National Weather Service Weather Forecast Office

• Home
• Site Map
• News
• Organization
• Search for: NWS All NOAA 

000
FXUS63 KLOT 232358
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
558 PM CST Tue Jan 23 2024

.SHORT TERM...
Issued at 307 PM CST Tue Jan 23 2024

Through Wednesday...

Key Messages:

- Widespread dense fog tonight into Wednesday morning

- Rain spreading northward tonight into Wednesday morning

- A slight chance of freezing rain late tonight into early-mid
  Wednesday morning across portions of interior northern Illinois

- Rain tapering to drizzle Wednesday afternoon with fog possibly
  becoming dense later in the day

- The rain, melting snowpack, and heavy lingering river ice may
  expand flooding issues, with a Flood Watch issued I-80 and south

A classic dense fog over widespread gradually melting high water
content snowpack scenario appears to be unfolding this afternoon.
Temperatures are in the mid to locally high 30s and dew points
primarily above freezing as well, with weak to calm boundary layer
flow. Already seeing some locations (including outside the office
as of this writing) tanking down to 1/4 mile or less visibility.
For the south half of the CWA, roughly I-80 and south, a Dense Fog
Advisory will be in effect as of 3 PM. There`s some uncertainty
on exact visibility trends farther north over the next few hours
toward and just past sunset. However, with high confidence in
dense fog developing area wide, the Dense Fog Advisory for the
rest of the CWA starts at 6 PM and will monitor observational
trends for the possibility of needing to start the advisory
earlier in these areas.

Lingering drizzle with northward extent this afternoon should
temporarily end into this evening. Waiting in the wings is the
next positively tilted short-wave that will spread rain northward
overnight into Wednesday morning. The highest rainfall totals
through Wednesday morning of near 1/2 inch to as much as 3/4 inch
in spots will reside generally near and south of the Illinois and
Kankakee River, tapering to the north and northwest. These higher
rainfall totals also correspond with the most extensive ongoing
ice jam flooding issues and prompted the Flood Watch issuance. For
in depth hydrologic information, see the Hydrology section issued
this afternoon.

The main points of uncertainty with the developing rain late
tonight into Wednesday morning are: 1) If any locations across
interior northern Illinois can drop to freezing prior to rain
onset and result in at least localized freezing rain; and 2) How
much, if any, the steady rain improves the dense fog.

On the first item, the guidance has generally been a bit too
cool, owing likely to the cold and snowpack across much of the
region going into this warm-up. Given the late afternoon
temperatures, socked in conditions, and generally above freezing
dew points, the thinking is that temperatures will be slow to cool
tonight and generally remain above freezing. However, can`t
completely rule out some spots north of I-80 and outside of
Chicago dipping down to 32F, so opted to mention a slight chance
of freezing rain and will let the evening shift assess trends in
this regard.

With respect to potential improvement in the dense fog as the rain
spreads northward tonight into Wednesday morning, suspect the
visibility will improve some, but to what extent is uncertain.
Typically in such a favorable setup for dense fog, wouldn`t expect
improvement tomorrow (Wednesday) morning, though with rain-aided
improvement wildcard, opted to have the Dense Fog Advisory expire
at 10 AM CST tomorrow/Wednesday for now.

As the weak surface low reflection lifts northward tomorrow
afternoon, forecast soundings indicate a loss of deeper moisture
through the mid-levels, but enough ascent through the saturated
lower levels (and sufficient depth of this saturation) to maintain
drizzle mention across the area. If the visibility does improve
due to the rain in the morning, it may very well tank again in the
afternoon in the evening. For more details on the dense fog
potential Wednesday night, see the long term section below.

Castro

&&

.LONG TERM...
Issued at 117 PM CST Tue Jan 23 2024

Wednesday Night through Tuesday...

Key Messages:

- Dense fog possible Wednesday night.

- Periods of rain Thursday.

- Low chance for rain, possible rain/snow mix Saturday night.

Primary forecast concern is the potential for dense fog again
Wednesday night into Thursday morning. Continued with low chances
for light rain, perhaps only in the evening, but much of Wednesday
night appears to be dry. Fog may be ongoing from Wednesday
afternoon and then continue to worsen into Wednesday night.
Surface winds look rather light, around/less than 5 mph and low
level winds, at 975mb, look in the 10 mph range. These light winds
should be favorable for at least widespread fog but there remains
uncertainty for how widespread dense fog will become. Have added
areas of fog to the forecast for this time period.

Another system will move across the area Thursday bringing another
period of rain to the area. Still some uncertainty for the exact
track of this system but there could be an axis of moderate to
perhaps briefly heavy rain, perhaps just east of the cwa as shown
by the ECMWF. After this rain ends Thursday evening, there could
be another round of widespread fog, perhaps dense fog Thursday
night into Friday morning but even more uncertainty for fog during
this time period.

