Forecast Discussions mentioning any of "ACARS" "MDCRS" "AIRCRAFT" "TAMDAR" "AMDAR" "WVSS" received at GSD on 08/27/21

Area Forecast Discussion

National Weather Service Chicago/Romeoville, IL

 959 PM CDT Thu Aug 26 2021

.UPDATE...
947 PM CDT

Evening updates include lowering pops overnight across parts of
the cwa. Convective evolution overnight continues to be unclear,
though it appears the best focus for much of the night will be
west and northwest of the forecast area across parts of IA and MN.
Have eased pops to generally slight chance across our far east and
southeastern counties and kept chances in the west, mostly after
midnight for the majority of the area.

Had a few isolated weak thunderstorms develop along the lake
breeze this afternoon, and along outflow boundaries across our
southeastern cwa. These have faded with the loss of diurnal
heating. Currently, local air mass is generally capped with
respect to near-surface based parcels. South-southwesterly low
level jet back to our west across central/western IA and MN will
likely be the driving feature for renewed convective development
overnight, with RAP mesoanalysis depicting the best organized
deep moist convergence from far northern NE across northwest IA
and southern MN. Some recent convective development noted across
northwest IA in fact within the low level jet axis. While CAM
guidance has not been very helpful of late, latest 00Z 3km NAM and
recent HRRR runs do focus on the IA/MN border region overnight for
best thunderstorm coverage. The LOT cwa will be on the far eastern
periphery of the low level jet, thus not expecting significant
coverage for the most part. Elevated showers/embedded thunder with
MCV across eastern IA will likely move into northwest/north
central IL later this evening/overnight, though better potential
for stronger convective development looks to remain off to the
west. Some potential may exist for remnants of southern
MN/northern IA MCS to push into the area Friday morning in a
decaying form. Based on this have maintained going chance pops
across north central IL from late evening into the overnight
hours, while have decreased pops across the east/southeast to
slight chances.

Otherwise, no significant changes made to forecast through Friday
morning.

Ratzer

&&

.SHORT TERM...
302 PM CDT

Through Friday night...

Main concerns are low confidence convective trends (especially
tonight into Friday morning) and heat and humidity on Friday
afternoon.

In the near term, main areas of concern are near the lake breeze
convergence and in the vicinity of the remnant MCV over the
east and southeast CWA. Have seen pulse convection into our
southernmost counties, with the somewhat more organized
convection farther south over ILX and IND CWAs. Recent AMDAR
soundings, visible satellite, day cloud phase RGB, and radar
trends indicate the cap has eroded or is in the process of eroding
near the lake breeze. Still think that coverage and organization
will be less than yesterday but expect a few widely scattered
storms until sunset. Lack of deep layer shear today vs the more
supportive shear with MCV yesterday will make lightning, heavy
downpours/localized flooding, and brief likely non-severe
downbursts the main threats.

Turning to the thunderstorm threat tonight, unfortunately
confidence continues to be quite low. Thus far, the upstream
convection from central IA to southern MN hasn`t really taken off
this afternoon. There appears to be a weak short-wave near the
southeast IA/southeast MN border assisting the ongoing convection,
with the RAP initializing this and passing it north of us through
Wisconsin this evening. If organization doesn`t increase with
signs of MCS development and cold pool formation from this
activity this evening, the chances of organized convection
tonight drop off drastically given modest height falls focused
north of the area.

It`s possible thunderstorms to the northwest from one of the
various waves over the Upper MS Valley and northern Plains
congeals and pushes in later overnight, but confidence is too low
to put stock in this scenario. Due to the low predictability
nature of convection evolution tonight, lowered PoPs a bit from
previous and show them increasing to mid to high chance overnight.
If any organized/semi-organized convection moves in tonight, main
threat would be wind gusts strong enough to down tree limbs. High
PWAT air mass and large MUCAPE reservoir would also be supportive
of at least localized flooding in the presence of deep moist
convection. Trends *should* become more clear with time by this
evening.

