Area Forecast Discussion
Issued by NWS Chicago, IL

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000
FXUS63 KLOT 291955
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
255 PM CDT Fri Mar 29 2024

.KEY MESSAGES...

- Periods of showers and thunderstorms expected tonight, some may
  produce:
-  Locally heavy rainfall, which may foster minor flooding in
       low-lying and poor drainage areas.
-  Some instances of marginally severe hail (1" in diameter).

- Another period of showers and thunderstorms is expected Sunday
  afternoon through early Monday morning especially south of
  Interstate 80 with level 1 out of 5 threats for damaging hail
  and flash flooding.

- A third period of showers and thunderstorms may occur on
  Monday, though exactly where and with what vigor are
  opportunities for refinement in future forecasts.

- After a brief period of cool weather Tuesday and Wednesday,
  warming temperatures are expected Thursday and beyond.

&&

.DISCUSSION...
Issued at 255 PM CDT Fri Mar 29 2024

Through Saturday Night:

While it is quiet at press time this afternoon, this will not
be the case tonight. In fact, cloud cover is already beginning
to increase just to our southwest. This cloud cover will be
heading our way into early evening as our next approaching mid-
level impulse and associated surface low begins to shift towards
the Lower Missouri Valley. As it does so, an intensifying
southwesterly low-level jet (45+ kt) will induce strong warm and
moist advection across much of the Midwest into the lower Great
Lakes region into tonight. Ultimately, this will set the stage
for about a 6-8 period of showers and thunderstorms across much
of our area tonight as the nose of this low-level jet sets up
right across northern IL.

While the prime time for the development of these showers and
storms into our area will be after 8 pm this evening (likely after
10 pm in northeastern IL and northwestern IN), a band of high
based light showers or sprinkles looks to shift overhead after 5
pm this evening. There is still a good amount of dry air in the
low levels, so we are not expecting much in the way of rain with
this initial push of mid and upper-level moisture. However,
don`t be surprised to see a few light showers around for a period
into early this evening.

Otherwise, once the development of showers and thunderstorms
onsets along the nose of the low-level jet early this evening,
easterly storm motions will foster periods of training cells
along the northward (cold) side of a sharpening surface warm
frontal boundary setting up near, or a bit south of, the I-80
corridor tonight. This thus adds some concern for localized heavy
rainfall amounts tonight. In fact, several HIRES models do
indicate narrow corridors of locally heavy rainfall amounts in
excess of an inch, and see no reason why this will not be possible
given the setup. While the locally heavy rain is not expected to
generate significant flooding concerns, we certainly cannot rule
out some instances of minor flooding, especially across the urban
areas in and around Chicago. We have advertised this threat with
a hydrologic outlook.

The other concern with some of these storms tonight is the
potential for some hail. Strong warm moist advection along the
low-level jet will act to steepen the mid-level lapse rates, which
in turn could support some hail production in some of the stronger
cores overnight. The SPS continues to highlight mainly the western
half of the NWS LOT CWA in a level 1 of 5 risk due to this
potential.

The shower and thunderstorms will end for much of the area by
daybreak Saturday morning as the surface low begins to depart to
our east. While the remainder of the day will be precipitation
free, cold air advection on breezy northwesterly winds will
result in a persistent deck of strato-cu, which should make for a
mostly cloudy day.

KJB


Sunday through Tuesday:

A frontal boundary is expected to be meandering from northern MO
through central IL and into central IN through Tuesday morning
when an area of low pressure will move east along this frontal
boundary. There still remains uncertainty for the location of
this boundary as well as the track of the surface low. However,
it will likely serve as the focus for periods of showers and
thunderstorms. Any drift north of the boundary will move more
of this activity across the local area. Severe weather will also
be possible near and south of the boundary with both the day 3
and day 4 SPC outlooks showing a severe potential in the
southern third or so of the cwa and this could be a few
different times, perhaps some strong storms Sunday afternoon/
evening across the southern cwa and then again Monday into
Monday night, with less confidence on timing.

Heavy rain and localized flooding will also be a concern,
especially Monday into Monday night when most of the area is
outlooked for a marginal excessive rainfall. The GFS and its
ensembles continue to indicate periods of showers and
thunderstorms with heavy rain across much of the area with a
little less support among the ECMWF ensembles but still many
members showing periods of heavy rain. The position/location of
the boundary will have a lot of influence on where the heaviest
rains occur. And there could be training where the same areas
see waves of showers/storms. Precipitable water values are in
the 1 to 1.25 inch range Monday. Thus there is plenty of
moisture to work with over an extended period of time. Model qpf
amounts vary considerably but in general, show the potential
for 2-3 inches across the southern cwa. If these trends
continue, some hydro product may be needed with later forecasts.

With the expectation of a frontal location either south of the
area or in the far southern cwa, high temps might only reach
the upper 40s/lower 50s north to upper 50s south on Sunday and
perhaps mid 50s north to near 60 south on Monday but there is a
large bust potential near the front both days.

By Tuesday, as the low shifts east, colder air will spread into
the region with highs possibly only in the 40s for the northern
half/two thirds of the area. Precip chances may be winding down
Tuesday but with the colder air, there could be a mix of
rain/snow Tuesday afternoon/evening but still quite a bit of
uncertainty for this time period. cms

Wednesday through Friday:

After this system departs Tuesday night, it becomes stationary
over the eastern Great Lakes Wednesday and then slowly weakens
and shifts east Wednesday night into Thursday. If this trend
pans out, Wednesday will likely be quite windy for the local
area with at least a low chance for showers, possibly with a few
snowflakes, depending on high temps which may stay in the low/
mid 40s. Wind gusts based on current soundings could easily be
into the 35 mph range. Thursday looks to be a transition day as
high pressure moves across the area Thursday night with highs in
the 50s Thursday and then highs possibly back into the lower 60s
Friday, away from Lake Michigan. cms


&&

.AVIATION /18Z TAFS THROUGH 00Z SUNDAY/...
Issued at 100 PM CDT Fri Mar 29 2024

Key Aviation Messages:
- TS late this evening and overnight, with periods of IFR
  visibility and the strongest TS potentially producing small
  hail.
- Varying wind directions around sunrise Saturday as a surface
  low tracks across the Chicago metro.
- IFR ceilings possible late tonight into mid-morning Saturday.

VFR conditions are expected through mid-evening while mid-level
cloud cover gradually increases and E/ESE winds remain around
10 knots. A few brief -SHRA or sprinkles are expected this
evening.

A SSW low-level jet impinging on a warm front will then result
in a period of training TS across northern Illinois beginning
late this evening and continuing to the pre-dawn hours Saturday.
The initial period of TS has the potential to produce heavy
rain (IFR visibility), with a few of the strongest storms
capable of producing small hail. While storm coverage and
intensity will wane after about 09Z, scattered TS should
continue into the area through 11Z and as late as 13Z.
Otherwise, winds will settle SE during the period of TS.

A surface low will cross the Chicago metro toward daybreak
Saturday, with the track passing very close to several of the
terminals. While wind speeds will diminish to under 10 knots
during this time, the wind direction could anywhere from NE to
SE to W for several hours depending on the exact low track.
Ultimately, winds will shift NW 10-15 knots after the low
passes by mid-morning.

Finally, MVFR ceilings will likely develop late tonight and
continue through much of the day Saturday. IFR ceilings are
possible along and north of the low track during the morning
hours.

Kluber

&&

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

&&

$$

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