764
FXUS63 KLOT 080001
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
601 PM CST Fri Feb 7 2025

.KEY MESSAGES...

- Freezing drizzle is expected to spread northward across the
  area beginning around sunrise toward central Illinois and late
  morning to early afternoon across northern Illinois and
  northwest Indiana. Slippery travel is possible on any
  untreated surfaces.

- There will be multiple opportunities for snowfall in the
  general region next week. Stay up to date on the forecast if
  planning to travel!

&&

.DISCUSSION...
Issued at 250 PM CST Fri Feb 7 2025

Through Saturday Night:

A mid-level trough with abundant Pacific moisture currently
centered over western Montana will track eastward across the
northern Great Lakes through Saturday. Meanwhile, a moisture-
starved low to mid-level wave over New Mexico will lift ENE
along a developing warm front while interacting with an
expansive area of low-level moisture to the east. This will put
the forecast area between the two systems during the initial
stages of phasing.

After the center of a surface high currently over northeast
Illinois shifts eastward through tonight, mid to upper-level
cloud cover will spread atop a persistent ESE feed of dry low-
level air. With limited mid to upper-level support to aid top
down saturation late tonight into Saturday morning, meaningful
precip may struggle to advance northward even as weak low-level
isentropic spreads across the area. By late morning, a mid-level
dry slot ahead of the northern system will limit the potential
for higher precip rates through the remainder of the day.
Residual forcing will quickly diminish from west to east late in
the afternoon, with only light precip potentially brushing the
far northern CWA through the evening.

With the exception of areas closer to the Wisconsin state line,
temps within the saturated layer are expected to be no colder
than -8C through the event. Therefore, precip over much of the
area will fall as drizzle, with the surface freezing line
gradually shifting to as far north at I-80 during the afternoon.
Total QPF should range from a hundredth or two of an inch north
to around 0.05" or slightly higher south. There are a few
factors that could limit impacts from any associated ice with
this system. These include, but are not limited to, less
efficient ice accretion from winds under 10 mph, timing of the
day (i.e. impact of midday sun), and overall weaker forcing
between the two systems. Given the potential for a low-end ice
event with limited impacts during the middle of the day, will
hold off on the issuance of a Winter Weather Advisory at this
point and employ the use of a Special Weather Statement to
highlight patchy slippery spots on untreated surface.

Finally, as noted earlier, thermo profiles closer to the
Wisconsin state line may support a mix with snow at onset of
precip before briefly changing to light snow late afternoon and
early evening across the remainder of northern Illinois and
northwest Indiana. Accumulations of snow are expected to be
under a half inch.

Kluber


Sunday through Friday:

In the wake of the wintry system moving through the Great Lakes
on Saturday, a surface high pressure system centered in western
Canada will expand southeastward and encompass much of the
northern United States on Sunday. As a result, Sunday and at
least the first half of Monday look relatively quiet. Highs will
range from the upper 20s to lower 30s on Sunday to the low to
mid 30s on Monday. Occasional mid to upper-level cloud decks
will stream through the area (thanks to the upper-level jet
being parked overhead).

Monday night into Tuesday, an upper-level shortwave emanating from
southwestern Canada will attempt to phase with an upper-level
shortwave lifting east-northeastward from the Baja California
region. As is often the case with phasing scenarios, forecast
confidence in how both systems will interact and potentially impact
our area is lower than average. With that said, the Monday night
through Tuesday timeframe will be one to watch for a swath of snow
somewhere in the general region. In the wake of whatever
materializes by the phasing waves, a surface high should slide into
the Great Lakes in the Wednesday timeframe perhaps accompanied by a
push of colder air. Thereafter, ensemble model guidance advertises
another upper-level wave diving southward toward the Four Corners
region and then northeastward toward the Great Lakes in the
Wednesday night through Thursday timeframe. Again, exactly how the
wave evolves is subject to considerable spread in guidance at this
range. Nevertheless, the Wednesday night through Thursday timeframe
will be another to watch for a swath of snow somewhere in the
general region. Finally, there is an unusually strong signal within
ensemble model guidance for yet another storm system with a threat
for snowfall in our general region toward the end of next weekend,
as well.

