605
FXUS63 KLOT 161653
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1153 AM CDT Thu May 16 2024

.KEY MESSAGES...

- Scattered showers and thunderstorms will develop this
  afternoon. A stray strong storm cannot be ruled out near the
  Wisconsin border this evening (5% or lower chance).

- Patchy to areas of dense fog may develop overnight, especially
  northwest of I-55.

- Friday will be pleasant with highs around 80 and nearly calm
  winds. A lake breeze will surge inland during the afternoon,
  and a stray storm cannot be ruled out south of Interstate 80
  (20-30% chance).

- Aside from isolated storm chances Saturday, mostly dry
  conditions are expected this weekend.

- An active pattern  is likely to develop next week bringing
  several opportunities for showers and storms.

&&

.UPDATE...
Issued at 1153 AM CDT Thu May 16 2024

Only update to the going forecast this morning was to add some
better time resolution to the shower and thunderstorm chances
through this afternoon.

In spite of the recent increase in mid and high-level cloud
cover, the low-level airmass continues to destabilize late this
morning as ~60 degree dewpoints continue to work northward into
the area in advance of low pressure shifting into the Upper
Great Lakes. With capping expected to be weak to non-existent
this afternoon, widely scattered showers and thunderstorms are
expected for much of the area this afternoon. Recent satellite
and radar trends even suggest that things are already getting
underway, and see no reason why this will not continue.

Modest mid-level lapse rates and deep layer shear on the order
of 25 to 30 kt suggests that pulse type storms and/or loosely
organized clusters will be the primary type. Accordingly, the
overall severe threat will remain on the lower side today. That
being said, we cannot rule out a couple instances of wind and
hail with some of the strongest storms.

Most of the activity is expected to wane with sunset this
evening. The only exception being a cluster of storms with a
cold front potentially persisting with a cold front for a
couple hours after sunset this evening, mainly across
northwestern parts of the area.

KJB

&&

.DISCUSSION...
Issued at 304 AM CDT Wed May 16 2024

Through Friday...

An early-morning hand surface analysis revealed a surface low
pressure system in central Minnesota along a cold front arcing
southwestward through western Iowa, southeastern Nebraska, and far
northwestern Kansas. A few thunderstorms were noted ahead of the
front in western Wisconsin and central Iowa. A second area of low
pressure (really a mesoscale convective vortex, or MCV) was noted
along the Oklahoma and Kansas Border along the backside of a
relative disorganized mesoscale convective system approaching
western Missouri. Ahead of the MCV, separate areas of showers and
thunderstorms, and cold front, an expansive area of low-level
moisture with surface dew points in the low to mid 60s and PWATs
ranging from 1.0 to 1.3" (re: 00Z TOP and SGF RAOBs). The northern
edge of the low-level moisture remains stark and near the Illinois
and Missouri border, thanks to continued, albeit light,
northeasterly flow on the backside of a departing high pressure
system moving into southern Ontario. Finally, a baggy upper-level
trough is evident across the Upper Mississippi River Valley water
vapor imagery, including a subtle embedded shortwave propagating
into northwestern Iowa at press time.

The forecast for today remains on track. Pressure falls across the
northern Great Lakes will cause surface winds to turn southwesterly
this morning, allowing the reservoir of moisture to our southwest to
creep northeastward. Forecast soundings depict minimal capping by
early afternoon which should allow for widely scattered showers and
a few thunderstorms to develop in somewhat random fashion. However,
heavy cloud cover should stunt MLCAPE from growing to more than 750
thereby limiting the intensity of any thunderstorm that develops.
With that said, any clearing that can take place late this afternoon
particularly near the Wisconsin border may allow for locally higher
instability and steeper low-level lapse rates ahead of the
approaching cold front. As a result, we can`t rule out a locally
strong thunderstorm from developing along the cold front this
evening between 4 and 8 PM, again, near the Wisconsin state line
(generally a 5% chance or lower). Overall, the concern for severe
weather today is pretty darn low.

Tonight, the cold front will decay into a surface ridge directly
overhead, leading to nearly calm winds. Thinning cloud cover,
especially northwest of I-55, will allow for efficient radiational
cooling within the still-moist airmass, supporting areas of fog.
Conceptually, it`s easy to envision pockets of dense fog (with
visibility less than one quarter of a mile) developing by daybreak,
particularly in areas where wet vegetation remains.

Tomorrow, any morning fog will erode and give way to partly cloudy
skies nearly calm winds and highs near 80 will make for a
spectacular and warm May day. The exception, of course, will be
along the Lake Michigan shoreline as a lake breeze surges inland
during the afternoon hours. Meanwhile, south of I-80, a few
showers or thunderstorms cannot be ruled out as a weak wave
passes overhead. Perhaps the best chances, generally between 20
and 30%, will be along any intersection of the southward-moving
lake breeze and any northward-moving outflow from thunderstorms
across central Illinois and Indiana.

Friday Night and Beyond...

