Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 200727

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
227 AM CDT Mon Mar 20 2017

922 PM CDT

No significant changes to going forecast though did nudge highest
PoPs a little farther south to reflect latest model trends and
expected placement of the low level jet. Scattered high based
showers are moving southeast across Wisconsin this evening ahead
of a cold front, and a couple thunderstorms have been able to
develop ahead of the front farther north in Central Wisconsin.
It`s possible to see some precip across northern Illinois within
the narrow and weak instability axis, but better dynamics are in
place across central and northern Wisconsin and think there is a
better chance for areas north of the I-88 corridor in Illinois to
remain dry this evening.

Main focus this evening and tonight though will be increasing
convection associated with a low level jet to our west/southwest.
Guidance has been somewhat consistent this evening indicating the
low level jet setting up across Missouri into far southeastern
Iowa this evening with convection increasing around or just after
midnight. Latest SPC meso-analysis and 00Z DVN Raob indicate
strong MUCAPE in excess of 1000 J/kg just west of the CWA.
Meanwhile, LLJ is progged to ramp up over the next couple hours
with strongest moisture convergence focused from far NE MO/SE IA
southeast into central IL. Expect this will be the region of
strongest convection with storm motions off to the ESE. Think that
greatest severe threat will stay closer to the LLJ convergence to
our west/southwest where the highest instability will be in place
which quickly decreases farther east. This should result in a
fairly minimal severe threat for the local CWA with elevated
hailers still the primary concern for any stronger storms that are
able to persist into the local area. As mentioned earlier, nudged
the highest PoPs farther south given the aforementioned thoughts
with support from latest runs of the HRRR and RAP which are
trending slightly farther south as well.



210 PM CDT

Through Monday...

After a pleasant and seasonal (though cool at the lake) Sunday
with partly cloudy conditions and modest winds, our focus will
shift to shower and thunderstorm chances tonight into Monday.

A lead shortwave riding the periphery of a flattening ridge across
the plains is leading to convection along a warm frontal boundary
across Missouri. This activity will dissipate, but additional
shortwave energy will propagate east along the frontal boundary
in an area of increasing moisture return ahead of significantly
increasing southwesterly low level jet. At this point the best
moisture return and theta-e advection will be across our
southwestern counties where the latest high- resolution guidance
is suggesting an expansion of shower and thunderstorm coverage
tonight. Initially this will be focused across Iowa this evening,
but the RAP/HRRR suggest enough moisture convergence north of the
front and some decent elevated instability to suggest a swath of
showers and storms in our area. The favored areas for storms will
generally be south as the night progresses toward daybreak. The
instability is enough to be concerned about some severe hail with
the instability extending through the hail growth region and
decent shear though marginal in the cloud bearing layer, and
therefore the threat is marginal and consistent with the SPC
outlook. The main focus will be after midnight into early Monday,
with better chance of hail largely southwest of the core of the
Chicago metro area based on latest convective allowing model
guidance, but it is certainly a non-zero threat farther north.

The shortwave trough axis will approach Monday morning with the
cold front, which will likely continue the chance of showers
during the morning hours, possibly redeveloping for our northern
areas including the Chicago area. The main warm frontal boundary
will generally slide east along our southern counties, thus we
never truly get into the warm sector ahead of the low. And while
the northern stream cold front will shift into northern Illinois
later morning, it will take a bit longer south which will linger
the shower chances for most of the day for these areas. Expect
cooling in Illinois, especially lake areas in the afternoon. The
instability axis gets shoved farther south but not completely out
of the area until the cold front arrives, so will need to hang
onto the thunder mention into the morning and it is possible the
marginal risk may have to shift north a bit on Monday.



259 PM CDT

Monday night through Sunday...

More energetic shortwave energy and associated precip will shift
south of the CWA by Monday evening, with high pressure expected to
build across the area Monday night into Tuesday. Cold advection
on the backside of this system will return a cooler/drier air mass
to the region, especially by Tuesday. Next chance of precip does
appear to be on Tuesday, although confidence is lower with this
chance owing to some placement and timing issues. Upstream
shortwave anticipated to drop southeast through the region during
this time, as baroclinic zones tightens up. With this setup, a
narrow axis of developing precip seems probable and at this time,
a favored location would be over the southern CWA. Expected
highs/boundary layer conditions would support all liquid at this
time, but will continue to monitor for at least a low potential
for a rain snow mix. With anticipated colder thermal profile
aloft, it looks like if enough forcing were to occur to cool the
entire profile, then snow could be possible. Too early to tell at
this time though, and have maintained only rain for Tuesday. Any
precip then sags southward Tuesday evening with further expansion
of the high to the north. Much colder air filters south and with
this high moving more overhead, Tuesday night is looking to be a
chilly night. However, the CWA may stay on the outer fringe of
this high and tighter gradient. This would allow the winds to
possibly stay more elevated and lessen the extent of the cooling
this night. Dry weather then expected to continue Wednesday into
Wednesday night. Large scale pattern begins to change though, with
upper level ridge building overhead and warmer air returning to
the region. Warmer temps expected Thursday and Friday, but
precip/rain and possibly thunder chances increase by late Thursday
into Friday.



For the 06Z TAFs...

Concerns and challenges with the Chicago area airports are:

- Overnight TSRA well to the northeast as well as some closer to
the southwest, possibly impacting the airports.

- Another round of -SHRA/TS in the region late Monday morning
into the early afternoon, most favored just to the southeast of

- IFR cigs and MVFR visibility possible at some point Monday
morning into early afternoon.

- South winds tonight becoming northeast early Monday afternoon,
likely overlapping IFR cigs.

Scattered TSRA are developing across central and eastern Iowa
at this time. They will translate east-southeast through the
overnight hours. Given where initiation of storms is occurring,
these should primarily track near or just south of the southern
Chicago area TAF sites overnight. While there is continued
uncertainty on how far north/northeast these will develop, there
could be showers to the north moving over the airports, so
continue mention of showers, and VCTS for southern TAF sites.

Wind speeds will diminish and possibly even become variable on
Monday morning, allowing for ceilings to lower. Confidence on just
how far these drop is low-medium and the rate in which they drop
has even lower confidence. Gradually lowering visibility is
likely during Monday morning, though dense fog is not favored. As
a cool front approaches, the possibility of showers and possibly a
few more thunderstorms increases in the 15Z-20Z window before
this activity shifts southeast. Cloud bases should see a raising
trend into Monday evening.



227 AM CDT

A cold front will shift southward over the lake through the
morning. Once this front front passes, expect a prolonged period
of northerly flow over the lake through Wednesday evening.
Initially, the northerly winds will be up around 20 kt later today
into early Tuesday. However, a stronger surge of northerly winds
up around 30 KT is expected to develop over the lake Tuesday and
Tuesday night in response to a strong surface high building
southeastward over south central Canada. This period of stronger
winds will likely result in hazardous conditions to small craft
over the southern Lake Michigan near shore waters late Tuesday
through Wednesday morning.

Once this surface high shifts over the eastern lakes by early
Thursday morning, the winds over the lake will shift southward
for a period again during the day Thursday. However, another cold
front is likely to shift down the lake Friday night into early
Saturday. Following this frontal passage, another potent surface
high is likely to build over the Upper Great Lakes and southern
Canada as a storm system tracks over the southern lakes region.
These two pressure patterns could result in a period of strong
northeasterly winds again on the lake for a period this weekend.






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