Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 250549

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1249 AM CDT Sat Mar 25 2017


739 pm...Several forecast concerns this evening with the first
being convection that developed a few hours ago across the
southwest cwa and continues to lift northeast. Short term/hi res
guidance has been handling this fairly well and continues this
activity through late evening as it lifts northeast. Possible that
additional development will occur to the south of the current
activity...affecting mainly the southeast half/third of the cwa
and trends will need to be monitored over the next few hours. Then
overnight...much of the model guidance is in agreement with
additional showers and thunderstorms developing through the center
of the cwa and while pops are in good shape during this time
period...may need to bump up thunder mention.

Cold front has pushed through about half of the cwa and guidance
has this moving into the southern cwa tonight before slowing and
eventually stalling. In addition...wind speeds/gusts behind this
front are higher than any guidance and have adjusted
accordingly. While there should be some diminishing in the winds
overnight...confidence is low as to how fast this occurs.

Temperatures have dropped into the lower/mid 40s behind the front
across much of the Chicago area...slightly warmer across the
northwest cwa. Have lowered temp trends this evening but so far
overnight low temps still look on track across the north...may
need some adjustments through the center of the cwa based on where
the front ends up.

Patchy fog has developed across southeast WI/far northeast IL this
evening and some patchy fog also seen on webcams over the
nearshore waters. Its likely that this fog will continue to
thicken tonight with dense fog possible. How fast this occurs is
uncertain so trends will need to be monitored...but patchy dense
fog along the shore is likely and may linger into Saturday
morning. cms


330 PM CDT

Through Saturday...

Near-record to record warmth is present across most of the area
this mid-afternoon on gusty southwest winds. The pneumonia cold
front surging southward is evident on the TMKE radar, just into
northern Lake County, Illinois. Given the strength of
baroclinicity with this boundary, and the weakness in the pressure
field ahead of it, this should continue to surge. Kenosha Airport
dropped 20F in one hour and that was likely in less than 30
minutes. Similar if not larger drops are expected in eastern Lake
and northeast Cook County prior to 5 pm, and in downtown Chicago
near 6 pm based on current trends. Temperatures should drop into
the 40s within several miles of the shore.

Dew points ahead of the front are not particularly high,
characterized by mid 40s in northern Illinois. So not expecting
fog to rapidly form, but do expect a gradual lowering in
visibility throughout the night with some dense fog possible late
into Saturday morning, especially near the lake front.

The closed well-developed cyclone responsible for today`s warm
sector is situated across western OK this afternoon and will
gradually move east-northeast tonight. While some isolated
showers/storms are possible early this evening, an uptick in
coverage will arrive under stronger upper diffluence and within
increasing precipitable waters. The better chances look to be
across the northern and western forecast area, with the east and
south possibly remaining dry for a large part of tonight.
Elevated lapse rates are modest, and would expect there to be
isolated storms late this evening into overnight across the area,
but not seeing any signs for widespread or severe storms given the
slow movement of the low center.

Saturday continues to look like periods of showers with some
light rain/drizzle in-between, in an overall murky day. An
occasional rumble of thunder is possible during the day. The cold
front is expected to stall near I-80 later tonight and hold
steady through Saturday morning before probably trying to inch
north in the afternoon. The temperature discontinuity of the
boundary will be tempered some by showers/drizzle on Saturday,
however the stout east winds off the lake should keep far
northeast Illinois in the lower-mid 40s, while along/south of I-80
reaches the lower to mid 60s.



351 PM CDT

Saturday night through Friday...

For the longer term forecast period, main concerns will be
associated with pcpn associated with a series of low pressure
systems lifting from the south high plains, through the Middle
Mississippi Valley.

Latest guidance is still trending toward the slower model
solutions, as is fairly typical with upper lows lifting out of
the swrn CONUS. The lead system should reach the Middle
Mississippi Valley by Saturday evening and then slow
significantly, meandering over the upper midwest as strong high
pressure builds over ern Canada. This low should eventually weaken
and move across central Lake Michigan by Sunday evening as a
second low develops over the south high plains. With a slower
progression to the system, the greater chances for any embedded
thunder will likely be confined to locations south of the I-80
corridor for Saturday night into Sunday, where forcing and
instability will be stronger. Through the day on Sunday, the back
door cold front that pushed south through the area Friday night
will lift back north as a warm front, allowing for a return flow
of warmer air, with temperatures expected to rebound back into the
lower 60s.

By late Sunday afternoon or sunday evening, there should be a lull
in the pcpn potential as weak shortwave ridging aloft builds
across the area ahead of the next system lifting out of the south
high plains. The general model consensus for this system is that
it should be a bit more progressive than the first system and also
take a more southerly track. However, the longer range guidance
develops a greater spread in solutions for the track and timing of
the system, introducing greater than normal uncertainty to the
weather impacts associated with this system. While the general
trend is expected to be for chances of rain for Monday through
Monday night, confidence in start and end times of the pcpn is
low. Confidence is a little higher in the temperature trends for
early next week and the exact track and timing to the system will
have a lesser impact on temperatures. The general trend should be
for above normal temperatures across much of the area as there
should be a period of swly flow in advance of the system. However,
as it tracks south of the local area, winds should turn to more
northerly which would bring cooler conditions to the lakefront and
a short distance inland. So, by as early as Tuesday, lakefront
locations could see temperatures only in the 40s, with temps in
the the middle to upper 50s inland. Towards the end of the period,
the longer range guidance is suggesting another upper low lifting
out of the swrn CONUS, which could bring a repeat performance
with chances for pcpn again late next week.


For the 06Z TAFs...

Main concerns are northeast winds/gusts today, IFR/LIFR ceilings,
periodic showers/drizzle which will likely provide reduced vis.

Ceilings remain VFR for DPA/ORD/MDW at this time, while RFD and
GYY observing low end MVFR and IFR ceilings. Do think the sites
seeing VFR ceilings will soon see this change, with IFR ceilings
likely moving in place over the next 1-2 hours. These lower
ceilings and even LIFR ceilings will then persist for much of the
day. There may be some slight improvement later this afternoon but
if this were to occur, it will be short lived with ceilings then
expected to fall once again. Light showers primarily affecting
RFD at this time but expect precip shield to the south to shift
north across the terminals later this morning and then likely
remain in place for much of the day. Could be a lull in the
activity by mid day, but precip should then fill back in by
early/mid afternoon with even a low chance for thunder.



351 PM CDT

A cold front is pushing down the lake this afternoon, and should
reach the southern tip of the lake early this evening and then go
stationary just south of the lake tonight as low pressure slowly
moves across the southern plains to the middle Mississippi Valley.
Strong southwest winds will quickly shift to nely-nly following
the fropa, with speeds of 25-30 kt. There is a chance for a few
gale force gusts, especially Saturday night when the gradient over
the lake will be strongest, but confidence is too low to include
mention of gales in the forecast at this time. The Small Craft
Advisory currently in place should be able to be dropped for the
Indiana nearshore waters on time this evening, but have extended
it for the IL nearshore waters into Sunday afternoon as an
extended fetch of brisk east winds should keep waves in the 3 to 6
ft range into Sunday afternoon. The low is expected to slow down
and weaken Sunday night as it eventually moves across the lake.
There should then be a break in the stronger winds for Sunday
night into early next weak as the low slowly lifts through the
Great Lakes region and weak high pressure builds over the upper
Mississippi Valley. Winds should remain comparatively light
through early next week and another weak low tracks across the
Ohio Valley and high pressure gradually builds across the western
Great Lakes region.


LM...Small Craft Advisory...IL nearshore waters until 5 PM Sunday.




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