Area Forecast Discussion
Issued by NWS Chicago, IL
FXUS63 KLOT 190740
Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
240 AM CDT Wed Oct 19 2016
140 PM CDT
A cold front continues to push across the region this afternoon,
and stretches from roughly Gary through Pontiac at 18Z. Out
ahead of the front, temperatures in the upper 70s with dewpoints
in the mid 60s will contribute to close to 1000 J/kg MLCAPE by mid
afternoon, so cannot rule out a shower or isolated thunderstorm
developing but better chances will be off to our east where there
is very modest upper support associated a sheared shortwave
pushing across the Great Lakes. Behind the front, winds are
turning northwest and easing while skies are clearing out. Latest
runs of the RAP and HRRR continue to suggest a lake breeze will
form and push inland across NW Indiana and a little into Cook
County mid to late this afternoon which will hasten temperatures
dropping back through the 60s for those areas. Otherwise, expect
quiet weather this evening and overnight as high pressure builds
towards the mid/upper Mississippi Valley.
High pressure will continue to dominate most of the day Wednesday
providing light northeast flow and primarily dry conditions.
Despite the cold frontal passage, we will continue to experience
above normal temperatures Wednesday with mid to upper 60s north of
I-80 and low 70s along and south of I-80. An upper level
disturbance currently digging across the Inter-mountain West is
progged to lift across the central plains Wednesday. Weak mid
level height falls will begin to overspread the local area by mid
to late in the afternoon with increasing mid and high clouds.
Models do generate some QPF up through roughly the I-80 corridor
by 00Z tomorrow evening, though soundings show dry air persisting
below 700mb. Could be some high-based light showers or sprinkles
moving up from the south into the local CWA, but better chances
for showers will be closer to the baroclinic zone stretching from
downstate Illinois into southern/central Indiana.
210 PM CDT
Wednesday night through Monday...
A transition back to more seasonal weather is on tap for the extended
NAM/GFS both depict fairly strong frontogenesis with the cold
front Wednesday night in the clash between our anomalously warm
airmass of late and the colder continental air arriving from the northwest.
This will be coupled with strengthening low pressure from the
left rear quadrant of an upper level jet stream and should result
in an increasing precipitation shield across the region Wednesday
night. At this point the better co-location of forcing and
moisture will remain draped south and east of our area, but there
should be enough for some bands of lighter to even moderate rain
across the I-55 corridor southeastward and possibly even extending
farther into northeast IL. This forcing continues through the day
Thursday with the jet in the region, an approaching pacific
shortwave trough axis and a strengthening surface low continuing
the deformation precipitation across much of Indiana with some
lower chances across Northwest Indiana. Expect more clouds than
While colder air slowly bleeds in during the day Thursday, a secondary
surge of colder air will be shoved southeastward by a portion of
the Canadian upper trough will reinforce the changing airmass.
Models depict this will be a largely dry frontal passage, but do
portray some lake effect rain showers across northwest Indiana.
The convergence signal is somewhat weak and progressive and
moisture is but the thermodynamic signal is robust to suggest
there will be some lake effect showers with fairly high
confidence, but lowered confidence on how widespread and how long
lasting. Areas away from the lake will have sunshine but cool
conditions and area wide highs in the 50s, some low 50s north.
High pressure returns Friday night and Saturday which look dry,
in spite of the EC painting some light precip along an elevated
warm frontal boundary as the high approaches. An upper wave will
pass by to our north on Sunday which will initially lead to
slightly warmer conditions, though there are better indications of
a backdoor cold front sometime later Sunday into Monday. The front
holds off till after sunset on the GFS, midday/afternoon on the
EC/GFS. This will setup a frontal zone across the local area which
may serve as a focus for increased precipitation chances Tuesday
or into the middle of next week.
For the 06Z TAFs...
There are no significant aviation weather concerns for the next
24 to 30 hours at the terminals, with conditions likely to remain
VFR through the period. The primary forecast focus/concern is northeast
winds and associated speeds at ORD/MDW this afternoon. High
pressure over the area will result in light/calm winds into this
morning. Winds on the larger scale will then become light north
and northeast, which should support modest lake enhancement at
ORD/MDW/GYY, though speeds are probable to remain primarily below
9-10 kt with medium confidence in this. There is a chance for
sprinkles or spotty light rain this evening and overnight, but the
lower levels of the atmosphere will be quite dry. Thus, have left
mention out of the TAFs and would anticipate little to no impacts
from the rain if it does reach the surface.
240 AM CDT
The primary concerns are a prolonged period of brisk northerly
winds tonight through Friday night and the potential for
waterspouts Thursday night and Friday. A cold front will move
down the lake tonight, with winds turning northerly and increasing
to up to 25 kt. Speeds/gusts will further increase to 30 kt on
Thursday on the south half of the lake and continue into Thursday
night. Winds will stay elevated with speeds/gusts up to 25 kt
through Friday night. Much colder air will move over the lake
Thursday night and Friday, creating a favorable setup for lake
effect showers and even isolated thunderstorms. With this,
waterspouts are also possible. Have added a slight chance of
waterspouts to the forecast, and the parameters appear favorable
for at least isolated waterspout potential.
Will need a Small Craft Advisory for the near shore waters
Thursday morning through Friday night for the strong northerly
winds and building waves. It`s possible the advisory will be
needed longest for the Indiana waters, as direction will turn more
northwesterly on Friday.
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