072
FXUS63 KLOT 240810
AFDLOT

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

.KEY MESSAGES...

- Expect several rounds of showers and storms today with some
  severe weather threat during the afternoon and early evening

- 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

&&

.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 /06Z TAFS THROUGH 12Z SATURDAY/...
Issued at 128 AM CDT Fri May 24 2024

Key messages/concerns:

- Couple rounds of gusty showers and thunderstorms expected
  from late morning through this evening.

The main forecast concern today revolves around the best timing
and impacts of thunderstorms in advance of a cold front that
will be moving across the area later this evening. Confidence
remains high that the area will experience one or two rounds of
storms today into early this evening. However, what remains
somewhat unclear to this point is the exact timing of
thunderstorms at each terminal, particularly with the potential
second round of storms from mid to late afternoon into this
evening.

A line of severe wind producing thunderstorms currently ongoing
across eastern NE early this morning will continue shifting
eastward across IA, while gradually weakening through the
predawn hours. While this line of storms will be in a weakening
phase as it approaches northwestern IL after 13z, it is likely
(60%+ chance) to hold together long enough to reach KRFD with a
period of gusty westerly winds during the mid to late morning
hours (15-17z). However, what is more unclear at this time is if
the storms will hold together long enough to reach the main
Chicago terminals. If they were to maintain themselves through
the morning, they could also impact the main Chicago terminals
around midday/early afternoon. Due to the increased
uncertainty with eastward extent, we have opted to just maintain
a PROB30 mention for TSRA in the 18 to 20z timeframe at the
Chicago terminals.

A second and more significant round of storms (severe storms
possible with damaging wind gusts in excess of 50 kt) is
expected to develop west of the area this afternoon ahead of the
approaching cold front. There is high confidence (70%+) that
this round of storms will move across the terminal airspace
sometime late this afternoon or early this evening, but as
mentioned earlier, the exact timing remains a bit unclear. The
current favorable time window for this later day round of storms
at the main Chicago terminals looks to be roughly in the 22 to
03z timeframe. This time window will have to be fine tuned with
future forecast updates as forecast information becomes more
clear.

KJB

&&

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

&&

$$

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