Forecast Discussions mentioning any of "ACARS" "MDCRS" "AIRCRAFT" "TAMDAR" "AMDAR" "WVSS" received at GSD on 09/14/21

Area Forecast Discussion

National Weather Service Chicago/Romeoville, IL

 946 PM CDT Mon Sep 13 2021

.UPDATE...
945 PM CDT

The main update this evening was to increase precipitation chances
a bit across northwest and northern Illinois and to ramp gusts up
late tonight and especially mid-late morning.

While regional evening RAOBs and recent AMDAR soundings reveal a
still-parched atmosphere overhead, upstream soundings indicate
recent moistening in response to a fairly potent mid-level wave
and attendant jet streak. This incoming feature, depicted nicely
in Simple Water Vapor RGB loops, is pushing into northwest Iowa
and western Minnesota at this hour. Evening model guidance does
show a continued cooling/saturation of the 800-600 mb layer
through the late evening/overnight hours as warm advection
increases in response to this incoming wave of ascent. There`s
still quite a way for our airmass to go before it`s able to
support elevated convection, but the magnitude of forcing suggests
that chances are certainly higher than last night. Have marched
PoPs up into the 30-40% range near Rockford and across far
northern Illinois after 3 AM or thereabouts. The main corridor of
forcing looks to push more solidly into southern Wisconsin, but
envision there is a potential for scattered convection to scrape
our northern counties late tonight.

Otherwise, have nudged wind gusts up after sunrise across the
region, and especially along the I-55/57 corridors where we`re
anticipating mixing into a waning but still robust low-level jet.
Forecast soundings support 35+ mph wind gusts for a time through
mid-late morning before the better flow pushes off to our east.
Thereafter, no changes to the thinking regarding thunderstorm and
any strong-severe chances later Tuesday afternoon with the
incoming cold front with the main corridor still looking to be
south and east of I-55.

Carlaw

&&

.SHORT TERM...
201 PM CDT

Through Tuesday night...

The focus remains on shower and thunderstorm potential through
Tuesday night. A warm front, which has been reinforced by outflow
from earlier thunderstorms over southern Wisconsin, is stretched
from near or just north of Rochelle across northern DeKalb and
Kane Counties and then southeast to downtown Chicago at 1 pm with
some southward movement still occurring thanks to the outflow and
some lake enhancement closer to the lake. The boundary will
probably continue to sag inland across the Chicago metro thanks to
the lake but western sections should lift northward. Southwest
winds will then spread area-wide tonight. An upper trough moving
east across the Dakotas this afternoon will help to strengthen a
surface low over southern Minnesota this evening which will then
track across the U.P. of Michigan Tuesday morning. A cold front
will trail the low to the southwest. Low level flow above the
surface strengthens in response to the developing system resulting
in some isentropic ascent developing tonight across the northern
tier or two of IL counties. Precipitation chances with this are
minimal but will maintain the slight chance overnight and first
thing Wednesday morning along/north of I-88. Otherwise the
shortwave and surface low are to the north and will keep best
precip chances out of the local area.

The cold front is expected to reach north-central IL late Tuesday
morning then steadily move east through the afternoon. The
tightening pressure gradient will result in increasing southwest
winds during the morning with gusts into the 30-35 mph range
expected by midday. Will go ahead and issue a small craft advisory
from late tonight through Tuesday afternoon. Winds will be
offshore so a beach hazards statement will not be needed. It will
be very warm with highs in the mid to upper 80s. Forecast
soundings show capping in place through much of the early to mid
afternoon which will keep thunderstorm potential low. Other than
the surface front large scale ascent is lacking through mid
afternoon reinforcing the low thunderstorm potential across the
area. By late afternoon, and more so early evening, another
shortwave approaches and the area becomes better positioned under
the favorable right entrance region of an upper jet. The front is
expected to be through much of the area by evening when this
better forcing arrives and capping weakens. While some isolated
shower/thunder activity cannot be ruled out through mid afternoon,
it is looking like there will be minimal activity at this point so
many areas west of a Chicago to LaSalle line may not see much if
any activity. Chances increase to the southeast late afternoon and
into the evening with decent confidence in at least scattered
thunderstorms along and east of I-57, especially the further
southeast you go. Central and southern Chicago metro locations and
areas along and just west of roughly I-55 are more of the wild
card depending on where the front ends up when capping weakens so
will keep these areas in the chance category. Severe storms are
possible during the late afternoon and evening, with damaging
winds and hail will be the main threats with a non-zero tornado
threat as well. The greatest severe potential lines up with the
SPC Slight/Level 2 risk which includes areas east of a line from
about Chesterton, IN to near Gibson City, IL. The front will
continue southward as the upper trough departs so activity should
largely be to the southeast of the forecast area overnight.

