Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 290237

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
937 PM CDT THU JUL 28 2016

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



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

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.




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 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/

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


For the 00Z TAFs...

Aviation weather concerns are primarily with the potential for
development of MVFR ceilings to develop tonight, along with
patchy IFR fog. TSRA threat appears to be fairly low for the
remainder of the evening.

A weak trough of surface low pressure stretches from near KDBQ to
about KDNV early this evening. Earlier convection across northeast
IL has stabilized the air mass there, with isolated-scattered
convection generally limited to outflow boundaries and
aforementioned surface trough well west/south of the terminals. A
slow-moving mid-level disturbance will move from the upper
Mississippi Valley this evening, southeast across the forecast
area on Friday, and will likely result in scattered SHRA/TSRA.
While isolated showers will be possible about any time through the
period, the greatest potential should be early this evening, and
again from late morning into the afternoon hours on Friday,
through northeast low level winds off of Lake Michigan will likely
have a stabilizing influence across the Chicago terminals by mid-
late Friday afternoon.

Low level winds somewhat convectively disturbed early this
evening, but should settle in from the east-northeast at less than
10 kt. Combination of moist low-levels after afternoon rains and
advection of low clouds from Wisconsin overnight is expected to
result in a period of MVFR cigs later tonight into Friday morning.
In addition, some fog is likely overnight, with the potential for
IFR visibilities away from center of Chicago.




353 pm...Broad weak low pressure over the southern Great Lakes
region will remain nearly stationary into Friday and then slowly
begin to move northeast across the eastern lakes Saturday into
Saturday night. A cold front extending across the south/central
portions of the lake will slowly sag south of the lake by Friday
evening with increasing northeasterly winds. These stronger winds
may turn more easterly at times but should remain in the 15-25 kt
range over much of the southern portion of the lake through
Saturday evening. High pressure currently over the northern lakes
will then build south across the western lakes Sunday into Sunday
night and then shift east on Monday. A large trough of low
pressure will develop over the plains and slowly move east early
to mid next week with southerly flow developing over the western
lakes. cms


IL...Beach Hazards Statement...ILZ006-ILZ014 UNTIL 1 AM Saturday.

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




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