Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 192341

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
641 PM CDT Thu Oct 19 2017

140 PM CDT

Through Tonight...

A quiet night ahead with the high pressure ridge shifting from
overhead this afternoon to east of the area by daybreak. The air
mass is quite dry with 40-45 degree temperature/dew point spreads
providing relative humidity of 15-25 percent, especially
along/north of I-80. The light winds, dry air, and clear sky on an
October night are a recipe for strong evening radiational
cooling, and have sharpened the slope of the early evening
temperature drop in the hourly forecast. The 9-10 pm readings
should already be down to within 5 degrees or so from the
nighttime lows. A slight south wind late will ease the rate of
cooling with lows forecast in the mid-upper 40s in outlying
locations, to the lower to mid 50s in the metro.



302 PM CDT

Friday through Thursday...

Quiet/dry weather continues Friday, with lingering boundary in
place expected to be north of the CWA at the start of the period.
Warmer air pushing across the region and sunny skies on Friday
will support above normal temps well into the 70s. Have maintained
similar forecast of mainly upper 70s, but could see some
locations rising to around 80 degrees. Southerly flow should help
keep conditions warm near the lake, but could see some slight
eastward component developing later in the day over far northeast
Illinois. Dry and above normal temps continue Friday night and am
still expecting a system to move across the region this weekend
and bring the next chance of precip.

No big changes with precip chances at this time, however, it does
appear as if guidance is struggling with the evolution of the
overall large scale pattern. This is more evident later in the
weekend into early next week though. Most of Saturday should
remain dry but with a stray light shower still not out of the
question over the far western CWA in the afternoon. This is a very
low chance though. Strongest forcing associated with approaching
upper level trough and surface trough/boundary doesn`t arrive
until Saturday night, with most locations still staying dry for
much of the evening. This forcing along with a likely strong LLJ
focused across the region, should support a decent precip shield
to move across the CWA late Saturday night into Sunday morning and
have continued higher pops. Developing instability axis to the
west of the CWA on Saturday does appear to shift east Saturday
night, with thunder still a possibility. An axis of higher
dewpoint air and this instability will support this possibility of
thunder and have maintained mention of thunder over much of the
area. With the mid/upper level trough axis still pushing through
the area on Sunday, will likely see precip now linger across much
of the CWA through midday.

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 appear to be on track as much of the CONUS will
see a large upper level trough/low in place.



For the 00Z TAFs...

Quiet VFR aviation weather continues across northern IL and
northwest IN terminals.

Winds are becoming light southeast this evening, as weak high
pressure drifts off to the east of the terminals. Dry air and full
sun will support deep mixing Friday, with south winds gusting near
20 kts at times by afternoon. Gusts will subside with sunset.



302 PM CDT

Lighter winds in place across the entire lake today, as gradient
has generally relaxed. This is most noticeable across the south
half where surface ridge axis has moved overhead. Across the north
half, fairly tight gradient still in place and allowing for
slightly elevated winds to persist. However, only anticipate these
winds to 30 kt to only last through this afternoon and then
diminish back towards the 15 to 25 kt range. Southerly winds
expected to be in place over the entire lake through much of the
weekend ahead of approaching surface trough/front. Expect winds to
increase back to 30 kt over the open waters and initially only
for the north half, but will see this trend observed over the
south half. Although currently not forecasting gales, am
monitoring the potential for gales late Saturday into Saturday
night mainly over the north half. Across the nearshore waters,
conditions hazardous for small craft are again possible by this






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