Forecast Discussions mentioning any of "ACARS" "MDCRS" "AIRCRAFT" "TAMDAR" "AMDAR" "WVSS" received at GSD on 08/21/21

Area Forecast Discussion

National Weather Service Chicago/Romeoville, IL

 847 PM CDT Fri Aug 20 2021

.UPDATE...
847 PM CDT

No significant changes to going forecast this evening. Did tweak
overnight mins up just slightly in a few spots based on higher
afternoon dew points, but otherwise going forecast is in good shape.

Warm and somewhat muggy conditions can be expected overnight, as the
forecast area remains in a region of weak southerly flow. Surface
low pressure of 998 mb along the ND/MN border will lift northeast
into Ontario through Saturday in association with a fairly deep
negative-tilt short wave, while a trailing cold front currently
across the mid-Missouri Valley moves slowly east. Height/pressure
falls to our north will induce a 15-20 kt south-southwesterly 850 mb
jet which will maintain warm/moist advection into the region. While
some patchy haze/light fog is likely in a few spots, significant
visibility reductions are less likely than the last few nights
thanks to the stronger winds above the boundary layer and light
south-southeast surface winds.

Broken line of thunderstorms across MN/IA and KS are progged to move
east overnight, weakening with time as they move toward the mid-
level ridge axis to the east, through some weakening showers will
likely reach the I-39 corridor by Saturday morning. Better
thunderstorm potential looks to be late morning (west) into the
afternoon hours as increasing low level moisture and approaching
cold frontal boundary coincide with diurnal heating/destabilization.
Ratzer

&&

.SHORT TERM...
235 PM CDT

Through Saturday Night...

A quiet night ahead with emphasis on Saturday afternoon`s
thunderstorm potential.

Agitated cumulus this afternoon was unable to penetrate through a
warming and already dry layer above 700 mb as noted on local
aircraft soundings. A stray shower or two is still possible this
afternoon along the confluence of the lake breeze or into
northwest Indiana but that should be the extent to it.

Convection is beginning to initiate over southeast South Dakota
and should continue to increase in coverage across the
Nebraska/Iowa border area into early evening. This is ahead of a
cold front extending nearly due south from a deep 999 mb low in
eastern North Dakota. Convection is likely to become organized
enough to outpace the front late evening into overnight in Iowa,
aided by a short wave impulse forecast to move eastward across
southern Minnesota/northern Iowa. This convection will also outrun
some of its better forcing and deep layer convergence for ascent,
so should both slow and gradually weaken into the morning.

The approaching primary upper wave and surface cold front will
bring a renewed chance for showers and thunderstorms late
Saturday morning into Saturday afternoon, although capping,
convective inhibition (CIN), and a fairly dry atmosphere above 700
mb should keep the morning storm activity lower in coverage.
Chances for showers and storms increase into the afternoon as the
mid-level moisture increases, CAPE increases (>1,500 J/kg), and
capping weakens. Storm coverage is expected to be scattered in
nature, with chances for showers and storms throughout northern
Illinois and northwest Indiana, mainly between noon and late
afternoon. With the previously mentioned decent CAPE, high PWATS,
weaker deep layer shear, a scenario for some storms developing
strong downdrafts exists, with isolated gusts above 30 mph
possible. The surface front moves through in the late afternoon /
early evening, shifting any lingering showers into mainly
northwest Indiana by mid-evening.

A slight warming at 925/850 mb temperatures would indicate slightly
higher high temperatures on Saturday, but increased cloud cover
may balance out or even keep highs a bit lower than today. With
continued southerly flow, expect dewpoints to increase into the
mid 70s, creating muggy conditions and adding to instability.
Outside of storms we can still see some gusty winds as we mix into
stronger flow off the deck, with gusts of 20 to 25 mph possible
in the late morning and early afternoon. Cloud cover will decrease
Saturday overnight, while dewpoints will drop slightly behind the
front into the 60s and even some 50s in the northwest. Low
temperatures Sunday morning will drop into the 60s.

MTF/BKL

&&

.LONG TERM...
139 PM CDT

Sunday through Friday...

