Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 202357

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
557 PM CST Sat Jan 20 2018

211 PM CST

Through Sunday...

Main concern for the short term forecast period will be drizzle and
fog overspreading the region.

In the lower levels, south to southwest flow is drawing copious
amounts of moisture northward from the Gulf of Mexico.  Low pressure
continues to slowly deepen over the South High Plains while a
persistent high sits over the ern CONUS.  Latest satellite imagery
shows the leading edge of the stratus deck has moved into the swrn
portions of the CWA.  The progression of the leading edge of the
stratus deck may be temporarily slowed as snow cover over the region
melts under areas that remain free of any cloud cover other than
some thin cirrus.  The combination of the warm, moist advection and
sunshine will allow temperatures to rise into middle to upper 40s
this afternoon.  Latest guidance suggests that the fog and stratus
will move over the Rockford and Chicago metro area by early this
evening.  As the low level moisture continues to increase overnight,
an elevated dry layer will set up over the low level saturation with
the saturated layer only reaching arnd 8-10 kft with much drier air
above.  This profile will be more conducive to drizzle, rather than
rain.  Also, as the snow quickly melts, the potential for dense fog
will be limited.  There is some potential for dense fog, with
visibility of 1/4 mile or less as the stratus deck lowers to the
surface, but this is likely not to occur until after the short term
forecast period.  Have removed mention of rain in deference to
moderate drizzle, but acknowledge that this may end up being a
matter of semantics as there will likely be some measurable pcpn
late tonight and through the day tomorrow, but none of the guidance
suggests that the moist layer will be deep enough to support
anything more than drizzle.  With increasing warm, moist advection,
have continued the trend of limiting overnight lows in the mid 30s,
so any pcpn should remain liquid.  The sfc low will finally begin to
lift out of the South High Plains and into the Central Plains
tomorrow afternoon.  This continues the trend of slowing the system
lifting newd, which is fairly common for the models to lift closed
lows out of the swrn CONUS too fast, with the guidance usually
slowing with each successive run.  The developing pattern is fairly
typical of this idea, with the latest model runs continuing to trend
a little slower, which not only supports the trend of delaying the
trend from drizzle to rain, but also allow temperatures to rise a
bit more tomorrow, with highs ranging from the low 40s near the
Wisconsin border to the upper 40s to around 50 F south of the I-80


308 PM CST

Sunday night through Saturday...

The main weather concerns are confined to the early portion of
the period, namely Sunday night through Tuesday. During this
period the weather concerns are:

-Areas of fog, some possibly dense over northern IL into Sunday
-Period of moderate to heavy rain with thunderstorms also possible
 Sunday night into Monday morning.
-Period of accumulating snow Monday night into Tuesday morning
 along with gusty northwest winds.

The period will likely begin foggy with drizzle across the area
Sunday evening as a storm system begins to shift eastward across
Kansas. As this occurs, a surface warm frontal boundary is
expected to set up near the I-80 corridor, and it may remain
nearly stationary here into Monday morning. As a result, expect
fog and drizzle/light rain to persist across much of northern IL,
with periods of dense fog certainly possible. The chances for
heavier rain showers will increase over the area after midnight
Sunday night as the warm-moist conveyor belt ramps up over the
Mississippi Valley in response to the approaching deepening area
of low pressure. This will result in the transport of an
unseasonably high amount of moisture over the area late Sunday
night. As the previous discussion mentioned, precipitable water
values up just over an inch are progged during this time, which is
within the 99th percentile for this time of year. Therefore, a
period of moderate to heavy rainfall is looking likely over much
of the area, especially late Sunday night through early Monday
morning. Embedded thunderstorms are also possible given the warm
air advection regime, which will induce some steeper mid-level
lapse rates potentially supporting some charge separation, and
locally heavier showers. This could cause some hydro issues over
the area, especially with the ground still frozen. Please see
hydro discussion below for more info.

