983
FXUS63 KLOT 201904
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
204 PM CDT Mon May 20 2024

.KEY MESSAGES...

- Scattered t-storms likely today, a few severe storms with
  large hail and damaging winds possible this afternoon.

- Another round of thunderstorms is expected Tuesday evening
  into the nighttime hours. Severe thunderstorms with damaging
  winds and brief QLCS type tornadoes are possible with this
  round.

- Another chance for showers Friday into Saturday

&&

.MESOSCALE...

Issued at 204 PM CDT Mon May 20 2024

The MCV, now centered near Madison, continues to pinwheel multiple
arcs of convection across northern Illinois this afternoon. While
the thermodynamic and kinematic set-up continues to support
organized severe convection, it appears low-level veering nearly
parallel to the main forcing axes this far south is not sufficiently
contributing to low-level convergence for maintaining larger cores.
There have continued to be several attempts at cells producing sub-
severe hail and wind, so have not dismissed the fact that some
strong to severe cores will remain possible through the remainder of
the Chicago metro through 4-5pm.

Kluber


Issued at 1052 AM CDT Mon May 20 2024

The warm front has now moved/mixed northward to near the I-88
corridor and across Chicago (12F temp spread at ORD/MDW).
Recent mesoanalysis indicates that areas south of the warm
front have generally become uncapped with MLCAPE values nearing
2000 J/kg. Meanwhile, effective shear profiles are becoming more
favorable ahead of the MCV across the western CWA. The first
area of interest noted earlier has spurred a renewed axis of
mainly elevated convection, though some of the southern cells
are attempting to become surface based near the warm front.
While this axis appears to continue to outrun the better
environment, the proximity of the warm front and increasing
potential for surface-based convection supports isolated severe
storms with a non-zero brief tornado risk over the next hour or
two into far northeast Illinois.

As for the second area to the southwest, we continue to keep a
close eye on the potential for convection to erupt in and around
the Peoria area over the next hour or so. So while convection is
slower to materialize than previously expected in this area, it
remains a favored area of a growing cluster of severe
convection into the Chicago metro 2-4pm.

Kluber


Issued at 806 AM CDT Mon May 20 2024

We continue to assess the impacts from a compact MCV over
southern Iowa (originating from a complex of convection over
Kansas last evening). While the low-level environment remains
stable this morning with the warm front still south of I-80 as
of 8am, the shallow nature of the front combined with 25-30 knot
winds above the frontal surface and abundant sunshine will
allow the front to mix/jump north across most of northern
Illinois over the next 2-3 hours. This will result in a quick
destabilization ahead of two areas of interest, one with a
broken line of showers and storms from around the Quad Cities
northward, and another with an area of agitated cloud cover
crossing the Mississippi River from around Keokuk to Quincy.
While the first will likely clip the western CWA through mid-
morning, destabilization may not occur early enough for
surface- based convection. However, we are honing in on the
second area for elevated convective initiation over the next
hour or two. This convection should eventually become surface-
based while nearing/over the far southwest CWA by around 11am.
With a locally enhanced mid-level speed max associated with the
MCV, the resultant effective shear combined with an increasingly
favorable thermodynamic environment may support a cluster of
severe convection through the central CWA into the Chicago metro
between 11am-2pm. Primary hazards will be damaging wind and
large hail, but a brief tornado cannot be ruled out with any
boundary interactions and marginally favorable 0-1km wind
profiles.

Kluber

&&

.DISCUSSION...
Issued at 325 AM CDT Mon May 20 2024

Today and tonight:

A pair of MCVs will likely be the significant players in our
weather today. MCV #1 located over northeast IA as of 07z will
continue to move northeastward and generally away from the area
this morning. A north-south line of convection extending south
of this vort across eastern IA should begin a weakening phase in
the next couple of hours as it progresses east-northeast into a
more stable air mass. Nonetheless, some showers and perhaps a
t-storm could affect our western and northwestern CWA early this
morning in association with this feature before dissipating
and/or moving north into WI by mid morning. Severe wx with this
activity is unlikely.

