Merrillville, Indiana 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

030
FXUS63 KLOT 201633
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1133 AM CDT Sun Jul 20 2025

.KEY MESSAGES...

- Thunderstorms capable of producing damaging winds toward
  central Illinois late this afternoon and evening.

- Another round of thunderstorms with an axis of torrential
  rainfall and potential flooding across central Illinois could
  reach as far north of a Pontiac to Fowler line tonight.

- Dangerous swim conditions for the Illinois beaches of Lake
  Michigan today into Monday morning.

- Dangerous heat and humidity expected Wednesday and Thursday.

&&

.UPDATE...
Issued at 1133 AM CDT Sun Jul 20 2025

Late morning GOES imagery showcases a defined circulation
cutting across our CWA just south of I-80, and an associated
plume of showers is making its way across the southern half of
the CWA. Just over the past hour or so have we seen coverage
quickly pull away from areas north of I-80. This circulation is
embedded on a quasi- stationary frontal boundary, but it appears
the more efficient baroclinicity associated with this system
resides just south of our CWA and along a low-mid level jet
where we find a long train of showers and storms extending from
northeastern KS across central IL. These showers that first
moved in late last night have been very efficient rain producers
and deceivingly heavy compared to how they look on radar. MRMS
points out a few spots of >1" 6-hourly accumulations around the
area, most notably a swath of 1-2" estimated totals extending
from the panhandle of Ford County east into Benton County. This
is thanks to a moisture-laden environment which featured 1.8" of
PWAT on the morning RAOB out of DVN, and just under 2" at ILX.

Expectations are that the surface front will drift south into
central IL following the departure of that circulation early
this afternoon. Through then, we could see decent shower
coverage as far north as around I-80. Once the front gets
shunted south, a majority of the precip coverage should follow
suit. From mid-afternoon through the evening, precip looks to be
focused near and south of that front which is anticipated to
park somewhere just south of our CWA boundary. The mid level
impulse moving across central IL will feed additional moisture
into the environment late this afternoon, possibly over 2.25" of
PWAT just south of the front according to latest RAP guidance.
Deep westerly shear vectors oriented roughly parallel to the
front with tall, skinny CAPE profiles amid such rich moisture
spells trouble from a flooding perspective given the possibility
of training convection. Conditions do look to dry out and
stabilize very quickly as you move north of the boundary, which
will hinder any flooding concerns across a big majority, but
maybe not quite all, of our CWA. A few CAMs do suggest that the
front could get hung up in our far south, but this does not
appear to be the likely outcome. A few showers and possibly a
couple of shallow thunderstorms may manage to go up north of the
front, especially along any outflow boundary moving across, but
coverage should be rather isolated.

As far as severe hazards go, damaging winds would be the
biggest concern, but again mainly only for areas near and south
of that front as conditions look to be too stable to present
much of a threat farther north. This may translate to a threat
area roughly defined as south of a Pontiac to Fowler line. A
low, localized potential for tornadoes may also exist along the
front where we should find ample uncapped SBCAPE and backed
surface winds may generate shallow vorticity sufficient for a
quick spin up or two. But again, this should really only be a
concern if the front remains in our local area, which at this
point does not appear all too likely but cannot be ruled out.
Wherever the front does decide to setup, model camps are in good
agreement that a long swath of at least 1-2" rain totals will
likely coincide through tonight. The expectation for now though
is that this should be found just outside of our local warning
area.

Doom

&&

.DISCUSSION...
Issued at 337 AM CDT Sun Jul 20 2025

Through Tonight:

A modest low-level boundary/theta-e gradient roughly along the
I-74 corridor early this morning has been drifting northeast
through the night as evident by a gradual expanse of scattered
3-5kft stratocumulus as close as the extreme southwest CWA.
Farther northwest on this axis over Iowa, associated isentropic
ascent on the nose of the LLJ has continued to initiate clusters
of convection. With modest WNW mid-level steering flow,
convection has struggled to extend much beyond the Mississippi
River, but should begin to develop ESE along the boundary in
response to the veering LLJ and corresponding moisture
transport. Meanwhile, a convectively enhanced wave centered near
Omaha is forcing an arc of convection southeastward toward far
northeast Missouri.

Like yesterday, the complex setup with multiple clusters of
convection and a mix of mechanisms for synoptic and mesoscale
forcing makes for a rather challenging forecast this morning
into the afternoon hours. 00Z guidance has been of little help,
with nearly every piece of CAM and global guidance already
diverging from radar trends as of 08Z. Conceptually, the wave
crossing into western Iowa should remain the driving force this
morning, with the stronger arc of convection over central Iowa
overtaking the developing arc along the theta-e axis in the next
few hours. More convective enhancement of the wave via
absorption of any current and future embedded MCVs should
provide a formidable mechanism to maintain convection across
much of central Illinois downstream. This would include areas
south of I-80 and especially across the southern tier of
counties through morning. An associated shield of rain with
embedded lightning may then extend as far north as the Wisconsin
state line while a cold front drifts southward across northern
Illinois. This overall activity should clear most of the CWA by
early afternoon with post-wave subsidence suppressing remaining
activity across at least the north half of the CWA this
afternoon. However, if the trailing outflow boundary and
incoming cold front do not clear the southern CWA by late
afternoon, regeneration of convection is likely along the
outflow boundary given an existing uncapped and unstable
environment. Seasonably strong deep-layer shear across central
Illinois will support multicell clusters capable of producing
damaging winds.

