-- Discussion containing changed information from previous version are highlighted. --
001
FXUS63 KLOT 232026 CCA
AFDLOT

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

.KEY MESSAGES...

- Scattered showers and 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

&&

.DISCUSSION...
-- Changed Discussion --
Issued at 320 PM CDT Thu May 23 2024

Through tonight:

Light winds this afternoon have allowed a lake breeze to push
inland with temperatures dropping into the upper 60s to lower
70s as it moves through. Meanwhile inland temperatures have
warmed into the upper 70s to lower 80s! Overall a nice late
spring day across the area! Quiet weather is in store through
tonight as surface high pressure gradually shifts east of the
area. Expect temperatures to drop into the upper 50s to around
60 overnight.

Petr

Friday through Sunday night:

Friday-Early Friday Evening:

A vigorous upper low over the northern High Plains will result
in strong to severe t-storm development over the central Plains
into this evening. This activity will likely congeal into an
MCS, tracking east across the Cornbelt tonight, and will likely
play a significant role in our weather on Friday.

The current more likely scenario is for the MCS timing to be
slow enough for either weakening convection or remnant MCV
arriving into our western counties later in the morning/mid day.
This would 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. While we certainly can`t take CAM simulated
reflectivity as gospel at this range, note that the most recent
extended HRRR runs generally favor the maintenance and/or
reinvigoration of ongoing convection.

This evolution is more concerning from a severe weather
perspective centered on tomorrow (Friday) afternoon, as the
steep mid-level lapse rate regime with dew points in the 60s
will be conducive for strong destabilization contingent upon
more solar insolation through the morning and early afternoon
allowing temps away from the IL north shore to reach the 80-85F
range. The aforementioned 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
(1500-2000 J/kg of MLCAPE, modest shear (up to 30 kt) would
likely still support multicell storms with an attendant threat
of damaging winds. In support of damaging winds as the primary
threat from a multi-cellular/quasi-linear mode will be steep,
near 9C/km low-level lapse rates and 1000-1500 J/kg of DCAPE
resulting in pronounced cold pool development.

Contingent upon embedded or lead supercellular mode, there may
be just enough deep layer shear given the progged steep mid-
level lapse rates for an isolated severe hail threat. Finally,
while brief tornadoes certainly can`t be rule out, it appears
that low-level shear will be lacking, amidst a more deeply mixed
(high LCLs) regime to keep the threat lower end. This being
said, if the anticipated MCS tonight develops into a more
organized MCS, spawning a stronger MCV, then that feature could
result in locally stronger shear and a localized more
concentrated meaningful severe weather threat, including
tornadoes, given a corridor of backed south-southeasterly and
breezy boundary layer flow.

The likelihood of robust pre cold-frontal convection Friday
afternoon-early evening does cast doubt on recovery and
redevelopment of deep convection toward and beyond sunset Friday
evening ahead of the true cold front. While there should be
enough forcing around for showers and scattered embedded
thunderstorms, coverage may end up even lower than implied by
the official forecast PoPs, which were already nudged lower from
the previous forecast. The front will sweep across the area late
Friday evening and early overnight, quickly drying things out.

Saturday:

For "unofficial start of summer" activities over the Memorial
Day Weekend, Saturday continues to look like the best weather
day with sunny skies, seasonable high temps in the 70s, and low
humidity (dew points in the 40s). Weak high pressure overhead
and initial light northwest winds under it will quickly "flop"
over to northeasterly synoptically and due to lake influence in
the late morning and early afternoon, keeping highs along the
lakeshore in the upper 60s to around 70F. Saturday evening will
be pleasant, dry, and seasonably cool. Timing of lead showers
and embedded thunderstorms has slowed enough that arrival in the
southwest 1/3 or so of the CWA appears to be toward dawn Sunday,
so for all intents and purposes, Saturday night will be a dry
night for most if not the entire area.

Sunday-Sunday night:

Another strong short-wave trough is progged to move across the
Plains Saturday and into the mid Mississippi Valley and western
Great Lakes Sunday. There remains some spread in timing and
track of the attendant surface low, which will dictate how far
north the warm front (and stronger instability) will get
following morning and mid day convection (which may be fairly
extensive). Concerningly, the EPS (ECMWF ensemble) has shown a
northward jog in the track of the deepening surface low (low-mid
990s mb at peak, quite strong for late May), with a majority of
members tracking the surface low across northern Illinois and
southern Wisconsin Sunday afternoon or evening. Only a few EPS
members showed a farther south track on the 12z run. This is in
line with the overall run to run trend of the operational
ECMWF, which has indicated a track across southern Wisconsin the
past few cycles. The GEFS has also shown a run to run northward
nudge, albeit with more north to south spread. Ultimately, it
is still 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, the global guidance suggests there will be a
coupled upper jet structure with strong synoptic ascent. The nose
of the mid-level jet has also trended farther north, and
regardless of its exact positioning, there should be 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). It does appear that instability will
be a limiting factor with northward extent, with higher instability
I-80 and south or thereabouts. Whether close to the surface low
and warm front or farther south in the warm sector, wind
profiles and low-level hodograph curvature are concerning from a
tornado threat perspective on Sunday.

The main issue is that if instability is indeed minimized farther
north, there may be primarily a brief tornado threat and not much
else farther north (with right-moving low-topped supercells).
Meanwhile, farther south, there may be enough instability for an
all-hazards type of threat, some of which could be significant.
While the outer range of the NAM model is typically less reliable
and prone to large run to run swings, its 12z solution can be seen
as a realistic, though less likely, outcome ceiling wise, if the
potentially available ingredients are maximized.

Our main message for Sunday is to pay close attention to
subsequent forecast updates regarding the potential for
thunderstorms and severe weather. SPC has 15% probs in its day 4
outlook up to roughly I-88, which appears reasonable. Based on
current thinking, this may be our first more favorably (or
unfavorably) timed strong synoptic system of this exceptionally
active severe weather spring across the greater region. While the
entire day probably won`t be a washout, most of the CWA should see
at least one or two rounds of showers and thunderstorms Sunday
through Sunday afternoon and evening. Regardless of the magnitude
of the severe weather threat, outdoor events will likely be
affected by lightning nearby. It does appear that the cold front
passage will be early enough Sunday evening to wind down the
thunderstorm coverage and attendant severe threat fairly quickly,
however. It will be breezy and cooler with diminishing shower
coverage behind the cold front.

Castro/Izzi

Monday through Thursday:

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
-- End Changed Discussion --

&&

.AVIATION /18Z TAFS THROUGH 00Z SATURDAY/...
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

&&

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

&&

$$

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