Evanston, Illinois 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

348
FXUS63 KLOT 191545
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
Issued by National Weather Service Lincoln IL
1045 AM CDT Sat Jul 19 2025

.KEY MESSAGES...

- A period of thunderstorms capable of producing damaging winds
  is possible late this afternoon into mid-afternoon.

- Several rounds of thunderstorms may occur early next week,
  with strong/severe thunderstorms possible along with
  torrential rain and localized flash flooding, particularly
  south of I-80.

- Dangerous heat and humidity becoming increasingly likely next
  Wednesday and Thursday.

&&

.UPDATE...
Issued at 1045 AM CDT Sat Jul 19 2025

A shortwave is progressing across northern Illinois, with a curl
evident in the radar loops centered near Stevenson County at
1030 am CDT. That wave will continue to advance east at 25 to 30
mph and produce widespread showers and storms through early
afternoon across NE Illinois and NW Indiana. Locally heavy
rainfall is also occurring with this line, with several areas
just west of our CWA reporting standing water on roads and
ditches full of water. 1 inch of rain in 20 mins was reported in
Henry County 2 NW of Geneseo at 940 am. Precipitable water
values are 1.8 to 2.1 inches in the main axis of this complex,
and upstream history does support rainfall rates of 3" per hour.
Will have to watch closely for any areas that see prolonged
heavy rainfall as we head through early afternoon.

High res model solutions are depicting the back edge of this
initial complex will advance across the area between 1 pm and 3
pm this afternoon, creating a sizable break in the rain and
storm chances the remainder of the afternoon into early evening.
A corridor of instability across out southern cwa could re-
initialize with some storms if any sunshine manages to break out
and destabilize that airmass.

The potential of severe storms appears low with this current
wave of storms, per SPC previous MCD and the impacts or lack of
wind impacts reported so far today in N Illinois. We can`t rule
out some severe weather with the overnight wave of storms that
could arrive later this evening into the overnight hours.Heavy
rain is more of a concern through early afternoon at this
point.

Shimon

&&

.DISCUSSION...
Issued at 328 AM CDT Sat Jul 19 2025

Through Tonight:

All eyes are on an axis of convection along the IA/MN line
early this morning and what that means for the downstream
evolution of potentially severe convection over our area later
this morning into this afternoon.

A long-lived supercell has anchored the growing MCS for the
past 4-5 hours, with a developing MCV becoming more evident on
KMPX 88D over the past hour. A gradually veering 40 knot LLJ has
continued to feed a moisture-rich airmass north of a weak warm
front into the ongoing complex. Meanwhile, backbuilding
convection has been associated with an incoming mid-level
impulse tracking within overall zonal mid-level westerlies.
Given a rather delayed growth of the MCS, the lack of any
notable cold pool has greatly delayed prior expectations of an
early-arriving decaying line of convection shortly after sunrise
today. So the main focus revolves around the downstream
trajectory of the MCS and its timing and intensity.

The current thermodynamic environment shows a notable
instability gradient drifting eastward from near La Crosse to
central Illinois as modestly steep mid-level lapse rates advect
eastward. Meanwhile, a theta-e ridge (local moisture boundary)
roughly along the I-74 corridor and the aforementioned weak warm
front along the I-64 corridor will both slowly advect northward
through the day. The next few hours will be more telling if the
MCS begins to accelerate and latch onto the southern front or
exhibits a slower track that could become more rooted along the
northern theta-e axis.

We then are faced with two favored scenarios today:

1) The MCS (and associated MCV) maintains a slower eastward
progress through sunrise and tracks toward the CWA while
persisting past the diurnal minimum in convection, then
intensifies roughly 10am-noon as surfaced-based rooting on the
northern axis begins and results in clusters or a line of strong
to severe convection across much of the forecast area. One
caveat in this scenario will be delayed surface-based heating
due to existing 4kft stratus over the northeast 2/3 of the CWA.

2) The main MCS dives southeast over the next several hours and
misses a vast majority of the forecast area to the southwest.
The trailing mid-level wave will then cross the area early/mid
afternoon in conjunction with a mostly uncapped and modestly
sheared environment across northern Illinois. This would yield
scattered strong to possibly severe convection over the northern
2/3 of the CWA.

Either scenario remains plausible at this time, with CAM
guidance providing little assistance in correctly initiating
ongoing convection. With that said, both scenarios support the
possibility of strong to severe convection sometime late this
morning into mid-afternoon. Damaging winds with multicell
clusters and/or segments are the primary risk, though seasonably
strong shear could yield some embedded or semi-discrete
supercell structures that would all produce large hail.

