864
FXUS63 KLOT 271951
AFDLOT

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

.KEY MESSAGES...

- Several waves of showers and thunderstorms are expected over
  the next 48 hours. A few storms may produce localized flooding
  and severe weather (mainly tonight).

- After a break on Tuesday, a stormy pattern will return toward
  the middle of next week.

- Above average temperatures will prevail through at least the
  middle of next week.

&&

.DISCUSSION...
Issued at 251 PM CDT Sat Apr 27 2024

Through Sunday Night:

A stout upper-level shortwave continues to dig into the Southern
Plains in the wake of a departing shortwave now lifting into Canada.
The resulting broad region of southwesterly flow across much of the
Mississippi River Valley has led to a summer-like day with partly
cloudy skies, temperatures in the mid to upper 70s, dew points in
the lower 60s, and breezy southwest winds gusting over 35 mph.

We are watching two areas for the development of thunderstorms this
evening. The first area is along a zone of low-level confluence
extending from near Topeka, KS to Milwaukee, WI, ahead of
locally pooling low-level moisture across northern Illinois.
Early attempts at the initiation of sustained convection have
thus far failed across Iowa and far northwestern Illinois,
likely owing to a strong cap sampled by an 18Z DVN RAOB based
near 850mb. While a storm or two may develop within the
confluence axis across eastern Iowa, northwestern Illinois, and
southern Wisconsin over the next few hours, a rapid expansion of
ongoing convection across northern Kansas into northern
Missouri and southern Iowa toward sunset appears to be more
likely as the aforementioned upper-level shortwave currently in
the Southern Plains lifts northeastward and lifts/cools the cap.

The second area we are watching for thunderstorm development is
across central Illinois, that is, south of US-24, within a gradually
destabilizing open warm sector. With nebulous forcing (e.g. offset
from the low-level confluence axis), the development of
thunderstorms in the second area will depend entirely the
erosion of a cap via warming and moistening of the boundary
layer. Based on AMDAR soundings from aircrafts
ascending/descending from ORD, temperatures around 78-79F and
dew points near 65-66F are needed for free convection to take
place. With that in mind, it may be several hours yet before
sustained convective attempts can take place, or at least until
the leading influence of the upper-level shortwave arrives.
Given impressively steep mid-level lapse rates across the warm
sector (>8 K/km from the base of the cap to around 500mb as
sampled by the 18Z DVN RAOB), convection will likely intensify
rapidly once the cap is breached in either area and pose a
threat for locally damaging wind gusts and hail. Note it`s
possible much of our area is completely dry through at least
sunset if the cap holds tight.

This evening, the expectation is for convection across northern
Missouri and southern Iowa to grow upscale and "meet" developing
free convection across the open warm sector across central Illinois
in tandem with the intensifying low-level jet. While the
pattern does not resemble more typical regimes for warm season
flash flooding, the prolonged forcing of the 925-850mb jet,
availability of PWATs of 1.25-1.5", and weak upper-level jet
diffluence may support a band of training convection across
central Illinois through the overnight hours. If convection can
repeatedly train over the same area tonight, 6-hour rainfall
amounts of 2-4" may fall supporting a threat for flash flooding
of low lying areas such as ditches, farm fields, and low-lying
roadways. The past two runs of the HREF and most recent
available HRRR/RAP/RRFS guidance continues to suggest the threat
zone for training convection will be near US-24, though
operational experience suggests it may end up just to the north
or (more likely) south of the advertised axis.

Now, for the fly in the ointment. Assuming the forthcoming MO/IA
convection does meet developing convection across central
Illinois this evening, any convection developing along the
Wisconsin state line would become "choked" off from the low-
level jet. However, if the central Illinois convection fails to
develop and meet the MO/IA convection, the confluence axis
across far northern Illinois would be the "hotpot" for storms
this evening and overnight given the replenishing low-level jet
will be able to lift much further northward. In such a scenario,
an axis of 2-4 inches of rain would be displaced closer to the
Wisconsin state line. Moreover, any upscale growth out of MO/IA
would support at least decaying complex of gusty thunderstorms
to move across most of our area after dark. At this point, it`s
not clear which scenario (axis across central IL or near the
Wisconsin state line), is most likely.