MCV #2 should be the more problematic feature today. Currently
over northwest Missouri, hi-res model guidance matches well with
simple extrapolation, both have this features moving into
southwest WI early this afternoon. A formidable mid level jet
accompanies this feature with the latest VWP from KEAX showing
50-55kt winds from 700-500mb. While this MCV will probably
weaken some this morning given the waning convective activity,
the RAP (which seems to have initialized this feature pretty
well when compared with observational data) maintains a 40-50kt
700mb jetlet and 35-45kt at 500mb into the early/mid afternoon
on the southeastern flanks of this MCV across far northern IL
into southern WI.

This stronger flow with the vort max results in some
respectable low-mid level hodographs for a time late this
morning through early-mid afternoon. While 0-6km bulk shear is
only progged to be around 30kt, almost all of the shear will be
focused in the 0-3km layer where hodographs are somewhat
elongated and a bit curved. Ongoing convection with MCV #2 is
expected to weaken/dissipate this morning and likely allow for
at least some mixed sunshine to heat the moistening boundary
layer as warm front lifts north across the CWA this morning.
Assuming convective debris isn`t more extensive than expected,
then MLCAPE of around 2000 J/kg could be realized by early
afternoon with minimal inhibition left. Assuming convection
develops over northern IL this afternoon, and that looks pretty
likely, the next question will be how soon. If robust convection
can develop by early afternoon, favorable shear profiles will
be in place to support an storm scale organization and including
maybe some messy supercell structures. Given the shear
maximized in the low levels with largely streamwise vorticity in
the 0-1km layer, there could even be a couple hour window with
some low end tornado threat. This would particularly be near the
northward retreating warm front and assuming robust convection
develops early enough to reap the benefits of the stronger mid-
level flow associated with the MCV.

Heading into the mid and especially late afternoon, shear
profiles will quickly weaken and hodographs collapse in on
themselves. This means our severe threat should become relegated
to mainly a pulse type severe threat with accompanying brief
marginal hail and localized microburst where any updrafts find
some untapped CAPE to feed on. Thunderstorms should dissipate
and then end this evening as the MCV lifts north out of the area
and the boundary stabilizes. Maintained some slight chance pops
late this evening and overnight as a nod to the RAP which has
another MCV and associated convective complex rolling across the
area. Not seeing any evidence of this feature upstream now and
this scenario seems unlikely, with most likely scenario being
mostly dry conditions tonight.

Tuesday into Tuesday night:

Guidance varies with where the warm front will be to start the
day, likely due to models` attempts to resolve convective
influences. If the warm front starts out in our area in the
morning, a strong and deepening cyclone to our west should drive
it quickly north into Wisconsin. Another MCS and associated MCV
should organize over the central Plains tonight with this
system likely moving toward the MS River late tonight into early
Tuesday morning. Can`t rule out some weakening form of this MCS
bringing some showers and storms to our NW CWA in the morning,
with the warm front and synoptic scale ascent shifting north
into WI, would tend to think the synoptics will overwhelm this
system and lead to it decaying Tuesday morning.

Barring any unexpected surprises from any meandering MCVs,
Tuesday afternoon looks to be windy, largely rain free, and
unseasonably warm to hot. Expecting high temps to reach at least
into the upper 80s with lower 90s a distinct possibility given
decent amount of sunshine during the afternoon.

Potentially explosive severe thunderstorms are expected Tuesday
afternoon across western or central IA south into northern
Missouri. This activity is expected to eventually congeal into a
QLCS that will march east and northeastward toward the MS River
by early to mid evening Tuesday. Gusty southerly winds should
keep the boundary layer somewhat mixed and temps warm Tues
evening, likely slowing the nocturnal stabilization of the
boundary layer. Even with that, still expect convection to be
encountering much weaker and weakening instability and growing
boundary layer convective inhibition as it moves into our CWA
mid-late evening Tuesday. Assuming the convection doesn`t outrun
the strong synoptic ascent, a still somewhat formidable, but
weakening QLCS could march across much of the CWA Tuesday night.


