Forecast Discussions mentioning any of "ACARS" "MDCRS" "AIRCRAFT" "TAMDAR" "AMDAR" "WVSS" received at GSD on 08/13/22

Area Forecast Discussion

National Weather Service Chicago/Romeoville, IL

 906 PM CDT Fri Aug 12 2022

.UPDATE...
Issued at 906 PM CDT Fri Aug 12 2022

No significant changes to going forecast this evening.

Evening GOES vapor imagery depicts a mid-level short wave
tracking southeast of the forecast area across MI/IN into
northwest OH. Showers over the western half of the WFO LOT cwa
have eroded significantly in the wake of the mid-level trough
passage, with veering and weakening mid-level winds and an
associated decrease into warm/moist advection magnitude since this
afternoon. Recent AMDAR aircraft ascent sounding from KMDW
continues to sample very dry air below 700 mb, which helped keep
rain from spreading east. While a few isolated sprinkles or light
showers can`t be ruled out through the remainder of the evening
along/west of the I-39 corridor, there should be a relative lull
in any organized precipitation.

Looking upstream, another mid-level wave was evident in water
vapor imagery over northern MN, and this wave is also progged to
propagate southeast across the western Great Lakes region through
Saturday evening with a better-defined lower level circulation.
Thus warm/moist advection is expected to ramp up across the area
again later tonight as this wave approaches. As this occurs, the
warm frontal zone (surface front currently from south central MN
to northeast MO) will shift eastward, with the resulting elevated
moisture convergence axis becoming focused largely north/northeast
of the WFO LOT forecast area mainly across WI/MI and Lake
Michigan. With better precip focus just clipping far
north/northeast IL and northwest IN, going forecast 20-30 percent
pops in these areas continue to appear reasonable. Better chances
of showers and a few thunderstorms locally look to hold off until
later Saturday/Saturday evening.

Otherwise for the remainder of tonight, expect partly to mostly
cloudy skies and temperatures gradually dipping to overnight lows
in the upper 50s/low 60s in most areas (mid-60s in the city).
Going forecast has all of the above mentioned trends in good
shape, and therefore have made no significant changes.

Ratzer

&&

.SHORT TERM...
Issued at 306 PM CDT Fri Aug 12 2022

Through Saturday night...

Mid and high level cloudiness continues to shift southeastward
over the area this afternoon in response to mid-level impulse
shifting into the region. An area of rain showers has also been
ongoing over parts of WI southward into far northwestern parts of
IL. While this area of rain is trying to shift southeastward, it
is running into a much drier airmass over eastern IL and
northwestern IN. AMDAR soundings out of MDW indicate that this
layer is rather substantial below 10,000 feet AGL, so the rain is
likely to continue to struggle to maintain itself as it continues
to the southeast. While I cannot rule out a light shower or
sprinkle into parts of the Chicago area yet this afternoon, it
appears the threat will be low with southeastward extent across
the area. Mainly cloudy skies should keep temperatures in the 70s
the remainder of the afternoon.

Warm air advection will begin to ramp up again tonight over WI in
response to the approach of another impulse shifting
southeastward over the Upper Midwest. While this will continue to
favor showers tonight, it appears the primary focus will remain
to our north in WI. Some of these showers may track into parts of
far northeastern IL Saturday morning, but the chances remain low
and any showers do look to be of low areal coverage this far to
the southwest. The better coverage of showers during most of the
day will remain north and east of much of the area.

While much of the daylight hours of Saturday looks to be free of
thunderstorms, there does continue to be some concern for the
possibly of a few strong storms Saturday evening, particularly
south of I-80. An MUCAPE instability axis initially to our west
early Saturday, is likely to shift east-southeastward into parts
of the area late Saturday afternoon and evening as lower level
Theta-E advection ramps up overhead, owing to the approach of the
next mid-level disturbance and an associated weak surface wave of
low pressure. Concurrently, veering deep layer flow under an
upper level speed max is expected to set up a well sheared
environment supportive of rotating updrafts Saturday evening.
This severe threat remains in question, however, due to questions
on the actual coverage of any storms that are able to develop over
northern IL with the approaching surface wave of low pressure. A
pronounced mid-level dry wedge is also forecast to be quickly
shifting into the area from the northwest into Saturday evening,
which would suggest that the coverage of any storms could be
limited. Nevertheless, the combination of shear and instability
makes for a conditional severe risk, mainly for areas south of
I-80 from 6 to 10 pm Saturday evening. The threat of showers and
storms will come to an end from northwest to southeast during the
evening and overnight.

