007
FXUS63 KLOT 271445
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
945 AM CDT Sat Apr 27 2024

.KEY MESSAGES...

- Summer-like conditions today with high temperatures in the
  upper 70s to low 80s and increased humidity levels.

- Breezy today with wind gusts as high as 40-45 mph from mid-
  morning through the early afternoon.

- Isolated showers and thunderstorms expected to develop this
  afternoon with coverage increasing markedly tonight. There
  are lower-end chances for damaging wind gusts with this
  evening`s storms and for localized flooding tonight.

- After a break Tuesday, an active weather pattern returns
  midweek with multiple periods of showers/storms expected
  through Friday.

&&

.UPDATE...
Issued at 945 AM CDT Sat Apr 27 2024

A recent hand surface analysis shows a pair of low pressure
systems, with the first near the Twin Cities and the second near
the Texas panhandle. Between both lows, broad south to
southwesterly flow is advecting moist and arguably humid air as
well as unseasonably warm temperatures into our region with
current temperatures already above the average for late April
and in the upper 60s to around 70.

Earlier this morning, both the 12Z ILX and DVN RAOBs sampled a
modest capping inversion based near 850mb. While continued low-
level warm air advection and subtle height rises (between the
parent upper-level shortwaves tied to each aforementioned
surface low pressure system) should reinforce the capping
throughout the day, aggressive low-level moisture advection in
tandem with forecast high temperatures in the upper 70s to
around 80 may nevertheless allow for free convection to develop
by mid-afternoon. Indeed, modifying the 12Z DVN/ILX RAOBs with
expected surface conditions this afternoon show little to no
capping with some 1000-1500 J/kg of MLCAPE. For this reason, it
appears prospects for at least isolated thunderstorms are
increasing across our area this afternoon.

Now, just how many thunderstorms develop this afternoon will
probably dictate the placement and coverage of thunderstorms
overnight. In the scenario where the capping inversion holds,
showers and storms would be poised to develop near a stationary
front draped across eastern Iowa and southern Wisconsin this
evening in tandem with the intensifying low-level jet. The
result of upscale growth of a cluster or two would then be
poised to move into northern Illinois and eventually
northwestern Indiana after dark. Recent iterations of the NAM3
show this idea. However, if the cap erodes fully this afternoon,
scattered to even widespread showers and storms may erupt near
the axis of highest low-level moisture somewhere near US-24 as
early as 3-4 pm, and continue more or less unabated overnight as
the low-level jet intersects composite west-to-east oriented
outflow. In this scenario, a localized threat for flash flooding
would materialize owing to repeated rounds of thunderstorms
over the same area. It`s worth noting the 00Z (overnight) run of
the HREF depicted a narrow axis of 24-hour LPMM of 2-4" near
US-24. While the 00Z WRF-ARW appeared to dominate the 00Z HREF
signal, the fresh 12Z HRRR and recent iterations of the
experimental RRFS depict a strikingly similar evolution with
training thunderstorms near or just south of US-24. Regardless
of where thunderstorms do form, the strongest will be capable of
producing locally damaging winds and damaging hail.

All things considered, opted to increase PoPs dramatically to
the 50-60% range along and south of I-80 this afternoon to
account for the threat that the cap breaks and coverage of
showers and storms is much higher than originally anticipated.

Updated products have been sent.

Borchardt

&&

.DISCUSSION...
Issued at 352 AM CDT Sat Apr 27 2024

Through Sunday:

After any lingering overnight convection clears the area this
morning, the already breezy southerly winds will get even
breezier by mid-morning as we begin to tap into 40 kt flow in
the lower part of an overhead low-level jet. This will likely
translate to 40-45 mph gusts being observed during a brief window
of time between the mid-morning and early afternoon before the
magnitude of the gusts lessens a bit as the low-level jet
continues to gradually wane going into this afternoon. These
southerly/south-southwesterly winds will also serve to advect in
an unseasonably warm and moist air mass, characterized by
summer-like dew points in the low to mid 60s and temperatures
that will likely climb into the upper 70s to around 80F with the
assistance of some filtered sunshine.

Height rises, large-scale subsidence on the backside of the
departing upper-level trough, and the development of a capping
inversion should ensure that most of our forecast area will remain
dry through the daytime hours. However, by about the mid-afternoon,
the capping inversion will likely be eroded as a result of surface
heating, and some combination of broad isentropic ascent and low-
level confluence should help isolated to scattered convection to
sprout. Confidence is highest in this occurring in our southern
counties, where high resolution models continue to suggest that
there may be a zone of more focused ascent, but can`t entirely rule
out a stray shower/storm or two popping up elsewhere, so continue
to carry slight chance PoPs across the entire CWA during the
mid-late afternoon. There will be a plentiful amount of instability
around come this afternoon (MLCAPE of up to around 2500 J/kg), so
would not be surprised if a couple of storms managed to grow deep
enough to support a threat for downbursts and/or hail, but the
latest thinking is that the relatively broad and modest forcing
for ascent should preclude the development of any overly
rambunctious convection capable of making full use of the available
instability.

