Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 251152
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
652 AM CDT Mon Mar 25 2024

.KEY MESSAGES...

- Strong wind gusts near 45 mph expected later this morning and
  afternoon, mainly south of I-80 and east of I-55. Elevated fire
  danger in the same area with afternoon RHs in the 30-40 percent
  range.

- Widespread, soaking rainfall arrives area-wide this evening.

- Threat for severe thunderstorms Tuesday morning and early
  afternoon, particularly east of about I-55.

- Additional period of strong non-thunderstorm wind gusts around
  45 mph on Tuesday morning east of I-55, and then again area-
  wide Tuesday afternoon.

- Brief period of cooler weather on Wednesday followed by a warming
  trend through the end of the week.

- Another period of active weather looking likely late week into
  next weekend.

&&

.DISCUSSION...
Issued at 303 AM CDT Mon Mar 25 2024

Through Tuesday:

A sprawling storm system is taking shape across the Central Great
Plains at this hour, with an analyzed sub-990 mb low pushing
across Kansas. Ahead of this feature, the low-level airmass across
our region remains very dry--notably drier than most guidance is
currently advertising--with low 20s and even upper teens dewpoints
stretching across northern Indiana and into Ohio. While dewpoints
are beginning to surge northward within this storm system`s
developing warm sector, persistent southeasterly winds and mixing
today look set to maintain a notable moisture gradient from NW to
SE across our forecast area. Arcs of mainly light showers will
start to stairstep their way across our northwest through the
morning and afternoon, while the lingering dry wedge to the east
looks to meaningfully curtail precipitation chances until very
late this afternoon and into the evening.

In addition to a continued tightening of the PoP/precip chances
today, have dropped dewpoints a bit south and east of I-55/57 and
also boosted high temperatures where at least some filtered
insolation will manifest from time to time. With the latest
forecast, afternoon RH values may touch about 30 percent across
our northwest Indiana locales, which will drive an elevated fire
danger given anticipated very strong and gusty winds.

Speaking of the winds today: there is a potential for gusts to
"pop" pretty notably through the mid-morning hours as mixing into
the base of an exceptionally robust low-level jet ensues,
particularly east of about I-55 where near-surface lapse rates
will be the steepest. While this LLJ will weaken in typical
diurnal fashion, there`s good agreement across the guidance suite
that we`ll at least ephemerally tap into near 40-knot flow around
900 mb through the morning. Latest indications are that gusts may
end up just a hair under formal Wind Advisory criteria on a
persistent basis, although would not be surprised to see a few
gusts over 45 mph late this morning and early afternoon.

Farther to the northwest, there`s been a pretty consistent signal
in the HRRR depicting a brief period of 50+ mph gusts at the
leading edge of a precipitation shield which will emerge out of
the ongoing convection across eastern Oklahoma (just another cue
that the wind field is exceptionally robust just off the deck).
Given this will be entirely contingent on the timing/placement of
convection/showers, there isn`t enough confidence to justify a
Wind Advisory at this time. It`s also possible this surge sets up
entirely outside of our forecast area.

Rain will become widespread into this evening and the early
overnight as very intense large scale forcing develops and
intercepts a plume of anomalous PWATs streaming into the region.
Swaths of locally heavy rainfall out of efficient showers appear
probable, particularly along and NW of I-55 where isolated
precipitation totals over 2 to 3 inches are on the table. We`ll
continue to highlight the potential for minor flood impacts and
attendant river rises into the middle of the week through a
Hydrologic Outlook (ESF).

The main focus of the short term continues to be on Tuesday, with
a steadily-increasing threat for strong to severe convection.
While timing differences remain (e.g. RAP/HRRR combo which are
much earlier), we`re seeing decent model agreement that arcs of
low-topped convection will develop at the leading edge the primary
shortwave which will eject across northern Illinois through the
late morning and early afternoon hours. While MLCAPE values are
not forecast to be particularly high (500 J/kg or less), the vast
majority of it will be present in the 0-3 km layer. Additionally,
most guidance suggests that 0-3 km lapse rates accompanying the
aforementioned short wave will be fairly steep, in the 7-8 C/km
range. This will act to reduce convective inhibition more quickly
than we typically see during the morning hours. With an extremely
robust low and mid-level kinematic parameter space in place (850
mb winds near 50 knots), a threat for low-topped bowing segments
exists across parts of our area Tuesday morning and afternoon.
Given the wind field in place, damaging winds and even a few
tornadoes appear possible. The specific timing still needs to be
worked out, but the current multi-model consensus suggests a rough
9 am - 3 pm timeframe (earliest in our SW, latest in our east).

Out ahead of any strong-severe convection, seeing a potential for
perhaps 50 mph gradient wind gusts materializing late Tuesday
morning/early afternoon mainly across our NW Indiana counties as
mixing deepens into the robust LLJ. Eventually, a cold front will
sweep across the region, resulting in temperatures falling into
the 40s and southwesterly winds gusting to around 40 mph through
the rest of the afternoon.

Carlaw

Tuesday Night through Sunday:

The potent storm system mentioned in the previous section will
be in the process of exiting northeast IL and northwest IN
Tuesday evening. While it appears that the bulk of the
precipitation will be east of us in Lower Michigan, guidance is
showing a developing upper low digging into the upper Midwest
and northern Great Lakes during this time which may allow some
light precipitation to persist through about midnight Tuesday
night. If precipitation is able to occur (currently about a 20%
chance) the most favored areas for such activity should be
across the northeast corner of IL into northwest IN with the
rest of the area expected to remain dry. Given that temperatures
aloft are forecast to be cooling Tuesday night, there is a non-
zero chance for a snowflake or two to mix in briefly, especially
across northern IL, with the aforementioned precipitation which
should otherwise fall as rain. Regardless, no snow accumulation
is expected.

