Forecast Discussions mentioning any of "ACARS" "MDCRS" "AIRCRAFT" "TAMDAR" "AMDAR" "WVSS" received at GSD on 09/16/19

Area Forecast Discussion

National Weather Service Chicago/Romeoville, IL

 929 PM CDT Sun Sep 15 2019

.UPDATE...
910 PM CDT

Main update this evening was to issue a dense fog advisory for
portions of north central IL for later tonight into Monday
morning. In the near term, area of scattered light rain showers
which moved through earlier this evening has diminished with dry
conditions now in place. Despite a boundary lingering across
northern IL, large scale support is really lacking and expect dry
conditions for the remainder of the night.

Turn attention to the likely fog development tonight, with once
again, a dense fog advisory in place for portions of north central
IL. Very moist conditions in place with high dewpoint air
overhead, on both sides of this front with upper 60s to low 70s
dewpoints in place over much of the region. Additionally, several
rounds of heavier rainfall across the area has added to this
moisture content. With a relaxed gradient overhead and with skies
expected to clear tonight, setup will favor fog development across
the CWA. Fog will likely be observed areawide tonight, but do have
some higher confidence at this time for more dense fog
development across northern IL where the higher moisture is
situated. These factors and the fact that dense fog has already
developed across parts of southwest WI where the skies have
cleared, has provided higher confidence for dense fog. Locations
within the advisory may have a couple of more hours until dense
fog develops, especially as stratus still remains in place.
However, this fog should really begin to develop around the
midnight time frame. Remaining areas outside of the advisory in
northern IL and even northwest IN once again will also likely
observe fog, and possibly dense fog, later tonight into early
Monday morning. This will be closely monitored, with expansion of
this advisory appearing probable later tonight.

Rodriguez

&&

.SHORT TERM...
254 PM CDT

Tonight...

Main questions, with the passage of a weak cold front tonight,
are additional precip chances and the development of fog in the
moist and stable airmass left behind from earlier widespread
rainfall. Given the lack of upstream development so far today,
the diffuse nature of the front, the low level inversion evident
in the latest aircraft soundings, and the meager upper level
support, chances for additional showers and thunderstorms appear
to drop considerably after this evening. These factors also
support the idea of fog development, mainly toward late evening
and the early overnight hours. There is a chance that slightly
better mixing with the frontal passage might support stratus
rather than fog toward morning, but will maintain a mention of at
least patchy to areas of fog for most of the overnight period.

Lows tonight, with the considerable cloud cover, should remain in
the middle 60s in many locations, cooler north and warmer south,
but relatively mild for mid September.

Lenning

&&

.LONG TERM...
201 PM CDT

Monday through Sunday...

Any lingering patchy dense fog Monday morning should readily scatter
out into a low stratus deck through the mid-morning. Low-level
moisture will likely remain trapped under a stout subsidence
inversion through Monday morning and possibly even into the
afternoon hours, especially in the vicinity of the lake front. As a
result, opted to knock high temperatures down quite a bit in the
post-frontal airmass for Monday afternoon. It looks like highs may
struggle to make much progress into the lower 70s near the lake
shore, while mid and upper 70s (possibly lower 80s) become more
prevalent the farther inland you go.

Light winds will prevail Monday night into Tuesday as high pressure
begins to scoot off to our north and east. Dewpoints in the lower
60s, coupled with some increase in high cloud cover through the
night should limit our ability to effectively radiate overnight, and
this will probably keep temperatures from tanking. Still, we`ll
probably see some upper 50s in our typical cooler spots. Patchy fog
will once again be possible, but the combination of a decent breeze
immediately off the deck and the aforementioned high cloud cover may
help curtail the potential for widespread dense fog issues.

The more quiescent stretch of weather looks to continue through the
middle of the week, although some low-grade chances may work their
way into portions of the region by Wednesday night/Thursday as the
next set of shortwaves eject out of the cyclonic flow across the
Great Basin and towards the International Border. Forcing for ascent
from these next waves look to remain displaced well to our north and
west, so we`ll cap PoPs at 20-30 percent for the time being. Some
upper impulses may start to inch a bit closer to our neck of the
woods by the end of the week and into the upcoming weekend, which
may bring somewhat better chances for precipitation and unsettled
conditions back to the area.

Carlaw

&&

.AVIATION...
For the 00Z TAFs...

Confidence has increased in low conditions, likely LIFR, at or
very near the Chicago airports Monday morning. Concerns for the
Chicago area airports are:

- Spotty light showers early this evening

- IFR to LIFR stratus likely overnight into mid-Monday morning,
  with IFR to LIFR fog possible

- Light winds at times variable tonight, becoming north-northeast
  near sunrise

- Potential for dense fog later Monday night into Tuesday morning

A weak cold front is moving through the area and dissipating as
it does. Spotty light showers along this will fade early this
evening. The MVFR clouds will fade across north central Illinois
while there is uncertainty across northeast Illinois. This is a
key as to whether stratus or fog will form later tonight. If this
current cloud cover can clear later this evening, then
fog...likely LIFR...is expected. Confidence is highest in this at
RFD, while lower further east toward Chicago where more of LIFR
stratus is expected.

Then there is the question on duration of stratus/fog into the
morning. For ORD and MDW, confidence is medium-high in winds
turning north-northeast by or shortly after daybreak. This will
advect in slightly modified, mixed air off of Lake Michigan, and
should help to lift conditions. How quickly though is somewhat
uncertain, and for inland locations such as RFD and DPA, it may
take most of the morning.

Eventual lifting and even scattering is likely into the afternoon.
Winds are favored to become even more northeasterly at the Chicago
airports through the afternoon. Residual low-level moisture will
allow for the likelihood of fog Monday night into early Tuesday
morning.

MTF

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...Dense Fog Advisory...ILZ003-ILZ004-ILZ008-ILZ010-ILZ011 until 10
     AM Monday.

IN...None.
LM...None.
&&

$$

VISIT US AT HTTP://WEATHER.GOV/CHICAGO (ALL LOWERCASE)

FOLLOW US ON FACEBOOK...TWITTER...AND YOUTUBE AT:
WWW.FACEBOOK.COM/NWSCHICAGO
WWW.TWITTER.COM/NWSCHICAGO
WWW.YOUTUBE.COM/NWSCHICAGO