Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 290715
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
215 AM CDT FRI JUL 29 2016

.UPDATE...
937 PM CDT

The main weather concerns tonight is for low clouds and/or fog
development overnight, along with the chances for at least some
additional scattered showers and storms overnight.

Water vapor imagery this evening indicates the presence of a mid-
level disturbance over northeastern Iowa, and this feature is
expected to gradually shift over northern Illinois overnight.
Although forcing for ascent with this feature is rather modest,
recent radar trends over the past hour have shown an uptick in
some widely scattered showers over the area within a weak, but
broad area of isentropic upglide. Additionally, recent AMDAR
soundings out of MDW indicate some weak uncapped elevated
instability. With this expected to continue over the area
tonight, it appears that at least some widely scattered showers
and storms could fester across portions of the area tonight,
especially in the vicinity of the inverted surface frontal trough,
currently over my western counties. With this in mind, I will
continue to mention low end chances for storms overnight. This
inverted surface frontal trough, could also act as a focus for
more scattered storms during the day on Friday.

Outside of thunderstorms, some low clouds and/or fog could become
an issue tonight. With low surface dew point depressions and light
winds in the vicinity of the surface trough, fog may become an
issue overnight, especially in areas that received heavy rainfall
earlier.

KJB

&&

.SHORT TERM...
306 PM CDT

THIS AFTERNOON THROUGH TONIGHT...A very complex pattern has set up
across nrn IL/IN this afternoon.  A quasi-stationary front extends
from near Rockford through the Chicago metro area and a cold front
extends from near Freeport through to Macomb.  Another
complicating factor is a strong outflow boundary extending from
Aurora to Joliet to Lansing, moving to the southwest.  While there
is little activity along the actual cold front and the outflow
boundary, as of 245PM CDT, there is a solid line of convection along
a pre-frontal trough extending from ern McHenry County through
southern LaSalle County. The quasi-stationary boundary will continue
to be a focus for thunderstorm development until the cold front can
push through the region. The factor which will control to the end
time of the convection is whether there will be any additional
development along the actual cold front, which currently is not
exhibiting any development. Currently, satellite imagery is
suggesting an area of subsidence behind the leading pre-frontal
trough, so have generally followed the back edge of the convection
associated with the pre-frontal trough as the ending time for
showers and thunderstorms. The latest timing suggests that the
organized lines of TS could be ending along the I-39 corridor btwn
20-21Z and btwn 23-00z along the I-55 corridor.  The lines of TS
would then push east of the nwrn IN counties of the CWA.  However,
some of the short range, high res guidance is suggesting the
potential for TS to fire along the outflow boundary as it pushes
south of the I-80 corridor.  While the thunderstorm activity has
been sub-severe so far, there is the potential for isolated severe
thunderstorms, with strong winds being the main severe threat.  The
storms have also been very efficient rain producers in spite of
pwats less than 1.5 inches.  At this point, any flooding potential
is marginal as thunderstorms are becoming more progressive.  Thus
far, urban and small stream advisories have covered any flooding
potential.  However, flash flooding is certainly possible is there
is any significant echo training or if individual cells become more
stationary.

Another fly in the ointment as far as shra/tsra is concerned is the
upper level shortwave initially over swrn WI.  Latest radar mosaic
shows a broad circulation associated with this shortwave and it
could still provide enough forcing for scattered shra/tsra through
the night.  This would largely impact the northern portions of the
CWA, so have gone with high chance PoPs over the nrn portions of the
CWA and low chance PoPs over the south. The strongest forcing should
be north of the WI border, so feel that chance PoPs/SCT coverage
wording will suffice for now.

Krein

&&

.LONG TERM...

Friday through Thursday...

235 pm...Forecast concerns include thunderstorm chances Friday
and Saturday and again next week along with a return of
heat/humidity next week.

Weak low pressure over the southern Great Lakes region will remain
nearly stationary through Saturday morning and then begin to lift
northeast across the eastern lakes region Saturday night into
Sunday. A frontal boundary will slowly sag into far northern IL on
Friday and perhaps a little further into the cwa Saturday. Expect
similar conditions to today on Friday and Saturday with scattered
thunderstorm development in the late morning...continuing through
the afternoon and dissipating in the evening. Best precip chances
may end up sagging south some on Saturday with the location of the
front. Main threat from these storms will be heavy rain and
localized flooding with expected slow movement. By Sunday...a weak
ridge of high pressure will move across the area with an afternoon
lake breeze. While most of the models suggest Sunday/Sunday night
will be dry...an isolated shower or thunderstorm is possible along
the lake breeze but confidence is too low to add mention just yet.

