Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 202050

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
350 PM CDT Fri Oct 20 2017

214 PM CDT

Through Saturday...

One more day of mild and dry left on Saturday before the pattern
starts to shift. Little in the way of note weather-wise through
the day Saturday besides gusty winds.

In the wake of a departing high and ahead of a northern Plains
deep low, the pressure gradient will strengthen over the area.
This will keep some wind tonight and provide lows several degrees
warmer than last night.

Strengthening winds on Saturday will provide gusts of 25-30 mph,
likely modulated some by whether there is sun within cirrus
breaks. Temperatures too may see slight dependence on sun, but
advection should mainly drive highs into the mid 70s given
850/925 mb temperatures near/just a tad cooler than today.



328 PM CDT

Saturday night through Friday...

Guidance still appearing to struggle with large scale pattern
later in the weekend into early next week. However, chances still
remaining high Saturday night into Sunday for precip as an
approaching upper level trough and surface trough/boundary arrive.
Most locations still staying dry for much of Saturday evening
though, as the stronger forcing still slowing and arriving later
in the night Saturday. This forcing along with a likely strong LLJ
focused across the region, should support a precip shield to move
across the CWA late Saturday night into Sunday. Developing
instability axis and higher dewpoint air to the west of the CWA on
Saturday still appearing to shift east into the CWA, with thunder
still a possibility. The previously mentioned slowing of the
system will be most noticeable on Sunday With most locations now
likely observing precip through much of the day Sunday. With the
potential for instability to still be in place along with the
potential for it to increase out ahead of this precip shield, have
added slight chance thunder for Saturday morning. Precip shield
should steadily move eastward later in the day Sunday.

On the backside of this system is when guidance really starts to
struggle, and have lower confidence going into early/mid next
week. Despite this variability, a cooler pattern with periods of
wet weather still looking to be the more favored patter through
mid week as much of the CONUS will see a large upper level
trough/low in place. A time frame that will need to be monitored
though is Sunday night into Monday night. Once again, guidance
varying during this time, but some guidance has been consistently
showing the potential for an extended period of rain for the
eastern half of the CWA, mainly northwest Indiana. Phasing issues
with energy to the south and a digging trough from the northwest
is likely the issue, but with this scenario not out of the
question. If this scenario were to occur, a developing low would
lift northeast across the region allowing for a steady stream of
moisture to push over the eastern half of the CWA. At this time,
confidence of this solution is low, but will continue to monitor.



For the 18Z TAFs...

Concerns with the Chicago area airport forecasts are:

- Wind direction backing slightly early this eve possibly to SSE

- Chance for low-level wind shear overnight

- Southerly wind gusts to near 25 kt Saturday afternoon

A pressure gradient will gradually tighten across the area through
Saturday leading to a strengthening wind field. Sporadic gusts in
the mid teens should be seen this afternoon before diminishing
around sunset. Winds will naturally want to back this evening and
could go as far as south-southeast in direction. Speeds are
favored to be 7-9 kt. Off the deck, a strengthening low-level jet
will provide southwest wind speeds of 40 kt around 1000 ft, and up
to 45 kt over RFD. So there is low-level wind shear potential.
Confidence is not high enough in true LLWS to include in the
Chicago TAFs at this time.

Southerly surface winds will strengthen on Saturday with gusts in
the 20-25 kt range being common Saturday afternoon. While clouds
will thicken, these will all be mid/high VFR.



350 PM CDT

Southerly flow will continue tonight through Sunday morning as
high pressure is situated over the eastern half the U.S., and
while a deepening low moves through the northern U.S. and southern
Canada. During this time, speeds will likely steadily increase
with 15 to 25 kt increasing to 30 kt occurring first tonight over
the open waters. These higher speeds will bring an extended period
of hazardous conditions for small craft, likely lasting into
Sunday. The 30 kt winds will be observed over both the north and
south half on Saturday, and am monitoring the potential for a
period of gales soon there after. Have lower confidence of this
possibility given stability issues over the lake, but with the
possibility of these higher speeds and the potential for at least
sporadic gales, have included occasional gales in the open water
forecast. This is from late Saturday through Saturday night. Will
continue to monitor the possibility for more prevailing gales
though. A diminishing trend will be the trend on Sunday, while
winds turn more westerly.



LM...Small Craft Advisory...LMZ740-LMZ741-LMZ742-LMZ743...9 AM
     Saturday to 4 PM Sunday.




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