893
FXUS63 KLOT 050817
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
317 AM CDT Wed Jun 5 2024

.KEY MESSAGES...

- Scattered thunderstorms this morning, especially across the
  southeastern cwa where locally heavy rainfall and minor
  flooding is possible.

- Period of showers tonight, followed by mainly dry and windy
  weather on Thursday.

- Below normal but comfortable temperatures through early next
  week, breezy daytime conditions through the weekend.

&&

.DISCUSSION...
Issued at 316 AM CDT Wed Jun 5 2024

Through Thursday:

Surface low pressure was analyzed over north central WI early
this morning, with a cold front trailing through eastern IA into
central MO. The low will lift north-northeast across the
northern Great Lakes through this afternoon in response to a
deep mid-level short wave trough which is progged to take on a
negative-tilt as it propagates across the region. As it does so,
the surface cold front will push east through about the I-57
corridor by midday, and eventually out of our IN counties
during the early afternoon hours. Scattered showers and
thunderstorm chances (discussed in more detail below) will end
with the passage of the front, with clearing skies and dry and
breezy conditions developing in its wake this afternoon.
Afternoon highs around 80 are expected, with lower humidity and
west-northwest winds gusting 25-30 mph at times especially
across the IL portion of the forecast area.

As for thunderstorm trends this morning, current radar depicts
a cluster of storms across Lake Michigan and the central/south
and southeast parts of the Chicago metro area as of 245 AM,
aided by an initial weaker lead short wave and a 35 kt
southwesterly low level jet focused into the IL/IN state line
and southern Lake Michigan area. Some of these storms had
produced downburst winds in excess of 50 mph earlier in the
night, though the primary threat at this time appears to be
mainly locally heavy rainfall across the southeast metro and
into northwest IN, where some training of storms has been
occurring. RAP mesoanalysis depicts 925-850 mb moisture
transport maximized on the nose of the low level jet and
precipitable water values around 1.80" across these areas. CAM
guidance suggests additional storms may continue to develop
across the southeast third of the cwa past sunrise within this
zone of better moisture advection ahead of the second deeper
wave and surface cold front, so we`ll be keeping an eye on this
area for the potential for additional heavy rainfall prior to
cold fropa. Farther west/northwest, scattered showers/isolated
thunderstorms will remain possible ahead of the front this
morning, but with conditions less favorable for heavy rainfall
than with those across the southeast cwa.

Farther upstream to our northwest, another quick-moving mid-
level vort is progged to quickly amplify as it digs southeast
across the upper Midwest this afternoon, transiting our area
this tonight before exiting just prior to sunrise Thursday
morning. This is expected to bring a band of showers and perhaps
a few isolated thunderstorms to the forecast area tonight,
while carving out an elongated upper low across the Great Lakes
region through Thursday. This will leave our forecast area along
the southern gradient of colder mid-level temps along the
southern periphery of the trough on Thursday, which may support
enough instability for a few diurnal showers/sprinkles as
indicated in some guidance roughly north of a Rochelle to
Valparaiso line in the afternoon. Strong mixing may lower
surface dew points enough to prevent precip and make for just a
stratocu field however, with better shower chances just off to
our north into WI and over Lake Michigan. Have maintained a dry
forecast for now, but will continue to monitor later guidance
trends in case a few showers/sprinkles need to be added later.
Of greater confidence are strong northwest winds on Thursday,
which could gust 35-40 mph or a little higher in spots in the
afternoon. Otherwise, Thursday will see temps ranging from the
the low-mid 70s north of I-88, to the upper 70s south of I-80.

Ratzer


Thursday Night through Tuesday:

A blocking pattern will effectively trap anomalously deep upper
level low pressure from today and tonight`s cold front passages
over the central and eastern Great Lakes through at least
Saturday. The upper low may finally become dislodged into
eastern Canada Saturday night into Monday. A west to east
oriented trough axis extending from the upper low will gradually
push south across the region during this time. Mid-level height
rises will build in to start next work week, possibly followed
by another trough digging into the Great Lakes by mid week.

All in all, atypically comfortable conditions for the second
week of June can be expected Friday through Tuesday, with below
normal high temperatures in the 70s away from lake cooling,
40s-50s dew points, and overnight lows in the 50s. Gusty west-
northwest winds will extend a bit past sunset Thursday and then
settle into a moderate, occasionally breezy westerly wind the
rest of the night due to cold air advection and a relatively
tight pressure gradient. Friday won`t be quite as windy as
Thursday, but still looking at northwesterly gusts up to 30-35
mph during peak mixing. Saturday and Sunday will be breezy (from
the WNW) but only up to 25 to perhaps 30 mph, followed by
onshore winds Monday and Tuesday as high pressure builds in.

The expected pattern configuration through day 7 (Tuesday) will
not be conducive for any meaningful rainfall, let alone any real
thunderstorm chances, during what is climatologically one of our
most convectively active times of year. Late day Saturday-
Saturday night and Sunday may offer some potential for isolated
to widely scattered showers as the earlier mentioned trough axis
pushes south, steering another pair of cold fronts across the area.

Castro

&&

.AVIATION /06Z TAFS THROUGH 12Z THURSDAY/...
Issued at 112 AM CDT Wed Jun 5 2024

Main Concerns:

- TS with temporary 1/2 to 1SM VSBY through 07-08z at the
  Chicago metro terminals

- A period of lower CIGs this morning

- Another round or two of SHRA possibly with embedded TS late
  AM-early afternoon, latest at GYY, prior to cold front passage

Last batch of TS with embedded +TSRA will move across the
Chicago metro terminals through 08z. During and behind this,
CIGs will dip to low MVFR with patchy IFR. A cold front will
approach from the west this morning, likely bringing additional
rounds of SHRA with occasional reduced VSBY. While there may be
isolated embedded TS, confidence was too low for any additional
mention in the TAFs. GYY will have the latest frontal passage
and likely a relatively higher chance for TS to develop ahead of
the front. The front will shift winds to westerly with gusts to
20-25 kt and improve lingering low CIGs. Scattered mainly VFR
SHRA will precede a secondary cold front late this evening
between about 02z and 06z.

Castro

&&

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

&&

$$

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