Slightly cooler air will spread into the region Friday and persist
into Saturday. This combined with expected mostly cloudy skies and
scattered precip will likely hold highs both days in the upper 70s
to around 80...perhaps a bit warmer if sunshine persists for a few
hours. Temperatures rebound back into the lower/mid 80s Sunday/
Monday.

The upper ridge begins to build back into the region Monday night
through Tuesday night next week. This period looks unsettled with
increasing chances for thunderstorms. Still some timing
uncertainty but Monday looks mainly dry right now with the focus
on Monday night or Tuesday. Heavy rain will certainly be possible
wherever thunderstorms develop. Maintained temps generally in the
mid 80s Tuesday...which could be a tad low but quite a bit of
uncertainty from this distance. While the edge of the ridge will
be nearby... it appears Wednesday may end up mainly dry and
potentially hot with highs in the lower 90s. A cold front will
arrive Thursday or Thursday night...bringing another chance of
thunderstorms and cooler weather into next weekend. Timing of this
front could lead to Thursday being dry and possibly hot/humid. cms

&&

.AVIATION...
For the 06Z TAFs...

Multiple concerns with the Chicago area TAFs today:

- LIFR cigs and IFR visibility early this morning including
possibly into daybreak.

- Isolated SHRA prior to 16Z, then scattered TSRA between 16Z and
23Z. Right now better coverage of TSRA is favored slightly west of
ORD and MDW.

- Northeast winds prevailing close to 10 kt at times today,
although any nearby SHRA/TSRA will be capable of briefly turning
the wind direction.

IFR and even some LIFR clouds are expanding southeastward across
northern Illinois as of 06Z. These seem to be expanding with
saturation as opposed to being advected, which points to clouds
likely sticking around through daybreak and have gone that way
within the TAF. However, northeast winds in the 1000-3000 ft
layer advecting in slightly drier air in that layer into far
northeast Illinois, so confidence is somewhat limited on precise
cloud base. Regarding visibility, the earlier a site clouds over
early this morning the less likely the visibility will drop
drastically. So right now this is mainly favored south of I-80,
although MDW and GYY have yet to go broken as of 06Z so they run
the higher risk of reaching IFR visibility, at least temporarily
early this morning. Confidence is low on visibility though, as
rain and wet ground from Thursday afternoon can come into play
too.

Expect that conditions will gradually improve after daybreak and
then more rapidly in the 13Z-15Z window. Confidence is medium to
high on improvement time of IFR though low on MVFR to VFR.
Isolated showers will also be around through mid-morning from a
broad upper low over the region. This low will again be a focus
for afternoon showers and storms. Morning cloud cover may somewhat
limit the intensity/depth of showers and storms, although
numerous coverage is expected across at least north central
Illinois and probably into the west/southwest metro of Chicago.
Some mention of TS or a TEMPO SHRA may need to be included in ORD
and MDW during the first half of the afternoon.

MTF

&&

.MARINE...
214 AM CDT

Mainly northeast winds will continue across the lake through
Saturday evening. The speeds will undulate through that time with
sporadic gusts exceeding 20 kt especially in the open water and
the western nearshores. The highest nearshore waves today can be
expected in the Illinois nearshore where they will be close to
Small Craft Advisory criteria of over a 4 ft average. Given that
waves at the Wilmette buoy are over 3 ft early this morning, may
end up leaning on issuing one but will watch trends for a couple
more hours. The winds turn a little more north-northeast on
Saturday spreading more of the 3 to 5 ft waves into the Indiana
nearshore.

High pressure will move over the lake during the last half of the
weekend into Monday before southerly flow resumes.

MTF

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...Beach Hazards Statement...ILZ006-ILZ014 UNTIL 1 AM Saturday.

IN...Beach Hazards Statement...INZ001 UNTIL 1 AM Saturday.

LM...NONE.
&&

$$

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