After a lull in convection this evening, broad low-level
isentropic ascent near and north of the warm front will likely
initiate an axis of elevated convection somewhere over central
Illinois. This includes the southern half of the CWA, where
steering flow parallel to the front, persistent elevated
instability, and PWATs around 2 inches could result in a couple
narrow axes of very heavy rain and localized flooding.

Kluber


Sunday through Friday:

A surface frontal boundary is expected to shift south of I-80
into Sunday morning, and eventually into central parts of IL
Sunday afternoon. As it does, another wave of showers and
thunderstorms is expected to develop tonight near this frontal
boundary along the nose of the low-level jet. Very heavy
rainfall is likely to accompany these storms given rich deep
moisture (precipitable water values 2"+). While it appears the
heaviest rainfall, and potential flash flooding, may remain west
of the area across parts of IA tonight into early Sunday, the
remnant MCS may slide eastward right across my southern areas
(mainly south of I-80) through Sunday morning. A locally heavy
rain threat thus exists south of I-80 Sunday morning.

Areas north of I-80 should largely remain dry through the day
Sunday as a cooler airmass moves in on northeasterly surface
winds north of the surface boundary. This should make for more
pleasant temperatures as high temperatures for Sunday are
forecast to remain in the 70s north of I-80! Why overall quiet
weather is anticipated for much of far northern IL, Lake
Michigan beach goers are urged to use caution on Sunday. This as
breezy northeasterly winds on the lake look to result in a high
swim risk due to large waves of 3 to 5 feet. A beach hazard
statement will thus likely be needed for Sunday, especially
along the IL shore.

The primary focus for additional periods of showers and storms
Sunday night and Monday should largely be remain south of our
area as the surface front sags into central and southern IL.
Accordingly, aside from a continued moderate swim risk along the
IL shore into Monday, no significant weather is anticipated in
our area on Monday.

Beyond Monday, concerns continue to increase in a period of
dangerously hot and humid conditions setting up across the area,
particularly on Wednesday and Thursday. Forecast guidance
remains consistent in building a 596+ dam 500 mb ridge that
becomes centered right across the Mid- Mississippi Valley on
Wednesday into Thursday, before deamplifying late in the week.
The building heat under this upper-level ridge axis is likely to
yield temperatures in the 90s both Wednesday and Thursday, and
readings could end up well into the 90s, particularly across the
Chicago urban heat island. This in combination with dewpoints
well into the 70s, to even the low 80s, could push peak heat
indices into the 110-115 range. Unfortunately, the offshore
southerly winds in this pattern will also result in little to no
lake cooling along the Lake Michigan shores. If these type of
conditions continue to look likely for Wednesday and Thursday a
heat headline is likely to be needed for the area in the coming
days.

Later in the week into next weekend, model and ensemble
guidance are in good agreement in deamplifying this upper-level
ridge and setting up a quasi- zonal mid-level flow pattern
across the region. This should in return allow a cold front to
shift southward down the Lake and bring at least some of our
area some relief from the heat humidity to close out the week.
However, it also looks to come at the cost of some potentially
active weather with strong showers and thunderstorms late Friday
into Saturday.

KJB

&&

.AVIATION /12Z TAFS THROUGH 18Z SUNDAY/...
Issued at 625 AM CDT Sat Jul 19 2025

Key messages:

- A period of strong heavy rain producing showers and
  thunderstorms continues to be the main weather threat today.
  Best timing is 15 to 18z at RFD and 17 to 21Z at the Chicago
  area terminals.

- Northerly wind shift with cold frontal passage expected
  overnight tonight with the development of IFR CIGs into Sunday
  morning.

The cluster of storms moving east-southeastward across eastern
IA early this morning will shift into the northern IL and
northwestern IN terminals later this morning into early this
afternoon. The only change to the going timing for each terminal
was to shorten the tempo group slightly, with an onset time
around 15Z at KRFD and closer to 17Z at the Chicago area
terminals. Some of the strongest storms today could produce
strong damaging wind gusts (locally in excess of 50 kt) and
torrential downpours.

The thunderstorm threat looks to wane later in the day. A cold
front is then expected to drop southward over the area later
tonight (Saturday night), likely after 06Z. In it`s wake,
expect a north-northeasterly wind shift and the potential for
the development of some IFR CIGs that should persist into
Sunday morning.


KJB

&&

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

&&

$$

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