662
FXUS63 KLOT 241152
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
552 AM CST Tue Dec 24 2024

.KEY MESSAGES...

- After a cloudy day today, tonight through Thursday night look
  damp and gray with persistent clouds and periods of light
  showers or drizzle and fog. Temperatures will be above
  freezing.

- Periods of rain expected Friday and again Saturday/Saturday
  night with well above normal temps both days.

&&

.DISCUSSION...
Issued at 238 AM CST Tue Dec 24 2024

Through Wednesday:

A surface ridge of high pressure across the Upper Midwest this
morning will slide eastward across the western Great Lakes
through the day. This will result in light (under 10 mph) east-
northeasterly winds and dry weather across our area today.
Sunshine is not in the cards today, however, as an expansive
low-level stratus deck currently encompasses a vast majority of
the Upper Midwest through the Great Lakes early this morning.
While this deck is rather shallow (only a couple thousand feet
deep), it remains trapped under a stout subsidence inversion
based around 850 mb. Typically, these type of setups lend to
persistent cloud cover with very little diurnal trend in
temperature, and we currently see no reason to expect otherwise
for our area today. Accordingly, under cloudy skies today
temperatures will only climb into the mid to upper 30s after
starting the day in the low to middle 30s.

Low-level cloud cover is not expected to go anywhere anytime
soon. In fact, it is likely that bases will become much lower
into Christmas Day as low-level moisture continues to stream
northward in advance of a pair of mid-level impulses tracking
from the southern Plains east-northward across the Lower and
Mid-Mississippi Valley regions. Christmas Day will thus remain
cloudy, with very low clouds, possibly even lowering into a fog
deck into the evening as a broad region of isentropic upglide
continues to moisten the lowest levels. Periods of light rain or
just area of drizzle are also possible through the day. So,
overall, it is looking to be a very dreary Christmas and not a
white one.

KJB


Wednesday Night through Monday:

Southerly flow will be slowly increasing aloft Wednesday night
into Thursday advecting even more low level moisture into the
region which should result in very low cloud cover and perhaps
some fog. Given the strength of the winds in the low levels,
dense fog seems unlikely, but trends will need to be monitored.
Pops during this period are generally 20-30% but its possible
that liquid drizzle becomes the main precip concern Wednesday
night into Thursday, with temps safely above freezing.

The first of two main systems looks to arrive on Friday,
bringing period of rain and likely pushing the warm front north
of the area with highs in the upper 40s north to lower 50s south
and dependent on the timing of the front, which could be later
in the day. A lull in the precip is possible Friday night into
Saturday morning, then a second system will lift north across
the region Saturday night/Sunday morning, though still
timing/location differences. Blended guidance did bring in a low
chance for thunder Saturday night and while this is possible,
it will be dependent on the track of the system and made no
changes at this time. Behind this system, cooler but still above
normal temps will move back into the area for early next week.
A short break in the precip is expected Sunday night into Monday
but the pattern looks to remain active going into the middle of
next week. cms

&&

.AVIATION /12Z TAFS THROUGH 18Z WEDNESDAY/...
Issued at 552 AM CST Tue Dec 24 2024

Forecast concerns include...

Ifr cigs this morning and again Wednesday morning.
Mvfr cigs this afternoon and this evening.

Ifr cigs are lifting into mvfr from north to south across the
area currently with cigs expected to be around 2kft for much of
today, once these lower clouds push south of the terminals.
Cloud cover is rather expensive but there are some holes across
WI and guidance suggests there could be some breaks in the mvfr
clouds this afternoon but confidence is low. Cigs are expected
to lower back to low mvfr tonight and into ifr early Wednesday
morning with some drizzle possible by midday Wednesday for the
Chicago terminals.

Patchy fog will also be possible for a few hours this morning
and patchy fog will also be possible late tonight into Wednesday
morning but with the expansive cloud cover, any vis reductions
from fog will likely remain mvfr.

North winds will turn northeasterly this morning and more
easterly later this afternoon into tonight with speeds generally
under 10kts. cms

&&

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

&&

$$

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