976
FXUS63 KLOT 241746
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1246 PM CDT Fri May 24 2024

.KEY MESSAGES...

- Expect multiple rounds of showers and storms today with some
  severe weather threat. A severe thunderstorm watch goes
  through early afternoon, and additional severe weather is also
  possible later today.

- The main threats for today will be damaging winds, hail, and
  even a few tornadoes

- There are increasing chances for another round of severe
  thunderstorms on Sunday

&&

.MESOSCALE...
Issued at 1148 AM CDT Fri May 24 2024

The line of storms that moved out of Iowa earlier today is
passing through the central portion of the local CWA as of late
morning. Generally this activity is staying below severe levels
but we are watching it closely for some local intensification as
well as possible embedded rotation. The recent cell warned in
Kendall/Will counties is an example of the continuing hail/wind
potential as well.  Deep layer shear in the 30-40 kt range and
low level helicity thresholds 100-200 m2/s2 are somewhat
marginal for widespread organized convection and rotation. The
air ahead of this line is uncontaminated by existing convection
and characterized by a ridge of SBCAPE values exceeding 2000
J/kg which would support sustained or new convection as the
activity continues east. IR satellite imagery shows warming
cloud tops for most of the existing convection, but new growth
along the southern and western edges, likely related to ongoing
support from a lingering low level jet and an approaching cold
front, respectively.

The question for later this afternoon into the evening is how
much redevelopment to expect in the wake of this initial round.
CAMs have generally not been handling this first line very well
at all, which raises concerns about putting too much trust in
their depiction of storms later today. But thinking of the
synoptic and mesoscale influences, we are looking at height
falls aloft in conjunction with a cold front approaching from
the west which is already supporting renewed activity across
central and eastern Iowa. This is despite not having the
advantage of peak heating, and also being closer to the back
edge of the morning activity. Additional support would be
provided from warm and moist boundary layer advection
essentially in the form of a warm front as the outflow from this
initial round stalls out and eventually gets pushed back north.

Better deep layer shear along the approaching cold front and
backing of low level winds along the stalled outflow
boundary and/or advancing warm front would support better
organization and rotation potential for storms that do develop
later in the afternoon or evening. Hail/wind and possibly
tornadoes would all be in play, but given the potential
lingering influence of this initial round through the early part
of the afternoon period, there is less confidence of the second
round being a widespread severe event.

Lenning

&&

.DISCUSSION...
Issued at 310 AM CDT Fri May 24 2024

Through Saturday...

After a fairly nice Thursday with warm temperatures and dry
conditions, a quick look at the satellite loops this morning
portends a bit of a change to kick off the Memorial Day weekend. A
sharp upper trough is working its way through the northern Plains
early this morning. Out ahead of the trough is a complex of strong
to severe thunderstorms, a bit more potent along and south of the
effective warm front, and there are some additional storms ahead of
the line as well. The surface pattern is a bit more complicated,
with a series of lows analyzed across northern Kansas from which
emanates a warm frontal boundary that extends east into central
Illinois and toward the eastern seaboard. A second low is across the
northern Plains, with a cold front to its south and a warm front
across the northern Great Lakes.

With time these lows will consolidate into NW Minnesota on Friday.
The upper trough will be moving to the northeast, thus the low level
mass response will drive the warm front will north this morning.
With the warm front lifting north, warm moist sector and its
associated instability will likely allow the thunderstorm complex to
intensify as well, adding some confidence that this line will be
ongoing toward daybreak. There will be some shortwave ridging in the
morning. Also, the warm sector will not quite be established across
northern Illinois in the morning. Therefore we have at least medium
confidence in some weakening to the MCS, and even some lifting to
the north this morning. Still, these will pose some threat for some
hail given forecast soundings show some low level inhibition may be
present, though there could still some lower end damaging wind
threat.

Things get a bit more complicated depending on where the outflow
from this complex will setup, and this will have implications for
the severe threat in the afternoon, and what types of hazards will
be in play. Conceptually, if there is no major convection in the
morning, the warm sector will become established and at least a
modest increase in the low to mid level wind fields, providing a
favorable instability-shear space.

