Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 220741

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
141 AM CST Mon Jan 22 2018


925 PM CST

While dense fog remains locked in across the area to the north of
a surface warm front over my southern CWA, attention is turning to
the maturing storm system over Kansas. This intensifying storm
system is inducing a 50+ kt southerly low-level jet across the
Ozarks and into the Mid-Mississippi Valley at this time. Strong
moisture advection occurring in association with this southerly
low-level jet, is resulting in the rapid northward transport of
an unseasonably moist airmass featuring in excess of 1"
precipitable water values. As a result, an area of showers and
thunderstorms is rapidly lifting northeastward over Missouri at
this time.

This area of showers and storms are expected to shift
into Northern IL after 2-3AM as the nose of the low-level jet
rapidly shifts northward over the area. It still looks like a good
bet for some thunderstorms over the area as mid-level lapse rates
steepen. Also, expect a period of moderate to heavy rainfall
through early Monday morning before the showers and storms shift
to the east and north of the area. Rainfall amounts up around an
inch will be possible with this active late tonight through mid-
morning Monday. Thereafter a break in the precipitation is likely
for several hours, before another line of showers shifts over the
area later in the afternoon in association with a cold front.

As for the dense fog, expect it to hang around across northern
Illinois for much of the night, while gradually improving from
south to north towards morning. The rainfall may help to increase
visibilities some as it moves into the area late tonight. We did
decide to extend the advisory for all but our far southern
counties through 12z Monday morning. While some fog may persist
past this time, especially over far northern IL, it does appear
that improvement will occur through the morning as the surface
warm begins to shift northward towards southern Wisconsin.



304 PM CST

Through Monday...

Little change to the going forecast. Main concerns remain fog and
drizzle for the remainder of the afternoon hours and then adding
the complication of thunderstorms with the chance for locally
heavy rainfall late tonight through early tomorrow morning.

Latest sfc analysis indicates that the deepening sfc low is
tracking through nern Kansas late this afternoon and will continue
to slowly track northeast, only reaching the IA/MO area by mid day
tomorrow. A warm front has set up from the low east across nrn
IL to the MI/IN line. The exact location of the boundary is a bit
diffuse, with ely flow continuing north of the boundary and sely
to sly to the south. Visibility has temporarily improved above
dense fog advisory levels this afternoon, but expect that
visibility will drop off again with sunset and the advisory is
being expended to cover the entire CWA. The dense fog duration is
a bit questionable and will depend on how fast the warm front
lifts north. Current thinking is that the boundary will lift north
through the night and that visibility will improve from south to
north through the overnight hours. As the warm front lifts north,
the low level saturated layer will deepen and increasing UVV at
the top of the saturated layer is indicated on RH/omega time-
height plots. Expect that pcpn will transition from drizzle to
more convective showers. A band of low level instability is
expected to lift through the area, in advance of the approaching
cold front, bringing a chance of thunderstorms to the region. In
combination with pwats in excess of 1 inch, which places deep
layer moisture well above seasonal levels, locally heavy rainfall
is likely with thunderstorms and even a period of moderate rain is
likely through the late night hours until the cold front push
through the region, bringing back more seasonably cool, dry air.
For temperature trends, given the persistent strong warm, moist
advection, there will likely be a flat temperature trend overnight
and could even rise a bit durg the overnight hours, with lows
remaining in the 40s. For tomorrow, temperatures should be able to
rise into the 50s across much of area. The the general trend of
the system continuing to slower that previous forecasts, it is now
likely that the true sfc cold front will not push across the CWA
until late tomorrow afternoon and not pushing east of the CWA
until shortly after 00Z tomorrow evening. Also, the sfc cold front
is expected to race ahead of the sfc low which now, is not
expected to lift into Lower Michigan, possibly, until late Monday
night. The net impact for the short term forecast is that the
cooler air will likely not move into the region until after the
short term forecast period.

With no significant changes expected for the heavy rainfall and
temperature trends and associated impacts, the the Hydrology
section of the AFD for details of the hydrologic impacts.


204 PM CST

Monday night through Sunday...

In typical see-saw La Nina fashion, the short lived
mild spell and active spring like weather on Monday will be replaced
by colder air. On Monday night, the deep upper low will pass right
through the area. The bulk of forecast sounding guidance suggests
that the low level cooling behind the front will initially be
slow, and therefore will continue to suggest a gradual change over
to a rain/snow mix and then all snow. As mentioned on the
previous shift, a very impressive TROWAL feature will pass through
Wisconsin, but closer to the IL/WI border there is still some
decent deformation and some lower level frontogenesis north of
the upper low to support some locally heavier precipitation
embedded in the generally lighter precip shield. The
NAM/GFS/EC/GEM all of a mesoscale feature/locally enhanced higher
precip shield (QPF from 0.15 to 0.4" or so) in the 6z-12z time
frame along and north of I-88 (mainly near NC IL) in this period
of low level cooling where enhanced precip rates could force some
wet snow to accumulate in spite of fairly marginal surface
temperatures. The forecasted lift during this period of higher QPF
is fairly impressive and consistent between the NAM/GFS just that
a few wet inches of snow are possible across northern/north
central IL, with much less to very little as you head toward
Chicago. Still, some slushy accums are possible. City of Chicago
temperatures will remain the most marginal. This feature will
weaken Tuesday morning. It will be cold enough for all snow
Tuesday morning but moisture will be shallow, so we would expect
some lingering lighter snow showers.