MDB

&&

.LONG TERM...
201 PM CDT

Wednesday through Monday...

As a surface high pressure system slides into the Great Lakes behind
the cold frontal passage on Tuesday, Wednesday will feature light
winds, clearing skies, and highs in the mid to upper 70s. Onshore
flow will keep lakeshore locations in the lower 70s. Wednesday night
will be the coolest of the week with lows in the low to mid 50s,
except locally warmer near urban centers.

The reprieve from above-normal temperatures will be short-lived as
the upper-level jet retreats northward in response to a digging
trough across the western United States. Accordingly, a warming
trend is expected toward the end of the workweek with highs in the
low to mid 80s Thursday and Friday, and upper 80s (to around 90?)
this weekend. A quick gander at climatological stats reveals that
record highs and warm lows should be safe, but the temperatures will
nevertheless be 10 or more degrees above normal.

Precipitation chances from Wednesday onward appear low (20% chance
or lower). The best chances may arrive Friday night into Saturday as
an upper-level wave moves through the northern Great Lakes, perhaps
sparking a few showers and storms along a remnant frontal boundary
along the IL/WI border. Such a scenario is supported by several
"boxes" generated by WPC Cluster Analysis, which nicely summarizes
ensemble model data into equally-likely outcomes. A better chance
for showers and storms may hold off until next week when the
aforementioned western US trough finally ejects eastward and
potentially interacts with remnant moisture from Tropical Storm
Nicholas.

Borchardt

&&

.AVIATION...
For the 00Z TAFs...
651 PM CDT

The concerns for the Chicago area TAF sites include:

* Lake breeze moving through MDW at or shortly after 00z
* Low chance for SHRA/TS later tonight
* Marginal LLWS at RFD overnight
* Southwest wind gusts possibly up to 35 kt Tue AM thru mid day
* Chance for TSRA near or more likely just east of ORD and MDW mid
  Tuesday afternoon, with a higher chance for GYY
* Low chance for a northeast wind shift late Tuesday afternoon

In the near term, only item of note is the lake breeze approaching
MDW after having passed through ORD during the 22z hour. Winds
will then come back around to the south and then become gusty out
of the southwest 06z and onward. With expectation of at least
occasional gustiness over the Chicago metro terminals, setup is
too marginal to maintain LLWS mention. RFD remains favored for
marginal LLWS. After sunrise Tuesday, more effective mixing into
the departing low level jet could yield gusts as high as 35 kt or
so through mid day, then easing back into 25-30 kt range ahead of
an approaching cold front.

Scattered TSRA are expected to form ahead of/along the front
eventually Tuesday afternoon, possibly over or just east of ORD
and MDW and then track southeast. Confidence is low for direct
impacts at ORD and MDW, though felt enough chance of VC/airspace
impacts to warrant PROB30 mention for planning purposes. GYY has
overall best chance for direct terminal impacts until the frontal
passage. If TS coverage is higher than expected, can`t rule out a
wind shift to northeast late Tuesday afternoon. In the TAFs,
continued with a wind shift to west-northwest and then northwest
behind the cold front with gusts up to and possibly over 20 kt
into the evening.

Castro

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...Small Craft Advisory...LMZ740-LMZ741-LMZ742-LMZ743-LMZ744-
     LMZ745...5 AM Tuesday to 6 PM Tuesday.

&&

$$

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