After the morning showers and storms, there may end up being a
several hour period of dry time on Monday as the surface low
begins to shift over portions of northern IL. This track is likely
to result in a dry slot shifting over the area during the day.
Monday also looks to be a fairly mild day, with highs expected in
the low 50s following the northward passage of the surface warm
front. This will change Monday night.

Monday night the surface low should begin to track over lower
Michigan, and as this occurs, expect a cold front to shift over
the area during the evening. Sharply colder conditions will set up
in its wake as west-northwesterly winds become gusty into Tuesday.
Given that the mid-level wave is expected to have a well developed
closed circulation, there is likely to be a decent deformation
zone along the back side of the surface low. This would support a
band of frontogenesis supporting a band of accumulating snowfall
over northern Illinois late Monday night into early Tuesday
morning. There remains uncertainty with the exact timing of the
change over to snow. However, at this time it appears that by very
late evening to around midnight the transition could be occurring
over north central Illinois, with an eastward transition into the
Chicago area after midnight. This is likely to have some impact on
the morning rush, but snow amounts should be wet on the lighter
side (perhaps an up to two inches near Rockford, and an inch or
less into Chicago).

A day or two of seasonal but dry conditions are likely in the
wake of this system through mid week before temperatures begin to
moderate again later in the week as another storm system takes
shape over the high Plains. This is likely to set up mild
southerly flow over much of the central CONUS by Friday. As a
result, temperatures could again be flirting with 50 degrees on
Friday. Increasing chances of rain will occur into next Saturday
ahead of this next storm system.




Snow melt has been ongoing the past 24 to 36 hours and will
continue tonight, with little snow cover likely remaining by the
time of main rainfall (Sunday night/early Monday). The rainfall
during that time period though could exceed one inch with an areal
basin average of 0.75" presently forecast. With many frozen
waterways this could create some issues. The rise in temperatures
and dew points could also cause some ice break ups. It should be
noted that this warm up is not as significant of a climb as seen
early last week that did cause some limited ice break up jams, but
will need to be monitored. Ice jams tend to be favored at bends
in area rivers and at choke-points and obstructions to free water
flow such as bridges.


For the 00Z TAFs...

Concern is on the timing of transition from MVFR to IFR
conditions, and the eventual deterioration of ceiling and
visibilities to LIFR or even VLIFR in what appears to be a
favorable pattern for low conditions. Have low confidence in
specific category changes, but high confidence that things will be
trending downward through the period. No big concerns at least for
the next 6 hours.

The problem in the near term, and for at least the next 12-15
hours is that a number of short term model guidance is really
hitting the area with IFR ceilings in the next few hours, or at
least by 6z along with IFR visibilities. A number of these sources
have not handled things too well today, and there is limited
snowpack and little IFR out there at all yet. Also there is not
great isentropic lift yet to get the drizzle going. Have trended
the TAFs arrival of IFR cigs overnight, and could even be too fast
still, but will need to keep an eye on short term trends for
lowering of the cloud deck.

The surface pattern tomorrow would suggest a more favorable time
for LIFR/VLIFR conditions will come mid to late morning as the
warm front will shift north to the I-80 corridor or so and then
remain parked there. We get a little better lift to support
drizzle at this point and then it will increase in chances during
the afternoon. Once things head downhill there seems very little
to suggest lifting we get to LIFR/VLIFR with the warm front
expected to be just to the south of the terminals through the
remainder of the period.



314 PM CST

No major changes in the current thinking. Please see previous
discussion below.

A low pressure system will develop over the Southern Highs Plains
tonight then track gradually northeast to northern IL/southern WI
by Monday. To the north of the low, the set-up appears favorable
for a period of east gales Monday over northern portions of Lake
Michigan. As the low moves across the lake winds will briefly ease
Monday night before northerly gales develop on the backside of
the low Tuesday. Northerly winds will ease later Tues night into
Wed as high pressure approaches the western Great Lakes. A couple
periods of strong southerly winds with at least some potential for
gales exists late in the week into next weekend.

- Izzi/KB





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