There is another system that will move across the Ohio Valley and
southern Great Lakes region Saturday night into Sunday and there
remains quite a bit of uncertainty for the track of this system.
There is more ensemble support among the ECMWF members for this
system to affect the local area than the GFS ensembles. If precip
were to occur, it would likely start as rain with some potential
to mix or change to snow but temps are marginal. Blended guidance
has increased pops, still into the chance category and that seems
fine from this distance. cms

&&

.HYDROLOGY...
Issued at 338 PM CST Tue Jan 23 2024

A complicated hydrologic forecast returns to the area this week
through the weekend due to uncertainty in temperatures, rainfall,
snow melt, and ice conditions. Snowpack across portions of
northern Illinois holds 0.5-2 inches of water equivalent, with
higher values generally near and along an axis from near the Quad
Cities east to near the western Chicago suburbs. Soils are
saturated and frozen near the surface. Significant ice cover
exists along most area rivers, with a few ice jams ongoing. The
snowpack is generally several degrees below freezing. Temperatures
are forecasted to remain above freezing for at least the next
several days, causing snow melt, with multiple chances for light
rain possible. Many factors which are evaluated for flood risk
are completing, with some suggesting a decreased risk and some
favoring an increased risk.

Light rainfall will at first cause little runoff as it will be
absorbed by the cold snowpack. Runoff will eventually occur as the
snowpack will warm to near freezing and begin to melt. Snow melt
will be at least partially blocked by frozen soils underneath,
increasing the runoff ratio, despite temperatures in the 30s which
would generally be favorable for slow melt and limited flooding.
The biggest forecast questions remain related to how quickly
runoff reaches area rivers and how quickly snow can melt.
Confidence in temperature forecasts is relatively high for the
next few days, but decreases slightly by Thursday through early
next week. If temperatures are able to warm well into the 40s,
enough melting may occur for rivers to rise to near flood stage in
some areas. Standing water would also be likely along small creeks
and other poor drainage areas, with shifting river ice along area
rivers leading to a risk for break-up ice jams. Downstream of ice
jams, including the significant ice jam located on the Kankakee
River near Wilmington, a sudden break up or shifting of ice could
cause rapid-onset flooding of downstream areas with a combination
of water and flowing ice. River gauge observations will be
monitored closely over the next few days to better assess the
speed of snow melt and runoff, which will impact the eventual
severity of river rises and possible flooding.

WSL

&&

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

Forecast concerns for the terminals...

* Periods of drizzle and fog tonight resulting in LIFR conditions

* Steady rain moves in late tonight into Wednesday morning, before
  transitioning back to drizzle fog Wednesday afternoon/evening

* IFR and LIFR conditions expected to persist through the period

Periods of drizzle and fog continue to be observed across the
terminals this evening with some pockets of improvement noted at
times. While guidance continues to struggle with fog and
resultant visibilities, the expectation is for the lower LIFR in
central Illinois to move back into the Chicago terminals later
this evening and persist through much of the overnight hours. A
period of VLIFR conditions is also possible after midnight as
temperatures cool which should result in a period of dense fog and
visibilities around 1/4 SM and sub 200 ft ceilings. Though the
exact length of these VLIFR conditions remains uncertain as a
disturbance will bring in more steady rain which should allow for
some improvement in ceilings and visibilities late tonight into
Wednesday morning.

The steady rainfall is expected to persist through the morning
hours before mid-level dry air advects into the area and
transitions the rain back to more of a drizzle or fine mist. As a
result, ceilings and visibilities are expected to lower back to
lower LIFR conditions Wednesday afternoon. Guidance does suggest
that the drizzle should taper late Wednesday afternoon into the
early evening hours which may allow for some minor improvement of
ceilings and visibilities (IFR conditions at best) until fog
redevelops Wednesday night. But confidence on exact ceiling and
visibility values is low. Otherwise, winds are expected to remain
light (speeds generally under 5 kts) and become east-northeasterly
on Wednesday.

Yack

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...Dense Fog Advisory...ILZ019-ILZ020-ILZ021-ILZ023-ILZ032-ILZ033-
     ILZ039-ILZ105-ILZ106-ILZ107-ILZ108 until 10 AM Wednesday.

     Dense Fog Advisory...ILZ003-ILZ004-ILZ005-ILZ006-ILZ008-ILZ010-
     ILZ011-ILZ012-ILZ013-ILZ103-ILZ104 until 10 AM Wednesday.

IN...Dense Fog Advisory...INZ001-INZ002-INZ010-INZ011-INZ019 until 10
     AM Wednesday.

LM...Dense Fog Advisory...IL and IN nearshore waters until 10 AM
     Wednesday.

&&

$$

Visit us at weather.gov/chicago

Follow us on Facebook, Twitter, and YouTube at:
www.facebook.com/NWSChicago
www.twitter.com/NWSChicago
www.youtube.com/NWSChicago