Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 121127

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
627 AM CDT Thu Oct 12 2017

234 AM CDT

Through Friday...

Primary short term forecast challenges include how long the
drizzle persists and when the stratus deck will clear out.

Large bank of stratus blankets the region with the nearest
clearing hundreds of miles away and not heading in this direction.
Recent ACARs soundings suggest the stratus deck is between 2-KFT
thick, so given the lowering sun angle, lack of dry air advection
and presence of frontal inversion trapping the low level moisture,
it is unlikely the cloud cover will clear out today. Tough call
on how long the drizzle will persist. Certainly no indications
that it will end in the next few hours, so plan to hold onto
drizzle through at least mid morning, but given how prevalent
drizzle was Wednesday afternoon, not confident that it will end

Low level flow will veer southerly tonight in response to low
pressure tracking east across central Canada. Opted to go with a
clearing trend from the south later tonight with the assumption
that the southern flanks of the stratus deck will erode diurnally
and then advection with the southerly flow will drag the clearing
line northward. Confidence isn`t high and should stratus linger
into Friday, it would result in lower high temps than current

- Izzi


234 AM CDT

Friday night through Wednesday...

A cold front trailing from the low pressure tracking across
central Canada will lose momentum and become more east-west
oriented, aligned with the upper flow Friday afternoon into Friday
night, likely stalling out over northern IL Friday night. Low
level jet will ramp up Friday night in advance of a lead impulse
ejecting out of the western trough with strengthening the low
level frontogenesis Friday night. Isentropic ascent will increase
as a 30-40kt low level jet develops likely resulting in blossoming
area of showers and thunderstorms, primarily near/north of the
strengthening warm front. While mid level lapse rates are not
forecast to be particularly steep, models are forecasting 500+
j/Kg of MUCAPE in the warm sector (source region for the LLJ).
Given the modest instability and tendency for nocturnal WAA
regimes to be more convectively active this time of year, have
increased the thunder coverage in the grids Friday night,
especially northern CWA.

Confidence is low in where the front will end up during the day
Saturday, with Friday night convection potentially giving the
front a nudge southward. While the location of the front Saturday
isn`t clear, the trend should be for the front to lift northward
as low pressure over the plains ride northeast along the boundary.
Some shower and thunderstorm activity is possible, mainly near and
north of the front Saturday, with there likely being a tendency
for a decreasing coverage in precip during the afternoon
(especially southern CWA).

Saturday night the wave of low pressure riding along the front
will move northeast into Quebec with the trailing front moving
across the CWA Saturday night. Despite strong shear, weak
instability along the front suggests the coverage of t-storms
along the cold front may not be all that great Sat night, but
still a fairly widespread and soaking rain is possible regardless
of coverage of storms.

Brief, transient shot of cooler (near average) air is expected
Sunday/Monday in the wake of the front, before WAA ramps up again
and upper ridge over the eastern CONUS strengthens and builds
north again. Medium range guidance continues to suggest we could
be heading into another period of unseasonably warm temperatures
starting the middle of next week with drier than average
conditions returning.

- Izzi


For the 12Z TAFs...

627 am...Main forecast concern remains cigs through tonight.

Generally low mvfr cigs over the Chicago area terminals with ifr
and lifr cigs further west/southwest. Cigs will likely trend just
a bit lower over the next few hours with ifr possible into the
Chicago terminals but confidence on specific heights is low. Ifr
is more likely across northwest IL and rfd. However...if drizzle
does materialize...the ifr cigs may become more prevailing. With
time...cigs will slowly trend a bit higher though still low mvfr
and with stratus widespread across the region...its likely that
mvfr cigs will persist well into this evening and maybe longer.

Easterly winds under 10kts will become southeasterly today...
southerly this evening and southwesterly Friday morning. cms



208 am...A ridge of high pressure extends across the upper lakes
into MO/IL early this morning. This ridge will slowly move east
today and tonight with winds turning southerly over Lake Michigan.
Low pressure will move across Hudson Bay Friday with a trailing
cold front moving south across the lake Friday night. There
remains uncertainty as to how far south this front will move
before becoming stationary as low pressure develops over the
plains. The front will likely make it to southern portions of the
lake and then the low pressure will move along this boundary
Saturday night into Sunday morning. The gradient is expected to
tighten behind the low and a period of gales is still possible but
confidence remains low. cms


LM...Small Craft Advisory...IL nearshore waters until 7 AM Thursday.




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