Forecast Discussions mentioning any of "ACARS" "MDCRS" "AIRCRAFT" "TAMDAR" "AMDAR" "WVSS" received at GSD on 09/13/21

Area Forecast Discussion

National Weather Service Chicago/Romeoville, IL

 926 PM CDT Sun Sep 12 2021

.UPDATE...
926 PM CDT

The 00z RAOBs and recent AMDAR soundings all nicely sampled the
very capped profiles across the region amidst exceptionally steep
mid-level lapse rates. Latest SPC/RAP mesoanalysis places the
MUCIN gradient along the WI state line, though based on lack of
any robust convection in the echoes streaming across Wisconsin,
it`s probably still more capped than indicated. With broad ascent
over the region, some widely scattered showers or even just
sprinkles could sneak into the northern tier. Overnight, while
confidence is low on thunderstorm occurrence, maintained slight
chance mention near the state line to account for sufficient
moistening to tap into the elevated instability. Perhaps a bit
better but still low thunder chance in the early to mid morning
hours before activity exits east to northeast.

Low-level moistening north of the back door cold frontal zone has
allowed for the development of lower clouds over southern Lake
Michigan, seen on recent webcam images from downtown Chicago and
scattered 700-1300 ft clouds on metars. How socked in conditions
remain north of the front in the morning will play a role in how
far north the front will mix back north as a warm front during
peak heating on Monday. It`s a very warm air mass that will again
have gusty south-southwest winds with it. Confidence is on the
lower side into Chicago proper and perhaps extreme northwest
Indiana regarding whether winds maintain an onshore component most
of the day, or if the breezy warm sector can lift north. South of
the front will be in the mid to upper 80s to locally around 90F,
with mid to upper 70s near the lake north of the front and upper
70s to around 80F inland.

Castro

&&

.SHORT TERM...
231 PM CDT

Through Monday night...

A shallow cold front has been drifting south across southern
Wisconsin through the day and is finally nearing the state line
while pushing inland with some lake enhancement across far
northeast Illinois. With little synoptic forcing to enhance the
front and surface winds rather oblique to the frontal zone, the
slow southward progress will continue through this evening while
the lake enhanced portion shifts southwest over the Chicago metro.
The front will ultimately stall in the vicinity of the I-80
corridor overnight. Ribbons of low-level stratus have formed
behind this boundary, with some chance of a shallow stratus deck
expanding below the frontal inversion across northern Illinois
tonight.

A train/cluster of small mid-level disturbances from central Lake
Michigan to eastern Nebraska will translate ENE within modestly
strong mid-level flow through Monday morning. The existing cloud
field within this corridor combined with forecast soundings
reveals a significant difference in the character of the low-level
air mass between northern Illinois and southern Wisconsin. The
stark contrast from a weakly to non-capped environment near
Madison to a substantial ML/MUCIN reservoir by around the I-88
corridor is not expected to change much through Monday as deep-
layer flow above the shallow surface front remains parallel to the
airmass gradient. This should keep nearly all convective activity
north of the state line tonight through Monday, though an
isolated shower or thunderstorm cannot entirely be ruled out for
the tier of counties along the WI state line.

Warm conditions will continue Monday south of I-80, with
decreasing confidence in the temp forecast with northward extent
owing to uncertainty with the rate or retreat of the surface
boundary. If low- level clouds are fairly widespread, a resultant
slow northward advancement of the boundary will produce a cloudy
day with seasonable temps for much of the Chicago metro. Less
cloud cover will support a northward jump of the shallow boundary
via mixing and result in seasonably warm conditions and gusty SSW
winds for much of northeast Illinois. Either way, strengthening
gradient flow into Monday night will ultimately force the front
well into Wisconsin by daybreak Tuesday.

Kluber

&&

.LONG TERM...
209 PM CDT

Tuesday through Sunday...