Held onto chance PoPs Friday morning for the possibility of
showers and thunderstorms lingering from overnight activity to the
extent that it occurs. Thereafter, Friday afternoon will be hot
and humid but otherwise could turn out more quiet as low level
winds will increase and likely keep lake breeze development at
bay, with unidirectional south-southwest winds also limiting low-
level convergence across the area. In addition, modest mid-level
height rises are forecast and a majority of the models advect in
some pretty dry air in the 700-500 mb layer. Forecast low level
thermal progs from 925 to 850 mb support highs near low to the
lower 90s, with some upside potential if the afternoon ends up
mostly sunny. With dew points back up in the lower to mid 70s this
yields forecast maximum heat indices of the upper 90s to lower
100s (with overperforming temps bringing values closer to 105F
advisory criteria).

Friday night, the support for thunderstorms will primarily be
across the Upper MS Valley and central and northern Wisconsin
closer to the stalled frontal boundary and where short-wave
forcing will be focused. Most locations have a dry forecast,
except the Wisconsin state line counties where some slight chances
are indicated to account for wiggle room if storms form a bit
farther south than expected. Any thunderstorms that form later
tomorrow-tomorrow night would have a lower end threat to become
strong to severe (mainly wind threat) over far north central
Illinois with up to 20-25 kt of deep layer shear forecast.

Castro

&&

.LONG TERM...
305 PM CDT

Saturday through Thursday...

Forecast thinking for the long term period has not changed.

Hot and humid conditions will persist through the weekend then
trending "cooler" next week with periodic shower and thunderstorm
chances through at least Monday.

The upper ridge continues to amplify in advance of an approaching
upper shortwave trough. In response to this, 850 temperatures
around 20C move overhead again, and this will support surface
temps into the lower 90s areawide. This combined with yet another
day of dewpoints in the lower to mid 70s will result in heat
indices around 100 degrees. There will be some breezes out there
which should help some with the heat. A few isolated to widely
scattered thunderstorms are possible on Saturday afternoon.

Details for Sunday into Monday are a little less certain and will
depend greatly on the timing of the arrival of the upper wave and
associated surface cold front. At this time, this looks to move
through late Sunday night into Monday. Thus have another warm day
forecast with highs in the upper 80s. Model guidance has
development out ahead of the main cold front Monday afternoon and
early evening with additional storms moving through with the cold
front overnight.

Once the cold front moves through, temperatures through at least
the first half of next week will be closer to normal in the lower
80s with east northeast winds off the lake keeping lakeside areas
in the upper 70s. The front does look to stall out somewhere
across central IL/IN on Monday with showers and thunderstorms
possible mainly south of I-80. We then trend cooler with generally
low chances for thunderstorms. Will note that uncertainty in the
upper pattern and precip chances increases by midweek with
potential influences from a tropical system in the Gulf.

Petr/KJB

KJB

&&

.AVIATION...
For the 03Z TAFs...

Update at 03Z:

Observational and forecast model trends have backed away from
convection overnight, favoring expansive thunderstorms along the
IA/MN border. Accordingly, we removed the TEMPO groups for
thunder at all terminals. Depending on how organized convection
becomes upstream overnight, some form of remnants may try to sneak
in late Friday morning or early afternoon, but will withhold on
any mention in favor of watching trends overnight.

Borchardt

Previous Discussion issued at 00Z:

The primary forecast concerns through the next 24 to 30 hours are
as follows:

* Isolated thunder chances through the next few hours
* Chances for additional thunderstorms overnight
* Potential for an easterly wind shift Friday afternoon

Isolated showers and a few storms continue to percolate along a
lake breeze stretching from near DPA/ORD/MDW to IGQ. With waning
instability (e.g. setting sun), coverage should continue to wane
over the next few hours, but opted to hold onto VCTS at the
Chicago terminals through 01Z just to be safe.

Attention then turns toward Iowa where thunderstorms are
developing along the intersection of the nose of a 30+ kt 850mb
low-level jet and a residual outflow boundary. Forecast model
guidance continues to perform poorly with convective evolution
tonight, though with continued low-level forcing (aided by the
approach of a subtle upper-level wave over the central Plains),
convection will likely continue to develop if not "grow upscale"
with time with eastward motion into at least southern Wisconsin.
The greatest uncertainty lies with how far south the convection
will develop, so for now opted to maintain the inherited TEMPO
for thunder at all terminals, but slide the timing back toward the
predawn hours of Friday. Should a more organized system develop
tonight, strong northwesterly winds would become an increasing
concern, especially at RFD.

