Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 212359
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
559 PM CST Sun Jan 21 2018

.SHORT TERM...
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.

&&

.LONG TERM...
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.

KMD

&&

.HYDROLOGY...

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

&&

.AVIATION...
For the 00Z TAFs...

Forecast concerns/challenges in the near term are with low
ceilings and vis/fog. Trends across all of northern Illinois and
northwest Indiana over the last couple of hours, have been for
rapidly falling vis in dense fog. Any locations still observing
vis at or above 1sm will see vis fall towards 1/2-1/4sm over the
next hour. Have fairly high confidence of these trends and the
adjustments to the TAFs, given the pattern in place. This dense
fog and low ceilings, as low as 200ft, will remain for much of the
night, as pattern won`t support any overall improvement. What may
likely improve this fog/vis later tonight/early Monday morning
will be the arrival of showers and possible thunder. However, vis
will still remain restricted under this precip. With the departure
of the more widespread precip by early/mid morning Monday, fog
could fall back down below 1sm but confidence is low. Additional
showers are likely later in the afternoon, that could linger into
Monday evening. Easterly winds will prevail tonight, with a
gradual shift to the east southeast. Don`t anticipate shift to the
south until midday Monday.

Rodriguez

&&

.MARINE...
204 PM CST

Low pressure moving out of the southern Plains will cross
the lake Monday night while gradually deepening. Ahead of this low a
warm front will lift into the southern part of the lake. This will
produce both variable wind conditions on either side of the front,
and a continued expansion of dense fog as better moisture moves over
the cold lake along with the front.

East-northeast winds north of the low will strengthen to gale force
by mid to late morning, with the strong winds continuing into early
Monday evening. After what may be a short lull Monday night as the
low passes over the center of the lake, winds across the entire lake
likely increase to near gales again by early Tuesday morning and
persist into at least Tuesday afternoon. An extended gale warning
will account for multiple periods across the north and a brief
break, with a gale watch for the south for the back side winds.
Winds will be more of a north to eventually north-northwest
direction at this point. Northwest winds will ease as high pressure
will pass over the lake midweek. Another lee-side low will approach
Lake Michigan late week, with gusty south winds ahead of it. This
low will take a more northern track near Lake Superior.

KMD

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...Dense Fog Advisory...ILZ019-ILZ023-ILZ032-ILZ033-ILZ039 until 9
     PM Sunday.

     Dense Fog Advisory...ILZ008-ILZ010-ILZ011-ILZ012-ILZ013-ILZ014-
     ILZ020-ILZ021-ILZ022 until 3 AM Monday.

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

IN...Dense Fog Advisory...INZ001-INZ002 until 3 AM Monday.

     Dense Fog Advisory...INZ010-INZ011-INZ019 until 9 PM Sunday.

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

     Gale Watch...LMZ777-LMZ779...3 AM Tuesday to 3 PM Tuesday.

&&

$$

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