Toward daybreak Sunday, the MUCAPE reservoir feeding the overnight
convection (wherever it is) will become exhausted, allowing the
low- level jet (e.g. the "advective" component offsetting any
southward propagation via consolidated outflows) to push
activity northeastward away from our area. Meanwhile, the
remnants of what will likely become an expansive squall line
across the Southern Plains will northeastward into northern
Illinois after daybreak in the form of thicker cloud cover or
decaying showers. Tomorrow will accordingly start out wet and
cloudy. Ongoing isentropic ascent may support isolated to
scattered showers throughout the day, but overall, there should
be dry hours tomorrow. A few breaks in clouds should
nevertheless allow for highs to reach the low to mid 70s,
especially southeast of I-55.

The next wave of showers and storms will arrive tomorrow night as
the well-advertised upper-level shortwave trough finally lifts into
the Upper Mississippi River Valley. Compared to tonight, instability
tomorrow night looks far more meager in spite of favorable
kinematics for severe weather. For this reason, we favor a broken
band of showers and storms (perhaps with gusty winds) to move
through the region during the overnight hours.

Borchardt

Monday through Saturday:

The upper trough currently ejecting into the southern Plains is
forecast to be lifting into the upper Midwest as an upper low
in conjunction with a surface low on Monday which will force a
cold front through northeast IL and northwest IN. Thus, shower
and thunderstorm chances are expected to persist through Monday
afternoon. While instability continues to look rather modest,
the trajectory of the upper low into Wisconsin will position a
stout 500 mb jet streak over the area generating around 30 to 35
kts of shear. Therefore, if the warm-moist advection ahead of
the front can destabilize the atmosphere I would not be
surprised to see some of the thunderstorms try to become better
organized Monday afternoon, especially east of I-55. However,
the progressive nature of the front does look to limit the
window for ample instability and shear to overlap in our area
and therefore confidence on any severe threat occurring is low
at this time. Regardless, showers and storms are expected to end
from west to east Monday evening as the front exits.

Heading into Tuesday, modest upper-level height rises are
expected in the wake of the aforementioned front which will
promote a period of dry weather for Tuesday. Though, the
reprieve looks to be short lived as a series of shortwave
troughs are forecast to develop and traverse across the north-
central CONUS. This will result in periods of showers and
thunderstorms beginning Tuesday night and persisting through at
least the end of next week. While details on exact timing and
intensity of storms remains fluid, the area is expected to
remain under in an unseasonably warm pattern with afternoon
highs forecast to be in the mid to upper 70s through Thursday.

Yack

&&

.AVIATION /12Z TAFS THROUGH 18Z SUNDAY/...
Issued at 1235 PM CDT Sat Apr 27 2024

Key Aviation Impacts for the current TAF period:

- Strong wind gusts out of the south to southwest between 30 to
  35 knots, occasionally as high as 40 knots through the early
  evening

- Pockets of MVFR cigs through sunset, otherwise VFR

- Showers and thunderstorms late tonight through Sunday morning
  with IFR conditions during periods of heaviest rainfall

- Lingering showers Sunday morning and MVFR conditions expected,
  but there is low confidence on the exact timing on when rain
  slowly diminishes

With prevailing southwest winds, a steady stream of clouds are
passing over the area. There are periods where cigs may lower to
MVFR conditions, but with afternoon mixing, it is expected to be
mostly VFR through much of the day. The bigger impact will be
the strong wind gusts at area terminals. ORD already had a peak
wind gusts of 38 knots earlier this morning. The expectation is
for winds to gust between 30 to 35 knots through the afternoon,
but having occasional gusts up to 40 knots is possible. Winds
will remain out of the south tonight diminishing down to 20 to
25 knots. There may be short instances of winds that switch to
the southeast, but confidence is too low to add it to the TAF.

After 00Z, higher moisture content and instability will advect
into the region increasing the risk for showers and
thunderstorms around area terminals as early as 02Z. Latest
guidance has not wavered to much, with the expectation for most
thunderstorms to pass over around and after midnight. Cigs are
expected to be MVFR prevailing, but conditions may reduce down
to IFR levels around midnight as stronger cells provide heavier
rain rates and reduce visibility.

Showers will continue into the early morning hours, though
coverage will slowly diminish. There is lower confidence in when
the breaks will occur, so it was decided to leave -SHRA in the
TAF from 14Z-19Z, and add VCSH through much of the afternoon,
though it would not be surprising if that was pulled in a later
TAF issuance when better guidance can inform. More impactful is
the expectation for cigs to hang around terminals. Given how
poor the models are depicting the current cloud cover trends,
TAFs were written more aggressive with lower MVFR conditions
Sunday morning, not lifting to 2000 feet until the afternoon
with better mixing. Lastly, another low level jet will increase
tomorrow afternoon. And while timing might be adjusted, there
was enough of a signal to add wind gusts again into the TAF
between 20-25 knots.

DK

&&

.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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