-- Discussion containing changed information from previous version are highlighted. --
994
FXUS63 KLOT 231735
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1235 PM CDT Thu May 23 2024

.KEY MESSAGES...

- Scattered showers and t-storms likely Friday with some severe
  weather threat during the afternoon and early evening

- Another round of showers and storms expected Sunday with some
  severe threat

- Period of below average temperatures appears likely first half
  of next week

&&

.DISCUSSION...
Issued at 335 AM CDT Thu May 23 2024

Today:

It`ll be sunny and seasonably warm today. A lake breeze this
afternoon will send temps back into the 60s near the lake,
elsewhere afternoon temps expected to generally top out in the
lower 80s.

Friday:

Vigorous upper low over the northern Rockies will emerge out
onto the northern High Plains this afternoon resulting in strong
to severe t-storm development over the central Plains later
this afternoon into this evening. This activity will likely
congeal into an MCS, tracking east across the Cornbelt tonight.
This activity will likely play a significant role in our weather
Friday, with at least a couple of plausible scenarios...

1) MCS will move more expediently eastward, entering our
western CWA in a weakening phase Friday morning, then continuing
east across our CWA, likely weakening, but with extensive
cloudiness and some rain likely limiting destabilization and
severe threat across most of our CWA.

2) MCS timing is a little slower, with either weakening
convection or remnant MCV arriving into our western counties
later in the morning. This would likely allow for more heating
and destabilization across our CWA and result in either a
reinvigoration of ongoing weaker convection or renewed
convective development during the afternoon.

If scenario 2, or some hybrid of scenario 2 ends up happening,
then there would be more of a concern for severe thunderstorms
Friday afternoon and early evening. Vigorous, negatively tilted
upper trough/closed low is expected to remain far enough
northwest to only bring our area a glancing blow of stronger
mid-upper level flow and resultant shear. However, given
moderate instability, modest shear would likely still support
multicell storms with an attendant lower end threat of locally
damaging winds and perhaps some marginally severe hail.

If tonight`s convection develops into a more organized MCS,
spawning a stronger MCV, then that feature in a scenario 2 set-
up could result in locally stronger shear and a localized more
concentrated meaningful severe wx threat Friday afternoon and
evening. As is often the case around here, we will need to wait
and see how convection evolves tonight, before getting a good
handle on magnitude and scope of any severe wx threat locally
Friday.

Saturday:

Definitely looks like the best weather day of the holiday
weekend with sunny skies with seasonable high temps in the 70s.
Northwest winds will probably be light enough to allow for an
afternoon lake breeze to provide cooler temps near the lake Sat
PM.

Sunday-Sunday night:

Another strong shortwave trough is progged to move across the
Plains Saturday and into the mid Mississippi Valley and western
Great Lakes Sunday. Still some spread in timing and track of the
attendant sfc low, which will dictate how far north the warm
front (and stronger instability) will get. GEFS and EPS both
have a majority of members tracking the sfc low across northern
Illinois Sunday afternoon or evening, with several members
farther south and some farther north. Ultimately, it is too soon
to say with much certainty exactly how far north the low and
warm front will get, there is always potential for prior
convection to keep effective boundary farther south.

Synoptically, GFS and ECMWF suggest there will be a coupled
upper jet structure with strong synoptic ascent. While nose of
mid level jet is progged to be south of our area, should still
see sufficient deep layer shear to support a meaningful severe
weather threat Sunday afternoon and evening in the warm sector
of the cyclone (however far north it gets). While the entire day
probably won`t be a wash out Sunday, most of our CWA should see
one, if not a couple rounds of showers and storms Sunday into
Sunday night.

Monday-Wednesday:

GEFS and EPS both suggest the upper trough responsible for
Sunday stormy weather will deepen and hang around the
northeastern U.S. early next week. There are growing indications
that this could result in our first several day period of
solidly below average temperatures in quite some time. Can`t
rule out a subtle northwest flow low amplitude shortwave
rippling down the backside of this trough and maybe bringing a
brief period or two of a few showers to the area, but overall
Monday-Wednesday looks to be fairly dry. The day to day chance
pops early next week are just slightly above climo pops and
reflect that small chance of a few brief showers at some point
early next week.

- Izzi

&&

.AVIATION /18Z TAFS THROUGH 00Z SATURDAY/...
-- Changed Discussion --
Issued at 1235 PM CDT Thu May 23 2024

Forecast concerns for the terminals...

- Lake breeze turning winds easterly at the Chicago terminals
  this afternoon

- Chances for showers and thunderstorms late Friday morning
  through the afternoon

A lake breeze continues to surge inland across northeast IL and
northwest IN this morning and is expected to move through the
Chicago terminals over the next couple of hours. Given the weak
winds aloft, I see no reason for the lake breeze to stall so
expect the passage to occur as forecast. Behind the lake breeze
winds will turn easterly with speeds increasing in the 8-10 kt
range. Outside of the Chicago terminals, winds should remain out
of the south-southwest with speeds generally in the 5-7 kt
range this afternoon before turning southeasterly overnight.
Otherwise, expect VFR conditions through the period.

Heading into Friday, winds will settle into a southeasterly
direction and begin to increase in speed as a storm system
approaches the area with gusts in the 20-25 kt range expected.
This storm system will also bring with it chances for showers
and thunderstorms late Friday morning through Friday afternoon.
However, there continues to be some uncertainties in the timing
and coverage of thunderstorms which will likely not be resolved
until the storm complex develops later today in Nebraska and we
are able to better track it overnight. Regardless, confidence
was high enough to introduce PROB30s to the ORD, MDW, and RFD
TAFs for this potential but was left out of DPA and GYY since
the arrival time is forecast to be after the current forecast
period. Depending on how quickly storms arrive Friday, a few
storms may become strong to possibly severe with gusty winds
and hail as the main threats.

Additionally, there is also the concern for a secondary line of
showers and storms to follow the aforementioned one. But given
the uncertainties in timing of the initial wave and how much
instability will remain in its wake, have decided to forego any
mention of this in the TAFs for now.

Yack
-- End Changed Discussion --

&&

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

&&

$$

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