Markham, Illinois Weather Forecast Discussion

725
FXUS63 KLOT 200841 CCA
AFDLOT
Area Forecast Discussion...CORRECTED
National Weather Service Chicago/Romeoville, IL
341 AM CDT Sat Sep 20 2025
.KEY MESSAGES...
- Scattered showers and embedded storms through early this
  morning, some with brief heavy rain and gusty winds.
  Additional threat for scattered gusty storms later this
  afternoon.
- Increasing signal for more widespread showers and embedded
  storms tonight/early Sunday morning, mainly north of I-80.
- Periodic chances for showers and some thunderstorms through
  early next week, though there will continue to be plenty of
  dry hours too.
- Afternoon temperatures will remain above normal through the
  Autumn Equinox (Monday at 1:19 PM CDT), though not quite as
  warm as they`ve been the past few to several days.
&&
.DISCUSSION...
Issued at 328 AM CDT Sat Sep 20 2025
Through Sunday:
Messy/tricky forecast continues today with a main upper low to our
northwest and several embedded lower-amplitude vorticity maxima set
to pivot through the general vicinity today. Model guidance
generally has exhibited poor run-to-run consistency as well, and
appears to be having a difficult time handling the degree of mid-
level saturation and precise placement and magnitude of embedded
vort maxes. All of this casts some extra layers of uncertainty on
the forecast for today and tonight, although some trends are
beginning to emerge.
Through the rest of this morning, a north-south axis of
increased 800-600 mb moisture coincident with broad, modest warm
isentropic upglide is resulting in the development of elevated
showers and embedded storms. Through daybreak, have focused
the highest precipitation chances east of about a Pontiac to
Woodstock line, and particularly focused across Chicago and the
south/western burbs, although isolated convection will be
possible farther west. Some showers and storms may linger
through mid-morning across far northeast Illinois and over the
lake prior upglide weakening and deeper moisture pushing off to
the east. Thereafter, expecting a brief lull in activity during
the late-morning.
Towards midday, the main 500 mb shortwave axis should be
pressing across the southwestern third of the forecast area as
the primary upper low shifts into Wisconsin. An additional
subtle 700 mb trough axis currently moving across central
Missouri will commensurately lift northward across central
Illinois. As all of this occurs, a zone of modestly-increased
mid/upper flow--presently scooting across southern Iowa--will
start to impinge on the region. The Des Moines, Iowa VWP has
recently indicated flow near 40-45 knots at 500 mb associated with
this westerly jet streak, which is a bit stronger than current
model guidance would suggest. While it`s possible these readings
are a bit high and contaminated a bit by ongoing precipitation,
this could play a role in the evolution of this afternoon`s
weather.
While it remains a bit unclear how much insolation we`ll see
today following the morning activity, trends suggest at least
filtered sunshine will develop, allowing temperatures to push
into the upper 70s/near 80 degrees. With dewpoints possibly
rising through the mid 60s and 500 mb temperatures near -15 C,
MLCAPE values around 1500 J/kg may materialize through early
afternoon, along with diurnally-eroding CIN. The arrival of the
aforementioned shortwave will likely serve as the main large
scale driving mechanism today, and if we destabilize as
currently expected, scattered showers and thunderstorms may re-
develop. At this time, the main focus area appears to be about
along/east of I-55, where the strongest DCVA is expected to
encounter the greatest instability. Accounting for perhaps a
little more mid-level flow than guidance currently indicates
(based on upstream radar observations), deep-layer shear values
near around 30 kts may be sufficient for at least ephemeral
updraft organization. Suspect the HRRR is too aggressively mixing
dewpoints out again, and resulting in deeper inverted-V soundings
than we`ll see in reality, but regardless, stronger cores today
may be capable of strong/gusty precip-loaded downbursts.
This evening and tonight, a trailing shortwave currently over
Nebraska will slide into the region. At the same time upper
divergence is forecast to increase under the right entrance
region of an anticyclonically-arcing jet streak. Given this
recent trend, while model guidance does not universally show
increasing precipitation coverage, based on upstream
observations and ongoing model trends, elected to boost PoPs
late this evening and overnight, mainly near and north of I-80,
with the expectation of increasing shower and embedded
thunderstorm coverage. Low-level flow again does not look super
concerning from a backbuilding/training perspective, but
some very localized training could materialize.
Precipitation may persist into Sunday morning, although expect
a gradual diminishing trend through the morning once again.
Model guidance has been all over the place regarding the
handling of follow-up shortwaves/convectively-augmented MCVs in
the vicinity, so forecast confidence on Sunday remains notably
low. Increased cloud cover could also end up muting insolation
to some degree as well, all of which suggests capping PoPs in
the 30-50 percent range for now. Can`t rule out the potential
for a few stronger storms given indications of some stronger
700-500 mb flow riding in overhead, but there are too many other
uncertainties to pin this threat down at this point.
Carlaw
Sunday Night through Friday:
Intermittent shower and thunderstorm chances will persist Sunday
night through Tuesday as additional disturbances ripple through
the west to northwesterly flow aloft. A chaotic upper pattern
into the middle and end of next week will likely continue to
result in lower-than-typical forecast confidence. Some signal
remains in the extended guidance for at least a brief return to
drier conditions (especially near/north of I-80) in the wake of
a cold front towards midweek, but depending on the influence of
another upper low towards the end of the week, this could end
up being short-lived. The latest GFS in particular would
indicate little/no break in the action with a Fujiwhara dance
taking place between several robust disturbances across the
central CONUS.
Temperatures this upcoming week will start out solidly above
normal (along with fairly humid conditions as well), with highs
well into the 70s to near 80F through Tuesday, then trending
closer to normal and a bit less humid in the mid to late week.
Carlaw/Castro
&&
.AVIATION /06Z TAFS THROUGH 12Z SUNDAY/...
Issued at 1232 AM CDT Sat Sep 20 2025
Key Aviation Messages for the current TAF period:
- Scattered showers and isolated thunder chances through
  daybreak
- Another round of showers with chances for thunder Saturday
  afternoon
An upper level low over Iowa that is gradually moving north and
east will continue to produce a showery regime over northern
Illinois and northwest Indiana through the current TAF period.
There will be periods of showers and storms, but model guidance
is struggling with handling it with lower confidence regarding
timing of cells passing over any one individual terminal.
Currently, clusters of showers and isolated embedded thunder in
Central Illinois are moving northward. With instability being
meager, there is low confidence in thunder perpetuating at any
one specific terminal. Confidence is higher for terminals to
receive showers, but chances for intermittent lightning has
increased therefore it was decided to add a VCTS at Chicago
terminals timing out when they are expected. Meanwhile, with
KRFD located in the gap of stability axes, maintained the
PROB30 for -SHRA. Thunder cannot be completely ruled out, but
was kept out at this time.
There is an expectation for a brief break this morning from the
weather with VFR conditions and southeast winds at or below 10
knots. However, a close eye is monitoring what is exiting
Nebraska and moving through southern Iowa as that should be the
next round for showers and storms. And with better instability
this afternoon, there is at least a better chance for thunder
and the potential for isolated wind gusts up to 20 knots
associated with a storm. Timing still looks good so the PROB30
was maintained for now, but if trends continue, switching to a
TEMPO during the 12Z package looks plausible.
Some models are suggesting another round of showers and storms
around 12Z Sunday morning, but given the low confidence in the
overall pattern, VCSH was maintained at present through the end
of the TAF window.
DK
&&
.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...None.
&&
$$
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NWS LOT Office Area Forecast Discussion