507
FXUS63 KLOT 172013
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
313 PM CDT Fri May 17 2024

.KEY MESSAGES...

- Isolated showers t-storms through early evening near & south
  of the Kankakee/Illinois River Valley.

- Areas of dense fog are possible tonight, particularly right
  along the northeastern Illinois Lake Michigan shore.

- Summer-like warmth inland through the weekend and then area-wide
  Monday and Tuesday along with more humid conditions

- A few isolated thunderstorms possible Sunday afternoon-evening.

- Waves of thunderstorms Monday afternoon through Tuesday night,
  with the highest coverage Monday and Tuesday evenings,
  including a threat for strong/severe storms and flooding.

&&

.DISCUSSION...
Issued at 310 PM CDT Fri May 17 2024

Through Saturday Night:

While dewpoints across much of northern IL have mixed out into the
upper 40s to low 50s this afternoon, boundary layer moisture
remains elevated across southern sections of the area (well south
of I-80) in close proximity to a lingering weak surface boundary.
Amble heating of this moist airmass has even recently resulted in
enough buoyancy to foster a few isolated showers and storms
across parts of central IL into IN. This activity is expected to
largely remain isolated in coverage and confined near and south
of line from roughly Bloomington to Gibson City, east-
northeastward to near Rensselaer, IN through early evening (~8pm).
Overall, weak flow and associated poor deep layer shear is
expected to curtail the threat of organized severe storms.
However, a few localized instances of gusty winds and/or small
hail will be possible with the strongest storms in the south
through sunset. Elsewhere, sunny and warm inland conditions will
persist through the afternoon. A lake breeze will result in a bit
cooler afternoon weather, and potentially some fog near the Lake
Michigan shore.

Dense fog has persisted across the open waters of Lake Michigan
today, and recently has began expanding into some of the
northeastern IL near shore waters. While these conditions have not
shown signs of shifting inland yet, things may change as we
approach sunset. We will thus have to keep a close eye on
observation trends into this evening as the fog could begin to seep
inland with sunset. If this does occur, visibilities within a few
miles of the Lake county IL and northern Cook county shoreline
could become very low (under a half mile) this evening.

An additional area of fog may develop late tonight across parts
of eastern IL and northwestern IL, where better lower-level
moisture is expected to persist. Some of this may also become
locally dense for period near daybreak as temperatures bottom out
in the light wind regime to near their crossover value. Any fog
that develops will quickly dissipate Saturday morning.

Following any early morning fog, Saturday looks to be mostly
sunny and even warmer than today. Forecast guidance continues to
advertise 925mb temps into the lower 20s Celcius. These temps are
warmer than the 90% percentile values for mid-late May and should
easily support high temps at least into the mid 80s. More of a
southerly gradient should result in far less pronounced of lake
breeze Saturday afternoon, likely confined mainly to the IL shore,
particularly the north shore communities.

While dry weather is expected in our area on Saturday, some
scattered afternoon storms are expected with a cold front west of
the area in parts of eastern IA. This cold front is expected to
shift into our area Saturday night. While it is not out of the
question that a few showers or storms maintain themselves long
enough into the evening to make it into parts of northern IL, it
appears the poor diurnal timing of the frontal arrival into our
area will limit our overall chances of precipitation. We thus,
for the time being, continue to advertise a dry forecast with the
frontal passage Saturday night.

KJB

Sunday through Friday:

On Sunday, the cold front discussed in the short term section
will slip south and take on backdoor characteristics, possibly
becoming somewhat lake enhanced given the large land-lake
thermal gradient. The lake cooling footprint (temps in upper
60s-mid 70s, coolest shore) will extend farther inland than
tomorrow. Locations inland of the lake cooling footprint can
expect temperatures similar if not a degree or two warmer than
Saturday`s highs. Dew points will reach into the 60s near and
south of the backdoor front and 50s north.

Weak flow aloft and neutral to rising heights fitting the
summer-like pattern will combine with very dry mid-level air
(substantial dry air entrainment into incipient updrafts) to
greatly limit the coverage of showers and thunderstorms Sunday
afternoon and evening. Suspect the NCEP guidance, particularly
the GFS suite, is too high overall with the dew point forecast
on Sunday. A combination of the high-biased Td forecast and not
adequately accounting for the likelihood of dry air entrainment
aloft appears to be resulting in an overly aggressive forecast
convective footprint Sunday afternoon and evening.