Finally, depending on convective coverage tonight and whether the
brunt passes to the north into Wisconsin, a northeasterly wind
shift off Lake Michigan cannot be ruled out particularity during
the early afternoon hours of Friday at ORD/MDW/GYY/DPA. After
collaboration with CWSU ZAU, opted to withhold formal mention for
now in place of watching convective trends over the next few
hours. Otherwise, light winds will prevail tonight, with a
southerly to southwesterly direction preferred tomorrow during
daylight hours.

Borchardt/Doom

&&

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

$$

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Area Forecast Discussion...UPDATED

National Weather Service Little Rock AR

 632 PM CDT Thu Aug 26 2021

.UPDATE...

Updated to include the 00Z aviation discussion.

&&

.AVIATION...

VFR conditions expected through the period with L/V winds
overnight, becoming E to SE around 10 kts by late morning Friday.
SCT to BKN cumulus aoa 4 kft will develop by afternoon and
dissipate near sunset.

&&

.PREV DISCUSSION...(ISSUED 235 PM CDT Thu Aug 26 2021)

SHORT TERM...Tonight Through Saturday

Currently, isolated showers and storms are developing in easterly
flow over southern/eastern Arkansas. These will continue to move
westward in the vicinity of a weak surface boundary, but should be
diminishing this evening as we lose the heat of the day.

Over the course of the short term periods, deep layer humidity
will increase, with winds becoming more southeasterly and the
gradient increasing as something comes this way.

This will result in cooler temperatures Friday and Saturday (and
I use that term loosely). While heat index values could approach
or briefly exceed 100 to 105, it doesn`t appear it will be
prolonged or widespread enough to warrant a Heat Advisory. So,
that being said, we`re planning on letting the current advisory
expire this evening.

With increasing humidity, we will see increasing clouds. Upper
level ridging will start to break down. So without further ado,
here`s info about that something I mentioned that`s coming this
way...or at least in the direction of the Central Gulf Coast...

LONG TERM...Saturday Night Through Thursday

To begin the period, an upper level ridge is expected to be
located to the east of the CWA. Afternoon convection is possible
through the period associated with diurnal heating. Temperatures
look to become more average through the long term period as the
upper level ridge pushes away from the CWA, as mentioned already.
High temperatures are expected to be in the mid 80s to lower 90s
across the state with lows temperatures in the lower 70s. Heat
index values look to stay in the lower 90s across much of the
state. Now for the main weather feature in the long term period.

Here is what we know...As we approach the the beginning and middle
of the period, all eyes will be on the tropics. Currently, Tropical
Depression Nine is located in the Caribbean, just to the west of
Jamaica. Latest model guidance continues to develop TD Nine into a
tropical storm then hurricane as it moves into the Gulf of Mexico.
The vast majority of major operational and ensemble guidance
develops TD 9 into to a hurricane, making landfall along the Gulf
Coast, with a bullseye around Louisiana.

Some uncertainties in the forecast are with the exact track and
intensity of Tropical Depression Nine. A major difference right now
is seen among some major operational models on track and timing.
Currently, the GFS is slightly faster than the Euro setting up a
Sunday night landfall rather than a early Monday morning landfall.
Part of this may be in part to the Euro hinting at a landfall
farther to the west than the GFS causing TD 9 to spend more time
over the Gulf waters. These differences are likely to be figured out
over the next couple of days as aircraft recon go out and sample the
storm in addition to more frequent upper air launches sampling the
environment ahead of the storm.

What does all of this mean for our CWA? The track will be key over
the next couple of days in determining if the CWA gets impacted and
if so...how. The farther to the east of the CWA that the storm makes
landfall would minimize impacts as the eastern half of a tropical
cyclone tends to be more impactful. At this point it is way too
early to pinpoint impacts (or not) to the CWA.

So what can you do now? Continue to monitor the National Hurricane
Center and local National Weather Service office for up-to-date
information regarding TD 9.

&&

.LZK Watches/Warnings/Advisories...NONE.
&&

$$


Aviation...DTC