318
FXUS63 KLOT 061123
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
623 AM CDT Mon May 6 2024

.KEY MESSAGES...

- Thunderstorms likely Tuesday morning, with at least scattered
  redevelopment in the afternoon especially across east and
  southeast portions of the forecast area. Severe weather
  threat will exist with afternoon storms.

- Potential of severe thunderstorms across our southern CWA into
  central Illinois Wednesday afternoon and evening.

&&

.DISCUSSION...
Issued at 336 AM CDT Mon May 6 2024

Through Tuesday:

Mainly dry and quiet weather conditions are expected through
this evening, as surface high pressure drifts slowly east across
the Great Lakes region. The only exception to this will be
across our far southern counties, where a few scattered shower
may affect areas south of about a Pontiac to Rensselaer line in
association with a mid-level short wave tracking across the
lower Ohio Valley and downstate IL/IN. Weak mid-level lapse
rates (5-5.5 C/km) should provide little or no lightning threat.
Otherwise, morning high clouds will move off to the east,
making for mostly sunny skies across the north and partly to
mostly cloudy conditions south later this morning and afternoon.
East-southeasterly winds 10-15 mph will maintain lake-cooling
along the Illinois shore of Lake Michigan, where temperatures
will only top out in the lower 60s, while areas farther west and
south of the lake should moderate into the mid-70s in most
locations.

Benign weather continues into this evening, before attention
turns to increasing shower and thunderstorm chances which will
ramp up from the west in the pre-dawn hours of early Tuesday
morning. An organized extensive north-south area of
thunderstorms is expected to be ongoing across the Plains this
evening, along a cold front trailing from an area of deep
surface low pressure over the northern Plains. This front is
expected to push east toward the Mississippi River through
Tuesday morning, as a stout mid-level short wave trough lifts
northeast into the mid/upper Mississippi Valley. Various CAM
guidance and HREF ensembles depict the Plains linear
thunderstorm complex spreading east ahead of the front, reaching
the WFO LOT forecast area pre-dawn Tuesday morning and reaching
the Chicago metro area just after sunrise. While severe weather
is expected well west/southwest of the area overnight,
indications (HREF calibrated severe probs, forecast soundings
indicated elevated inflow bases) are that the primary severe
threat area will remain well west/southwest of the cwa through
Tuesday morning. However, sub- severe hail and wind gusts will
be possible with the stronger cores here. Current CAM/HREF
guidance suggests storms will be largely through our IL counties
by mid-morning and our northwest IN area prior to noon, though
some scattered redevelopment appears possible on the trailing
flank of the complex due to a southwesterly 35-40 kt low level
jet.

Convective redevelopment and coverage then becomes less clear
during the midday and afternoon hours on Tuesday. Both the
footprint of the morning cold pool and the eastward movement of
the now-occluding cold/warm fronts across the area will likely
have an impact on if/where redevelopment occurs through the
remainder of the day. Current high-res guidance indicates that
the better low-level moisture and instability will be
increasingly focused across the far east/southeast parts of the
forecast area (and into IWX, IND and ILX`s areas) during the
afternoon, with shallower boundary layer moisture and mid-level
drying (and likely clearing) farther to the west. That said, the
area will remain beneath a favorable diffluent exit region of
the upper jet, with steepening mid-level lapse rates associated
with cooling provided by the mid-level trough axis and strong
mid (40-50 kts) and upper level (70+ kts) flow providing
substantial, mainly unidirectional deep-layer shear. HREF/HRRR
develop 1000-1500+ J/kg MLCAPEs during the afternoon (highest
generally from the south Chicago burbs south and southeast),
potentially supporting robust updrafts and a severe weather
threat. Forecast hodographs are generally long and straight
given the unidirectional wind/shear profiles, with wind/hail the
primary severe threats. However, there would also be a non-zero
tornado threat where boundary interactions or bowing segments
might alter the low-level shear profiles. SPC has adjusted their
day 2 outlook to pull the slight risk a little farther into our
southeast cwa with a marginal risk for the remainder, and this
looks appropriate.

Ratzer


Tuesday Night through Sunday:

While deep-layer drying behind Tuesday`s rounds of potential
convection is expected to greatly limit or end precip potential
by Tuesday evening, a trailing mid-level wave along with steep
mid-level lapse rates could result in a couple widely isolated
showers or storms across the northern CWA through mid evening.
Otherwise, dry conditions amid mid-level ridging is expected
across the area Tuesday night into Wednesday morning.

A mid-level low digging across the central Great Plains on
Wednesday in conjunction with a departing right entrance of an
upper jet streak and associated enhanced low-level flow will
support another period of convection across the Ohio Valley
Wednesday afternoon and evening. Robust low-level moisture
advection ahead of an elongated and partly occluded surface low
from western Iowa into central Illinois will result in a warm
front nearing or possibly shifting northward into the southern
CWA by late afternoon. Around and south of the warm front, ample
instability and impressive shear profiles support discrete
supercell structures capable of all severe hazards. We will need
to closely monitor trends in our southern CWA over the next
couple days given the proximity of the warm front and triple
point and diurnally favorable time of the day.

Beyond Wednesday evening, a mid-level trough extending
southwest across Quebec and southern Ontario will begin to phase
with Wednesday`s low. While some differences in guidance remain
with regards to the exact degree of phasing and timing of
trough and low, scattered showers and perhaps some diurnally
driven thunderstorms are favored on Thursday. After a quick-
moving ridge yields dry and seasonable temps on Friday, another
upper-level low is progged to cross the western Great Lakes on
Saturday. Like Thursday, scattered diurnally-enhanced showers
and a few storms appear likely on Saturday. Diverging solutions
with regards to the residence time of the upper-low over the
region next weekend may support a continuation of showers on
Sunday.

Kluber

&&

.AVIATION /12Z TAFS THROUGH 18Z TUESDAY/...
Issued at 623 AM CDT Mon May 6 2024

Key Aviation Messages:
-  Line of TSRA with potentially gusty winds in the 12-15Z
   window Tuesday.

VFR  conditions with winds between ENE and SE around 10 knots
are expected through at least late tonight. Shortly after
sunrise Tuesday, a line of TSRA with potentially gusty winds
will cross northern Illinois. Confidence is high enough to
include TEMPO TSRA from 12-15Z at ORD/MDW. While the TS will
move quickly across the area and affect a given location for
only two hours or less (favoring 13-15Z for ORD/MDW), a slightly
longer TEMPO period was included for the potential of faster
storms. After the initial TS period, isolated -SHRA may persist
through late morning before another period of isolated to
scattered strong TSRA just beyond the end of the TAF period
(18Z).

Kluber

&&

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

&&

$$

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