Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 221915
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
215 PM CDT THU SEP 22 2016

.SHORT TERM...
215 PM CDT

Through Friday...

The area of showers across northwest Iowa will continue to drift
east along a lingering stationary front. The airmass ahead of
this area is not super conductive for this activity to remain
quite as intact as it currently is, and there is very little
lightning activity with it as well. Think that some showers or
sprinkles will enter Northern Illinois as the afternoon continues.
Coverage should diminish with eastward extent.

The front will remain across the same area tonight. Several
shortwaves in wnw flow coupled with a strengthening low level jet
will trigger additional showers and thunderstorms tonight across
Central Iowa and into southwest Wisconsin, and some of this looks
to extend into our area overnight into Friday. Moisture and
instability across most of the area remains benign, but this area
could form into a mesoscale convective system overnight. It could
certainly turn SE through our area tomorrow, but the better
instability to our southwest tends to suggest thunder would remain
both north closer to the elevated warm front, and also west closer
to the LLJ, which could then support the case for highest pops
along and north of i-88 and more western areas. This may be
another situation of monitor observations as the convection
develops as model handling of these near term details sometimes
remains poor until storms form.

In addition...a secondary front across WI will move south tonight
as high pressure builds to the north. Low level winds will turn
northeast with this front. Stratus and fog will accompany the front
as it moves through northern Illinois tomorrow morning. There will
likely be a day with a decent temperature gradient across the area
with far northeast IL in the lower 70s to solidly into the mid 80s
into central IL. With a lingering frontal boundary across the area
and weak waves traversing the flow, there could be additional
showers, but instability should be weak with some cloud cover and
with an upper ridge building in, therefore will keep some pops
north of I-80.

KMD

&&

.LONG TERM...
342 AM CDT

Friday night through Wednesday...

Boundary looks to stall NW to SE across the region Saturday before
slowly washing out Sunday. Over the weekend, mid-upper level
heights will be rising as upper ridge amplifies over the region
and likely limits precip chances despite the frontal boundary in
the area. Upper ridge would also allow for more sunshine and
warmer temps on both sides of the front over the weekend compared
with today and Friday.

Strong shortwave/closed mid level low will eject northeast out of
the mean western trough over the weekend, but it appears that
the strong upper ridge over our area should cause this s/w to be
largely deflected northward. Guidance continue to paint some QPF
over the area Sunday/Sunday night and while that cannot be ruled
out, really thinking that the rain chances don`t look all that
great with upper ridge and drier air looking likely to pose a
substantial obstacle to overcome.

Medium range models typically struggle big time in patterns with
significant blocking and this is definitely the case heading into
next week. Tremendous differences exist in the medium range
models early next week with ensembles also not painting a clear
picture either. The range in possibilities is very large with a
huge spread in solutions. Opted to leave blended model cocktail as
is in the grids due to low confidence, but could be quite a bit
warmer or colder and largely precip-free or a rainy period.

Izzi

&&

.AVIATION...
For the 18Z TAFs...

Forecast concerns center around thunderstorm chances and a wind shift
to NE tonight into Friday. Additional concerns exist regarding
some low stratus or fog across northern/northeast Illinois
overnight as well.

The area of showers across northwest Iowa will continue to drift west
along a lingering stationary front. The airmass ahead of this
area is not super conductive for this activity to remain quite as
intact as it currently is, and there is very little lightning
activity with it as well. Think that some showers or sprinkles
will enter Northern Illinois as the afternoon continues. Coverage
should diminish with eastward extent, and therefore have only
mentioned some vicinity showers at KRFD at this point.

The front will remain across the same area tonight. Several
shortwaves in wnw flow coupled with a strengthening low level jet
will trigger additional showers and thunderstorms tonight across
Central Iowa and into southwest Wisconsin. Moisture and
instability across most of the area remains benign, but this area
could form into a mesoscale convective system overnight. It could
certainly turn SE through our area tomorrow, but the better
instability to our southwest tends to suggest thunder would remain
both north closer to the elevated warm front, and also west closer
to the LLJ. Therefore have again mentioned vicinity thunder at
KRFD. This may be another sedition of monitor observations as the
convection and any potential cold pool develops.

In addition...a secondary front across WI will move south tonight
as high pressure builds to the north. Low level winds will turn
northeast with this front. If there is convection to the north it
could drive the front through with stronger wind speeds than
currently in the TAF. There is also some suggestion that there
could be some stratus and fog with and behind the front, with
northern IL favored (UGN, DPA, RFD) but these details may need
refinement as well.

KMD

&&

.MARINE...
342 AM CDT

A larger area of high pressure is building across srn Canada while
a warm front and associated trough extends from low pressure over
the western plains across the northern portion of Lake Michigan.
Showers and thunderstorms north of the warm front will track
across the central portions of the lake, disrupting the wind
field again today. Expect that there will be periodic chances for
showers and thunderstorms across the lake as long as the front
remains over the lake and the larger scale wind pattern will, at
times, been representative of local conditions. However, the
large high will begin to build to the south and east, across the
ern Great Lakes, forcing the front to the south of the lakes by
Friday night or Saturday, setting up generally easterly flow over
the lake. Through the weekend, the high will continue to sag to
the southeast while low pressure developing over ern Colorado on
Friday will track north to southern Manitoba by Sunday morning,
turning flow over the lake to sely, while dragging a cold front
across the plains. The low is expected to remain over the nrn
plains area into early next week while the cold front pushes
across the lake.

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...NONE.
IN...NONE.
LM...NONE.
&&

$$

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