Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 251558 AAA

Area Forecast Discussion...UPDATED
National Weather Service Chicago/Romeoville, IL
1058 AM CDT THU AUG 25 2016

1055 AM CDT

Have made some adjustments to specific timing and to magnitude of
rain/storm chances, but overall the forecast theme continues with
potential for afternoon and early evening storms in the southern
CWA. Cannot rule out some stronger to possibly severe storms in
the far south toward Gibson City...Watseka...and Fowler. Many
communities south of I-88 will remain more cloudy today than
initially forecast as well, but overall highs still fall out
pretty close to forecast.

The surface cool front bisects the CWA from near downtown Chicago
to LaSalle-Peru and continues to ooze southeastward with little in
the way of a push behind it. A fairly wide band of low clouds
exists along this and looks like it could hold for several more
hours before high clouds also move over. Along and immediately
ahead of the front an atmosphere with lower 70s dew points is
destabilizing, and more so over central Illinois where sun is
prevalent. The lapse rates sampled this morning on the ILX
sounding were much steeper than the DVN sounding, so a fairly
decent instability gradient is setting up near the southern CWA

A short wave trough and speed max as well as a convectively-
enhanced MCV are propagating east-northeastward in northern
Missouri. The MCS associated with this has shown weakening trends
on satellite and radar and would expect this to continue along its
northern periphery, however a re-enhancement will probably occur
along its south as it works into the more unstable air in central
Illinois during the mid-afternoon. Even if the current
convection/precip shield from this feature does not make it to
the southwest/southern CWA...the feature combined with the
confluence of the boundary probably will trigger at least
scattered convection in the far southern CWA. Any organized
storms will have the possibility of some wind threat in the far
southern CWA from a combination of precipitation loading and DCAPE
around 1,000 J/kg. Propagation speed tied to 35-40 kt mid-level
flow should be enough to limit any broad flash flooding threat,
however the precipitable waters are once again around two inches
so efficient rates on wet ground could cause localized flooding



328 AM CDT

Through tonight...

Today through Tonight:
Early this morning a few ragged lines of thunderstorms began to
strengthen across the southern portions of the forecast area and
Northwest Indiana. The instability axis began to shift further south
towards Central Illinois, which was helping to pull the focus prior
to daybreak to the south for the stronger storms; however, there was
still modest instability further north and expect a few scattered
showers/storms to linger through daybreak across Northwest Illinois
stretching through Southeast Wisconsin. Will also expire the flash
flood watch early, as confidence in pockets of heavy rainfall has
dissipated. This was oriented along the better vorticity channel,
and was expected to sag southeast after daybreak and then become
slightly better organized midday/early afternoon from the Chicago
metro area stretching southwest as the frontal boundary approaches
from the northwest.

Some of the hi-res solutions are leaning towards minimal
convection/rain development early this afternoon from any
frontogenetical forcing in the mid-levels; however, given the
continued elevated Td`s in the upper 60s to lower 70s ahead of the
boundary and some lingering boundaries, it`s difficult to completely
remove POPs for at least areas along and east of a LaSalle County to
Chicago line. With the continued warm/moist axis overhead and the
southeast, temps will likely easily warm back into the mid 80s.
Meanwhile further northwest temps will generally be around 80 with
winds shifting to the northwest as high pressure pushes drier air
into Northern Illinois this afternoon.

Mid-lvl ridge will drift further south tonight towards the Southeast
CONUS, allowing the surface ridge to become centered over the
western Great Lakes and allowing the frontal boundary to transition
towards a quasi-stationary boundary across Kansas to Southern Il
orientation late tonight. Expect clouds to dissipate with temps
falling into the upper 50s to mid 60s. Expect enough spread in the
T/Td`s to prevent much fog from developing tonight, although could
see some patchy fog develop in areas closer to area streams prior to
daybreak Fri.



328 AM CDT

Friday through Wednesday...

Friday through Saturday: Surface ridge will be overhead to start the
day Fri; however, the quasi-stationary boundary will be starting to
drift north across Kansas/Nebraska Fri aftn. Flow aloft appears to
be more fluid, allowing a more progressive pattern to setup. Expect
clouds to be returning late Fri aftn/eve from the southwest, with
some of the operational solutions indicating precip could return to
Northern Illinois as early as Fri eve. Current thinking is that with
the upstream 500mb trough axis remaining over the Northern Plains,
and weak height rises Fri eve then falling overnight into Sat, that
any precip will hold off until the overnight hours. A lobe of
vorticity lifts northeast across Illinois Sat morning, which may
require POPs to trend further up towards likely or possibly
categorical for the first half of Sat. Temps Fri will be more
seasonal, then as clouds thicken late Fri aftn/eve this will hold
warmer conds overnight. Have nudged temps towards the low/mid 60s
Fri ngt, but this may end up needing to be further warmed into the
upper 60s to around 70 given the warm/moist axis returning. Sat high
temps will be tricky due to the abundance of clouds limiting surface
heating, so have held onto temps around 80 to the low 80s.

Sunday through Wednesday: The extended periods continue to be
favored towards a quasi-zonal flow based off of the latest ensembles
forecasts. 500mb flow will initially feature a mid-lvl trough
weakening Sun across the upper midwest, then shifting the jet
further north into Canada. Expect periodic chances for showers and
thunderstorms through the extended periods; however, trying to
highlight a particular period or two remains a challenge given the
lack of a defined shortwave in the extended forecast. Temps will
generally be seasonal in the low 80s to perhaps middle 80s.



For the 12Z TAFs...

A weak cold front will move across the Chicago area terminals
within the next few hours. This will result in winds shifting
northwesterly by late morning. The threat for thunderstorms has
ended, though a few widely scattered showers are still possible
through the morning as the front moves over the area. It also
appears that a short period of low CIGs (down around or just below
1,000 FT AGL) will be possible early this morning, but will
quickly improve later this morning.

Winds this afternoon are expected to be north-northwesterly.
However, lake breeze development is possible along the lakefront
by this afternoon. While the current forecast does not include a
wind shift at KORD or KMDW this afternoon, there is a small chance
that an east-northeasterly wind shift could occur after 21-22 UTC
this afternoon. Confidence is too low at this time to include this
in the forecast.



137 AM CDT

Low pressure over western Quebec is pushing a cold frontal trough
across Lake Michigan early this morning. Expect the lighter west-
northwesterly winds to set up over the lake today in its wake. A
secondary front is expected to push over the lake tonight as high
high pressure builds into the upper Midwest. This looks to tighten
the pressure gradient for a few hours tonight to support a period
of stronger northwest winds (up around 20 KT) into early Friday
morning. The surface high is expected to build over the lake on
Friday. This should result in light winds Friday, with lake
breeze influences likely to produce onshore flow for the
nearshore waters.

The winds should turn southerly and increase again by the weekend
as the surface high shifts east and another area of low pressure
takes shape over the Upper Midwest.






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