NWS Chicago Decision Support Briefing - Forecast

044
FXUS63 KLOT 240759
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
259 AM CDT Sat May 24 2025

.KEY MESSAGES...

- The pattern through next week will be relatively stagnant with
below to near-average temperatures for this time of year and
limited opportunities for widespread, soaking rainfall.

&&

.DISCUSSION...
Issued at 259 AM CDT Sat May 24 2025

Through Monday night:

The prevailing synoptic-scale weather pattern across the CONUS
continues to be characterized by ridging over the central/southern
Plains and troughing over the northeastern CONUS. A jet stream
remains sandwiched in between these features and will continue
to serve as an eastward conveyor for a couple of shortwave
ripples originating from the Rocky Mountain and Great Plains
over the next 48 hours. At the surface, an expansive region of
high pressure extends from Hudson Bay down through the Great
Lakes, and the expectation is that this will largely not change
through Memorial Day.

With the more appreciable large-scale forcing for ascent and
quality column moisture remaining fenced off to our southwest,
the daytime hours today should be dry and quiet. Today`s high
temperatures will be comparable to yesterday -- perhaps a hair
warmer in most locations, which would allow more locations
across mainly southwestern portions of our forecast area to
reach the 70 degree mark this afternoon. Another lake breeze
will also develop and keep temperatures in the 50s closer to the
lakeshore.

Tonight, a lobe of planetary vorticity meandering along the
western flank of the northeast CONUS longwave trough will
approach our forecast area from the north. As an associated
cold pool of air aloft presses southward, it will induce a
strengthening mid-level frontogenetical circulation over
northern Illinois. On the whole, model guidance has generally
trended towards a sharper f-gen circulation compared to 24-48
hours ago, and NCEP-based guidance in particular is actually
now fairly bullish on our forecast area seeing light to modestly
steady rain tonight into Sunday as a shortwave trough traversing
the aforementioned jet stream augments the lift provided by the
f-gen circulation. ECMWF-based guidance is not quite as on
board with this idea yet, however, with only a minority of
ensemble members in the 00Z EPS outputting QPF in our forecast
area (though it is worth noting that the EPS has still gradually
trended towards greater precipitation chances here with each
successive run over the past 24 hours). This appears to be
because it generally favors the aforementioned vort lobe taking
a more southeastward trajectory and/or getting yanked into
Michigan`s Lower Peninsula quicker, thus resulting in weaker
frontogenesis occurring over northern Illinois.

When considering the dry low-level air that will be place over
the area along with the large-scale subsidence that will be
occurring in the region, it will likely take a fairly robust
frontogenetical circulation developing in order for any sort of
precipitation to develop and make it to the ground here any time
tonight through Sunday. Thus, if the ECMWF camp has the right
idea regarding the evolution of the aforementioned vort lobe,
then our forecast area will likely remain dry throughout this
time frame. However, if what most NCEP-based guidance is
suggesting out has credence to it, then rain showers or
sprinkles will likely occur in our forecast area at some point
tonight into Sunday, with the potential rain shower corridor
shifting southward with time in tandem with the f-gen
circulation. With this forecast package, felt that introducing
slight chance PoPs was the best course of action when
considering the current spectrum of solutions advertised across
the guidance suite and the existing uncertainties, but
additional adjustments to these PoPs will likely need to be made
as guidance starts to come into better agreement and
observational trends start to become more apparent.

Aside from the potential rain chances, Sunday should feature
onshore flow once again and similar temperatures to Saturday
(perhaps a smidge cooler if the rain and increased cloud cover
do end up materializing). A similar story should then play out
again on Memorial Day Monday as the weather pattern across the
region remains stagnant, though there is greater confidence that
the daytime hours will remain dry with the next upper-level
disturbance of interest remaining well to our southwest over
the course of the day.

Ogorek


Tuesday through Friday...

A blocky upper-level pattern is likely to set up and persist
across North America next week as anomalous upper ridging
dominates across the high latitudes of Canada. This larger scale
pattern will generally favor slow eastward moving and evolving
weather systems across the CONUS through the week. The main
system we will be keeping any eye on for our area is the upper
trough currently shifting onshore across the California coast.
This feature is expected to gradually shift eastward through the
holiday weekend before potentially interacting/merging with a
northern stream wave somewhere over the northern Plains early
next week. As this occurs, ensemble guidance continues to favor
the development of a slow east moving closed upper low from the
Upper Midwest into the western Great Lakes during the middle to
latter half of next week. With this being said, the overall
complexity of this evolving weather pattern makes for lower than
average confidence with the the exact track and timing of this
closed low feature over the Upper Midwest next week.
Nevertheless, chances for showers will return to our forecast
for Tuesday afternoon and night as this system begins to track
into the Upper Midwest. Additional diurnal showers and perhaps a
few thunderstorms will then be possible (20% chance) through
the end of the week as the upper low tracks over the western
Great Lakes.

Temperatures through the period are expected to generally be in
the low to mid 70s, particularly for inland areas. However, the
cooler by the lake theme will persist through next week as
persistent onshore flow continues. Accordingly, expect
conditions to remain several degrees cooler along the lakeshore
each day.

KJB

&&

.AVIATION /06Z TAFS THROUGH 12Z SUNDAY/...
Issued at 1153 PM CDT Fri May 23 2025

VFR conditions will prevail through the period as a surface
ridge settles over the area. Winds will remain calm or VRB
under 5 knots later through sunrise. Winds will settle NE on
Saturday, with lake-enhancement increasing speeds to over 10
knots in the afternoon and early evening. Finally, periods of
mid to upper-level cloud cover will prevail.

Kluber/Borchardt

&&

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

&&

$$

Visit us at weather.gov/chicago