Area Forecast Discussion
Issued by NWS Chicago, IL

Home | Current Version | Previous Version | Graphics & Text | Print | Product List | Glossary Off
Versions: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50

FXUS63 KLOT 221537

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1037 AM CDT THU SEP 22 2016

1023 AM CDT

With the lack of instability across Illinois the morning MCS is
collapsing. Convection will likely continue to fire in vicinity of
the lingering stationary front across Iowa today as shortwave
energy continues east across the boundary. We will be slow to
destabilize and if so only weakly, but it is possible some of this
activity sneaks into north central Illinois later today though in
a weakening form.

Additional thunderstorms are expected to develop overnight across
Central Iowa eastward into southwest Wisconsin as the low level
jet ramps up. The majority of guidance keeps the bulk of this
north and west, and this will be the forecast concern for the
afternoon package.



342 AM CDT

Through Friday...

Another low confidence forecast today as convective and mesoscale
features, which tend to not be well resolved by models, likely
have an impact on forecast. Large MCS continues to dump copious
amounts of rain on portions of WI, MN, and northern IA, with
regional radar imagery depicting a well defined MCV west of
Madison, WI early this morning. The convection over the eastern
portions of this MCS and close to the MCV is struggling as
instability is lacking over WI. While convection is likely to
continue weakening the next couple hours, some showers could flirt
with far northern portions of the CWA as strong/well developed
cold pool continues to nudge effective boundary a bit farther

Upstream, much stronger instability is present over IA and feeding
convection into southern MN ahead of another shortwave moving into
western IA this morning. Short range guidance has picked up on
this wave and moves it eastward and into northern IL this
afternoon. Cloud cover is extensive, both high cloudiness blowing
off the MCS and a thick OVC mid level deck over IA, both should
work against temps getting as warm as previously thought today,
which also will keep instability down. Still, worried that some
showers and perhaps a storm could meander into northern IL this
afternoon as depicted by last several runs of the HRRR. Will be
beefing up cloud cover and lowering temps for today.

Outflow enhanced frontal boundary could end up farther south than
earlier thought tonight, and while the strong WAA and support for
nocturnal convection should be to our west, cannot rule out some
showers and perhaps an isolated t-storm over northern IL tonight.
The farther south positioning of the front should also set the
stage for northeast winds off the lake coming out of the gate
Friday morning, which should keep temps near the lake a bit
cooler. Still cannot rule out some showers or maybe a t-storm
Friday near, and particularly to the cool side of the boundary,
which would confine best chances to our northern CWA.



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.



For the 12Z TAFs...

Main concern for this morning will focus on whether any of the
ongoing convective activity over srn WI and nern IA will move
south/east far enough to to impact any of the terminals. Latest
radar trends suggest that the line moving through ern IA has some
potential to impact RFD should it maintain integrity long enough.
However, the direction of motion is southeast, which would keep
it southwest of RFD. Any other pcpn north of the WI border is
moving east, so RFD is the terminal site that has the highest
probability of seeing any pcpn, though the probability is still
low. The greater impact from the convective activity may be more
on winds with outflow boundaries causing temporary increases in
wind speed/gusts as well as direction shifts. Otherwise, today,
expect winds to gradually increase through the morning and trend
toward swly and increase to arnd 10-12kt. Some gustiness is
possible by early afternoon, with the potential for gusts of 15-20
kt if mixing is deep enough.


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
showersand thunderstorms across the lake as long as the front
remains overthe lake and the larger scale wind pattern will, at
times, beunrepresentative of local conditions. However, the large
high willbegin 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.





WWW.YOUTUBE.COM/NWSCHICAGO is the U.S. government's official web portal to all federal, state and local government web resources and services.