926
FXUS63 KLOT 051110
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
610 AM CDT Sun May 5 2024

.KEY MESSAGES...

- Active pattern with periodic shower/thunderstorm chances
  expected Tuesday onward. Severe weather and locally heavy
  rainfall threat likely for parts of the area Tuesday and
  Wednesday.

&&

.DISCUSSION...
Issued at 353 AM CDT Sun May 5 2024

Through Monday:

Last evening`s storm system and trailing cold front have
shifted well east of the area early this morning, ushering in a
period of tranquil weather today and Monday ahead of another
period of active weather mid-week. Residual post-frontal stratus
across roughly half of the CWA should gradually drift southeast
through early afternoon while being replaced by scattered
cirrus. Max temps will range from the upper 60s well inland to
the mid 50s along the shore amid modest NE flow through the day.

Persistent NE flow within a very dry air mass tonight will
allow dew points to fall through most of the evening. Shallow
decoupling under generally clear skies within the dry airmass
will support a relatively cool night with min temps in the low
to mid 40s. It wouldn`t be surprising to even see some rogue
upper 30s readings in the typically coldest spots of interior
northern Illinois.

Ongoing robust convection across central Texas early this
morning has generated a fairly substantial mid-level wave that
is expected to drift ENE across Ohio River Valley by Monday
afternoon. The wave will remained trapped under a pseudo-Rex
block traversing broader mid-level westerlies across the central
CONUS, with forecast guidance generally supporting a gradual
degeneration of the wave with time. Slowly veering low-level
flow ahead of a large trough across the Rockies may begin
advecting better moisture into the far southern CWA late Monday
afternoon, ultimately interacting with the northern periphery of
the decaying wave to generate some sparse showers toward
central Illinois. Otherwise, seasonably warm conditions away
from Lake Michigan are expected Monday.

Kluber


Monday Night through Saturday:

Surface low pressure is progged to be lifting northeast across
the Dakotas Monday evening, in association with a deepening
level trough/closed low across the region. Within the
south/southeast periphery of the trough, a strong negative-tilt
short wave is expected to propagate northeast into the mid-upper
Mississippi Valley and western Great Lakes region overnight and
into Tuesday, in association with a strong 80-90 kt mid-level
jet. Increasing forcing for ascent with this wave, combined with
isentropic ascent atop a northward-advancing warm front and
diffluent upper flow ahead of the jet streak, is expected to
allow showers and thunderstorms to spread into the IL portion of
the forecast through Tuesday morning. The initially largely-
elevated nature of these storms, and only modest mid-level lapse
rates suggest little/no significant severe threat with these
storms as the continue to lift northeast across the area Tuesday
morning/midday.

A weak surface wave/triple point is indicated passing north of
the forecast area in WI by midday, along what will then be an
occluding warm/cold front pushing east into the cwa. This looks
to shunt the better low-level moisture and conditional
instability axis across our far east/southeast cwa by afternoon,
during the more favored diurnal timing for renewed surface-
based convective development. The extent of cloud cover and
residual cold pool stabilization from the morning activity makes
Tuesday afternoon and evening thunderstorm
redevelopment/evolution somewhat uncertain, however. Provided
enough afternoon clearing and heating can occur, a severe
weather threat will likely exist. Forecast soundings suggest
1500-2000 J/kg of MLCAPE and nearly 70 kts of mainly
unidirectional 0-6 km bulk shear (nearly 50 kt 0-1 km shear) by
mid-late afternoon across our far east/southeast cwa, where the
low-level moist axis looks to be ahead of the occluded front.
Thunderstorms would likely be less numerous across our western
cwa, away from the stronger low-level instability. SPC Day 3
outlook (marginal for most of the area, slight far southeast)
appears reasonable at this point.

The upper trough/low over the northern Plains is progged to
wobble east-southeast Wednesday into Thursday, with the main
vort moving across the forecast area later Wednesday/Wednesday
night. This is expected to result in a surface low pressure wave
tracking across the region, with a renewed shower and
thunderstorm and conditional severe weather threat ramping up
again Wednesday afternoon/night, especially across our southern
cwa.

The upper trough is then expected to amplify across the eastern
CONUS through the end of the week. Cooler mid-level
temperatures and a series of smaller scale short waves rotating
through the trough should maintain a periodic
shower/thunderstorm threat, focused on the afternoon/evening
peak diurnal heating hours. After daytime temperatures in the
70s to near 80 Tuesday and Wednesday, cooler 60s are expected
for the second half of the week beneath the upper trough.

Ratzer

&&

.AVIATION /12Z TAFS THROUGH 18Z MONDAY/...
Issued at 606 AM CDT Sun May 5 2024

Aviation Key Messages:

- MVFR ceilings linger around KGYY early this morning,then VFR.

- NNW winds shift to NE by mid-morning.

Weak surface high pressure will build across the area today,
then shift east by Monday morning. Patchy MVFR ceilings will
linger over Lake Michigan and northwest IN through mid-morning,
otherwise VFR conditions are expected. Breezy north winds 10-15
kt will settle to around 10 kt from the northeast this morning,
and eventually east-southeast on Monday.

Ratzer

&&

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

&&

$$

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