Sunday night
into Monday as this low first moves across the lake, and then
departs to the east.

Rodriguez

&&

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

&&

$$

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