Between all of the storm systems (Monday night into Tuesday,
Wednesday night into Thursday, and end of next weekend), the
signal for accumulating snow in our general region among
ensemble guidance at some point from Monday February 10 through
Monday February 17 is quite strong. In fact, the probability for
at least 1" of snow (meaning the development of a snowpack)
from the 00Z Grand Ensemble (all 100 members of the EPS, GEFS,
and CMCE) by February 17 is >90% within a broad region from
eastern Colorado through the middle Mississippi River Valley all
the way to the northeastern United States. Of course, this does
not guarantee our area will see snow (as the evolution of each
system will ultimately dictate eventual snow swaths), though our
"luck" in dodging impactful snow systems thus far this winter
may soon come to an end. Finally, there is an emerging signal in
ensemble guidance for a period of much below average
temperatures in the middle of the month, likely in response to
what may be a formidable snowpack across the northern US by
then.

Borchardt

&&

.AVIATION /00Z TAFS THROUGH 06Z SUNDAY/...
Issued at 601 PM CST Fri Feb 7 2025

Key Messages:

- A period of freezing drizzle, possibly mixed with snow at
  times, remains possible tomorrow, primarily during the
  afternoon hours.

- MVFR ceilings are likely to accompany the precipitation
  tomorrow afternoon and may linger into the night after
  precipitation ends.


A surface high pressure system continues to slide through the
Lower Great Lakes and is leading to generally light and variable
winds. As the surface high shifts east this evening, winds
should eventually adopt an easterly component and gradually
increase toward 5-10 kt by daybreak while mid- to upper-level
clouds continue to pass overhead.

Tomorrow, a pair of upper-level storm systems will attempt to
merge directly overhead, allowing for precipitation to blossom
over the terminals. Forecast soundings appear problematic from
the perspective of precipitation type, as a developing mid-level
dry slot will scour ice particles (and even mid-level clouds)
atop increasing low-level moisture advection via isentropic
ascent. The net result should be the development and expansion
of "warm" or primarily liquid low-level clouds, contributing to
a threat for freezing drizzle. Perhaps a few snowflakes may
sneak in from time to time as well, particularly as
precipitation begins and ends.

Keeping in mind no two weather systems are the same, the
forcing for ascent tomorrow appears weaker compared to the
weather system that delivered freezing drizzle and even outright
freezing rain earlier this week. In addition, the system
tomorrow will be developing directly overhead, rather than
upstream and moving into our area like a few days ago. Then
comes the question of timing, as forecast soundings depict a
wedge of dry low-level air, which may initially limit the
ability for precipitation to reach the ground. When also
considering relatively muted signals for ceiling and visibility
restrictions advertised by high resolution model guidance (a
signal for largely MVFR rather than IFR conditions), do have to
wonder if precipitation tomorrow may be more of a transient mist
rather than prevailing freezing drizzle.

All things considered, opted to maintain prevailing 6SM BR with
a TEMPO group for FZDZ during a 4-hour window from 18-22Z at
ORD, MDW, DPA, and RFD as this is the time period when forecast
confidence is currently "highest" (50-60% chance) in
precipitation being observed. Cooler temperatures at RFD will
allow for a greater chance for snow to be observed there during
this time window, but anything more than a dusting of snow
accumulation appears unlikely to occur. Confidence in prevailing
FZDZ and lower ceilings and visibilities is higher with
southward extent, so have the FZDZ mention prevailing in the GYY
TAF, though it`s possible that the steadier drizzle will end up
remaining south of GYY. Temperatures are currently forecasted
to be within a degree or two of freezing as precipitation falls
tomorrow, though should start to drop as precipitation begins to
wind down going into the evening. This should allow for a
better chance for snow to mix in or briefly become the sole
precipitation type before the precipitation ceases for good, so
have maintained the inherited PROB30 groups for FZDZSN.

Lastly, the MVFR ceilings that accompanied the precipitation
may linger into the evening after precipitation has ended. It`s
possible that VFR conditions may be attained again prior to the
end of the current TAF period at ORD and MDW, but confidence in
this is low.

Borchardt/Ogorek

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...None.

&&

$$

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