263
FXUS63 KLOT 271742
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1242 PM CDT Tue May 27 2025

.KEY MESSAGES...

- Scattered showers return tonight into Wednesday, with shower
chances then persisting through the end of the week as a slow-
moving storm system meanders across the Midwest.

- A pattern change is anticipated during the first week of June
and will likely result in the arrival of more summer-like
temperatures.

&&

.DISCUSSION...
Issued at 330 AM CDT Tue May 27 2025

Through Wednesday:

A Rex block remains locked across North America, as characterized
by anomalously strong upper-level ridging draped across
northern Canada to the north of a broad upper-level low centered
over the northern US. Water vapor imagery shows several
distinct upper-level vorticity maxima pinwheeling clockwise
within the broad upper-level low, including one maxima moving
southward in South Dakota and another moving westward into
southern Manitoba. Several auxiliary shortwaves are also
propagating along the outer-perhiperhy of the upper-level low,
such as one moving across the Southern Plains, another lifting
across the middle Mississippi River Valley (which is responsible
for upstream radar returns across Iowa and west-central
Illinois), and a third, comparatively stronger, wave lifting toward
the central Appalachian Mountains. The main forecast challenge over
the next 36 to 48 hours remains identifying the time periods and
locations with relative highest chances for showers within this
relatively benign pattern.

During daylight hours today, a pocket of dry air extending from
the surface to about 10kft should prove resilient to incoming
radar returns thanks to a steady feed of reinforcing dry low-
level air along the southern side of a surface high pressure
system parked over southern Canada. So, still believe that
almost all areas will stay dry today outside a few locations
picking up a stray sprinkle or two. With a warmer start to the
day (current temps some 4 to 6 degrees warmer than yesterday
morning), high temperatures this afternoon should be fairly
similar to yesterday and in the upper 60s to lower 70s away from
the Lake Michigan shore even with heavy cloud cover limiting
solar insolation.

Tonight into tomorrow, an ensemble of high resolution and
global guidance advertises the aforementioned South Dakota
vorticity maxima arcing around the base of the upper-level low
across the central Plains and then shooting northeastward into
the Lower Great Lakes. As this occurs, a pool of low-level
moisture (PWATS > 1") across the middle Mississippi and Ohio
River Valleys will finally get pulled northward into the
Southern Great Lakes, setting the stage for showers to develop
basically over our heads. Even at such a limited lead time,
confidence on where the highest coverage of showers will develop
remains an item of low confidence owing to very subtle
differences amongst model guidance in the structure of the
incoming shortwave and any potential interaction with an
auxiliary shortwave lifting east-northeastward from the
Southern Plains. Regardless, the conceptual pattern of an
approaching vorticity maxima into an airmass characterized by
PWATS > 1" supports broad-brushed 50-60% PoPs across much of our
area from tonight through much of Wednesday. Lows tonight will
be in the low to mid 50s, and highs Wednesday will be stunted to
the low to mid 60s by any ongoing showers and heavy cloud
cover.

Finally, will have to watch for any focused area of mid-level
cyclonic convergence to support pocket or two of healthy
rainfall rates and totals particularly Wednesday morning. Even
with differences in placement in rainfall advertised by high
resolution and global guidance, most do show a pocket of 1-2" of
rain somewhere with this system (HRRR/RAP being most aggressive
in magnitude and area). With the ongoing abnormally dry to
drought conditions at least east of I-39, any rain we can get
would be welcome.

Borchardt


Wednesday Night through Monday:

The forecast for the latter half of the week remains complex
and quite uncertain with respect to precipitation owing to
continued uncertainties regarding the evolution of the sprawling
closed-off upper-level low over the Great Lakes/Midwest. Medium
range guidance has come into better agreement that the upper
low will become more disheveled by Thursday, and likely get
split in two with one chunk of it departing in the direction of
the Canadian Maritimes, while the other portion straggles behind
and remains in the Midwest for at least one more day. How this
trailing disturbance evolves will be paramount for determining
the coverage of any potential precipitation in our area on
Thursday, and unfortunately, there are still some notable
differences in its evolution between the main global ensemble
systems.

The 00Z deterministic GFS and a majority of 00Z GEFS ensemble
members suggest that this disturbance will quickly get
reorganized into a closed-off upper-level low that slowly drifts
eastward across the lower Midwest and delivers a good swath of
soaking rainfall to at least part of our forecast area Thursday
into Friday. It is worth noting that support for such a solution
was much less popular in the 12Z GFS/GEFS suite, but gained
more backing in the 18Z suite that has since persisted. On the
other hand, the majority of 00Z EPS and CMCE ensemble members,
along with their corresponding deterministic counterparts, keep
this disturbance weaker/more disorganized and/or shunt it
farther south faster. The 12Z, 18Z, and 00Z runs of the NAM were
also in line with this thinking. In this kind of forecast
solution, weaker forcing for ascent and poorer moisture quality
would likely result in the precipitation coverage across our
forecast area being no greater than widely scattered on
Thursday with the best precipitation chances and coverage
favored to end up across our western CWA. The latest NBM PoPs
came in more or less aligned with what the majority of available
guidance (i.e. not the 00Z GFS/GEFS) favored, but were still
probably a bit lower than ideal. Made some modest adjustments to
them to increase them slightly and to expand the mentionable
PoP footprint, but if other guidance starts to trend in the
direction of the 00Z GFS and GEFS, then they will need to be
boosted in forthcoming forecast packages.

Friday into Saturday, a pronounced northern stream trough will
dive southward from Hudson Bay towards the northeastern quadrant
of the CONUS. While there is still some uncertainty with how
its interaction with the aforementioned disturbance will play
out, there is pretty good ensemble agreement that it will help
send a backdoor cold front our way. The timing of this front`s
arrival, the front`s strength, and how far south it will get are
all still points of uncertainty as a consequence of this complex
pattern evolution, so confidence in temperature trends from
late Friday through Saturday remains low at this time. There is
also a fair ensemble signal for showers and maybe even
thunderstorms developing near/behind this front, though a
diurnally unfavorable timing of the front`s arrival could limit
precipitation coverage and/or the likelihood of this activity
getting into our forecast area.

A pattern change is then expected across the eastern half of
the CONUS next week as the mean upper-level troughing over the
northeastern quadrant of the CONUS is pushed eastward and
replaced by an amplified upper-level ridge. As the ridge sets up
over the region, a warmup to more summer-like temperatures
appears likely to occur early next week.

Ogorek

&&

.AVIATION /18Z TAFS THROUGH 00Z THURSDAY/...
Issued at 1242 PM CDT Tue May 27 2025

Forecast concerns include...

Low mvfr cigs, possible ifr cigs Wednesday.
Periods of rain overnight/Wednesday.

A few showers will be possible this afternoon, mainly south of
the terminals, but trends will need to be monitored. Otherwise,
a few sprinkles will be possible for the rest of the area.
Periods of rain are expected to develop across western IL
tonight and move east across the Chicago terminals around or
just after daybreak Wednesday morning. The rain will continue
into the afternoon and then is expected to taper off and end in
the mid/late afternoon.

Guidance is in decent agreement that as the rain spreads across
the terminals Wednesday morning, cigs will lower to low mvfr,
which is expected to continue into Wednesday afternoon. Ifr cigs
are also possible but confidence is low for how widespread they
may become and for now have only included scattered mention.

Northeast winds will continue through the period with speeds in
the 10-15kt range today and then likely under 10kts for the rest
of the period. cms

&&

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

&&

$$

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