Following the departure of this system, we will have several quiet
days. A modest push of colder air will shift into the area Tuesday
into Wednesday, bringing temperatures closer to normal. Any
lingering snow showers will be across Michigan snow belt and
likely just to the east of Porter county. By Wednesday night, the
upper level pattern will feature a western trough (that will
become our late week weather maker), center of the country ridge,
and eastern trough. Mid-upper level heights will begin rising as
more anomalous upper ridging across the center of the country
builds east over the region.

The pendulum heads upward as strong southerly flow ahead of the late
week system, with Thursday and Friday to feature well above seasonal
temperatures again. The upper level jet will strengthen in our near
our area due to the increased baroclinic nature (shaper temperature
contrast) with this system, but the main upper forcing appears that
it may be disjointed from the deeper gulf moisture, and thus the
better better rain would tend to be favored southeast of our area.
Either way, expect a mild start to the weekend with some rain
chances, then a bit more potent though likely progressive cold snap
will take hold for the remainder of the weekend.




Majority of the snow cover in the area has melted and there has
been some small response on some area rivers. Significant, and
potentially locally heavy, rainfall is expected late tonight into
Monday morning. This rain will be falling on largely frozen
ground, resulting in the majority of the rain going straight into
runoff. Given the potential for rainfall totals to exceed an inch,
should see significant rises on many area rivers, with some mostly
minor flooding possible on main stem rivers. In addition, expect
ponding of water to be a problem, particularly in poor drainage
areas. Generally expect that any flooding will be more of the
nuisance variety and probably could be handled with flood
advisories. Given the expectation of mainly minor flooding, opted
to hold off on a flood watch and will just issue an ESF and
continue to highlight the threat in the HWO.

The quickly increased stream flow on frozen rivers and rising
temperatures will likely dislodge ice on frozen and partially
frozen rivers. The flowing ice could jam up and result in an
increased ice jam flood threat in addition to the potential for
minor areal flooding. Ice jams tend to be favored at bends
in area rivers and at choke-points and obstructions to free water
flow such as bridges.

- Izzi


For the 06Z TAFs...

Concerns in the near term continue to be with the low ceilings and
vis in dense fog. As warm front slowly lifts north, will see some
slight improving trend from south to north, with some of these
trends already reaching the GYY and MDW terminals. Vis had been
at/below 1/4sm in these two locations, but have now seen some
slight improvement to 1/2sm to 1sm. The other terminals will
likely remain in the lower vis until the arrival of more
widespread showers early this morning. Still anticipate showers,
with embedded thunder, to reach the terminals early this morning
with upstream trends appearing to be on track, including current
reported thunder. Once again, some slight improvement in vis and
even ceilings with the arrival of this precip until this precip
exits by early/mid morning. Confidence is lower, but vis and
ceilings could fall once again with its departure. Winds will turn
more southerly by the afternoon with an improving trend to the
ceilings/vis, but with shower chances returning once again.
Forecast gets complicated by late afternoon into the evening,
owing to the approaching system. Periodic showers will continue to
be likely, that could end up being light snow showers before it
is over. Ceilings and vis will also continue to need to be



141 AM CST

There are multiple concerns through Tuesday as a strong storm
system impacts the region. Northeasterly 35-40 kt gales will
develop on the north half of the lake this morning as the low
pressure moves into southern IA, tightening pressure gradient
between it and high pressure north of the lakes. Meanwhile, a warm
front lifting north spreading unseasonably mild and moist air
over the south half will keep favorable conditions in place for
dense fog through mid evening. Northeasterly gales will continue
north of the surface low path, which will be over northern
Illinois this evening to Lake Huron by Tuesday morning. Once the
low shifts east, gales will turn northerly on the north half of
the lake, with north-northwesterly gales spreading down the south
half. A Gale Warning is in effect from 9am CST this morning
through 6pm Tuesday for the north half and 3am CST to 6pm CST
Tuesday for the south half.

For the nearshore waters, expecting the dense fog to dissipate
this morning with steady rain moving in and warm front lifting
north. A brief period of near hazardous southerly winds will occur
this morning through early afternoon for mostly the Indiana
nearshore. East-southeast winds north of the warm front could
also produce waves up to 3-5 feet along the Illinois shore from
mid morning through early afternoon. Will hold off on Small Craft
Advisory issuance with some uncertainty on the speed/gust
magnitude and very short duration of the higher winds for the
Indiana shore and waves just shy of criteria for the Illinois
shore. A Small Craft Advisory will then be needed Tuesday into
Tuesday evening for the Illinois shore and through Tuesday night
for the Indiana shore during the strong north-northwest winds. A
brief period of gale gusts appears possible for the Indiana shore
during the mid morning through early afternoon timeframe, but do
not have enough confidence for an additional gale headline.

The next period of strong wind concern is south-southwesterlies
Friday through Friday night, with a period of gale force winds
currently appearing probable over the open waters. Winds will
shift westerly and diminish but remain elevated behind cold
frontal passage on Saturday.



IL...Dense Fog Advisory...ILZ023-ILZ032-ILZ033-ILZ039 until 3 AM

     Dense Fog Advisory...ILZ003-ILZ004-ILZ005-ILZ006-ILZ008-ILZ010-
     until 6 AM Monday.

IN...Dense Fog Advisory...INZ001-INZ002 until 6 AM Monday.

     Dense Fog Advisory...INZ010-INZ011-INZ019 until 3 AM Monday.

LM...Dense Fog Advisory...nearshore waters
     until 6 AM Monday.

     Dense Fog Advisory...LMZ777-LMZ779 until 9 PM Monday.

     Gale Warning...LMZ777-LMZ779...3 AM Tuesday to 6 PM Tuesday.




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