Forecast Discussions mentioning any of "ACARS" "MDCRS" "AIRCRAFT" "TAMDAR" "AMDAR" "WVSS" received at GSD on 08/11/23

Area Forecast Discussion

National Weather Service Chicago/Romeoville, IL

 642 PM CDT Thu Aug 10 2023

.SHORT TERM...
Issued at 200 PM CDT Thu Aug 10 2023

Through Friday night...

The primary forecast points are as follows:

* Threat for an isolated shower or two this afternoon with low
  (<10%) chance for thunder

* Expectation for a mesoscale convective system to dive from the
  northern Plains toward central Illinois tomorrow morning with a
  threat for damaging winds primarily along and in close vicinity
  to a line from Davenport, IA to Peoria and Springfield, IL

* Conditional threat for a second round of thunderstorms across
  northern Illinois and northwestern Indiana after sunset tomorrow
  with a threat for all severe hazards. The threat is conditional
  on low-level moisture to our west remaining in tact.

Technical Discussion:

Regional surface observations depict a zone of low-level confluence
near the Illinois/Wisconsin borders which in combination with the
moist and diurnally-heated airmass is supporting healthy cumulus
development. Recent AMDAR soundings from MDW, ORD, and MKE depict a
shallow unstable layer from about 850 to 600mb atop the low-level
confluence zone, which in time may support the development of an
isolated shower or two. With equilibrium levels well below the -20C
level, the chance for a lightning strike this afternoon and evening
seems exceedingly low (<10%). Nevertheless, keep an eye to the sky
if planning to be outdoors.

A pair of upper-level shortwaves are evident in water vapor imagery
upstream of the Great Lakes, with the first moving eastward over the
Dakotas and the second racing southeastward through Alberta. The
leading shortwave will soon "kick off" an episode of thunderstorms
across the northern Plains, which through cold pool amalgamation
should lead to the development of a mesoscale convective system
overnight. Using the roughly north to south instability axis
draped across eastern Iowa as a guide, the MCS should track
southeastward toward the Mississippi River toward daybreak and
into central Illinois Friday morning. If the cold pool associated
with the MCS is sufficiently deep to lift through the near-surface
stable layer, a local damaging wind threat may materialize along
the leading edge of the system. At this point, we favor the most
vigorous convection with the morning MCS tracking west of our
area, roughly along a line from the Quad Cities to Peoria and
Springfield. Even north of the most vigorous convection,
stratiform rain and embedded convective cores with occasional
lightning strikes can be expected roughly along and south of a
line from Rockford to Rensselaer (from daybreak to noon or so).

Behind the morning MCS, a remnant 20-25kt southwesterly 925-850mb
low-level jet may support regenerating showers and thunderstorms
atop a residual outflow boundary into the early afternoon hours.
Assuming the MCS does track into central Illinois tomorrow
morning, the position of the remnant outflow boundary should
remain south of our area. Nevertheless, we will carry low (20%)
chances for storms from mid-morning onward in case the boundary
ends up further to the north. Increasing MLCIN owing to increasing
capping at the base of an encroaching EML should cut off any
regenerating convection (if it`s even there) by late-afternoon as
southwesterly winds increase (augmented by a wake low?), opening
the door for rapid airmass recovery. Temperatures are expected to
climb into the lower 80s by mid-afternoon, which with surface dew
points around the 70 degree mark and mean 850-200mb lapse rates
of around 7 K/km will support a burgeoning area of MLCAPE of 1500-2000
J/kg across the southwestern Great Lakes. Meanwhile, increasing
upper-level flow at the nose of the approaching secondary upper-
level shortwave will support increasing deep-layer shear along and
just ahead of the approaching cold front. Taken together, the
ingredients to support severe thunderstorms may align for our
neck of the woods tomorrow evening.

Now, confidence lowers in exactly where and at what coverage
thunderstorms will develop tomorrow evening. Interestingly, CAM
guidance differs on the arrival timing of an approaching surface
cold front, with several solutions depicting the front outrunning
the increasing upper-level jet max. In such scenarios, increased
mixing behind the cold front (and head of the jet max) leads to a
notable reduction in low-level moisture, and hence instability,
tomorrow afternoon in eastern Iowa and indeed upstream of our area.
In fact, several individual CAMs have a distinct "hole" of zero
instability near the IL/IA border tomorrow as the upper-level
shortwave moves overhead, which would pretty much axe the chances
that storms develop and move into our area after sunset. It`s
difficult to discern whether this is a realistic scenario, given the
earlier arrival of the cold front and area of notable low-level
drying appears to overlap with ongoing D2/Severe drought conditions
across eastern Iowa. In other words, it`s unclear if improper
feedback of drought conditions is leading to an artificial decrease
in forecast low-level moisture. Interestingly, observed dew points
this afternoon are running some 3-5 degrees higher than in CAM
output across eastern Iowa. Moreover, forecasters at NWS La Crosse
(in the heart of the drought) reported that observed dew points have
been verifying much higher than advertised in CAM guidance in
recent weeks. For this reason, we have cautious confidence that
the earlier arrival of the cold front and rapid reduction in low-
level moisture (and relatively low coverage of simulated
reflectivity/storms) may be unrealistic. However, we`d be remiss
to ignore this possibility that severe weather does not
materialize here tomorrow evening owing to meager instability
(best case scenario?).

