100
FXUS63 KLOT 041130
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
630 AM CDT Tue Jun 4 2024

.KEY MESSAGES...

- Warm/humid summer-like weather again today.

- Scattered thunderstorms develop this afternoon, and increase
  in coverage this evening. A few strong to severe storms with
  gusty winds and hail are possible, in addition to locally
  heavy rainfall and a localized flooding threat.

- Following isolated to scattered showers Wednesday evening,
  mainly dry and comfortable (seasonably cool) through Sunday.

- Breezy to windy on Thursday with gusts up to 30-40 mph.

&&

.DISCUSSION...
Issued at 351 AM CDT Tue Jun 4 2024

Through Wednesday:

Early morning surface analysis depicts 993 mb low pressure over
north central South Dakota, with a cold front trailing
southwest through the central Rockies and a warm front extending
east- southeast across northern IA to the IL/WI border. A broad
warm sector was noted south of the warm front across the Plains
and Corn Belt, with current temperatures in the upper 60s to
lower 70s and dew points in the low-mid 60s across the forecast
area. Goes vapor imagery indicated a convectively-enhanced short
wave drifting northeast from eastern WI into lower MI, with a
30-35 kt southwest low level jet enhancement lingering across
far northern IL. Warm/moist advection has resulted in isolated
convective showers attempting to develop overnight near the warm
front/outflow boundary just north of the IL/WI border, and
while an isolated shower or thunderstorm can`t be completely
ruled out near the state line over the next hour or so, the
departure of the mid-level wave and weakening of the low level
jet should end any such threat by sunrise.

Quiet weather is then expected through midday, with sunshine
filtered by increasing high clouds from the west allowing
another warm and somewhat humid summer-like day. Afternoon highs
are expected to range from the mid-80s across our southwestern
counties where cloud cover will thicken earlier, to the mid-
upper 80s farther east and northeast into the Chicago area and
northwest IN. Scattered thunderstorm chances then increase from
the southwest by early this afternoon, as a mid-level wave
currently over the lower Missouri Valley lifts slowly north into
IL in an environment characterized by MLCAPEs of 1000-1500 J/kg
(1500-2000 J/kg across our western cwa where surface dew points
will likely be a bit higher). Modest mid- level lapse rates
(6-6.5 C/km) and relatively weak (~20-25 kts) deep- layer shear
look to limit widespread severe potential, though isolated
downbursts and marginally severe hail (quarter sized) are
possible. SPC day 1 marginal (level 1/5) covers most of our IL
cwa for this possibility.

Greater thunderstorm coverage is likely to develop from the
west this evening through midnight or so however, as the mid-
level short wave trough axis shifts across the area. Showers and
embedded thunderstorms likely will persist into the overnight
hours as well, as a second, deeper upper trough and surface cold
front approach from the west. Greatest thunder coverage after
midnight may end up focused on the southeast part of the cwa,
where the southwesterly low level jet will be initially be
focused. Locally heavy rainfall will be possible with storms
tonight, as deep moisture increases (precipitable water values
increasing to >1.75") and the increasing low level jet slows
forward propagation of storms. The shower and thunderstorms
threat continues in to early Wednesday morning, before the main
upper trough axis and surface cold front push across the area.
Precip may linger in northwest IN into early afternoon before
the front clears our eastern-most counties.

Behind the front Wednesday, winds will become breezy from the
west with decreasing cloud cover. It will still be seasonably
warm with highs in the low-80s, but with lower humidity.

Ratzer


Wednesday Night through Monday:

A secondary cold front associated with the unusually deep (for
June) low pressure system and upper trough over north-central
North America will move across the area Wednesday night. A short
window of column moistening ahead of the front, but more
importantly, strong upper jet support, will result in a period
of isolated to scattered showers north of US-24. The highest
shower coverage should be near and north of I-88. Forecast
soundings depict fairly low freezing levels for this time of
year, though equilibrium levels will also be low (at or under
h5) with temps generally warmer than -10C, only yielding 100-200
J/kg of MUCAPE. Thus, while thunder chances with the evening
and early overnight shower activity aren`t zero, they`re fairly
slim and only warrant slight chance mention. Confidence is
higher in shower occurrence, hence coverage wording in the
official forecast. Skies will clear out in the pre-dawn hours of
Thursday, allowing temperatures to drop into the upper 50s to
lower 60s.

Deep mixing in a dry (dew points 45-50F) post-secondary frontal
air mass on Thursday along with a tight pressure gradient will
result in gusty west-northwest winds peaking during the afternoon.
Official forecast gusts are up to 35 mph and upside potential from
model soundings suggests at least sporadic 40 mph gust potential.
The strongest winds will be over our Illinois counties. High
temperatures will reach the mid to upper 70s thanks to the deep
mixing and dry air mass in place.

A blocking pattern will effectively trap anomalously deep upper
level low pressure (from the Wednesday and Wednesday night cold
front passages) over the central and eastern Great Lakes through
the weekend. Our area will be in dry northwest flow on the
southwest flank of the upper low, resulting in very little if any
rainfall during our what is typically our wettest time of year.
Atypically comfortable conditions can be expected Friday through
Sunday, with below normal high temperatures in the 70s away from
lake cooling, 40s-50s dew points, and overnight lows in the
50s. Low rain shower chances may return early next work week if
the upper low stays in close enough proximity to our east as it
finally shifts south from its weekend home. Below normal
temperatures will likely continue until the mid to latter part
of next week.

Castro

&&

.AVIATION /12Z TAFS THROUGH 18Z WEDNESDAY/...
Issued at 630 AM CDT Tue Jun 4 2024

Main Concerns:

- Scattered TS probable near the terminals in the late afternoon.

- SHRA with another round or two of TS possible in the evening
  and overnight.

- Wind direction in the late afternoon and early evening.

- Lower CIGs likely overnight into Wednesday morning.

Another challenging convective forecast through this TAF cycle,
though confidence remains higher in at least one round of TS
causing some impacts. Quiet VFR conditions with southerly winds
(15-20+ kt gusts by the early afternoon) will persist until
~19-21z. Instability will build out ahead of a subtle upper
level disturbance lifting northward, which should be enough to
kick off widely scattered TS in the late afternoon (~20-24z) in
the vicinity of the terminals. While on station impacts can`t be
ruled out, TS coverage with this first round still appears to
be sparse enough to maintain VCTS mention.

A stronger upper disturbance will pivot northeastward this
evening and early overnight. Expecting TS to be fairly
widespread with this in a portion of the ZAU airspace, with the
main question being if the higher coverage ends up a bit south
of the TAF sites. PROB30 groups for this possible second TS
window were left unchanged for assessment of trends today.
Finally, a cold front will approach overnight and move across
the area by mid Wednesday morning, likely preceded by SHRA and
isolated TS (confidence too low for another PROB30 mention). CIG
guidance continues to support low MVFR to patchy IFR CIGs
during this time, with steady improvement behind the cold front.

Regarding wind direction uncertainty later today, it`s a bit
unclear if south-southeast or south-southwest will be favored at
ORD and MDW during the strongest winds this afternoon, so the
TAFs split the difference. In addition, some previous guidance
indicated a close approach of the lake breeze toward 00z this
evening, still worth monitoring.

Castro

&&

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

&&

$$

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