fxus63 klot 182349 

Area forecast discussion 
National Weather Service Chicago/Romeoville, Illinois 
549 PM CST Thu Jan 18 2018 

Short term... 
120 PM CST 

Through Friday... 

The clipper system to our north will quickly exit the region 
tonight. Southwest winds have ushered in some more seasonal 
temperatures getting close to the freezing mark. High pressure 
aloft will shift across the Great Lakes tonight. At the surface 
high pressure will to the southeast USA, while a second low will 
dig into south central Canada. This will maintain breezy southwest 
winds through tonight and Friday. Expect lows to hold near or even 
above yesterday's highs, with lows tonight mostly in the 20s. 
Continued warm advection on Friday will steer high temperatures up 
near 40. We will have some higher clouds due to the low passing by 
to the north and from a system well out in the west. 



Long term... 
326 PM CST 

Friday night through Thursday... 

Warm trend continues at the start of the period, with dry 
conditions also persisting. Initial zonal flow will transition to 
more ridging as upper level low/trough moves through the central 
Continental U.S. This weekend. In this pattern, warm air advection will be the trend with 
lows Friday night in the 20s rising to highs around 40 on 
Saturday. However, more pronounced moist advection will be delayed 
some, with dewpoints not really rising until late Saturday into 
Sunday morning. This will result in increasing low clouds and fog, 
with drizzle becoming more likely Sunday night. Similar concerns 
with the low chance of freezing drizzle continues, and so have 
added mention to the forecast primarily for far northern Illinois. 
At this time, it does not appear to be overly concerning but 
enough to warrant mention. 

Fog concerns really increase into Sunday, with the arrival of 40+ 
dewpoint air. Increasing forcing will help to continue precip 
chances throughout the day Sunday, with light rain becoming more 
likely. However, strongest forcing associated with the now closed 
low and surface low doesn't arrive until late Sunday night into 
Monday. This should help a more widespread precip shield to spread 
across the region, with a period of steady rain likely during 
this time. Guidance still indicating some elevated instability 
through this period, with rumbles of thunder still a possibility. 
Will continue to monitor flood issues/concerns, with the current 
snow pack and this possible rainfall, especially if there ends up 
being more convective type precip. Cold returns on the backside of 
this system later in the day Monday, with any lingering precip 
likely transitioning to snow. 



for the 00z tafs... 

Continued fairly quiet VFR conditions over the next 24 to 30 
hours. The main issue this evening is low level wind shear at Florida ~020, with west- 
southwest winds around 45 kt. Since wind gusts have already died 
down and become sporadic, added low level wind shear to the tafs and expecting it 
through about 06z when winds aloft subside a bit. Southwest winds 
will pick up again on Friday, with gusts in the 20-25 kt range 
starting in the late morning or early afternoon and likely through 
Friday evening. The low level jet will ramp up Friday evening, 
causing southwest winds at Florida ~020 to increase to about 50 kt. If 
wind gusts are less frequent than expected, low level wind shear may again need 
to be added to the taf. 



326 PM CST 

Southwest winds to 30 kt still in place this afternoon as low 
pressure pushes east through southern Ontario. Expect these winds 
to continue into this evening, but with some weakening expected as 
the low weakens. This trend should also continue for the 
nearshore waters, but with winds remaining elevated, have extended 
the Small Craft Advisory into this evening. A new low will move 
towards the Great Lakes region out of Canada on Friday, with winds 
expected to increase over the entire lake. With gales appearing 
likely later in the day Friday into Friday evening, have replaced 
the gale watch with a Gale Warning. Speeds increase over the 
nearshore waters as well, but have lower confidence for the 
potential for gales during this time. Have maintained winds to 30 
kt but did include mention of gale force gusts. Will need to 
monitor the potential for more prevailing gales over the nearshore 
waters Friday afternoon into Friday evening. 



Lot watches/warnings/advisories... 
Lm...Gale Warning...lmz777-lmz779...2 PM Friday to 3 am Saturday. 

Small Craft Advisory...nearshore waters 
until 6 PM Thursday. 



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