263
FXUS63 KLOT 051220
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
620 AM CST Thu Mar 5 2026

.KEY MESSAGES...

- Expect periods of dense fog through Friday morning. The
  longest duration of very low visibility will be near the
  lakeshore, where there may not be much if any improvement.

- Scattered showers and perhaps an isolated thunderstorm may
  move across the area later this morning through midday,
  mainly south of I-88.

- Friday through Friday night will be windy and unseasonably
  warm, with a period of showers and embedded thunderstorms
  during a portion of the daytime hours and then more
  widespread showers and scattered storms late Friday night into
  early Saturday. Some of these storms may be strong to even
  severe, though confidence is low.

&&

.DISCUSSION...
Issued at 330 AM CST Thu Mar 5 2026

Through Tonight/Early Friday:

Early this morning, most of the region (including the entire NWS
Chicago forecast area) is "socked in" with dense fog. Stagnant
flow through most of the column paired with low-level moisture
trapped beneath a strong inversion has resulted in extensive
fog. Widespread 1/4 mile or less visibility near/north of I-80,
including frequently 1/8 mile at both ORD and MDW overnight will
probably be steady state past sunrise (and deterioration where
visibility is >1/4 mile as of this writing). During the 2 AM
hour, there was some temporary improvement in spots farther
south, but suspect this will be short-lived as well. As noted in
the update yesterday evening, the main axis of widespread
showers and embedded storms has primarily remained just south
of the CWA, grazing areas south of US-24. This will pave the way
for drizzle to likely expand northward with time, causing
visibility to deteriorate where it temporarily improved, and
reinforcing the very low visibility farther north.

We`re watching an area of showers and isolated thunderstorms
near the Iowa-Missouri border associated with the cold core of
a 500 mb short-wave. This feature tracking east-northeastward
through the morning may yield some widely scattered showers and
perhaps a few isolated thunderstorms over roughly the southeast
2/3 of the CWA. Any heavier showers able to materialize may
theoretically be able to stir up the low levels enough for
modest visibility improvement. The most likely contributor to
improvement in the fog today, where this occurs, will be the
stronger early March sun. Interior sections of the CWA,
especially with south and southwestward extent will probably be
the best candidates for noticeable improvement. With light
reinforcing light onshore flow from the cold marine layer of
Lake Michigan, the stagnant surface pattern will likely be
conducive to dense fog maintenance across northeast Illinois and
perhaps near the Indiana shore as well. Have made no changes to
the current dense fog advisory expiration times today, with the
day shift using observational and web cam trends to inform
where the advisory needs to be extended (possibly into Friday
morning).

A continued stagnant surface pattern tonight through pre-dawn
Friday will result high boundary layer moisture trapped beneath
an inversion. Where the fog is actually able to erode today,
which seems unlikely to a meaningful extent near the lake, will
once again be conducive for fog expansion and areas of dense
fog. Southerly flow off the deck will increase towards daybreak
Friday north of an advancing warm front, which may start the
improvement process with southward extent.

Castro/Borchardt


Friday Morning through Wednesday:

Forecast attention the rest of Friday into early Saturday
continues to focus on the threat for potentially two periods of
showers and thunderstorms Friday and Friday night.

A mid/upper-level trough is forecast to traverse the central
and southern Rockies early Friday morning. Thereafter, a sharp
mid-level impulse, and accompanying mid and upper level speed
max, is expected to eject northeastward across the Plains into
the Upper Midwest Friday into Friday night. Meanwhile, at the
surface, a cold front, along the southwest periphery of a
surface low, will shift southeastward from the northern and
central Plains into the western Great Lakes by early Saturday
morning.

Into Friday morning, strengthening low-to-mid level warm air
advection, atop a northward shifting warm frontal boundary,
should result in an uptick in convection to our west-southwest.
We will be watching the evolution and extent of this activity,
as its focus should generally spread northeastward towards our
area later Friday morning. With no real change in forecast
thinking with this activity, we continue to focus the highest
chances for showers and scattered storms across northern IL
during the late morning and early afternoon period
(approximately the northwest 1/2 or so of the CWA).

Outside of the morning-early afternoon shower and thunderstorm
chances, temperatures Friday afternoon will soar well into the
60s and even 70s (near record territory for ORD and RFD which
is 70F at both sites on Friday) as the surface warm front
surges northward across Illinois and Indiana. However, as is
typical this time of year, onshore flow ahead of this warm front
across far northeastern IL (including parts of Chicago) will
keep conditions much cooler through much of the day as the warm
front struggles (at least initially) to overcome the colder
marine airmass over southern Lake Michigan. Patchy fog would
also be likely to hang on near the Illinois shore until surface
flow turns offshore sometime later in the day.

