078
FXUS63 KLOT 171126
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
626 AM CDT Wed Jul 17 2024

.KEY MESSAGES...

- A few isolated showers/thunderstorms early this morning mainly
  southeast of I-57. Slight chance of a few showers or sprinkles
  along the lake breeze in northeast IL this afternoon.

- Potential for hazardous swim conditions at Lake Michigan
  beaches tonight into Thursday.

- Period of dry, seasonably cooler and less humid weather Thursday
  into the weekend.

- Pattern trends toward slightly warmer temps and a return of
  slight shower/thunderstorm chances by early next week, though
  does not look particularly stormy at this distance.

&&

.DISCUSSION...
Issued at 348 AM CDT Wed Jul 17 2024

Through Thursday...

Early morning GOES water vapor imagery depicts a rather deep
upper trough (by July standards) across Ontario and the upper
Midwest. A couple of short wave disturbances were noted within
the trough, one low-amplitude impulse and mid-level jet streak
tracking across northern IL/IN, and another more amplified wave
rotating southeast across the upper Mississippi Valley. At the
surface, the weak cold front which sagged across the forecast
area on Tuesday appears to have become nearly stationary from
west-central IL into northern IN, while a subtle surface
trough/cold front stretched from northeast WI into central IA. A
fairly moist low-level air mass lingers in the vicinity of the
stationary front, depicted by 70 degree surface dew points
lingering across the southeast half of the WFO LOT cwa. Farther
northwest, dew points gradually decrease into the mid-upper 50s
ahead of the cold frontal trough to our north.

A few showers and isolated thunderstorms have festered
overnight within the moist low-level air near the southern
front, where objective analysis depicts an axis of MLCAPE ~500
J/kg and weak moisture convergence from south of the Chicago
metro into northern IN. These may continue to bubble in isolated
fashion through sunrise or so, before eventually shifting off
to the east-southeast. The morning hours otherwise should be
dry, with light northwest winds providing modest dry advection.
Isolated shower chances do look to increase again by this
afternoon however, as the upper Mississippi Valley short wave
(and larger scale trough axis) shift across the region. Guidance
soundings and CAM output vary on degree of shallow instability
and shower coverage, though there`s some signal that isolated
showers may focus along the lake breeze boundary over the
Chicago area. Even the more bullish model soundings suggest
equilibrium levels to around -5C, suggesting little/no thunder
threat with these. Have kept inherited slight (<25%) pops for
the afternoon hours inland of the lake in northeast IL along the
lake breeze.

The upper trough axis shifts east of the area later tonight,
with strong height rises developing across the western Great
Lakes region into Thursday. At the surface, this area of
subsidence manifests as a broad area of surface low pressure
which will settle across the area through the end of the week.
Temperatures will cool from the upper 70s/low 80s today to the
mid-70s in most spots Thursday, with slightly cooler readings
along the lake with onshore winds. In fact, increasing north-
northeast winds on the southeast flank of the surface high will
lead to building waves on southern Lake Michigan tonight into
Thursday, which will likely make for hazardous swimming
conditions at Lake Michigan beaches, especially from Cook county
southeast into IN.

Ratzer

Thursday night through Tuesday...

The upper pattern is forecast to remain fairly amplified into
the weekend, with a deep upper trough across the eastern CONUS
and a strong upper ridge across the west. We`ll be in between
these features, in a region of northwest flow aloft, and
lingering surface high pressure ridging settling across the
Midwest/Ohio Valley. This will maintain a period of dry and
pleasant weather conditions across our area into the weekend,
with temperatures generally in the upper 70s to lower 80s.
Models continue to be in decent agreement in a mid- level trough
eventually developing southeast across the region by later in
the weekend and early next week, with indications of a weak
surface trough perhaps allowing at least some chance of precip
to return as well as a gradual moderation in temps by Monday and
Tuesday.

Ratzer

&&

.AVIATION /12Z TAFS THROUGH 18Z THURSDAY/...
Issued at 625 AM CDT Wed Jul 17 2024

The primary aviation weather concerns for the 12Z TAF period
include:

* Uncertainty regarding the lake breeze today in Chicago

Light NNWs will should begin mixing into some ~10 kt winds not
long after 12Z eventually gusting as high as 20 kt later
today. While not anticipated, a few light, non-impactful showers
may fall on the Chicago sites during the afternoon. This
evening, a lake breeze looks to move onshore, although it
remains unclear if the boundary will make it across the Chicago
sites. If it does, it likely won`t be until early evening. While
it won`t mean a big shift in direction (NNW to NNE), winds
could continue to gust to near 20 kt through as late as 03Z.
Light, near-northerlies will take us through the late evening
and night with NEs under 10 kt expected during the day on
Thursday. Expect VFR throughout the period.

Doom

&&

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

&&

$$

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