Sat Feb 10 Meteorology for Chicago

[Brett Crapser]

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1118 AM CST Sat Feb 10 2024

KEY MESSAGES...

- Temperatures will trend cooler, but remain above normal.
Otherwise, no impactful weather is expected through the
weekend.

- An active pattern will keep occasional systems passing
nearby, but the next chance for precipitation (25 to 35
percent) looks to be mid week.

&&

DISCUSSION...
Issued at 325 AM CST Sat Feb 10 2024

A large swath of stratus clouds seen on nighttime microphysics
is slowly descending southward out of Wisconsin and southeastern
Minnesota behind a weak front. A period of mostly cloudy to
overcast conditions will occur for some this morning, but
uncertainty exists in how far south this widespread stratus will
spread, especially with pockets of scattering already
developing upstream. With forecast soundings showing the stratus
layer is relatively thin, increasing mixing after sunrise may
help scatter out the leading edge of the stratus deck.

Ahead of the front in south-central Illinois and Indiana are
thunderstorms that are still firing at the time this discussion
was published, moving east. With lingering weak instability,
there is still a slight chance (less than 20 percent) for
isolated showers in southern Iroquois or Benton Counties before
day break as they are closer to the elevated moisture source,
but it is more likely that the forecast area remains dry through
the day.

Surface high pressure will build into the area today leading
to quieter and drier conditions. A cooler air mass behind the
front will limit high temperatures into the low 40s, potentially
only the upper 30s along the Wisconsin border, with similar
conditions forecasted on Sunday. It will be slightly cooler than
recent days, but afternoon high temperatures are expected to
remain above normal as morning low temperatures drop below
freezing each night.

There is a deeper, longer wave trough currently over the
intermountain west that will move eastward through the day. Both
GEFS and EPS ensemble guidance suggest a sub-550 dam closed low
aloft may materialize as it takes a southerly track toward the
Ohio River Valley through Sunday into Monday night. Despite the
southern track of the closed low aloft, a positively tilted
trough is expected to move over northern Illinois and Indiana on
Monday. Even with fairly robust forcing associated with the
trough, the forecast remains dry through Tuesday due to the main
reservoir of moisture remaining well south tied to the
aforementioned closed upper low. If the upper low`s track
shifts a little more northerly, PoPs may need to be re-added for
areas south of I-80. For now, it appears that the precip will
pass nearby, but remain to the south.

As that trough moves east and stalls over the North Atlantic and
a weak ridge aloft develops over the Four Corners, northwest
flow develops over much of the CONUS. Models have been
consistently showing a weak wave developing mid week (maybe Wed
or Thurs?) with a noticeable f-gen circulation associated with
its passage. However, models are still disagreeing on exact
timing and impacts. One limiting factor, again, will be the
amount of moisture available. The GEFS appears to be a bit more
moist than its Euro counterpart and the present moment. But with
steeper lapse rates aloft, it looks like chances are that the
next potential for precip should arrive mid week.

While air temperatures are still forecasted to be above
freezing, model wet bulbs are still leaning sub-freezing. For
now, maintained the chance (20 to 40 percent) for a rain/snow
mix from late Wednesday into Thursday, but that will need to be
refined early next week. With northwest flow aloft, additional
fast-moving disturbances may bring intermittent precip chances
to the region into next weekend, but the devil will be in the
details as the time period draws closer.DK

&&

AVIATION /18Z TAFS THROUGH 00Z MONDAY/...
Issued at 1117 AM CST Sat Feb 10 2024

Forecast concerns for the terminals...

* MVFR stratus will continue to scatter out this afternoon

* Chance for some MVFR clouds to redevelop Sunday morning, but
confidence is low

The 2000 to 2500 ft stratus deck continues to show signs of
eroding which should allow VFR conditions to return to the
terminals around 19z this afternoon. While VFR conditions are
expected to prevail through the remainder of the forecast
period, there is a signal in guidance that MVFR clouds may try
to redevelop Sunday morning. Given that the moisture depth looks
to be very shallow on Sunday confidence on a true MVFR ceiling
is low, but I did decide to add a FEW025 mention to account for
this chance.

Otherwise, expect north-northwest winds through the period with
speeds generally around 10 kts. Though a few gusts around 15 to
18 kts may be seen at times this afternoon.

Yack

&&

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

&&

$$