This is where messaging the best thunderstorm, and severe
thunderstorm potential, becomes more challenging.  In the afternoon
it does appear that forcing is a bit more nebulous, more associated
with warm advection and shortwave activity and suggestive of
scattered festering convection.  However, the instability will be
the highest in the afternoon, and would be the most likely timing
for severe weather. Plus, any convectively induced vort max could
put a wrinkle in things, and would certainly increase the convective
coverage potential.  Our highest concern area is in initially toward
the Rockford area and I-39 corridor closer to the better forcing,
with this spreading closer to the Chicago area late in the
afternoon. Deep layer shear is slightly higher to our northwest,
thus instances of damaging winds and hail would be concerns, but
anywhere close to the warm front or composite warm front/remnant
outflow boundary from morning convection would also pose a tornado
threat. With some uncertainty to this location, this threat would
exist farther south into IL, which is what prompted the SPC
expansion of the tornado hazard in the Day 1 outlook. Also, discrete
cells or clusters appear the most likely mode, given the shear
vector more normal to the advancing cold front.

Finally in the evening, better height falls will overspread the area
from west to east along with the cold front. This appears to be
another period for thunder getting into the Chicago area and
especially south of I-80, depending on the footprint of the
afternoon convection.  The severe threat is still highest to the
west where instability may not get pinched off as early.

Behind the front tonight, the upper low will exit northeast of the
area, bringing a period of shortwave ridging/surface high pressure.
Temperatures will drop back a bit, especially lakeside with onshore
flow, but modest temperature recoveries will occur inland to
support mid 70s given plentiful sunshine and very dry conditions.


KMD


Saturday Night through Thursday:

Another period of active weather is likely (60%+ chance) for the
area on Sunday as another potent short-wave trough drives a
deepening sub-995 mb surface low northeastward into southern WI
during the day. Ensemble solutions continue to show some spread
in the actual track of the surface low across northern IL into WI
Sunday afternoon. However, while this is the case, the overall
synoptic setup of a deepening sub 995 mb surface low tracking
northward into southern WI through the afternoon is certainly one
that is concerning for the potential for severe thunderstorms in
our area. For this reason, this is certainly a period we will have
to monitor closely.

The main weather message at this point for Sunday is to pay close
attention to the later forecast updates regarding thunderstorms,
and in particular severe thunderstorms. The SPC currently has much
of the area highlighted in a level 2 out of 5 for severe weather,
with southern sections of the area in a level 3 out of 5. The
entire day will not be washout, but the potential continues for a
couple of rounds of thunderstorms, one potentially early in day,
and a potentially more significant round of severe thunderstorms
in the afternoon and evening.

Following Sunday`s storm system, a building upper-level ridge
across the Rockies is expected to setup a cooler, but still
active, northwesterly upper-level flow pattern across the Great
Lakes region early next week. Ensemble guidance suggests that
this pattern will act to steer a few smaller scale impulses
southeastward into our area for Monday and Tuesday. Accordingly,
there are chances (30-50%) for some scattered afternoon showers
(and possibly a few non-severe storms) both days.

A period of dry (and rather pleasant) weather is expected mid to
late week (especially Wednesday and Thursday of next week) as
surface high pressure sets up shop over the Great Lakes region.
Daytime temperatures are forecast to be in the low to middle 70s
both days under mainly sunny skies.

KJB

&&

.AVIATION /18Z TAFS THROUGH 00Z SUNDAY/...
Issued at 1246 PM CDT Fri May 24 2024

Main Concerns:

- TS over MDW and GYY exiting east, with winds returning to
  east-southeast behind them

- Potential for additional SHRA/TS late this afternoon into the
  evening

- Wind shift to easterly on Saturday

In the near term, on station TS impacts at 18z at MDW and GYY
will end by 19z. Expecting north-northwest winds immediately
behind the convection to shift to east-southeast within the next
hour or so.

Confidence is quite low regarding the next potential round of
SHRA/TS late this afternoon into the evening ahead of a cold
front. Maintained previous TAF VCTS and TEMPO timing and plan to
update as confidence in trends increases.

Winds will shift to northwest behind the front tonight and then
a lake breeze will push inland on Saturday, shifting winds to
easterly near 10 kt at ORD and MDW. Confidence is medium in
the lake breeze wind shift timing.

Castro

&&

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

&&

$$

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