-- Discussion containing changed information from previous version are highlighted. --
652
FXUS63 KLOT 281746
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1246 PM CDT Fri Jun 28 2024

.KEY MESSAGES...

- Periods of showers through the afternoon, decreasing in areal
  coverage for a period this evening.

- Increasing potential for a period of thunderstorms tonight
  after 11 pm, some could be strong and produce locally heavy
  rainfall.

- 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.

&&

.UPDATE...
Issued at 1205 PM CDT Fri Jun 28 2024

The main updates to the forecast for today was to raise PoPs and
go with areal coverage wording for the rain showers expected
through the day. We also lowered high temperatures across far
northern IL, where the highest coverage of showers are expected
through the day.

Warm air advection will continue to foster numerous showers this
afternoon in advance of a mid-level impulse tracking across far
northern IL. The highest coverage and more persistent showers are
expected primarily along and north of I-80, with more isolated to
scattered activity to the south. This is expected to hold
temperatures this afternoon several degrees cooler than
previously thought. We have therefore lowered high temperatures
across far northern IL, with readings expected to top out in the
70s this afternoon. Fortunately, poor lapse rates will curtail the
threat of an thunderstorms with this activity through the day.

Late this afternoon (after 5 or 6 pm) we may begin to experience
a break in the shower activity for a few hours into this evening
as the first mid-level impulse begins to shift east of the area.
While this is the case, we cannot rule out some continued isolated
to widely scattered showers, but the coverage of them should be
lower than those expected through the afternoon.

A second round of showers (along with a better chance of
thunderstorms) is then expected with another impulse shifting
into the area late this evening (likely after 11 pm) into the
overnight hours. Strengthening theta-e advection along an
intensifying 40 kt southwesterly low-level jet ahead of this
disturbance will steepen mid-level lapse rates through the
evening, thus supporting a better chance of thunderstorms with
this overnight activity. A low end threat for a few strong storms
with hail and strong wind gusts even exists with these overnight
storms given the favorable kinematic environment. However, the
best threat for any severe storms looks to largely remain to our
west-southwest.

Very high precipitable water values (in excess of
2", or 200% of normal) will support very efficient and high
rainfall rates (1+" per hour) with these overnight storms. This
could result in some localized instances of minor flooding.
However, the storms do look to be rather progressive, which
should limit their duration at any given location. Nevertheless,
with high rain rates, even a short period of training storms
could result in localized flooding, so this is something that will
need to be monitored.

KJB

&&

.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 /18Z TAFS THROUGH 00Z SUNDAY/...
-- Changed Discussion --
Issued at 1246 PM CDT Fri Jun 28 2024

Key Aviation Messages:

- Showers will continue through the afternoon, becoming more
  scattered in nature this evening.

- TSRA chances appear low until late this evening, increasing
  after midnight.

Current radar imagery shows a broad area of precipitation moving
across the area. Southerly flow has allowed dew points to be on
the rise through the morning. This will continue into the
afternoon promoting a continuation of the showers that are
currently in the area, though they may scatter out a bit into
the evening. Rain may bring periods of reduced visibility this
afternoon resulting in on and off MVFR conditions at all of the
terminals. Thunder chances remain quite low through the
afternoon.

Thunderstorm chances increase late this evening, especially
after midnight. Coverage will likely be more scattered than
numerous, thus have opted for a tempo group for TSRA rather than
prevailing. However, any storms that impact the terminals, will
deteriorate conditions with rain reducing visibility to
potentially IFR conditions. Winds could also get a little
squirrelly during any storms at the terminals, however,
southerly winds should prevail through the night.

Confidence is not as high in thunderstorm coverage continuing
into the morning, so have decided to continue with the PROB30
for this time frame. Once showers and storms move out of the
area, some low clouds will likely hang around through the
morning, keeping the area in MVFR conditions. Conditions will
begin to improve during the late morning and into the afternoon
hours with winds turning more westerly than southerly.

Carothers
-- End Changed Discussion --

&&

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

&&

$$

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