KJB

&&

.LONG TERM...
Issued at 303 PM CDT Fri Aug 12 2022

Sunday through Friday...

Main concern:

* Threat for dangerous swimming conditions at Lake Michigan
  beaches Sunday afternoon through late Sunday night.

In the wake of Saturday night`s cold front passage, stronger high
pressure nosing southward and modest cold advection down the lake
will result in brisk northeast flow setting up (15-20 kt sustained
winds over southern and southwestern Lake Michigan). This favors
building waves and resultant dangerous currents at the beaches.
Guidance has trended upward the past few cycles with winds over
the lake, so if this holds tonight and tomorrow, expect to need a
Beach Hazards Statement Sunday afternoon and night for most of if
not all our shoreline counties. Weather wise, Sunday should be
partly cloudy to partly sunny on balance in disturbed northwest
flow aloft. Can`t rule spotty PM sprinkles, though pronounced dry
wedge aloft should preclude anything more than that.

A stagnant mid and upper level pattern will result in relatively
quiet conditions next week, albeit turning a bit more unsettled by
Thursday and Friday. Amplified mid-level ridging over the High
Plains and interior West will gradually retrograde westward by mid
to late week. Eastern Canada and northeastern US troughing will
persist through the week. For the first part of the week, parched
mid-levels and lacking forcing given mid-upper pattern configuration
amidst surface high pressure will keep us primarily dry and pleasant.

By Thursday-Friday, retrograding of the 500 mb ridge will allow
for stronger disturbances to pivot southeastward in continued
northwest flow aloft, along with higher column moisture. This
could set up the threat for primarily diurnally driven isolated to
widely scattered showers and maybe a few thunderstorms. From this
vantage point, instability appears it will be weak as forecast
mid-level lapse rates will be weak and dew points will remain
somewhat muted due to cut off moisture trajectories. High temps
Sunday-Friday will range from the upper 70s to lower 80s inland
(possibly mid 80s in spots Tue-Thu) and generally mid 70s near the
lake due to daily onshore flow. Night time lows will be pleasant
from the upper 50s to mid 60s.

Castro

&&

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

Forecast concerns for the terminals...
*Winds becoming southwesterly and breezy Saturday

*Isolated chance for a light shower/sprinkle during the day on
 Saturday

*Additional showers and possibly a thunderstorm Saturday evening

Broken to overcast VFR skies continue to persist across
northeastern Illinois and portions of northwest Indiana in the
wake of an upper-level wave that pushed through the area earlier
this afternoon. These conditions will linger throughout the night
and through much of the day on Saturday as another upper-level
wave moves into the western Great Lakes region. The current
easterly winds will gradually become southeasterly overnight with
speeds remaining light in the 5 to 10 kt range before turning
southwesterly and increasing in speed Saturday morning. Winds are
expected to gust into the mid to upper 20 kt range throughout the
day on Saturday before easing during the evening hours.

The aforementioned upper-level wave will also bring with it the
chance for a few isolated light showers/sprinkles through the day
on Saturday. Forecast soundings continue to show sufficient dry
air in the subcloud layer through Saturday afternoon which should
limit the amount of precipitation able to reach the surface,
therefore I have decided to maintain a dry forecast through
Saturday afternoon. However, heading into Saturday evening a
surface boundary is expected to move through the area which should
allow for ample low-level moisture to return ahead of it and
provide better chances for showers and possible a few
thunderstorms. There is still uncertainty in the coverage of these
evening showers and thunderstorms due to the expected dry air
aloft which could limit the storm potential until the boundary
gets south of the terminals. For now I have decided to add a VCSH
mention Saturday evening at the Chicago terminals and RFD for
this potential, but a more formal thunderstorm mention may be
needed in future TAF issuances if conditions trend more favorably.

Yack

&&

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

$$

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