Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 230758

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
158 AM CST Fri Feb 23 2018

157 AM CST

Through tonight...

Scattered showers are approaching the I-39 corridor and more
widespread showers will spread east across the forecast area early
this morning.  0.1-0.2 inches of rain are expected from the showers,
but the additional rainfall should have little to no impact on area
rivers. Areas of fog are also being reported at most locations, but
dense fog is not expected.

The weak surface low will shift north over the northern Great Lakes
while the low`s front shifts northeast. Rain will shift out of the
forecast area late this morning, but cloudy conditions will
continue. High temperatures will be in the mid 40s to around 50.

High pressure spreads over the upper Midwest tonight as a frontal
boundary and weak upper level disturbance shifts north into central
IL. Guidance differs on how far north the precip associated with
this front will get, but I think precip will be along and south of I-
80 by sunrise Saturday morning.

A wintry mix of precip including freezing rain is possible. Forecast
soundings feature a deep warm layer and very little saturation in
the DGZ.  Therefore, any precip that falls should be liquid prior to
hitting the ground. Surface temperatures will be key, and right now
surface temps are forecast to be right around freezing along the I-
80 corridor and points north.  Therefore, there could be slick spots
Saturday morning, especially on colder, elevated surfaces.
Temperatures will warm to above freezing by mid to late Saturday
morning, ending the ice threat.

I`m not expecting much ice accumulation so I will not be issuing any
ice related headlines.



157 AM CST

Saturday through Thursday...

Overview...A wintry mix is followed by more rain and thunderstorms
Saturday and Saturday night. Rain from this system could cause rises
on local rivers.  Strong winds, possibly gusting to around 40 MPH,
are possible Saturday night. Drier and relatively mild conditions
are expected early next week, but rain may return mid to late week.

The aforementioned Saturday wintry mix turns to all rain by mid to
late Saturday morning. Scattered light showers could reach the IL/WI
state line, but I have higher confidence in measurable precip along
and south of I-88.

The next surface low takes shape over the high Plains Saturday, and
reaches the upper Mississippi River Valley Saturday evening.
Scattered showers are possible Saturday afternoon, although some
models suggest we could have a dry period. As the low itself moves
into the region, rain, potentially hard at times, will spread across
the region. Similar to earlier this week, embedded thunderstorms are
possible Saturday evening.  While not as high as earlier this week,
PWATs are forecast to be well above normal at around 1 inch.

Rainfall totals from late tonight through Saturday night will be 0.3
inches or less north of I-80 and 0.3-0.6 inches south of I-80 with
locally higher amounts. The additional rain could lead to minor
rises on rivers, but nothing as significant as what we experienced
earlier this week.

Gusty west winds are expected Saturday night into Sunday. The GFS
would suggest gusts of 40-45 MPH are possible while the NAM has
gusts of 30-35 MPH.  Either way, Saturday night looks windy.

The beginning of next week looks drier with a warming trend.
Additional systems could bring rain to the region Tuesday night
through mid next week, and models are hinting that the system next
Thursday could bring another round of moderate to heavy rain.



For the 06Z TAFs...

The main forecast concern overnight into the morning continues to
focus on the LIFR CIGS and visibilities across the area.

An area of low pressure over will shift into Wisconsin during the
day. As it does so, expect an associated frontal boundary to move
across the area around daybreak, with winds becoming westerly in
its wake for the remainder of the day. Ample low-level moisture
over the area ahead of this boundary will continue to result in
deteriorating CIGS over the next few hours. Overall, expect the
developing IFR CIGs to to fall into the LIFR category overnight
into early Friday morning ahead of the approaching boundary. There
is also likely going to be a period of low visibilities overnight
in fog and some light rain showers. It still appears that the
lowest visibilities, down around 1/2 mile, will be favored mainly
west of the main Chicago area terminals. For the 06z TAFs I have
changed little in regards to the timing and magnitudes of the
conditions through the morning.

Conditions should gradually improve across the area during the day,
though some MVFR CIGS may hold on over the area into the



356 PM CST

The current east northeast winds will become all easterly early
this evening while winds increase, as low pressure approaches the
Great Lakes region. At the start of the weekend, high pressure
will move through the region and expect winds to be on the lighter
side. This quickly changes though as deepening low pressure is
expected to move across the region Saturday into Sunday. Continue
to monitor this period, Saturday night through Sunday, as gales
with varying direction will be possible over much of the
lake/nearshore. Included with this are the possibility for higher
end gales to 45 KT. Also will need to monitor the possibility for
storm force winds for a time Sunday morning, mainly for the north
half of the lake. Do have growing confidence for gales, with lower
confidence on storm force winds. Given the time frame of these
gales and possible storm force winds, have not issued a watch yet.






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