Thu Feb 15 Meteorology for Chicago

[Brett Crapser]

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1124 AM CST Thu Feb 15 2024

KEY MESSAGES...

- Increasing confidence for a narrow bands of light snow to set
up tonight into Friday morning for areas south of I-88

- Colder temperatures to start the weekend, before becoming
more mild next week

&&

DISCUSSION...
Issued at 317 AM CST Thu Feb 15 2024

Rain is currently moving eastward along a surface warm front.
Temperatures over Illinois remain above freezing allowing rain
to be the dominant precip type. However, colder temps reside
just over the state line in Wisconsin, where it is currently
snowing. Most of the precip is expected to be rain, but given a
tight baroclinic gradient, there remains a slight chance for
some wet snow to mix in along the northern state border. While
lightning was observed in Hancock County, instability over the
CWA remains fairly limited. An isolated thunderstorm cannot be
completely ruled, but the confidence was too low to mention it
in the forecast. The system should move northeast of the city
of Chicago by mid morning, with clouds clearing in its wake
which will provide a brief respite before the next system moves
in.

The next upper level disturbance approaches northern Illinois
Thursday night. Strong frontogenesis at midlevels will increase
as the surface baroclinic gradient tightens. As Friday
approaches, an upper level jet will strengthen and loosely
couple with the left exit region of a southern stream jet
providing additional synoptic left, with a secondary mid level
f-gen circulation developing across central Illinois, growing
to become the dominant circulation.

With sub-freezing thermal profiles through the column, confidence
in snow bands developing was such that PoPs were increased to
above 75 percent south of I-80. With modest ascent through a
highly saturated and fairly deep dendritic growth zone, snow
ratios can be expected between 13 to even 17 to 1. Over the last
three days models have gradually nudged this system slightly
southward. But now that high-res guidance is keying in on the
southern part of the cwa, confidence is growing in most of the
area receiving at least a light dusting. However, the greatest
uncertainty is the exact location of the dual f-gen
circulations. Where that develops could receive over an inch of
snow; however, it wouldn`t be surprising to see locally over 2
inches where the strongest forcing develops given relatively
high SLRs and pockets of slantwise instability noted on regional
cross sections. The strongest signal for this currently is
south of about the Kankakee River.

Current projections show this system exiting through Friday
afternoon, with a colder air mass descending in behind it. Low
temperatures Saturday morning can be expected in the teens, with
highs only rising into the low 30s. Post frontal winds will
also increase with wind gusts up to 25 mph, netting wind chill
values down to the single digits Saturday morning and into the
teens in the afternoon.

By Sunday, upper level ridging begins to develop aloft with
higher pressure developing at the surface. Warm air advection
will guide temperatures back above normal, and there`s even a
chance for some 50 degree readings by Tuesday. There is a weak
signal for maybe another chance for some rain mid to late next
week, but this far out it is not enough to put stock in it.

DK

&&

AVIATION /18Z TAFS THROUGH 00Z SATURDAY/...
Issued at 1124 AM CST Thu Feb 15 2024

Aviation key messages during the period:

- MVFR cigs improve early this afternoon, with VFR conditions
expected after 20z this afternoon.

- Occasional northwest wind gusts around 30 kts early this
afternoon, gradually easing in magnitude through the
afternoon.

- Potential for a period of -SN Friday morning.

MVFR cigs linger across the Chicago terminals early this
afternoon. However, improving conditions (to VFR) are already
approaching the western suburbs, and expect this improving trend
to work across the the main Chicago area terminals into the 19-
20z timeframe. Otherwise, strong/gusty northwesterly winds
will continue this afternoon, but speeds are expected to gradual
ease through the afternoon and evening hours as the surface
gradient relaxes. Winds will becoming light (under 10 kt)
overnight tonight and the directions may turn north-
northwesterly for a period after midnight. However, winds will
settle back into a west-northwesterly direction by daybreak
Friday.

The next disturbance will bring a chance for snow to the
terminals Friday morning. Uncertainty in both the system`s track
and the depth of low-level dry air remain. Also, with the best
chance of the more persistent snow looking to remain south of
most of the terminals, we have opted to go no higher than a
PROB30 mention for -SN at the terminals for Friday morning.

KJB

&&

LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...Small Craft Advisory until 9 PM CST this evening for Winthrop
Harbor IL to Gary IN.

Small Craft Advisory until 3 AM CST Friday for Gary to Burns
Harbor IN-Burns Harbor to Michigan City IN.

&&

$$