The main focus will be shower and thunderstorm potential Tuesday/Tuesday
evening. A fairly low amplitude flow pattern will be in place
Tuesday morning with an upper shortwave pushing a strengthening
surface low east-northeast across the upper Great Lakes. A
deepening larger scale upper trough will be following behind which
will help to amplify the upper flow. A surface cold front will
trail the surface low and move across the area Tuesday into
Tuesday evening. From a large scale perspective, the overall setup
is not too dissimilar from the cold frontal passage that produced
thunderstorms last Tuesday, but there are some differences. The
surface low will pass closer to the local area this time around
but be weaker overall. The upper trough is less vigorous and does
lag the surface front by a ways. Moisture looks to be more
abundant with this system and the right entrance region of the
upper jet looks to pass overhead as well. Forecast soundings show
good instability with little capping/inhibition. Overall, the
dynamics are less impressive but there is higher confidence in
stronger instability being present. Still too early for details
and timing/placement of best thunderstorm potential still needs to
be worked out. It also appears that outflow from storms to the
north Monday night may play a role on the evolution of things
Tuesday. For now, will continue with highest chances south of a
Chicago to LaSalle/Peru line later in the day and into the
evening. Expect at least an isolated severe wind threat along with
heavy downpours so it remains a period to keep watching. Highs
will be in the mid to upper 80s with humid conditions thanks to
dewpoints around 70.

The progress of the front will probably slow Tuesday night leading
to continued chances for showers and a few storms across most of
northwest Indiana and areas east of I-57 Wednesday morning. Some
guidance wants to keep chances going through the afternoon but
suspect that the main chance will end up southeast of the forecast
area by then. North to northeast winds will move in as surface
high pressure approaches from the west. Highs around 80 are
expected with decreasing humidity. High pressure shifts to the
east Thursday leading to a return of southerly winds and warming
temps. Another upper trough moves across the Canadian Prairies and
northern Plains Thursday night and Friday with the attendant
surface low well to the north, though a trailing cold front does
cross the area. Guidance has backed off on precip potential with
this front which makes sense conceptually so will carry lower pops
than previously. Weak high pressure passes with modest upper
ridging spreading in by later Saturday. This will support warmer
and more humid conditions for next weekend with little if anything
to support precipitation.

MDB

&&

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

Aviation weather concerns consist of:

* Uncertain potential for low-MVFR or near-IFR cigs tonight and
  Sunday morning

* Shower potential INVOF RFD tonight and Monday morning

* Complicated wind trends Monday afternoon with a potential for a
  gusty southwest wind shift at the Chicago-area terminals

* Returning southwest winds Monday evening and an outside chance
  of LLWS

A composite cold front/lake breeze is currently pushing through
the Chicago-area terminals and will result in northeast winds
prevailing through tonight and into at least Monday morning. SCT-
BKN VFR clouds will prevail this evening in the wake of the front.
There is some potential for these cigs to build down into a low-
MVFR or IFR cloud deck, but wildly varying model solutions cast a
good deal of uncertainty here. Based on upstream conditions, have
continued to favor more optimistic TAFs, but there may be a window
roughly 13.11-13.18z or so for low-MVFR cigs at the area TAF
sites. We`ll continue to monitor observations and guidance trends
this evening.

The next uncertainty is regarding wind trends later Monday morning
and afternoon. The current front is forecast to lift back north
through the day on Monday. How quickly and how far north this
occurs is contingent on how much low cloud cover develops tonight
and Monday morning. Currently, it appears there`s perhaps a 20%
chance for winds to turn southwesterly Monday afternoon at ORD,
and closer to 30-40% at DPA, MDW, and GYY. If winds do switch
southwesterly, they`d likely gust towards 20 kts at times. A lake
breeze casts additional uncertainty on wind trends through the
rest of Monday afternoon. Southwest winds look to return through
Monday evening.

Finally, there`s a potential for showers INVOF RFD overnight
tonight and into Monday morning. Probabilities currently look to
be under 30 percent and have left a mention out of the RFD TAF at
this time as a result.

Carlaw

&&

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

$$

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