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Area Forecast Discussion (AFD)

Issued by Chicago, IL (LOT)

FXUS63 KLOT 180750

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
150 AM CST Sat Nov 18 2017


832 PM CST

A fairly active weather period is expected through Saturday. The
main forecast concern for tonight is the threat for a period of
dense fog overnight. However, on Saturday the main concerns will 
be the developing strong northwest winds as well as the chance 
for a period of rain changing to snow.

Surface low pressure over Kansas this evening will shift eastward
across the Mid-Mississippi Valley early Saturday morning. As this
occurs, an associated frontal trough will shift into northern 
Illinois overnight. This is likely to result in a period of light 
and variable winds before they shift northward by early Saturday 
morning. Given the high low-level moisture in place over the area,
and the fact that visibilities and CIGS to our west are dropping,
I think portions of north central IL could experience a period of
dense fog overnight. The good news is that any fog should improve
quickly early Saturday morning as the winds becoming northerly. 
We currently have no plans for any dense fog headline, but trends 
will continued to be monitored to see if one may be necessary.

It appears for the most part that precipitation for the area for
most of the night will likely be either a light rain or drizzle as
we await the arrival of the main storm system for Saturday
morning. As the system approaches the area on Saturday it appears
that good large scale forcing for ascent will overspread northern
Illinois during the morning and become augmented by increased 
mesoscale forcing for ascent associated with a band of 
strengthening low and mid level frontogenesis. Therefore it 
appears likely that a more steady area of moderate rain will 
develop over area early Saturday morning. Thunderstorms will be
possible south of I-80. The main concern then is if this 
precipitation is able to change over to a wet snow before ending. 
At the present time, it appears that the thermal profile may 
dynamically cool enough to support a change over to wet snow 
across portions of northern IL north of I-80 during the late 
morning hours into the early afternoon. If this does occur, there 
is a small chance of some minor wet snow accumulation, which would
likely be limited to grassy surfaces. 

Otherwise, expect northerly winds to become quite gusty during 
the day. Wind gusts of 35 to 40 mph appear likely for most areas. 
However, even stronger winds up to 45 to 50 mph appear possible 
for areas near Lake Michigan in northwestern Indiana due to the 
strong onshore flow off the lake. This may end up requiring the 
need for a wind advisory for this area. Will also have to watch
for the possibility for some minor lake shore flooding/erosion 
due to the high waves over 10' expected on southern Lake Michigan.



228 PM CST

Through Saturday...

A complicated forecast tonight through Saturday, which includes 
potential for thunder area wide this evening and then south of 
I-80 later tonight into early Saturday morning, an extended period
of rain/drizzle tonight through Saturday which will briefly 
change over to a rain snow mix or even all snow very briefly late 
Saturday morning, and then potential for Wind Advisory criteria 
winds Saturday afternoon for locations south of I-80.

In the near term, narrow band of rain continues to progress 
through portions of northeast Illinois and northwest Indiana 
riding along WAA in place across the region. Had noted earlier 
reports of UP with this precip, mainly across northwest Illinois. 
Although initial saturation/cooling to wetbulb could support a 
brief period of a wintry mix, trends and most guidance supportive 
of lacking crystals this afternoon. This would support liquid, 
along with a continued warming column. So, all rain still expected
in the near term. Later this afternoon into the evening, will see
this initial rain exit, and then already present rain over 
eastern IA expand in coverage as it moves into northern IL and 
northwest Indiana. This will occur as speed max and surface trough
approach, and while WAA increases. Instability is still trying to
increase across the region, with a ribbon of 100-250 J/kg in 
place. With this in place and while forcing increases late this 
afternoon into the evening, can't rule out some embedded thunder. 
Steadier rain should depart mid/late evening, but will see large 
scale ascent remain overhead. This should support periodic light 
rain or drizzle over much of the CWA. Continued warming through 
tonight will support all liquid, with frozen precip not a concern 
during this time.

Surface trough/low will continue to approach the southern CWA 
late tonight into early Saturday morning, ahead of deeper trough 
digging across the region. Although forcing will be in place over 
much of the region, focus will likely be on sagging boundary which
by late tonight, will be along/south of I-80. Additional rain and
thunder chances are expected for this location especially into 
early Saturday morning, as forcing really ramps up with the 
arrival of this upper level trough and deepening low. Still not 
anticipating any severe weather on Saturday with these thunder 
chances, as most guidance fairly consistent with keeping any 
surface based instability just south of the CWA Saturday. Thunder 
chances should quickly depart by late morning and midday though.