With the above being said, the more realistic ECMWF still features
weakly capped to uncapped soundings Sunday afternoon, so even
with the very dry air aloft, can`t rule out a few isolated showers
and thunderstorms primarily inland of the marine layer push. If
the ECMWF depiction ends up being on the right track, a low but
relatively better chance for a couple showers and storms may end
up focusing south of I-80 where the higher dew points are forecast.
Barring some sort of convectively enhanced short-wave Sunday
evening, diurnally driven convection should dissipate with time
and the overnight may end up primarily if not entirely dry.

The backdoor cold front bisecting the area on Sunday will surge
back north of the area as a warm front on Monday, putting us in
the more humid warm sector amidst highs likely well into the 80s
once again. The magnitude of large scale forcing is somewhat
questionable as the main short-wave lifts into the northern Great
Lakes. However, with the erosion of MLCINh and up to 1000-1500
J/kg of MLCAPE, the departing short-wave and proximity of the warm
front to the north may provide enough of a trigger. There remains
enough of a signal across the guidance for 30-60% PoPs across
northern Illinois and most of central Illinois Monday afternoon,
particularly in the mid-late afternoon.

Should convection initiate during Monday afternoon, it could
result in the effective front sagging back south, presenting a
focus for higher thunderstorm coverage Monday evening, when PoPs
come up to 60-70% near/northwest of I-55. Marginal deep layer
shear looks to be a limiting factor for a more appreciable
severe threat late day Monday through the evening, though
wouldn`t be surprised at a level 1 of 5 (marginal risk) threat
for parts of the area. In addition, PWATs up near or upwards of
1.5" (150-200%) will probably support ponding on roads, with
flash flooding possible if training convection occurs.

Tuesday continues to be the most "interesting" day next week
convectively speaking. Another more substantial short-wave trough
and associated cyclogenesis over the Plains (tracking northeastward)
introduce the prospect for a more synoptically classic set-up for
thunderstorms, severe weather, and flooding over the region late
day Tuesday, and especially Tuesday evening. As is typical at this
range of the forecast, there`s plenty of uncertainty in the
timing and evolution of key features and mesoscale influences of
multiple potential waves of convection. There has been enough
slowing in the approach of the main short-wave that after any
Tuesday morning convection, there *may* be somewhat of a minima
in coverage Tuesday afternoon, especially with southeastward
extent. Continued to feel comfortable in high PoPs (up to 80%)
and likely thunderstorm wording Tuesday evening, when the severe
threat may maximize, perhaps with areas near/NW of I-55 a bit
more favored given the (current) timing.

The system`s cold front will likely sweep across the area
sometime between Tuesday night and mid day Wednesday, which would
entail a quieter day on Wednesday with a bit more seasonable
temperatures. Can`t rule out a further slowing of the cold front
passage though, with slower guidance members helping explain the
chance (30-50%) range PoPs Wednesday PM. Thursday at this extended
range appears the most likely to be quiet, dry, and cooler,
followed by a highly uncertain evolution into the start of
Memorial Day Weekend.

Castro

&&

.AVIATION /18Z TAFS THROUGH 00Z SUNDAY/...
Issued at 1228 PM CDT Fri May 17 2024

Key messages:

- Easterly winds at the Chicago area terminals this afternoon
  following the midday lake breeze arrival.

- Threat of fog tonight near the Lake Michigan shore and at
  KGYY. Main threat area looks to remain out of ORD and MDW at
  this time.

Surface high pressure centered over the western Great Lakes is
aiding in an inland lake breeze surge over northeastern IL early
this afternoon. With no good westerly wind push to hold/slow
its inland progress, it is expected to move across ORD prior to
18z, and shortly after 18z at MDW. Easterly winds will then
prevail for the remainder of the afternoon at the Chicago area
terminals, before becoming light and somewhat variable in
direction tonight. Winds will settle into a south-southeasterly
direction (190-200) around 8-10 kt Saturday morning following
the eastward passage of the surface high.

Dense fog is likely to expand across southern Lake Michigan
into this evening, and it remains possible that some of this
fog could work onshore across parts of northeastern IL.
However, it remains a bit unclear if the fog would be able to
reach far enough inland to directly impact ORD and MDW. For this
reason, I have continued with a more optimistic forecast for
the main Chicago terminals late tonight into early Saturday
morning. KGYY stands the best chance to experience any
significant reduced VSBYs due to fog tonight.

KJB

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...Air Quality Alert from midnight tonight to midnight CDT
     Saturday night for INZ001-INZ002-INZ010-INZ011.

LM...Dense Fog Advisory until 4 AM CDT Saturday for Winthrop Harbor
     to Northerly Is. IL.

&&

$$

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