The upper trough progged to bring us rain chances on Friday is
expected to be traversing along the southern Ohio River Valley
as we head into Saturday. Guidance has come into notably better
agreement that the main core of the trough should be well east
of our area by Saturday morning allowing mid-level height rises
(higher pressure) to build into our area. However, forecast
soundings do show some modest instability developing across the
southeastern corner of our forecast area (i.e. areas south of
the Kankakee River and east of I-57) which in conjunction to
the closer proximity to the trough may allow for a low chance
(<20%) of an isolated shower and/or thunderstorm Saturday
afternoon. Additionally, guidance continues to depict a northern
stream shortwave moving across the Upper Midwest toward the
northern Great Lakes which is expected to push a cold front
through northern IL Saturday night. Since the anticipated
arrival time of the front is expected to be after sunset in our
area, guidance continues to favor any showers/storms along the
front dissipating prior to reaching the I-39 corridor. Thus, I
saw no need to change the dry POPs offered by the NBM but still
caution that a stray shower or rumble of thunder near I-39 and
the IL-WI line Saturday evening is not completely out of the
question just yet.

Aside from these minor rain chances, dry conditions are largely
expected for the weekend as the aforementioned height rises
pivot overhead on Sunday. Despite the passage of the cold front
Saturday night, temperatures for both Saturday and Sunday are
expected to be in the seasonably warm category with highs in the
lower to mid-80s and overnight lows in the 60s. Though, the
light surface winds will allow daily lake breezes to develop and
surge inland keeping high temperatures cooler (in the upper 60s
to lower 70s) for locales near the lake.

Heading into Monday and the beginning of next week, guidance
continues to be in strong agreement that a more active pattern
is likely to develop across the central CONUS. While there are
some subtle differences in timing and intensity, the general
idea is as follows. The northern jet stream is forecast to dig
into the Pacific Northwest and phase with a developing sub-
tropical jet across the southwestern US on Sunday. This phasing
is expected to develop a broad trough across the western CONUS
on Monday allowing the aforementioned sub-tropical jet to
intensify and nose into the southern Plains and mid-Mississippi
Valley. Not only will the sub-tropical jet aid in enhancing the
southwest low-level flow over the Plains and Midwest supporting
seasonably warm and humid conditions, but it will also allow
any shortwaves that break off to eject towards the Midwest and
southern Great Lakes. This combination of synoptic forcing,
warm-humid air, and potential wind shear should promote the
necessary ingredients needed for period of rather stormy weather
in the region next week including the threat for severe
weather.

As stated above, there are some subtle timing issues as to when
any shortwaves will break off and where exactly they will
track. However, the 00z guidance suite has started to highlight
a couple periods of potential interest for our area. The first
of these periods is within the Monday through Wednesday timeframe
with the second favored toward the end of next week into the
following weekend. Obviously the coverage and severity of any
potential storms in our area remains uncertain given we are
still 5-7+ days out, but this will be a period to monitor
closely. Therefore, we continue to recommend keeping an eye on
subsequent forecasts over the coming days for any changes. It
may also be wise to review your severe weather plans just in
case our area does become favored.

Yack

&&

.AVIATION /12Z TAFS THROUGH 18Z FRIDAY/...
Issued at 640 AM CDT Thu May 16 2024

Forecast concerns for the terminals...

- Scattered showers and thunderstorms this afternoon through
  early evening

- Potential for patchy fog to develop late tonight into Friday
  morning, with a growing signal for dense fog in spots

- Weak lake breeze to push through the Chicago terminals early
  Friday afternoon


VFR cirrus continues to stream into the terminals this morning
ahead of an upper-level disturbance pivoting through the Upper
Midwest. As this disturbance gets closer to northern IL later
this morning it will force winds to settle into a southwesterly
direction and therefore aid in increasing low-level moisture.
This moisture increase will in turn allow instability to build
and allow scattered showers and thunderstorms to develop around
midday and persist into the early evening hours. While
widespread severe weather is not expected, an isolated storm or
two could become strong along the IL-WI line this afternoon and
produce localized instances of gusty winds and/or small hail.

As showers and storms come to an end this evening (by 03-04z)
winds will briefly become southerly before veering towards a
west-northwest direction overnight. Though, speeds are expected
to remain light around 5 kts. These prevailing southerly winds
will also aid in keeping low-level moisture elevated through the
night and allow for the development of patchy fog after midnight
(even at ORD and MDW). Guidance has continued to be in good
agreement on timing and widespread coverage of fog, but
continues to vary on the fog`s density. That said, there is a
growing signal that IFR visibilities are possible (around 30%
chance) but given the lower confidence have decided to go with
MVFR visibilities (4-5 SM) in the TAFs for now. Regardless, the
fog is expected to dissipate within an hour or two after sunrise
Friday morning leaving VFR conditions through the rest of the
period.

Finally, a weak lake breeze is expected to develop early Friday
afternoon as temperatures warm. The aforementioned light west-
northwest winds look to offer little (if any) resistance and
should allow the lake breeze to surge inland across the Chicago
terminals between 16-18z Friday. In the wake of the breeze wind
directions will become easterly with speeds increasing into the
6 to 8 kt range where they look to remain through Friday
afternoon.

Yack

&&

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

&&

$$

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