Still monitoring an increase in precip by mid morning Saturday 
across the entire CWA, including much of northern Illinois and 
northwest Indiana. As this upper level trough and surface low 
swing through, guidance still indicating the high likelihood of 
more banded precip to push into northern Illinois through mid/late
morning. This deformation/fgen supported band of precip will 
swing through, as much colder air pushes in behind this system. 
Although lower confidence with the exact trends, there is a 
definite possibility for at least a small window of a rain snow 
mix. This low confidence is due to the variability to the extent 
of the deeper moisture/crystals in place and timing of cooling of 
the column. It's possible that the pattern will only support a 
rain snow mix briefly before the precip completely departs, but 
it's also very possible for even all snow to occur briefly. With 
the highest chances of a mix of precip across northern Illinois, 
have included mainly rain/snow but do have a couple of hours of 
all snow beginning by mid morning and going into midday. If all 
snow were to occur slightly longer, it's possible to see some 
minor snow accum mainly on grassy surfaces.

Warmest temperatures will occur in the morning Saturday, but with
increasing northerly winds and CAA behind the system, will see 
temps fall into the afternoon. Strongest winds expected in the 
afternoon with gusts around 40 MPH. It's possible that sustained 
speeds and gusts may be higher, and will need to monitor the 
possibility for an Advisory Saturday afternoon.



211 PM CST

Saturday night through Friday...

Primarily dry conditions are expected throughout the extended time 
frame, but temperatures will be quite variable as a series of lows
move across the northern Great Lakes region.

Saturday night, deep area of low pressure will be over the central 
and eastern Great Lakes region with precipitation now east of the 
local area but cold air arriving behind the cold front. 850mb 
temperatures will bottom out around -10C Sunday morning but the 
thermal trough will quickly shift off to the east with 
temperatures moderating through the day Sunday. Strong subsidence 
behind departing 500mb trough axis will overspread the area 
through the day. Forecast soundings indicate stratus will be in 
place early in the day Sunday, but subsidence helping to dry the 
column and surface ridge axis building in from the west may allow 
some sunshine later in the day. 

Monday, another vort is progged to dig across the Canadian
Prairies and reach the Great Lakes region midday Tuesday. 
Attendant surface low will stay over Canada, and precipitation 
remains favored north of the local area so will maintain a dry 
forecast through this time frame. Southwest flow out ahead of the 
wave will help temperatures further moderate, reaching back into 
the mid to upper 40s Monday and Tuesday. Expect a reinforcing 
shot of cold air to push across the region Tuesday evening and 
Tuesday night putting the area back into a deep freeze Tuesday 
night with colder air remaining in place through early Friday. 
Another clipper-like system will take a similar though slightly 
more southerly track across the region Friday into Saturday with 
temperatures undulating once more. This system may also provide 
the next decent chance for precipitation in the area.



For the 06Z TAFs...

There are multiple aviation concerns through Saturday. The 
overnight/early Saturday morning hours will be characterized by 
drizzle and occasional light rain. CIGs and VSBY will continue to 
trend downward as low pressure approaches and winds diminish. 
Concern continues for fog potential overnight, with highest 
confidence at DPA and RFD where temporary 1/2SM VSBY was 
maintained. Increasing north winds by or a bit after daybreak 
today should end the fog threat. Expecting CIGs to drop to LIFR 
area wide over the next few hours, with some uncertainty on 
improvement time on Saturday, likely mid day or thereabouts.

Additional TSRA development this morning through mid day should 
stay south of the terminals. However, an area of steady rain will 
develop and move into the area on the northwest side of 
strengthening low pressure during mid morning. Periods of low MVFR
to IFR VSBY are likely with the rain. As this occurs, northerly 
winds will increase with gusts in the 25-30 kt range, and 30-35 kt
range at GYY. Colder air moving in could potentially allow snow 
to mix in, with a changeover to snow possible before the precip 
ends, but confidence is low. Confidence is also low-medium on 
exact precip end time. Only maintained PROB30 for RA/SN at RFD. 
Will need to monitor trends for this at the other terminals. 
North-northwest to northwest winds will stay strong and gusty 
through Saturday evening, with gusts likely to 30 kt or even 
higher at times, especially at GYY. CIGs are expected to remain 
in MVFR range, possibly improving sometime by early Sunday.




150 am...A cold front will move south across Lake Michigan early 
this morning shifting winds northerly. The gradient will quickly 
tighten as low pressure moves from Missouri this morning to near 
Lake Erie this evening. These northerly winds will increase to 
gales by late morning with 45kts likely on the south end of the 
lake this afternoon. The gales will slowly diminish from north to 
south late tonight into early Sunday morning. A large area of high
pressure will move across the southern U.S. Sunday into Monday. 
As it passes south of the lake...northwesterly winds Sunday 
morning will shift southwesterly Sunday night. Low pressure will 
move across Ontario Monday night and the gradient will tighten 
ahead of this low with a period of southwesterly gales expected 
late Monday afternoon through Tuesday morning. A trailing cold 
front will move across the lake Tuesday afternoon and then large 
high pressure will move across the region next Wednesday. cms


LM...Small Craft Advisory...nearshore waters until 3 AM Saturday.

     Gale Warning...LMZ740-LMZ741-LMZ742-LMZ743-LMZ744-LMZ745-LMZ777-
     LMZ779...9 AM Saturday to 6 AM Sunday.