574
FXUS63 KLOT 282003
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
303 PM CDT Sun Apr 28 2024

.KEY MESSAGES...

- Showers and storms to redevelop this afternoon/evening, a few
  of which could become strong to severe near and west of I-39.

- Locally heavy rainfall is also possible with stronger storms which
  may yield instances of ponding

- Additional shower and storm chances Monday afternoon, mainly
  east of I-55.

- After a break on Tuesday, a stormy pattern returns midweek
  along with continued unseasonably warm temperatures.

&&

.DISCUSSION...
Issued at 303 PM CDT Sun Apr 28 2024

Through Monday night:

Northeast IL continues to reside in the warm sector of a double
barreled surface low which is centered over IA. The warm front
has made little progress and still resides near the IL-WI line
which has allowed for a lake enhanced surface boundary to surge
inland across Lake, IL and Cook Counties. As a result, onshore
winds behind this boundary have maintained much cooler
temperatures (readings in the 50s to lower 60s) and has been
advecting in fog off of Lake Michigan. Recent radar trends
continue to show the lake boundary gradually pushing northward
as south-southwest winds in the warm sector have increased and
this is expected to continue through the rest of the afternoon.
Therefore, the advection fog in Lake, IL County and over the
open waters of the lake should begin to erode over the next few
hours as winds turn and deeper mixing is achieved.

In addition to the lake boundary, the position of the warm
front has also trapped mid-level moisture overhead and allowed
broken to overcast stratus to prevail. Thus, instability has
struggled to increase this afternoon despite the continued warm-
moist advection as recent aircraft soundings out of MDW show a
weak cap has actually developed instead. While the reduced
instability may serve as a hindrance to shower and storm
coverage this afternoon, additional showers and thunderstorms
are expected to develop this evening and persist through the
night as the upper low pivots into the upper Midwest placing
northeast IL and northwest IN in the right entrance region of an
associated upper jet streak. Since any deeper instability will
be waning after sunset the severe weather threat with any storms
overnight continues to look low. However, mesoanalysis
continues to show 30-40 kts of effective shear in place which
could aid in generating an isolated strong to possibly severe
storm if sufficient destabilization can occur prior to sunset.
The main threat with any severe storm would be gusty winds and
hail and should be largely confined to the I-39 corridor and
points west.

Additionally, there is also a growing concern (20-30% chance)
for instances of locally heavy rainfall tonight as PWATs in the
1.3 to 1.5 inch range continue to remain in place. Obviously the
main driver for this threat will be the intensity of any storms
tonight which as previously mentioned is uncertain. However,
any more robust cores should be able to produce locally heavy
rainfall and may lead to instance of ponding in lower-lying and
flood prone areas. Given the lower confidence in amounts and
exact location of any heavier rainfall have decided to hold off
on any flood products.

The showers and storms overnight will exit the area on Monday
as a cold front begins to move into the area. While another dry
period is expected for the middle part of Monday, a final round
of showers and storms is forecast to develop Monday afternoon
along and east of I- 55. Given the short duration of showers and
storms in our area and the marginal instability, no severe
weather is expected. The wet weather will finally come to a
close Monday evening with dry conditions Monday night into
Tuesday. Otherwise, seasonably warm temperatures will prevail
through Monday with highs once again in the 70s.

Yack

Tuesday through Sunday:

Ensemble model guidance favors the development and persistence of
broad upper-level cyclonic flow centered along the central
US/Canadian borders throughout the week, placing the Great Lakes
along the western periphery of a synoptic-scale ridge. As a result,
above-average temperatures are poised to return to the region with
highs climbing back into the 70s by Tuesday, and even the lower 80s
Wednesday and Thursday. As is typical this time of year, cooler
temperatures can be expected along the Lake Michigan shoreline
Tuesday and especially Wednesday due to lake-induced onshore flow
(and moreso the passage of a backdoor cold front Wednesday
afternoon). However, increasingly breezy southwest winds on Thursday
should allow for the warmth to spread as far as the lakeshore.

Embedded shortwaves within the slowly-approaching upper-level
cyclonic flow will also present opportunities for showers and
thunderstorms next week. The first opportunity will be Tuesday
night into Wednesday. At this point, it appears that our area
may remain just on the outside of the influence of the first
shortwave propagating along the eastern periphery of the
steering cyclonic flow aloft. As a result, the highest chances
(30 to 40%) for showers and storms Tuesday night into Wednesday
are focused west of I-39 (and increase further west of the
Mississippi River). The second opportunity for showers and
storms will be in Thursday night into Friday timeframe. With the
parent upper-level low closer by then, confidence in showers
and storms impacting our area is higher with the second
shortwave (chances for showers and storms are currently around
60-70% Thursday night into Friday). The forecast for high
temperatures on Friday will depend entirely on when the second
shortwave and associated showers and storms move through the
area.

Behind the second shortwave and as the core of the upper-level low
makes its closest approach, ensemble model guidance advertises a
signal for brief cool-down toward seasonal temperatures this
weekend. Thereafter, there is a signal for the return of above
average temperatures and a generally stormy pattern toward the
second week of May.

Borchardt

&&

.AVIATION /18Z TAFS THROUGH 00Z TUESDAY/...
Issued at 1239 PM CDT Sun Apr 28 2024

Key Aviation Impacts for the current TAF period:

- Scattered showers northwest of Chicago with the potential for
  some thunderstorms around KRFD this afternoon

- Widespread showers and thunderstorms expected this evening
  and through Monday morning

After a brief window of drier, VFR conditions, the next round
of showers moves into the area this afternoon ahead of a front
tonight. The risk for thunder was low enough around Chicago
terminals to leave it out of the TAF currently; however, a TEMPO
for -TSRA was placed into the KRFD TAF given the higher
instability aloft expected to the west. Winds are slowly
becoming out of the southwest as the warm front drifts
northward. As cloud cover erodes, stronger gusts around 25 knots
are expected to return to area terminals, though the occasion
gust up to 30 knots is not out of the question.

After 00Z, a cold front will start its movement eastward over
Illinois through the overnight hours. This will bring a renewed
threat of showers and thunderstorms at terminals that will
provide another opportunity for MVFR/IFR conditions from lower
cigs and vis. Current expectation is for the cold front, and
associated heaviest rain, to be over the lake after 12Z Monday
morning. However, post frontal showers are expected to linger
through the morning hours. While there is lower confidence on
the exact timing for when rain cuts off, drier conditions are
expected Monday afternoon as southwest winds gust around 20
knots and gradually become slightly more west to southwest by
the late afternoon.

DK

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...Dense Fog Advisory until 4 PM CDT this afternoon for Winthrop
     Harbor to Northerly Is. IL.

&&

$$

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