Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 061719

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1119 AM CST Wed Dec 6 2023

Issued at 257 AM CST Wed Dec 6 2023

Through Thursday...

Key Messages:

* Precip-free conditions with warmer temperatures on Thursday.

The main forecast issue for today centers around cloud trends
which remain a bit unclear. Expansive low stratus is sloshing back
eastward across the region, while a relatively defined back edge
is apparent in nighttime microphysics RGB loops approaching the
Mississippi River. While this clearing line should continue to
march eastward, persistent modest warm advection/upglide on the
back side of the stratus shield may yield stratus re-development
through the day. Copious high-deck will also stream overhead in
advance of an incoming disturbance embedded within the northwest
flow aloft. Have continued to side with the cloudier guidance as a
result, although suspect some holes in the stratus layer will
evolve with time, particularly across our west. High temperatures
should be similar to yesterday.

A robust LLJ will briefly drift across the region tonight, likely
yielding a period of southwesterly breezes with gusts to 20-25
mph. These will ease towards daybreak Thursday. With an increase
in moisture/mixing ratios just off the deck, some signal exists
for some light mist (BR) developing Thursday morning. Mechanical
mixing associated with lingering 25 kt flow towards 925 mb will
curtail any fog threat, however.

Through Thursday afternoon, 850 mb temperatures will jump towards
+10C although this will remain solidly out of reach with a
formidable subsidence inversion in place. 925 mb temperature
climatology does, however, support highs rising towards the 50s.



Issued at 257 AM CST Wed Dec 6 2023

Thursday night through Tuesday...

Key Messages:

* Unseasonably warm temperatures expected for Friday and possibly
  again on Saturday.

* Storm system forecast to move into the Great Lakes this weekend
  and bring more precipitation to the area.

* Seasonable temperatures return next week.

The upper-level ridge will be in the process of exiting the area
Thursday night into Friday as a deepening trough begins to eject
into the central CONUS. This will put our area solidly under the
influence of stout warm advection that will support at least one
more day of unseasonably warm temperatures for Friday where high
temperatures are forecast to be in the mid to upper 50s and
possibly near 60 in spots. While the current forecast remains
under the daily record highs for Chicago and Rockford of 64 and 60
respectively, if the combination of warm advection and diurnal
heating allow temperatures to over preform then we could get close
especially at Rockford. However, cloud cover is expected to
gradually increase through the day on Friday as the trough inches
closer which could limit diurnal heating and keep temperatures a
tad cooler if clouds arrive quicker than forecast. Given that
winds are expected to be breezy on Friday (gusts of 20 to 25 mph)
from the southwest and the anticipation for the thicker clouds to
hold until Friday evening, I have decided to once again trend the
temperature forecast towards 75th percentile of guidance.

Beyond Friday, our attention turns to the aforementioned trough
that is forecast to continue to progress eastward through the
Great Lakes over the weekend. As the trough enters the area, it
will force a frontal boundary through the area sometime on
Saturday which looks to serve as the breading ground for an area
of low pressure to develop along with its associated area of
precipitation. The 00z guidance suite continues to be in overall
good agreement on the evolution of this system through the
weekend, but varies greatly on the speed of the surface frontal
boundary and therefore the exact track of the developing low
center which will largely dictate precipitation duration and type.
While the general consensus is for most of the precipitation to
fall in the form of rain as temperatures should still be within
the 40s and 50s ahead of the front on Saturday, there is the
potential that a transition to a rain/snow mix or even all snow
could occur in the wake of the front if temperatures can cool
quick enough prior to drier air arriving. Given the uncertainty in
the speed and track of the system I have decided to maintain the
NBM forecast for rain transitioning to a rain/snow mix at this
time, but may be able to refine with future forecast packages if a
more consistent solution arises.

Additionally, there is also the chance for some thunderstorms to
develop within the warm sector of this system if the system
dynamics are able to overlap with favorable diurnal heating and
increasing low-level moisture to generate sufficient instability.
Not to mention that if instability is able to materialize it does
appear that a strengthening kinematic field would overlap the warm
sector and could support more robust updrafts and possibly
stronger storms. Obviously there are a lot of conditions that
have to come together for any thunderstorms to materialize, but I
do feel confident enough in this potential to maintain the near
20% thunder chances offered by the NBM though confidence in storm
intensity is low.

Depending on the ultimate speed of the aforementioned system, dry
conditions could return to the area as early as Sunday and look
to linger into at least early next week. However, the upper-level
pattern is forecast to favor northwesterly flow through much of
next week and possibly beyond. Thus, periodic chances for
precipitation could materialize at times when disturbances and
sufficient moisture overlap. Regardless, more seasonable
temperatures for early to mid December are expected through next




Issued at 216 PM CST Tue Dec 5 2023

Unseasonably warm temperatures are expected Thursday through
Saturday and here are the record high temps for Rockford and

Friday, December 8th.....60 in 1980
Saturday, December 9th...55 in 2020

Friday, December 8th.....64 in 1946
Saturday, December 9th...62 in 1879

The record warmest low temperatures for the calendar day, December
8th are,

Rockford...37 in 1987

Chicago....53 in 1946


For the 18Z TAFs...

Forecast concerns for the terminals...

* MVFR ceilings persist into this afternoon before improving
  prior to sunset

* Threat for LLWS for about a 6 hour period mid to late evening
  into the overnight

MVFR ceilings with bases as low as 1800 feet will continue to
blanket northeast Illinois and northwest Indiana into this
afternoon. We likely are not to see much improvement until after
21z this afternoon. Shortly thereafter, VFR conditions are
expected to return for the remainder of the forecast period.

Westerly winds this morning will become southwesterly through the
afternoon as a surface high settles in across the Tennessee
Valley. Wind speeds should generally remain around 10 kts through
the remainder of the day. Speeds are expected to increase this
evening as a strong 45 to 50 kt low-level jet develops about
1500-2000 ft off the surface. While some surface gusts of 20 to 25
kts are possible as this low-level jet develops through the
evening, the gustiness may be more occasional in nature. For this
reason, we have opted to add a formal mention of LLWS to the TAFs
for about a 6 hour period from 03-09z tonight for the Chicago
area terminals, and a couple hours earlier at KRFD. The threat of
Low- level wind shear and gusty surface winds will wind down
after 09z, with a light (up to around 10 kt) southwesterly
surface wind expected through the day on Thursday.



LM...Small Craft Advisory...IL and IN nearshore waters...6 PM
     Wednesday to 3 AM Thursday.



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