Meanwhile, to our west and southwest, a frontal zone is expected to
light up with convection during the afternoon and evening as large
scale forcing for ascent increases in advance of another upper-level
trough ejecting into the Great Plains. Convection that originates in
Iowa and Missouri is likely to grow upscale with time and track in
our general direction, with most of the latest CAM runs envisioning
that at least a semi-organized multicell storm cluster will be
knocking on the doorstep of our western CWA by mid-evening. While
these storms are expected to lose steam with time and eastward
extent, forecast soundings indicate that the degree of available
buoyancy here this evening could still be adequate to support the
development of new, deep updrafts along the storm cluster that
would carry a threat to produce strong to severe downburst winds,
particularly across our northwestern CWA. This threat largely
hinges upon this storm cluster developing a robust consolidated
cold pool that would be able to overcome increasing nocturnal
boundary layer stability, which does not appear to be a given at
this time.

Of additional interest with tonight`s thunderstorms is the potential
for localized flash flooding. It does not have the look of an ideal
flash flood setup by any means, but it does appear that storm
orientation could somewhat align with storm motion vectors for a
time and promote some potential for training convection. The
nose of a strong low-level jet to our southwest will also pivot
in our direction tonight and provide a continuous replenishment
of moisture into the region, which will likely keep thunderstorms
and heavy rainfall ongoing in at least part of our forecast
area throughout the night. Thus, some potential for localized
minor flooding does seem to exist in our forecast area tonight,
and WPC`s Marginal Risk in their Day 1 Excessive Rainfall
Outlook continues to look appropriate for highlighting this
possibility.

Shower and storm coverage is likely to diminish after daybreak on
Sunday as the low-level jet diurnally weakens. However, with surface
low pressure and an associated cold front still to our west, warm
conveyor belt showers and storms will likely continue in some
capacity in our forecast area during the day on Sunday, with
the highest chances for precipitation remaining across our
western counties through sunset.

Ogorek


Sunday Night through Friday:

Heading into Sunday evening upstream convection is likely
across Iowa and Missouri and pushing east of the Mississippi
River. This activity reaches the area by late evening. While
instability will be on a weakening trend, the arrival of better
forcing and a strengthening low-level jet would support a
localized damaging wind and large hail threat, especially west
of a McHenry to Kankakee, IL line.

A continued slower eastward progression of the surface cold
front will likely allow for showers to persist into the day on
Monday. Instability is much more muted compared to over the
weekend, though a few thunderstorms can`t be ruled out,
potentially lingering into the afternoon and early evening hours
east of I-57. Severe weather is not expected. Once showers exit
to the east late Monday, skies clear out as weak mid-level
ridging builds in allowing overnight lows Monday night to dip
back into the upper 40s to lower 50s. Tuesday will be noticeably
less humid as we remain between weather system. Ample sunshine
and a return to southwest flow will still allow temperatures to
warm well into the 70s again.

The break in the precipitation appears to be short lived as
ensemble guidance continues to support a return to an active
weather pattern midweek with multiple chances for showers and
thunderstorms Tuesday night through Friday as multiple
disturbances move within the broader northern stream quasi-zonal
upper jet.

Petr

&&

.AVIATION /12Z TAFS THROUGH 18Z SUNDAY/...
Issued at 640 AM CDT Sat Apr 27 2024

Key Aviation Messages:
- SSW wind gusts over 30kt through early afternoon with sporadic
  gusts near 40kt
- Patchy MVFR stratus this morning
- SHRA/TSRA redevelops this evening through early Sunday AM

SSW winds will quickly increase this morning with gusts over
30kt expected through early afternoon. Sporadic gusts in the
upper 30kt to near 40kt can`t be ruled out. Winds ease toward
sunset remaining SSW. Patches of MVFR stratus may move into the
area this morning.

Isolated showers/storms may attempt to develop as early as this
afternoon, especially near and south of I-80, though confidence
remains too low for a formal TAF mention of TS during this
period. The thunder potential increases mid-to-late evening,
however, when model guidance continues to depict a complex of
showers and thunderstorms expanding across the area. While
confidence on the start time is on the lower side (and will be
dependent upon where and when upstream convection develops),
confidence in thunderstorms is high enough to prevail VCTS
through the overnight hours with a targeted TEMPO groups
overnight when the greatest coverage is anticipated along with
any associated IFR/near-IFR vsby/cig reductions.

Petr

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...Small Craft Advisory until 10 PM CDT this evening for the IL
     and IN nearshore waters.

&&

$$

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