595
FXUS63 KLOT 281156
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
656 AM CDT Fri Jun 28 2024

.KEY MESSAGES...

- Periods of showers this morning becoming more widespread this
  evening and overnight with increasing thunderstorm chances.

- Potential for a few storms to be strong to severe this
  evening into Saturday morning with a threat of gusty winds.

- Cooler and dry Sunday and Monday, then warmer and more humid
  for the remainder of the coming week with shower and
  thunderstorm chances returning on Tuesday.

- Moderate to High Swim Risk expected Sunday with onshore
  winds.

&&

.DISCUSSION...
Issued at 340 AM CDT Fri Jun 28 2024

Through Saturday:

The area of high pressure centered over the Great Lakes
overnight continues to get pushed east as a mid-level shortwave
trough propagates across eastern NE. Ahead of the shortwave a
broad area of showers has materialized across IA which will
continue to make steady progress eastward into northern IL and
eventually northwest IN later this morning. Initially the
showers will be more along the line of sprinkles due to copious
amounts of dry air in the lower atmosphere, but as winds turn
south-southwesterly this afternoon the combination of moist
advection and the top down saturation will increase the shower
intensity. However, instability is expected to be rather weak
this afternoon which should keep thunder chances low.

The aforementioned mid-level shortwave will exit to our east
this evening which will allow a broader trough, currently over
the northern Plains, to pivot into the upper Midwest and
northern Great Lakes overnight. In doing so, a developing
surface low will move northeast into Wisconsin forcing a cold
front into the mid- Mississippi Valley. The continued warm-moist
advection ahead of the front in combination with cooling upper
temperatures should aid in some gradual destabilization of the
atmosphere overhead tonight despite the lack of diurnal heating.
Therefore, shower and thunderstorm coverage (70-80%) is
expected to increase areawide through the night into Saturday
morning. While deep layer wind shear is expected to remain
modest (20-30 kts at best) overnight, a 40-45 kt low-level jet
looks to develop after midnight which will increase the low-
level shear. Where the strongest low-level shear and modest
instability overlap a few storms may become more organized and
possibly strong to severe with a threat of gusty winds. Thus,
the area remains under a level 1 of 5 risk for severe weather.

Heading into Saturday, the cold front will be in the process of
moving through the area which will gradually taper shower and
thunderstorm chances from northwest to southeast through the
day. Though, guidance has started to trend a bit slower with the
cold front which may allow shower and thunderstorm chances to
persist longer for areas further north than originally
anticipated but the greatest coverage is still expected to be
southeast of I-55. Furthermore, the later frontal passage may
also allow diurnal heating to destabilize the air mass ahead of
the front which could then increase the potential for another
period of strong to possibly severe storms especially if storms
can take advantage of the better shear forecast to be along the
front. If a severe threat was to materialize the main threat
would still be gusty to damaging winds, but some hail may also
be possible given the better instability.

Otherwise, seasonable temperatures can be expected to close out
the week with highs in the upper 70s to mid-80s today followed
by mid to upper 80s for Saturday.

Yack


Saturday night onward:

After the cold front clears the area Saturday afternoon, cooler
and drier air settles in along with rising heights near the
surface and aloft. The result will be a quiet end to the
weekend, with a beautiful Sunday in store. Given onshore flow
Sunday, highs near the lake may not escape the 60s.
Unfortunately, the northerly flow will also build dangerous
waves at most of the southern Lake Michigan beaches, with
moderate to high swim risk conditions expected.

Monday also looks seasonably cool and dry before warmer and
more humid air gradually works back into the area. The surface
high that will be overhead on Sunday and Monday will push into
the Ohio Valley and eastern Great Lakes by Monday night. Gulf
moisture will not make an appreciable return to the area until
the high pushes farther east during the day on Tuesday.

By midday Tuesday into Tuesday afternoon, warm and moist
advection at low levels along with height falls aloft on the
back side of an upper ridge will support increasing chances for
showers. Midlevel lapse rates look to remain fairly modest on
Tuesday, so thunderstorms may be less of a concern initially.

Better support for thunderstorms looks to arrive by Wednesday
as lapse rates increase, winds strengthen aloft, and a cold
front approaches from the west. The chance of precipitation then
appears to persist through July 4th and the remainder of the
week as warmth and humidity remain in place despite the frontal
passage Wednesday. With flow aloft becoming more zonal and the
Gulf remaining open, embedded shortwaves would likely be able to
provide sufficient forcing to support periods of showers and
thunderstorms. If Wednesday`s frontal zone stalls out near the
area, that would provide an additional focus for development.

Lenning

&&

.AVIATION /12Z TAFS THROUGH 18Z SATURDAY/...
Issued at 631 AM CDT Fri Jun 28 2024

Key Aviation Messages:

- Southeast winds this morning, then becoming breezy from the
  south-southwest by early afternoon.

- Scattered showers through midday, most likely at KRFD with
  lower coverage farther east. TSRA chances appear minimal until
  late evening, and increase after midnight.

Regional radar early this morning showed a broad swath of
showers moving into northern Illinois, but surface observations
suggested a lot of this activity was not yet making it to the
ground. Ceilings were still in the 6000-10000 ft range,
also reflecting the drier air at lower levels. The activity to
our west appears to be supported by a low level jet that
gradually is expected to weaken as it slides east today. Thus
the thinking that shower activity will continue to be off and on
rather than persistent across the local airspace. Elected to
continue handling this with a VCSH for most of the day, until a
brief break expected for the afternoon. Confidence is higher
that activity increases during the overnight hours, with periods
of thunderstorms likely, especially after midnight. This would
be supported by a somewhat reintensified low level jet along
with an approaching cold front.

Lenning

&&

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

&&

$$

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