In the wake of the cold front responsible for scattered showers and
storms Saturday afternoon, Sunday continues to look pleasant with
"relatively" low humidity levels, plenty of sunshine, and highs in
the low to mid 80s (upper 70s along the lakeshore). Forecast
guidance has ever-so-slightly slowed on the timing of the frontal
passage, which may help clouds overstay their welcome in northwest
Indiana. However, Sunday afternoon looks great areawide. With
healthy pressure rises of 3-5 mb/6 hours (highest across northern
Lake Michigan) and a long northerly fetch, waves may become choppy
enough for at least a Moderate swim risk along Lake Michigan
beaches especially during the morning hours. So, it may be a good
idea to keep an eye on the forecast if planning to head to the
beach.

The reprieve from hot and humid conditions will be short lived as
an upper-level ridge builds across the central United States and
a surface high pressure system across the southeast reestablishes
unimpeded moist low-level flow from the Gulf of Mexico to the
Great Lakes. Accordingly, muggy conditions will return Monday and
last at least through the middle of next week. Forecast ensemble
mean 850 mb temperatures of +20 to 21C will be supportive of daily
highs around 90F with surface dew points in the low to mid 70s
making it feel closer to 100F. Overnight lows will drop no further
than forecast surface dew points -- generally in the lower 70s.

Of course, building heat and humidity levels will also increase the
chance for waves of gusty thunderstorms, each individually tied to
subtle upper-level impulses as well as remnant mesoscale convective
vortices from prior convective episodes. Accordingly, timing out
when and where thunderstorm chances will be highest can be a futile
effort at this range forcing a forecast with nearly continuous
but low chances for precipitation in spite of the expectation for
many hours to remain dry. Ensemble model guidance appears to favor
Tuesday night into Wednesday as the time period of peak
thunderstorm coverage in the Great Lakes, but really anytime
Monday onward will carry a threat for thunder. Should
thunderstorms move through some or all of our area during daylight
hours, observed temperatures may end up much lower than
advertised. Altogether, the pattern next week looks similar to
last week and the message of increasing heat, humidity levels, and
thunderstorm chances remains appropriate.

Borchardt

&&

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

Aviation Forecast Concerns:

* Breezy south-southwest winds Saturday.

* Scattered thunderstorms in the area Saturday afternoon.

* Cold frontal passage Saturday evening with a wind shift to the
  northwest.

Southerly winds late this afternoon were just strong enough to
push a northwest-southeast oriented lake breeze boundary north of
ORD and MDW. Some moderate TCU along the boundary produced a few
isolated showers, though these have largely dissipated and diurnal
cu should continue to fade with sunset this evening. Surface winds
should settle to a light south-southeast this evening/overnight.
While some haze remains across the region, not expecting a repeat
of overnight fog development as we have seen the past few nights.
Stronger south winds aloft should aid in mixing drier air into the
top of the boundary layer and high clouds will likely increase
later tonight, both of which should help limit fog development in
addition to slightly stronger surface winds.

To our west, a cold front was pushing east across the upper/mid
Missouri Valley early this evening, with a broken line of
thunderstorms within a relatively narrow pre-frontal moisture
axis. Guidance continues to indicate that these will weaken as
they move east of the Mississippi River Saturday morning and out
run the frontal forcing and low level instability. Thunder chances
diminish eastward by this time, with mainly an area of decaying
showers expected to move into the KRFD area after sunrise. Low
level dew points and instability will increase later in the day
however as the front continues to move east into IL, and this
should set the stage for at least scattered thunderstorm
development by early-mid afternoon across the forecast area.
Window of thunder potential may be relatively short-lived for KRFD
as the front looks to pass by 23-00Z. Frontal approach and
associated convergence and instability in the moist axis ahead of
it will line up better with peak diurnal temps for the Chicago
terminals however, with 19-24Z looking to be the time of highest
TSRA threat for the metro terminals. Some differences, both in
timing and coverage, exist in CAM guidance. Reasonably high
confidence in some convection around the area though have
maintained a VCSH/prob30 for thunder mention at this distance for
uncertainty in coverage. While coverage may decrease, a few
showers or isolated TS could linger into the evening prior to the
passage of the cold front.

Winds ahead of the front are expected to be from the south-
southwest during the morning/early afternoon hours, with some
gusts near 20 kts likely. Lower confidence in wind directions
later in the afternoon/early evening in convection, though west-
southwest is most likely outside of any immediate outflow. Surface
cold front looks to pass KRFD around 23-00Z and KORD/KMDW 02-03Z
which should produce a wind shift to the northwest.

Ratzer

&&

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

$$

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