Assuming low-level moisture and hence instability remains in tact
tomorrow afternoon, thunderstorms would be poised to erupt in
eastern Iowa between 5 and 8 PM along the leading edge of the
secondary shortwave and cold front, and then expand in coverage as
they move southeastward into northern Illinois after sunset.
Somewhat unidirectional southeasterly low- to mid-level shear
profiles oriented obliquely to the initiating cold frontal
boundary would support a semi-discrete supercell storm mode with
increasing chances for consolidation into a compact complex or two
with time. Accordingly, any thunderstorm would have a threat for
all hazards as they move across our area after dark. We time the
highest threat to be between roughly 8 PM and 2 AM, give or take
an hour. Again, this threat is "conditional" on low-level moisture
remaining preserved to our west tomorrow afternoon.

Borchardt

&&

.LONG TERM...
Issued at 200 PM CDT Thu Aug 10 2023

Saturday through Thursday...

Friday`s storms system will continue to track east of the Great
Lakes on Saturday, but a weak front boundary will likely stretch
back westward through Central Illinois and Missouri. Drier dewpoints
will filter in for Northeastern Illinois, but with precipitable
water amounts over an inch and potentially over an inch and a half,
there is still a chance for limited showers Saturday, but current
thinking is that it would be confined along or souther of I-88 and
Northern Indiana. The front would act as the lifting mechanism, but
confidence is low as the gradient will gradually peaked as it
tracks eastward limiting the amount of forcing over the region. So
lingering showers are in the forecast but most areas should see
mostly cloudy skies and seasonal temperatures.

The next shortwave trough starts to develop over Saskatchewan
Saturday night and starts to follow the northwest flows
southeastward toward the Chicago metro area, deepening on Sunday
almost two and half to three standard deviations below the normal.
Winds are expected to turn to the southwest and advect low to mid 70
degree dew points northward into North and Central Illinois, with
upper 60 dew points through the Wisconsin border. With surface front
developing and pushing through area, that should be enough lift to
tap into the elevated amounts of instability higher up in the
atmosphere. Currently, models are keeping the higher amounts of CAPE
in SE Iowa and Central IL, a slight trend downwards from yesterday`s
model runs with the addition of models trending toward the front
moving through later in the night and potentially overnight into
Monday. The being said, with sufficient shear through the column,
the chance for strong and potentially severe thunderstorms will
remain in the Sunday PM forecast leaning into Monday morning. While
the risk for severe conditions will be monitored, the signal from
p-wats makes for the potential for flooding concerns, however,
the current thinking is the bigger risk for that will be for the
southern portions of the area. Additionally, with models trending
toward this event impacting the area, it is a good time to double
check how to get warning alerts in case something needs to wake a
person up. Last but not least, surface winds will likely increase
and be out of the northwest as the front pushes through and models
are suggesting it will remain in that direction to midweek, and
increase. The risk for elevated swimming conditions for Lake
Michigan would liking be focused in northwest Indiana in this set
regime, but will be monitored closely.

As the trough slowly moves eastward on Monday, lingering showers and
the low probability for a storm are forecast, but low confidence
in this forecast as it will largely depend on what and how fast
things transpire on Sunday. Beyond that, ensemble models are
suggesting the 500 mb trough moves eastward and expands into an
longer wave trough and stalling over eastern Canada, with a
growing ridge over the desert southwest with its pinnacle
stretching northwestward to Seattle. If this transpires, this
would keep northwest flow planted over Chicago for much of next
week. With this pattern, it will depending on a smaller, mesoscale
feature that would transverse into the area to provide inclement
weather or just general diurnally-driven isolated pop-up summer
time showers. There is a little signal of a small wave moving
toward Illinois for the Thurs/Fri timeframe, but does not seem to
be terribly significant at this time. Temperatures currently would
be forecast in the mid to upper 80s for much of the weak.

DK

&&

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

Isolated SHRA/TSRA south and southwest of the terminals should
diminish this evening. VFR conditions expected tonight and if
any shallow ground fog develops it should be mostly in low lying,
rural areas. Cluster of SHRA/TSRA from storms developing over the
Plains could be crossing the MS River early-mid morning hours
Friday. Most guidance keeps the brunt of this activity to the
southwest, but there remains a small threat of it tracking farther
north and impacting the terminals. If it does go farther north
than expected, it will likely be on a weakening trend and mostly
SHRA, but confidence at this distance isn`t high enough to justify
removing the PROB30s. Gusty southwest winds are expected Friday,
likely easing by early evening. Cannot rule out a second round of
SHRA/TSRA Friday evening, but confidence is too low to introduce
precip.

- Izzi

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...Small Craft Advisory...LMZ740-LMZ741-LMZ742-LMZ743-LMZ744-
     LMZ745...11 AM Friday to 8 PM Friday.

&&

$$

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