As we have mentioned for the past couple of days now, the
combination of the very warm temperatures and unseasonably high
dew points into the upper 50s to lower 60s (locally mid 60s?),
will support a weakly to moderately unstable and strongly
sheared warm sector supportive of a conditional threat of severe
thunderstorms Friday afternoon and night. The main question at
this point is if any of these early afternoon storms will be
able to become surface based near the quickly approaching warm
front before the main focus for them shifts to our northeast
later in the day. If they do become surface based, then severe
storms would be more of threat for a period Friday afternoon and
early evening, particularly near the surface boundary. However,
at this time confidence on the severe threat with these early
storms remains low. However, the currently more likely scenario
appears to be the convection remaining elevated/just north of
the front, possibly yielding a threat for mainly sub-severe
hail. Ultimately, it is going to come down to the timing of the
northward frontal surge into northern IL Friday afternoon.

A period of nearly storm free weather (PoPs 15-25%) still
appears plausible for a time late Friday afternoon and evening
due to capping and/or a general lack of large scale forcing
(neutral height tendencies) following this initial impulse,
though south-southwesterly winds do look to become gusty (35+ mph)
in the warm sector Friday afternoon and evening. Assuming these
capped/primarily dry conditions come to fruition, it will feel
like a warm, slightly muggy, and windy later spring season night
for those out and about. Expect shower and thunderstorm chances
and coverage to ramp up again late Friday night into early
Saturday morning in association with a cold front sweeping
eastward across the area.

The currently unfavorable diurnal timing of the cold front
passage late Friday night into early Saturday does raise
questions as to the threat for organized severe weather with
this activity locally. However, the combined presence of near
record warm temperatures and dew points deep into Friday night
within a strongly sheared kinematic environment certainly will
support at least a conditional risk for some stronger storms
capable of damaging winds late Friday night. Unless confidence
increases in surface based warm sector storms Friday afternoon
and early evening during peak instability, the higher severe
threat may focus farther west than the current level 2 of 5
threat area in the SPC day 2 outlook (ie. west of the Chicago
metro, east-central IL, and far northwest Indiana).

Rain chances will come to an end following the cold frontal
passage on Saturday, at least for a few days. Temperatures will
cool down a bit immediately behind the front Saturday PM, but
generally remain well above average through early next workweek.
For areas that don`t receive much rain in the upcoming stretch,
we`ll need to monitor for elevated fire danger on Sunday and
Monday due to mild temps paired with fairly low dew points and
gusty southwest winds. Both next Monday and a part of Tuesday
look quite warm (highs possibly 70+ for a good chunk of the
area). Thereafter, recent medium to long-range ensembles have
continued point towards a stronger cold front passage in the
wake of a possible formidable mid-latitude cyclone with
widespread precipitation mid next week, bringing a return to
colder conditions. Tuesday PM may have a threat for thunderstorms
ahead of the cold front, followed by widespread rain into
Wednesday (non-zero chance of mixing with wet snow in spots
before ending).

Castro/KJB

&&

.AVIATION /12Z TAFS THROUGH 18Z FRIDAY/...
Issued at 620 AM CST Thu Mar 5 2026

Main Concerns:

- Fog and very low stratus trends through Friday morning

- Potential for thunderstorms mid-late Friday morning

After spending the entire night at 1/8 to 1/4 mile VSBY, ORD and
MDW (along with other Chicago metro sites), have recently
improved a bit to 1SM. A bit wary of this only being temporary,
so we did hold onto prevailing LIFR VSBY in the ORD and MDW
TAFs for now until the late morning. All in all, given the
recent variability, confidence is on the lower side regarding
specific VSBY trends at the Chicago airports. GYY, with its
proximity to the lake, will be slower to improve, if at all.
Meanwhile, conditions never got quite as low at RFD, though it
still appears possible for temporary LIFR VSBY to be observed
there. Confidence is higher in CIG trends, with only minor
improvement at best with time today, particularly in RFD area,
and possibly none at all at GYY.

The setup appears favorable for VSBY to tank once again this
evening, particularly at Chicago metro TAF sites, with under
1/2SM certainly possible. If this occurs, wouldn`t expect
improvement until after sunrise Friday, when strengthening
southerly winds aloft may finally be able to scour out the fog.
Finally, at the end of the ORD and MDW TAF period, a broken
arc/line of showers and at least embedded storms will lift
northeastward in the mid to late morning, with a PROB30
included for now.

Winds will be light to calm through tonight, primarily from an
east to northeast component when there is a direction, shifting
to southeast 10+ kt by mid Friday morning.

Castro

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...Dense Fog Advisory until noon CST today for ILZ003-ILZ004-
     ILZ005-ILZ008-ILZ010-ILZ011-ILZ012-ILZ019-ILZ020-ILZ021-
     ILZ023-ILZ106-ILZ107-ILZ108.

     Dense Fog Advisory until 3 PM CST this afternoon for ILZ006-
     ILZ013-ILZ103-ILZ104-ILZ105.

     Dense Fog Advisory until 9 AM CST this morning for ILZ032-
     ILZ033-ILZ039.

IN...Dense Fog Advisory until 3 PM CST this afternoon for INZ001-
     INZ002.

     Dense Fog Advisory until 9 AM CST /10 AM EST/ this morning for
     INZ010-INZ011-INZ019.

LM...Dense Fog Advisory until 6 AM CST Friday for the IL and IN
     nearshore waters.

&&

$$

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