Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 261805

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
105 PM CDT Sun Mar 26 2017

1116 AM CDT

Apart from some light showers and patchy drizzle the main body of
heavier precipitation has lifted north of the area as of late
morning. Additional periods of isolated to scattered activity are
possible through the afternoon, especially into northwest Indiana.
A stacked low pressure system gradually lifting through the area
is supporting these continued precip chances, though short term
model guidance suggests the strongest shortwave energy with this
low is directly overhead or has already moved a bit to our east.
So there remains plenty of cyclonic flow aloft to support weaker
disorganized activity through the afternoon, but focused areas of
stronger forcing aloft and moisture flux at the surface would
generally support a better chance for organized showers and
thunderstorms over eastern portions of the area.



245 AM CDT

Through tonight...

Another day of showers on tap for northern Illinois and northwest
Indiana, with precipitation finally winding down late this
afternoon into the evening.

Slow moving upper low is crossing the Mississippi River early this
morning near far northeastern Missouri and the 500mb circulation
is expected to lift across the Chicagoland area mid afternoon.
Meanwhile, at the surface, a frontal boundary is draped from PNT
to RZL early this morning and expected to lift north to near the
IL/WI state line this afternoon. This will allow warmer air to
overspread most of the CWA with afternoon highs expected to be in
the low 60s. Southwest winds behind the front should bring the
warmer temps all the way to the lake front. NAM guidance indicates
the front may stall over the northern tier of counties which
would result in a temperatures staying cooler for areas within
several miles of the state line, but as of now the NAM is a
southerly outlier.

Isentropic ascent continues to be fairly broad across the CWA
today but there is better pressure packing that lifts across the
area between 12Z and 18Z today which coincides with a lobe of
DCVA that lifts across the eastern half of the CWA. Instability
has shifted farther east today, and is mainly over the northwest
Indiana counties which will limit the thunder threat for Illinois.
With this in mind, have the highest PoPs and expected the
greatest precipitation coverage east of the I-55 corridor today
with more spotty activity expected to the west. Precipitation
chances should wind down from the west this afternoon as the 500mb
low/trough axis moves to Lake Michigan mid afternoon then to
southwestern Michigan early this evening.

Fog may be an issue throughout the day within the lighter flow of
the frontal trough with some improvement expected as southwest
flow increases from the south today. Overnight, a trailing surface
trough and eventually a col area will move over the CWA which
will continue to provide light winds. Expect cloud cover to stay
locked in so we won`t radiate very well, but the combination of
high dewpoints and light flow will promote fog redevelopment



245 AM CDT

Monday through Saturday...

Upper low that provided wet conditions over the weekend is progged
to open up and shift to New England early next week with a brief
lull in precipitation first half of Monday as a transient upper
ridge builds across the Midwest. Meanwhile, water vapor imagery
this morning indicates another upper level disturbance over the
Desert Southwest which will eventually lift across the Plains and
Midwest through the day Monday. A modest surface reflection will
move from Oklahoma late tonight to central/downstate Illinois
Monday afternoon with precipitation overspreading the CWA
associated with a corridor of mid level WAA out ahead of the low.
Soundings indicate fairly shallow moisture this far north with
weak instability only clipping areas east of the I-57 corridor.
This system will quickly depart to our east Monday night with high
pressure building from the Canadian Prairies across portions of
the Upper Midwest Tuesday and Wednesday. Expect dry conditions
during this timeframe but north to northeast winds will suppress
temperatures near the lake front, keeping those areas in the low
to mid 40s while farther inland temperatures warm into the 50s.
Models continue to struggle handling track and timing of low
pressure late in the week. The GFS has a southerly track that
keeps some of the local area dry late in the week while the ECMWF
and GEM lift the low across portions of the midwest which would
result in showers and possibly thunderstorms locally. While the
details are still way out of focus at this distance, the GEM and
ECMWF have been more consistent over the past few days and will
lean on those solutions with at least high chance PoPs late in the



For the 18Z TAFs...

The midday radar shows a few isolated showers across northern
Illinois, with more widespread activity across northwest Indiana.
Trends generally should be on the dry side this afternoon, though
some brief showers certainly are possible. There is even a low
chance of an isolated thunderstorm, but confidence on timing and
location are far too low to warrant mention in the TAF at this
time. As for ceilings, despite some hint of clearing or at least
VFR skies, most locations remain solidly in MVFR and given the low
pressure system parked overhead, that is the favored trend
through the afternoon.

The remaining question is how winds will evolve. A fairly light
gradient has been allowing for some variability in direction so
far today, but the overall flow supports a southwesterly trend
going foward, with turning to northwest overnight and eventually
around to northeast tomorrow ahead of the next approaching system.

Rain and low conditions appear to be on the way again for tomorrow
afternoon. Confidence in timing and exact cig/vsby trends is not
high at this range.



405 AM CDT

Low pressure continues to slowly lift northeast through the mid
Mississippi valley this morning, with more of an easterly wind in
place. This low will move into northern Illinois today, and then
across the lake early this evening. This will provide highly
variable winds over much of the lake today into tonight, however,
do expect speeds to diminish today into tonight. The higher speeds
which provided conditions hazardous for small craft across the
Illinois nearshore have diminished. Higher waves are still in
place, and so have continued the Small Craft Advisory today. Will
monitor for a possible early end time though. Continue to monitor
fog over the south half of the lake, and at this time, it still
appears that the more dense fog is situated over the western
shores of the lake. It does look like this dense fog will continue
for much of today, and quite possibly expanding over much of the
southern half of the lake. Will monitor the possible need for
expansion of the dense fog advisory to remaining areas over the
south half of the lake.



LM...Dense Fog Advisory...LMZ740-LMZ741-LMZ742-LMZ777-LMZ779 until 7
     PM Sunday.




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