036
FXUS63 KLOT 282334
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
634 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 /00Z TAFS THROUGH 06Z TUESDAY/...
Issued at 621 PM CDT Sun Apr 28 2024

Key Aviation Messages:

- Gusty south-southwest winds easing early this evening.

- Scattered showers increasing in coverage overnight. Lower TSRA
  coverage expected than earlier, with highest chances from
  south Chicago metro southward.

- Period of MVFR ceilings late tonight/early Monday morning.

- Breezy west-southwest winds Monday.

Early evening surface map shows low pressure along the NE/IA
border, with a warm front extending east along the IL/WI border.
Warm sector south of the front remains mild and somewhat humid
with gusty breezy south winds, though fairly extensive cloud
cover has helped to limit the degree of destabilization and
convective shower/thunderstorm coverage across the region this
afternoon. An upper-level disturbance over the Plains will lift
northeast across the upper Midwest tonight into early Monday
however, and deep ascent associated with this disturbance is
expected to result in increasing SHRA/TSRA coverage across the
area tonight. While isolated thunderstorms can`t be ruled out at
all TAF sites tonight, various guidance including the HREF/SREF
ensembles suggests that thunderstorm coverage will be greatest
from eastern MO/central IL northeast across northern
IN/southwest MI. Thus have decreased mention of thunder in TAFs
with mainly VCTS for KDPA/KORD/KMDW while maintaining a TEMPO
for KGYY during the pre-dawn hours coincident with the best
forcing. Showers and thunderstorms will move east of the area
around sunrise.

MVFR ceilings will likely linger into Monday morning, until
mixing/warming can lift bases to VFR mid-late morning. Also with
diurnal warming, steepening low level lapse rates will make for
gusty west-southwest winds by late morning, and could support a
few scattered showers into the afternoon. A cold front will move
through late in the afternoon, which will shift winds to the
west and scatter out clouds heading into Monday evening.

Ratzer

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...Dense Fog Advisory until 10 PM CDT this evening for Winthrop
     Harbor to Wilmette Harbor IL.

     Small Craft Advisory until 10 PM CDT this evening for
     Northerly Is. IL to Michigan City IN.

&&

$$

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