Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 281219

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
619 AM CST Tue Feb 28 2017

333 AM CST

Through tonight...

An active and possibly dangerous evening/night of thunderstorms
will finish what has already been an atypical February of weather.
The season`s first widespread severe chances will overspread the
CWA later this afternoon and through the night. A lot has to come
together for severe storms and even more so for significant
severe weather this early in the season and after dark, but there
are a majority of the ingredients that will be in place over our
area...including 1.) a deep low just to our west that will be
strengthening, 2.) very strong upper and mid-level winds, and 3.)
rapidly increasing moisture/instability to near the top of
climatological values just ahead of the storms. Whether they can
come together just right though, and over a several hour period
and across much of the area as some guidance shows, still has some
noteworthy uncertainties.

Through early-mid arc of warm advection induced
showers and some thunderstorms are likely to progress east during
the mid-morning hours. While small hail certainly is possible,
larger hail potential should remain low with these as the leading
edge of the elevated mixed layer EML with deeper instability will
likely not push in until early afternoon. Scattered elevated
storms, if they can develop during the early and mid-afternoon
will have an increasingly more favorable air mass for large hail
with strengthening deep layer shear and growing -10C to -30C CAPE
(only at 16000 ft to 23000 ft AGL).

Later this afternoon...The western upper trough will shift east
on a powerful 150 kt upper jet this afternoon and evening. It is
this upper jet, along with mid-level winds of 80+ kt at 500mb
winds and 60+ kt at 700mb, that will nose into northern Illinois
late afternoon into this evening and serve as an impetus for
increasing convective coverage. The northwest half of the CWA
seems favored for the highest coverage late this afternoon in
what would likely be elevated strong to possibly severe storms
with a large hail threat. However, a more theta-e rich low-level
air mass to the south and east could support any scattered severe
storms there becoming rooted lower late this afternoon, and
several CAM solutions do indicate what appear to be isolated
supercell structures along/south of I-80 after 4 pm. RAP and NAM
soundings still indicate that these storms would likely be
slightly elevated late this afternoon. Nonetheless, the potential
for large hail will certainly be there.

Tonight...The severe threat increases sharply after dark. At the
surface, a somewhat elongated yet still deep 998 mb low is
expected along the IA/MO border at 6 pm and this will move to near
or just north of Chicago by midnight. Guidance is in fairly good
agreement with this and feel comfortable with the high-resolution
models at this close proximity and little in the way of upstream
convection to impact mass fields much. This low track will
continue low-level warm and moist advection after dark on 15-25
mph winds with higher given forecast NAM soundings, and a very
strong 65 kt low-level jet above this. These low-level winds are
key as they will continue to destabilize the low-level air mass
despite after dark, and is almost always a needed ingredient for
nocturnal wind and tornado severe weather potential in our area if
convective mode is ideal, and that`s an uncertainty.

Storms should increase in coverage during the early-mid evening
over the CWA immediately upstream in the IL/IA/MO border region
right ahead of the surface low/convergence and the aforementioned
upper ascent. CAMs indicate little in the way of convection
further south (southern MO/southern IL) through early evening,
which should allow uninhibited dew points around 60 degrees to
advect into the southern CWA and keep the area`s storms more
dominant. The mode is a bit of a challenge to forecast as there
are certainly stout shear and crossover parameters for rotating
updrafts and supercells, and believe there will be some in the
region, possibly embedded in multi-cell or quasi-linear
structures, especially by late evening into overnight. Whether or
not there are discrete structures present when the true effective
warm front moves into the region this evening is uncertain, but if
so, they would be likely to have a tornado threat. Where this
potential is highest is highlighted well by the SPC enhanced risk,
though the most rich low-level instability could reach further
north. It it stressed though that even in a quasi-linear structure
with a wind threat, which is presently favored in our south later
tonight, a tornado threat would continue given extremely strong
0-1 km shear of 45 kt and helicity in excess of 250 m2/s2. The
storms should shift southeast of the area just before daybreak

Hazards/timing...While the entire area has a risk for large hail
throughout this afternoon and tonight, the area most favored for
surface-based/low-level rooted storms is south of I-88 and
primarily after 6 p.m. and through 3 a.m. The northward extent
could be modulated some by convection this afternoon, and again
the mode of storms moving into the CWA this evening, but feel
comfortable with that area given the consensus in surface low and
wind field parameters. CAM ensembles also add support to this
area, especially along/south of I-80, and this is the area that
has better parameter space for a significant tornado or two if
again ingredients come together with the proper mode. SPC`s
outlook captures this well with their hatched tornado risk.