Our area will remain under the influence of cold advection
through the day on Wednesday as the previously mentioned upper
low drifts across the northern Great Lakes. In turn this will
keep temperatures on Wednesday and Wednesday night on the cool
side with highs in the 40s and lows in the 20s to lower 30s.
However, mid-level ridging is forecast to build into the area
on Thursday which is expected to gradually warm temperatures
back towards more typical late March readings (in the 50s and
60s) by the end of the week.

Unfortunately, the warming temperatures do look to be
accompanied by another period of active weather heading into
next weekend and possibly next week. Guidance is in decent
agreement that a baroclinic zone should establish in the
vicinity of the Great Lakes on Friday into the weekend which is
expected to serve as the breeding ground for showers and perhaps
even some thunderstorms as an upper wave moves through the
region. However, there is still uncertainty as to the strength
and track of this upper wave which will obviously dictate the
coverage and location of any showers and/or storms. Therefore,
shifts in the forecast may occur in the coming days so stay
tuned for additional details.

Yack

&&

.AVIATION /12Z TAFS THROUGH 18Z TUESDAY/...
Issued at 652 AM CDT Mon Mar 25 2024

Forecast concerns for the terminals...

- LLWS threat early this morning as 50-55 kt low-level jet resides
  overhead

- Gusty southeast winds today with gusts in the 30 to 35 kt
  range expected

- Storm system will bring rain into the area late this
  afternoon/evening through Tuesday morning resulting in reduced
  visibilities

- Ceilings lowering this evening to MVFR with IFR conditions
  expected near RFD

- Chance for thunderstorms Tuesday morning into early afternoon


An active period of weather is expected at the terminals through
this TAF cycle starting with a threat for LLWS early this
morning as a 50-55 kt low-level jet remains overhead. While this
jet is expected to weaken later this morning, winds will
increase as the storm system over the central plains moves
closer. This system along with diurnal mixing is expected to
generate wind gusts in the 30 to 35 kt range through this
afternoon. Though, occasionally higher gusts to 40 kts are
possible especially this evening. Gusts are expected to ease
into the 20 to 25 kt range overnight, but will increase once
again Tuesday morning with similar values to today.

Aside from the winds, this system will also bring widespread
rainfall some of which could be locally heavy. The main axis of
rain is expected to get underway late this afternoon and evening
at RFD and the Chicago terminals respectively and persist into
the predawn hours Tuesday morning. While there continues to be
the chance (around 20 to 30%) for scattered showers to develop
ahead of the main rain axis this afternoon, the dry low-levels
noted this morning may limit the coverage especially with
eastward extent. Therefore, I have decided to remove the PROB30s
at the Chicago terminals for this potential but did leave the
TEMPOs in at RFD and DPA where slightly better moisture is
forecast to reside. Regardless, any visibility reductions are
expected to occur with the main wave of rain tonight with
visibilities in the MVFR to possibly IFR range.

In addition to the visibility reductions, ceilings are also
expected to lower as the rain moves in this evening. Guidance
continues to be in good agreement that most areas will see MVFR
ceilings tonight but IFR conditions will also be present
especially near RFD. Once these lower ceilings arrive they do
look to persist through the remainder of the forecast period,
though any IFR ceilings should rise to MVFR Tuesday morning.

Lastly, there is a growing threat for thunderstorms Tuesday
morning into early afternoon as a secondary disturbance pivots
through the area. While forecast soundings do show at least
modest instability developing areawide on Tuesday, guidance
continues to very on the exact timing. Therefore, given this
lower confidence on timing and intensity I have decided to
introduce a PROB30 at ORD and MDW for now.

Yack

&&

.MARINE...
Issued at 303 AM CDT Mon Mar 25 2024

Several periods of gale force winds continue to be possible
through Tuesday evening. The first is later this morning and
towards midday with intermittent 35 kt gales, mainly in the first
few miles of the nearshore waters south of LMZ740. The second is
overnight tonight. Given the expected relatively brief duration
and intermittent/occasional nature to gales during these time
periods, the Small Craft Advisory will continue, mentioning
occasional gales.

Thereafter, there is a better signal for more persistent/regular
35 to 40 kt gales developing Tuesday afternoon as a cold front
sweeps across the lake. Uncertainty remains in gales developing
earlier in the morning, however. As a result, the Gale Watch
remains in effect, but an upgrade to a Gale Warning may be
needed later today.

Carlaw

&&

.FIRE WEATHER...
Issued at 303 AM CDT Mon Mar 25 2024

There is an elevated fire danger today, mainly for locales east of
about I-57, and particularly across northwest Indiana.
Temperatures in the mid and upper 60s are now expected which,
when combined with dewpoints in the upper 30s/near 40 will
result in afternoon relative humidity values as low as 30 to 35
percent. With southerly winds gusting as high as 45 mph, any
fire starts have the potential to spread quickly. A widespread
wetting rainfall will occur tonight.

Carlaw

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...Small Craft Advisory until 3 AM CDT Tuesday for the IL and IN
     nearshore waters.

     Gale Watch from late tonight through Tuesday evening for the
     IL and IN nearshore waters.

&&

$$

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