000
FXUS63 KLOT 121116
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
616 AM CDT Fri Apr 12 2024

.KEY MESSAGES...

- Gusty northwest winds (up to 40 mph) develop this morning and
  continue through the afternoon.

- Small chance for an isolated afternoon shower or thunderstorm
  with localized stronger wind gusts across parts of northwestern
  IN this afternoon.

- Lake enhanced front will bring an abrupt temperature drop to
  northeast IL and northwest IN on Sunday afternoon-evening
  following summer-like warmth

- Waves of showers and a good chance (30-40%+) for thunderstorms
  Monday night through Tuesday night, some of which may be severe,
  particularly on Tuesday afternoon and evening

- Warm and windy on Tuesday with southerly gusts potentially up
  to 45 mph

&&

.DISCUSSION...
Issued at 335 AM CDT Fri Apr 12 2024

Through Saturday:

The primary weather story today will be the strong and gusty
northwest winds expected as a strong and deepening surface low
(sub 980 mb) occludes this afternoon over far southeastern
Ontario near the Georgian Bay. This strong storm system will
drive strong gusty winds across much of the Great Lakes eastward
into New England today. In our area, we are expecting northwest
wind gusts to ramp-up quickly after daybreak as boundary layer
mixing ensues. Frequent gusts of 35 to 40 mph are expected, with
peak gusts looking to be roughly from 10 am through 3 pm this
afternoon. The gusty winds should begin to ease some late in the
day, but the cold air advection regime will allow for gustiness
of 25 to 30 mph to persist into this evening before abating
overnight. While some isolated higher gusts up to 45 mph are
possible, it appears the threat for frequent gusts above 40 mph
are not high enough to warrant the need for a wind advisory at
this time.

Rain chances this morning will drop quickly with daybreak as a
fast moving impulse continues to exit the region to the
southeast. This is expected to result in a mainly dry day for
about 80-90 percent of the area. The only exception to this
being this afternoon across parts of far northeastern IL
southward into northwestern IN. In this corridor lower level
moisture looks to remain modestly high enough to support the
development of some isolated afternoon showers and storms along
what looks to be a low-level zone of confluence along the
southern end of Lake Michigan. While severe storms are not
likely, notable DCAPE in excess of 500 J/kg could support some
locally stronger wind gusts (perhaps up around 50 mph) in and
near these showers and storms.

A surface ridge axis will shift overhead into Saturday. This
will result in a period of quiet and very mild weather during
the day. Sunny skies and southwest winds are expected to support
temperatures near 70 degrees Saturday afternoon. Wind speeds
may become light enough right along the IL lakeshore to support
a lake breeze Saturday afternoon. While this could thus support
slightly cooler weather right along the lakeshore, it currently
appears unlikely that the lake breeze would make significant
inland progress.

KJB


Saturday Night through Thursday:

Saturday Night-Monday:

A plume of very steep mid-level lapse rates (elevated mixed
layer/EML) will advect eastward on Saturday night as a wave of
low pressure tracks across the northern Great Lakes. The deeper
layer moisture will be collocated with the much stronger large
scale ascent off to our northeast, closer to the surface low
pressure. Thus, it appears likely that strong capping from the EML
should hold and result in a dry, breezy (sporadic southerly gusts
to 30 mph), and unseasonably mild (lows 55-60F) night.

The aforementioned surface low pressure will drag a cold front
toward our area which will lay out on Sunday and take on back-door
characteristics. This will yield a classic early to mid spring
lake enhanced cold front (sometimes called a "pneumonia front")
passage Sunday afternoon. Out ahead of the front, the warm start
to the day and unseasonably mild air mass aloft will set the
stage for summer like warmth with highs well into the 70s and even
around 80F over the southwest half or so of the CWA.

Model guidance has come into better agreement in the lake enhanced
front surging in from the lake around 12-2pm CDT and then making
steady inland progress through the afternoon. Expect temperatures
to drop from the 70s to the 50s and then 40s within a few hours
near the lake, with a quite noticeable but less pronounced drop
with farther inland extent. Northerly winds will temporarily gust
up to 25 mph to perhaps briefly 30 mph behind the frontal passage
closer to the lake.

A strong EML will still be in place on Sunday afternoon as the
front moves inland. Thus, it appears more likely than not that
the boundary layer moisture pooling near the front will be
insufficient to overcome to the stout capping and very mid-level
air advecting in. Only have slight chance PoPs east of I-65 in
portions of interior northwest Indiana during the afternoon. That
being said, if convection is able to develop, it will likely be
capable of producing lightning.

Following the lake enhanced cold front passage, Sunday night will
be a cool interlude (coolest near the lake) amidst an otherwise
unseasonably mild pattern through the early part of the week.
Monday should be fairly similar temperature wise to Sunday`s
high temps away from the lakeshore with plenty of sun.
Meanwhile, near the lake, southeasterly winds will likely back
more easterly due to lake influence and keep temperatures
markedly cooler than the mid-upper 70s inland, particularly
along the Illinois shore.

Castro

Monday Night-Thursday:

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,
packing 60+F dew points.

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. In addition,
plentiful large scale forcing and less capping could make for a
rather messy setup and curtail destabilization at least somewhat.
A 15% contour that includes our CWA was in the Storm Prediction
Center`s Severe Weather Outlook for Tuesday continues to seem
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.

In addition to the chance for thunderstorms and associated
severe threat, Tuesday will be warm (highs in the 70s) and
windy, with uncertainty on the top end magnitude due to cloud
cover, precipitation, and mixing depth question-marks. Potential
does exist, however, for southerly gusts to flirt with if not
exceed advisory criteria. Showers may linger in the blustery
cold advection behind the system on Wednesday, especially in the
morning, with a sign of yet another short-wave bringing
additional waves of showers and possibly a few thunderstorms on
Thursday, albeit with cooler temps by then. Signs are currently
pointing toward a period of below normal temps to close our next
work week and into the following weekend.

Castro/Ogorek

&&

.AVIATION /12Z TAFS THROUGH 18Z SATURDAY/...
Issued at 616 AM CDT Fri Apr 12 2024

Main Concerns:

- Strong and gusty northwest winds today with a period of
  occasional 35-40 kt gusts late morning-early afternoon

- Isolated SHRA possible this afternoon, primarily
  east/southeast of the near lake Chicago area terminals

Deep low pressure will slowly lift north through the eastern
Great Lakes region into this evening, resulting in strong and
gusty northwest (300-320 deg direction) winds. While gusts will
be primarily in the 30-35 kt range from ~14-02Z, forecast
soundings continue to point toward a period of gusts up to 35-40
kt from ~16-19Z. Included temporary upper 30s kt gusts in the
ORD and MDW TAFs, which may cause minor crosswind issues on
west-east runways. Gustiness will likely continue a bit beyond
sunset and then abate by the mid evening.

An upper level disturbance may touch off a few isolated SHRA and
possibly even a TS or two this afternoon, mainly over interior
northwest IN, so no mention in the TAFs. Aside from lingering
MVFR CIGs at GYY early this morning, expect VFR conditions the
rest of the period.

Castro

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...Small Craft Advisory until 9 AM CDT this morning for Winthrop
     Harbor IL to Gary IN.

     Gale Warning until 6 PM CDT this evening for Winthrop Harbor
     IL to Gary IN.

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

&&

$$

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