Wind profiles will be plenty favorable for a continued threat
of damaging winds and perhaps a QLCS tornado threat. These
threats appear to be greatest over our western CWA, with the
threat lessening with eastward extent. The earlier the storms
arrive, the more mesoscale organization that they can attain,
the more significant the severe threat will be in our CWA.
Certainly, the severe threat should be highest west of our CWA
across Iowa and there are scenarios where we dodge a bullet with
little or no severe weather in our area. But given the strength
of the synoptic system and associated kinematic fields, Tue
evening warrants close watching, especially across our western
CWA.

Wednesday and Wednesday:

GFS is finally starting to cave and give up on the idea of
extensive post frontal convection Wednesday afternoon and
evening across our CWA. The 00z NAM came in dry, in line with
the very consistent ECMWF and GEM models which have been
steadfast in their dry forecast for our area for days now. NBM
pops are still being weighted down by the 12z runs and the GFS
and its ensemble members. Have coordinated a significant
reduction in pops with neighboring offices and removal of NBM
QPF with WPC for Wed into Wed night.

- Izzi

Thursday through Sunday:

Brief upper level ridging should allow for better height rises
on Thursday for much drier conditions setting up. Winds should
be out of the west and light with the occasional afternoon burst
to 15 mph. Afternoon temperatures should warm into the 70s,
with slightly cooler conditions in Northern Indiana along the
immediate shoreline.

The next upper level trough ejects out of the Rockies Thursday
night and starts tracking toward southeastern Manitoba. Despite
the system`s track being farther to the northwest than earlier
in the week. As the reflected surface low passes through the
Dakotas and Minnesota, a warm front will slowly enter Illinois
on Friday. Southerly winds will slowly advect precipitable water
amounts over an inch back into the area. The best instability
will be in the warm sector behind the front in a Rockford to
Fowler, IN line, but even then forcing does not look impressive.
So PoPs for scattered showers were kept at the chance
threshold.

Overnight and into Saturday, the low exits into Canada as a
secondary surface low strengthens over the Southern Plains.
Friday`s surface front will be come a NW/SE oriented cold front
and move over the region on Saturday. With it comes better
forcing and instability to allow widely scattered thunderstorms
to develop through the day. This far out, timing can be adjusted
closer to the event. At this juncture, model soundings have
weaker mid level lapse rates and deep layer shear to keep the
word severe out of the forecast. There may be isolated showers
that stick around through Monday, but there should be breaks in
the rain. However, confidence remains to low this far out for
specifics at this time.

DK

&&

.AVIATION /18Z TAFS THROUGH 00Z WEDNESDAY/...
Issued at 1234 PM CDT Mon May 20 2024

Aviation Concerns:

- Multiple rounds of storms moving though the region today.

- Gusty south winds expected through 00z, diminishing overnight
  into Tuesday morning. Increasing again Tuesday afternoon.

- Lower ceilings possible overnight towards Tuesday morning
  (~10-14z)...remaining VFR throughout much of the period.

A large MCV system continues to track through northern Illinois
into southern Wisconsin heading into the afternoon. This
developed showers and storms across the northern terminals late
morning, with the leading moisture surge. Additional showers
and storms are developing and intensifying across northern
Illinois and southern Wisconsin as the core of the MCV interacts
with a lifting warm front across the region. Strong to severe
storms will be possible this afternoon across the northern
terminals as this system tracks ene.

Winds will continue to be gusty this afternoon ahead of the
frontal boundary trailing to the south south-west of the MCV.
Sustained winds are forecast in the 15-18kt range, while gusts
upwards of 25-30kts will be possible at DPA, ORD, and MDW. As
this system exits tonight, winds will decrease overnight,
becoming easterly once again less than 8kts. Winds turn
southeasterly Tuesday morning, then become southerly once again
midday, also increasing.

Ceilings will remain VFR throughout the period, while some
SCT010 will be possible in the 10-14z time frame Tuesday morning
as a weak wave of showers potentially lifts across northern
Illinois. Not as confident in the shower activity, so left as a
PROB30 group until late model guidance.

Baker

&&

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

&&

$$

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