Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 200602

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
102 AM CDT Thu Oct 20 2016

227 PM CDT

Tonight and Thursday....

Through late afternoon...Surface low pressure is moving east
northeast from northeast OK/southwest MO this afternoon while a
cold front drops southeast toward the local area. Mid level
frontogenetical forcing has been steadily increasing northeastward
into central and now northern Illinois bringing the thickening mid
cloud canopy that is now overhead. Deeper moisture is available
across south central and east central IL where numerous showers
and thunderstorms have been occurring. Far southern sections of
the local forecast area from far southern Livingston Co across
Ford and Iroquois counties are seeing radar echoes move in but dry
air near the surface may preclude more than sprinkles occurring
over the next hour or two before wetting rain becomes more likely.
Newton, Jasper and Benton Co. will see this increase in echoes and
possible light precip as well. Temps have leveled off and will
probably cool somewhat prior to 4 pm thanks to the increased cloud

This evening and tonight...The northwest extent of precip remains
the main challenge through the evening. The aforementioned forcing
will be strong overhead of most of the area through late evening
but lower level/sub cloud moisture decreases as you go north. Have
been concerned about needing higher pops further northwest towards
areas from a Peru to Chicago line and even slightly north of that
and have gone ahead and increased pops up into this corridor.
Radar mosaic has shown an increase in returns in a narrow band
from near Kirksville, MO to Quincy, IL and points northeast,
typical of this type of forcing. A few elevated returns have even
appeared over areas near Joliet. Thus supports that the forcing is
capable of producing precip but the dry air may continue to limit
how much reaches the ground. Do expect radar returns to continue
to spread northeast and expand across central portions of the
forecast area including much of the Chicago metro into early
evening. Otherwise, areas further southeast will see the most
persistent, though still intermittent, precipitation and maybe a
few isolated storms through the overnight. The cold front will
cross the area bringing a steadier northerly wind, especially

Thursday...Rain chances will linger...mainly across northwest
Indiana and east of I-57 in Illinois with a decrease from
northwest to southeast through the afternoon. Breezy north winds
will persist through the day with speeds easing during the
afternoon. Cloud cover will be abundant through the morning but
will likely become a mix of clouds and sun through the afternoon.
Much cooler air will be spreading in as high pressure arrives from
the northwest. Expect highs in the mid to upper 50s with a few
southern areas possibly touching 60.




Thursday night through Wednesday...

227 pm...Forecast concerns include potential for frost Thursday
night/Friday morning and again Friday night/Saturday
well as lake effect rain showers Friday morning.

A large area of high pressure extending from Ontario to the
southern plains will be moving into the area Thursday night into
Friday. As colder air spreads in with this high...across the
warmer waters of Lake Michigan...some lake effect rain showers
are expected to develop Thursday night and persist into Friday.
Wind direction will be important and initially will be more
northerly with convergence expected but winds will likely turn
more to the north/northwest or northwest during the day Friday.
Porter County appears to be the most likely location for these
showers with a slow drift east out of the area by late Friday
afternoon or Friday evening.

Away from the lake...diminishing northwest winds and clearing
skies should allow for temps to drop into the 30s...with some of
the usual cold spots possibly dropping into the lower 30s. Frost
appears likely across much of the western half of the cwa and if
trends continue a frost advisory will probably be needed with
later forecasts. Frost will again be possible Friday night into
Saturday morning as the ridge slides southeast of the area.
Despite low temps likely in the mid/upper 30s...confidence is a
bit lower as there could be some cloud cover with weak warm air
advection beginning overnight. Favored areas would be the central
and eastern areas of the cwa.

An area of low pressure will move across the Great Lakes region
Sunday or Sunday night pushing a cold front south across the area
and there remains some timing differences. A slower arrival would
mean southwest winds and temps into the 60s on Sunday but changes
are possible if the front speeds up. Differences remain through
next week with GFS maintaining high pressure across the southeast
U.S. which stalls a frontal boundary near or just north of the
area. Ecmwf is faster and deeper with a surface low moving across
the region midweek with a stronger cold front passing across the
area. Too early for specifics but given these trends...possible
that at least one day...perhaps a few days...could be rather warm
with southwesterly flow. cms


For the 06Z TAFs...

Concerns today include low CIGs, the duration of these CIGs and
gusty northerly winds. A cold front will move south early this
morning, reaching the terminals shortly after daybreak.
Accompanying the front will be stronger northerly winds and low
CIGs. There may be a small lag until the onset of the strongest
winds after frontal passage, with speeds of 10-15 kt and gusts of
15-20 kt. GYY will likely increase to 15-20 kt sustained with
gusts around 25 kt. The direction in the morning should vary
between 350-010, and then could shift to more NNW to N in the
afternoon at ORD/MDW/DPA/RFD (330-360). Winds will then diminish
tonight, except at GYY.

Satellite and METAR obs show the lower clouds behind the front,
so have high confidence in them occurring. Main uncertainty is
whether there will be a brief period of IFR. In this TAF, added a
scattered IFR mention and will monitor trends. Confidence in IFR
occurring is low, with high confidence in MVFR. The rain should
stay southeast of the terminals today, but closest to GYY.
Parameters will become favorable for lake effect clouds and
showers tonight, though GYY would likely be only terminal to be




307 pm...A cold front will move south across Lake Michigan
tonight as high pressure builds across the upper midwest and low
pressure moves northeast across the Ohio Valley. The gradient will
steadily tighten behind the front with northerly winds increasing
to 30 kts Thursday into Thursday night...especially on the
southern half of the lake. The high will slowly move east Thursday
night into Friday with the gradient also slowly weakening.
Northerly winds will gradually diminish Thursday night into Friday
as the high moves southeast of the region. Colder air spreading
across the lake will create enough instability for the a chance of
waterspouts and thunderstorms...late Thursday night into Friday

There remains some uncertainty regarding the timing of the next
low pressure which will move across the lakes region Sunday or
Sunday night. Eventually winds will shift northerly and may end up
in the 20-25kt range Sunday night into Monday morning...but ahead
of that time period...wind directions are quite uncertain. A weak
ridge will likely move across the lake Saturday or Saturday
night...shifting northerly winds to southerly...but how long those
southerly winds persist...if they develop is uncertain from this
distance. cms


LM...Small Craft Advisory...LMZ740-LMZ741-LMZ742...7 AM Thursday TO
     10 AM Friday.

     Small Craft Advisory...LMZ743-LMZ744-LMZ745...10 AM Thursday TO
     4 PM Friday.




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