Merrillville, Indiana 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

949
FXUS63 KLOT 042345
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
645 PM CDT Sat Jul 4 2026

.KEY MESSAGES...

- Showers with isolated thunderstorms will slowly come to end
  this evening.

- Sunday will feature a few showers and storms largely south of
  I-80. Coverage and intensity will be far less compared to the
  past few days.

- Drier and more seasonable conditions expected to start the
  week before stormier weather returns late week.

&&

.DISCUSSION...
Issued at 1251 PM CDT Sat Jul 4 2026

Showers and thunderstorms are beginning to expand in coverage
across western Illinois ahead of a notable mesoscale convective
vortex (MCV). The airmass ahead of the MCV is very moist and
unstable with mean 1km mixing ratios in excess of 15 g/kg,
PWATs nearing 2", and MUCAPE nearing 3000 J/kg. Unlike
yesterday, mid-level flow today is notable weaker with the DVN
VWP sampling only around 25 to 30 kt at 4 to 6 km. As a result,
the main threats with thunderstorms today will be locally
damaging winds in wet microbursts and very heavy rainfall (rain
rates >2"/hr) with a threat for flash flooding.

In terms of the evolution of thunderstorms, current
expectations are for the roughly north to south-oriented band of
storms from Rockford toward Peoria at press time to intensify
and shift eastward with time. At the same time, also anticipate
storms to develop in a west-to-east oriented band across
northern Illinois associated with modest warm-air advection
ahead of the MCV atop a remnant frontal boundary and approaching
lake cold front. The net effect should be a fairly high
coverage of thunderstorms across the area, especially across
northern Illinois and the Chicago metropolitan area. In terms of
timing, thunderstorms should expand in coverage and cross the
I-39 corridor in the next few hours, move across the Chicago
metropolitan area generally between 3 and 6 PM, and then move
through northwestern Indiana in the 6 to 9 PM time window.
Unlike yesterday, we do not anticipate additional storms after 9
PM outside a few lingering cells here or there.

As previously mentioned, thunderstorms today will be capable of
producing very heavy rainfall with hourly rain rates in excess
of 2 inches per hour (instantaneous rates in excess of 4 inches
per hour). Of particular concern is the potential for several
hours of such rain rates in the western and southern Chicago
metropolitan area where observed two-day rainfall amounts are
in the 3 to locally 6 inch range. It is thus no surprise that
many rivers and creeks in the aforementioned area are at
bankfull or flood stage. With the potential for several more
inches of rain today, continue to think there is a localized
threat for significant flash flooding today primarily across
portions of Kane, DuPage, Cook, Will, and Lake (IN) counties.

A Flood Watch and Severe thunderstorm Watch are in effect for
portions of the area through this evening.


Tonight into Sunday:

Winds will turn northeasterly overnight in the wake of the
aforementioned backdoor lake front. With cooling temperatures
within the residually moist airmass, areas of fog may develop
overnight especially near the Lake Michigan shoreline.

Tomorrow, a weak area of low pressure is expected to pass
through central Illinois encouraging another backdoor front to
move onshore from Lake Michigan. A period of low clouds or fog
may follow the secondary front. With forecast soundings
depicting largely uncapped profiles by tomorrow afternoon,
suspect a few showers and storms will develop along the inland-
pushing lake breeze favoring areas south of Interstate 80.
Compared to the yesterday and today, coverage of storms tomorrow
should be far less (isolated to scattered), and be tied to the
peak heating of the day (not lingering after sunset). Highs
tomorrow will be in the upper 70s to lower 80s. Finally, will
have to watch winds in the wake of the secondary lake front
closely as they may cause waves to rise into the 3 to 4 ft
range, making swimming dangerous. At this point, we favor waves
moreso in the 2 to 3 ft range. Will proactively advertise a
Moderate Swim Risk tomorrow just to be safe.

Borchardt


Monday onward:

By Monday the upper trough will be ejecting eastward and taking
with it the chances for showers and thunderstorms as a surface
high settles into the western Great Lakes. It should be noted
that our official forecast does have some 20-30% POPs in our far
southern CWA Monday afternoon where some slower guidance has a
stray shower/storms trying to develop, but the overall
environment locally should support dry conditions areawide so
suspect these POPs will get removed in future updates.
Nevertheless, temperatures on Monday will be on the seasonable
side with highs once again in the lower to mid-80s inland with
70s expected near the lake due to onshore winds. Tuesday will
feature similarly dry conditions, but with slightly warmer
conditions (highs in the mid to upper 80s) as winds turn back
southerly.

Unfortunately the tranquil weather is not forecast to last long
as guidance continues to show the return of west-southwest flow
aloft for the middle and latter half of next week. While
temperatures look to remain seasonable, humidity will be on the
increase which will also bring the return of shower and
thunderstorm chances.

Yack

&&

.AVIATION /00Z TAFS THROUGH 06Z MONDAY/...
Issued at 645 PM CDT Sat Jul 4 2026

Forecast concerns include...

Showers with isolated thunderstorms ending early this evening.
Low mvfr, ifr, possible lifr cigs overnight into Sunday morning.
Chance of showers Sunday morning.

A large area of rain with a few embedded thunderstorms will end
across the Chicago terminals between 00z and 01z this evening. A
few showers, possibly an isolated thunderstorm will remain
possible across northwest IL this evening. There will be another
chance of showers and possibly an isolated thunderstorm across
the Chicago terminals and northwest IN Sunday morning into early
Sunday afternoon. Confidence for this potential is low and opted
to maintain a dry forecast for now.

There is quite a bit of uncertainty for cig trends tonight. Low
mvfr cigs are possible this evening with a better chance for low
mvfr or ifr cigs overnight and have maintained scattered mention
for now. The best timing for prevailing low mvfr cigs, possibly
ifr cigs, appears to be from around sunrise Sunday morning
through early Sunday afternoon. Trends will need to be monitored
and changes are possible with later forecasts.

Northeast winds are expected this evening, turning more
northerly or north/northwest overnight, then back to northeast
Sunday morning. Speeds may increase into the 12-16kt range
Sunday afternoon, along with a few higher gusts possible. cms

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...Flood Watch until 10 PM CDT this evening for ILZ011-ILZ012-
     ILZ013-ILZ019-ILZ020-ILZ021-ILZ103-ILZ104-ILZ105-ILZ106-
     ILZ107-ILZ108.

IN...Flood Watch until 10 PM CDT this evening for INZ001.

LM...None.

&&

$$

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