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Area Forecast Discussion
Issued by NWS Chicago, IL
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FXUS63 KLOT 112333
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
633 PM CDT Thu Apr 11 2024

.KEY MESSAGES...

- Narrow band of thunderstorms possible along/near Interstate-39
  late this afternoon and early evening. Main hazards: lightning,
  small hail/graupel, gusty winds, and brief funnel clouds.

- Strong northwest winds develop Friday morning through the
  afternoon.

- Unseasonably warm this weekend into early next week.

- There is a potential for strong to severe thunderstorms to
  occur in the region on Tuesday.

&&

.DISCUSSION...
Issued at 327 PM CDT Thu Apr 11 2024

Through Friday Night:

Two areas of precipitation are ongoing across the area this
afternoon. The first of which is across northwest Indiana where rain
has persisted for much of the day. Rainfall rates have been fairly
light, but the prolonged nature of the rain has resulted in amounts
upwards of 1.5" in spots and would support ponding in poor drainages
areas and rises on area rivers/creeks.

The other area of rain/drizzle is along/west of I-55. Embedded
within this area of rain a more focused axis of showery
precipitation resides closer to I-39 where a weak surface
convergence axis has set up. A secondary convergence axis resides
just to the west of our area along the Mississippi River in Iowa.
This axis set up in an area that has had more sunshine and is closer
to a vort max moving through northeast Iowa. This has resulted in
the development of a narrow line of scattered showers and
thunderstorms. This band will be capable of small hail/graupel,
gusty winds, and even a few funnel clouds. The expectation is for
the pool of low-level instability to slosh east into the area over
the next couple of hours, focusing along and near the I-39 corridor
late this afternoon into early evening. There is a short window this
afternoon where low-level instability and surface vorticity (surface
convergence) align where any funnel clouds might be able to briefly
touch the ground. Have a Graphical Nowcast out already highlighting
this potential.

As we get later into the evening precipitation will expand east
across the rest of the area and through the overnight hours as a
pair of vort maxes pivot across the region. Expect precip to then
gradually end from northwest to southeast toward daybreak Friday.

Behind these showers strong northwest winds begin to develop early
Friday morning and continue through much of the afternoon.
35-40 mph wind gusts are likely during this time. Some soundings mix
deep enough to support higher gusts to near 45 mph. Confidence is
not high enough for a Wind Advisory headline with this update,
however. Temperatures are forecast to warm to around 60 across the
area. With the strong winds, if we mix a bit more deeply, slightly
warmer temperatures and lower dewpoints would result in minimum RH
values dipping into the 30-40 percent range, which would create
elevated fire weather concerns depending on how much rain falls
through tonight.

Petr


Saturday through Thursday:

The large-scale pattern from this weekend into the first half of
next week will generally be characterized by upper-level troughing
across the western CONUS and ridging across the eastern CONUS.
Southerly winds/warm air advection will thus be observed in our area
through much of this time frame and will help facilitate a warm-up
to summer-like temperatures going into early next week. After a day
of mid 60s to mid 70s temperatures on Saturday, the warmest
temperatures of this upcoming warm stretch are currently expected on
Sunday and Monday, when 70s will be prevalent across most or all of
our forecast area and some locations (primarily in the southwestern
half of our forecast area) could very well see their first 80 degree
readings of the year. Lake breezes could occur on either of these
days (especially on Monday) and help keep temperatures along the
Illinois lakeshore a little cooler. The synoptic southwesterly/
westerly winds on Sunday may be strong enough to keep the cooler
marine layer at bay for most or all of the daytime hours, but a lake-
enhanced front should eventually drop through the area late in the
day and send temperatures tumbling going into the evening. Above
normal temperatures are then favored to continue through the middle
of next week.

With the warm-up will come chances for showers and storms,
potentially as early as this weekend as an EML builds into the
region. Still think that most or all of the area should remain dry
through Monday owing to a combination of capping and overall limited
large scale forcing for ascent. However, with the expected presence
of at least one surface boundary in the area on Sunday, it would
not entirely be surprising if surface convergence along the
boundary (or boundaries) provided enough of a spark to get some
isolated convection going. Instability profiles look plenty deep
to support lightning with any more coherent updrafts, so have
maintained slight chance thunder probabilities to go along with
the slight chance PoPs that we have going for some areas on Sunday.

A much better chance for showers and thunderstorms will come Monday
night through Tuesday as a deep upper-level trough ejects out of the
southwestern CONUS towards the Upper Midwest, spurring a fairly
robust episode of lee cyclogenesis to our west in the process.
Conceptually, a deepening surface low passing to our northwest this
time of year usually spells trouble in the form of severe weather
for our area, and there is a fair amount of support in medium range
deterministic and ensemble guidance for strong to severe
thunderstorms to occur in or near our forecast area on Tuesday as
the low pressure system`s warm sector spreads over the region. That
said, with this potential event still being five days away, there
is still unsurprisingly a relatively large amount of spread in
guidance regarding the exact strength, track, and timing of the
incoming surface low and its associated fronts, which will
influence whether and to what extent the severe weather threat
materializes in our forecast area. A 15% contour that includes our
CWA was introduced in the Storm Prediction Center`s latest Day
6 Severe Weather Outlook, and this seems appropriate for now when
considering the overall setup and the additional support offered
by both CIPS Analog guidance and CSU`s Machine Learning
probabilities for severe weather occurring in the region.

Ogorek

&&

.AVIATION /00Z TAFS THROUGH 06Z FRIDAY/...
Issued at 633 PM CDT Thu Apr 11 2024

Forecast concerns include...

Scatter showers this evening.
Mvfr cigs tonight.
Strong/gusty northwest winds Friday.

Scattered showers and isolated thunderstorms extend from near
RFD southeast to near BMI and will continue moving slowly to the
southeast this evening. This activity may remain southwest of
ORD/MDW/GYY with the models showing new shower development by
mid evening across the Chicago area terminals. Best thunder
potential, and still isolated, is expected to remain southwest
of the Chicago terminals with the current activity and likely
only through sunset. Shower activity is expected to be moving
south of the terminals by the predawn hours Friday morning with
perhaps a few lingering showers across northwest IN Friday.

Guidance is still struggling with the cig forecast tonight and
continues to slowly back away from mvfr cigs. Confidence remains
low and while prevailing mvfr cigs are possible, they may be
dependent on shower coverage this evening. Changes are possible
with later forecasts. Cigs are expected to lift to low vfr
Friday morning and then scatter out from west to east Friday
afternoon into Friday evening.

Current northerly winds are expected to turn northwesterly in
the next few hours when speeds and gusts will begin to increase.
Northwest winds will continue to increase overnight. Shortly
after sunrise Friday morning, forecast soundings support a
period where gusts into the lower/mid 30kt range will be
possible. Gusts may diminish slightly into the 30kt range by
early Friday afternoon and then speeds/gusts are expected to
steadily diminish with sunset early Friday evening. cms

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...Small Craft Advisory until 7 AM CDT Friday for Winthrop Harbor
     IL to Gary IN.

     Gale Warning from 7 AM to 6 PM CDT Friday for Winthrop Harbor
     IL to Gary IN.

     Small Craft Advisory until 10 PM CDT Friday for Gary to Burns
     Harbor IN-Burns Harbor to Michigan City IN.

&&

$$

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