Message...With this event being the first probable severe weather
event of the season in the region, and coming so early prior to
Preparedness Week, it is encouraged to freshen up on your severe
weather plans today. Uncertainty exists in the dominant mode of
storms this evening, but the highest threat or wind/tornadoes
certainly looks to be after dark, which brings challenges for
awareness and should be stressed. Finally, with individual storm
motions forecast to be around 65 mph, and the timing of after
dark, presents challenges/dangers to storm spotters, and
vigilance and safety is stressed.



345 AM CST

Wednesday through Monday...

With the surface low deepening even more rapidly on Wednesday to
our east, cold advection is expected with temperatures falling
through the day Wednesday. Forecast profiles continue to look
favorable across northern parts of the CWA, especially north of
I-88, for a change over to snow with any precipitation Wednesday
morning. Given the speedy movement of the surface low, there
remains high uncertainty on how far south the deformation area
will reach and for just how long. Do have snow accumulation of
around an inch near the WI state line, but it cuts off fast south.

Beyond, the pattern is unsettled under northwest flow Thursday and
Friday. A better indication in global guidance that a clipper-like
system could bring convective snow or rain/graupel showers on
Thursday afternoon. That will need to be looked at further in

It continues to look mild later this weekend into Monday with a
strong system moving over the region early next week. Have added
the possibility of thunder late Sunday night into early Monday
given the forecast synoptics and instability.



For the 12Z TAFs...

Main forecast concerns across the terminals are with periodic
chances for thunderstorms this morning through tonight some of
which could become severe later today, low ceilings, and then
southerly winds becoming northwest late in the period.

Latest radar imagery depicting a continued diminishing trend with
the ongoing showers across northern Illinois. This lull in
precipitation will likely persist for several hours this morning
but then should see scattered showers develop and move back over
the terminals by mid to late morning. This additional precip is
currently focused over Iowa, but will then spread east into
northern Illinois. Have only showers mentioned this morning as
instability will still be on the weaker side, but can`t rule out
an isolated thunderstorm during this time. After another lull,
should see more numerous shower and thunderstorm development later
this afternoon. Do have higher confidence with thunder with this
precip. After a two to three precip window, confidence does lower
with additional shower/thunderstorm trends later tonight.
Although confidence is lower with trends and timing, do have high
confidence for additional shower/thunderstorm development
overnight tonight. Thunder should exit late in the night, but with
showers likely lingering into early Wednesday morning.

Ceilings have not been as low as guidance continues to indicate,
and have continued MVFR ceilings for much of today. Will continue
to monitor for the possibility of IFR ceilings later this morning,
especially as a warm front lifts north towards the terminals. IFR
ceilings becoming more probable with precip moving overhead later
today and tonight, and especially as low pressure moves overhead.



423 AM CST

With low pressure developing over the central Plains and then
tracking northeast towards the southern end of the lake, will see
higher winds in place across much of the lake today. These higher
winds will be observed beginning early this morning. With the
stronger winds today, hazardous conditions for small craft are
likely and will continue into tonight. Have capped speeds in the
20 to 25 KT range today, but it is possible for higher speeds to
occur in the nearshore. These higher speeds will definitely be
possible later tonight with winds to 30 KT likely. Will continue
to monitor later this evening as it will be even possible for
higher speeds than 30 KT to occur. However, confidence is low with
this possibility at this time. Do expect warmer and more moist
air to move at least over the nearshore and southern half of the
lake later today into this evening. This will provide an increased
chance for fog, possibly dense at times. As the low moves across
the lake later tonight, will see a brief lull in the speeds but
with northerly winds to then increase on the backside of this
system late tonight into Wednesday. This will likely bring another
period of hazardous conditions for small craft Wednesday. Some
uncertainty to the strength of the winds over the open waters on
Wednesday, but do think gales are possible and so have issued a
gale watch for the open waters Wednesday into Wednesday evening.
Although the strongest winds look to be over the north half at
this time, gales remain a possibility over the south half.



LM...Small Craft Advisory...NEARSHORE WATERS
     UNTIL 4 AM Wednesday.

     Gale Watch...LMZ777-LMZ779...9 AM Wednesday TO 9 PM Wednesday.




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