Development of a LLJ across northern Missouri this evening is
expected to expand NE while veering late evening and overnight.
This will provide an impetus for isentropic ascent across the
stalled front and/or outflow. Given mid-level flow generally
parallel to the front, an uncapped environment, and a moisture-
rich airmass characterized by PWAT values over 2.2" across
central Illinois, a corridor of very heavy rain is possible
somewhere across central Illinois tonight. While it is difficult
to trust much guidance given the poor performance early this
morning, some guidance does support that the anticipated
evolution of this morning`s convection and any severe storms
across the southern CWA late this afternoon should push the
front far enough south to keep the axis of heavy rain and any
potential flooding southwest of the forecast area. Reassessment
will be needed based on trends through the day, though, as a
Flood Watch could be needed for the southern tier of counties
(south of a Pontiac to Fowler line) if the expected position of
the effective front is realized farther north.

Beside all of the convection, a seasonably cool day is in store
with highs in the 70s given morning cloud cover and the passing
cold front. Northeast winds around 15 mph and resultant waves
will yield dangerous swimming conditions for Illinois beaches of
Lake Michigan through at least tonight, with moderate swim risk
conditions for the Indiana beaches.

Kluber


Monday through Saturday:

Forecast concerns during the period continue to focus on the
increasing threat for at least a 2 day period of dangerously hot
and humid conditions Wednesday and Thursday.

Following another seasonably cool day on Monday, with highs in the
lower 80s, forecast guidance remains consistent with the building
of a 596+ dam 500 mb ridge right across the Mid-Mississippi
Valley late Tuesday into Wednesday. Building heat and humidity
under this ridge will begin to shift into our area on Tuesday, but
the worst conditions locally are expected Wednesday and Thursday.

Overall, cannot rule out the possibility for a few storms on
Tuesday as the heat and humidity begin to return, but overall
rising mid-level heights and capping should keep any activity
isolated in nature at best. Temperatures on Tuesday will rebound
back into the upper 80s, but as dewpoints climb into the lower
70s it will feel like its in the 90s. Slightly cooler conditions
may persist right along the IL Lake Michigan shore for most of
Tuesday due to a southeasterly wind component. However, the low-
level flow will begin to shift offshore here sometime either
late in the afternoon or in the evening.

As the center of the 500 mb ridge builds over the region on
Wednesday, subsidence and capping will keep convective clouds to
a minimum, thus resulting in mainly sunny conditions.
Temperatures in this pattern will climb to around 90, likely as
early as midday, then top out in the low to mid 90s during the
afternoon (highest over the Chicago urban heat island). However,
as is typical with dangerous periods of heat in our area, its
not just the about the temperatures, but also the humidity and
the associated heat indices. In this case, the influx of
moisture with this airmass will push dewpoints well into the 70s
(low 80s around the corn fields), and result in extreme heat
indices peaking in the 105 to 115 range Wednesday afternoon.
Overnight temperatures will offer little relief, especially in
the Chicago urban heat island where heat indices may drop no
lower than 85 early Thursday morning.

The dangerous heat and humidity will continue on Thursday, and
temperatures could even end up a degree or so warmer than those
observed on Wednesday. Unlike on Wednesday, however, capping may
end up weakening later in the day as the mid-level ridge axis
begins to deamplify across the area and an impulse tracks
northeastward into the Upper Midwest. Accordingly, a small (20%)
shower and thunderstorm chance will exist late in the day and
evening. However, current indications place the better chances
for storms to our north in closer proximity to the track of the
mid-level impulse.

With the increasing threat of this dangerous heat, a heat
headline (either a heat advisory or an extreme heat
watch/warning) will likely be needed for Wednesday and Thursday.
While it is still too early to consider issuing such a headline
at this time, we plan to continue to strongly message this
threat in our forecast graphics.

Later in the week into next weekend, model and ensemble
guidance are in good agreement in setting up a quasi-zonal mid-
level flow pattern across the region. This may help ease the
extreme heat across the area into next weekend. However, it may
also end up placing our area right in the target area for
potentially a couple of severe MCS`s tracking across the area
sometime Friday into next weekend. While the timing and
placement of these potential MCS`s is largely unclear at this
timescale, it is certainly another period we will have to keep
an eye on.

KJB

&&

.AVIATION /12Z TAFS THROUGH 18Z MONDAY/...
Issued at 634 AM CDT Sun Jul 20 2025

Key messages:

- IFR/LIFR CIGs persist this morning, with gradual improvement
  expected this afternoon.

- Main threat of thunderstorms to remain south of the terminals
  today, through scattered showers will move across the
  terminals through the morning.

IFR to LIFR CIGs have developed across the terminals in the wake
of the early morning cold frontal passage. Expect prevailing
northeasterly winds and IFR CIGs to persist through the morning.
CIGS will then gradually improve this afternoon, with VFR
conditions likely by this evening.

Showers and thunderstorms continue to move eastward into IL
this morning. Given the recent trends, we have opted to convert
the PROB30 to a tempo group for showers this morning. However, we
suspect the best threat of thunderstorms will remain to the
south of the terminals. Nevertheless, a 10-20% chance for a
thunderstorm exists at the terminals this morning. Otherwise, some
widely scattered shower activity could fester across parts of
northern IL this afternoon into tonight. However, the main focus
for additional heavy rain producing showers and storms  later
today and especially tonight will be setting up south of the
area. For this reason, we continue with a dry TAF after 19Z.


KJB

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...Beach Hazards Statement through Monday morning for ILZ006-
     ILZ103-ILZ104.

IN...None.
LM...None.

&&

$$

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