Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 251930

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
230 PM 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 18Z TAFs...

The challenges with the TAFs are present MVFR cloud cover as of
18Z that should shortly lift, a probable wind shift with a lake
breeze this afternoon, and finally a small chance for showers
this afternoon.

A surface cold front has slowly moved just south of the Chicago
area TAF sites with a light northwest wind in its wake. The light
wind flow has allowed the low clouds to persist, although these
should lift and scatter shortly. The light wind flow also has
allowed for a lake breeze to develop. This has been hugging the
Illinois shore between 11 am and 1 pm, but should make some
progress inland. Confidence is highest in this reaching MDW and a
little less for ORD. The later the wind shift arrives at
ORD...assuming it does arrive...the higher confidence that speeds
will be under 9 kt behind it.

The area of showers that has been moving northeast along the
IA/MO border has been gradually weakening. While we do expect an
uptick in scattered to numerous storms across central Illinois
this afternoon ahead of this feature...possibly as far north as
just outside the southern TRACON area...the chance for thunder at
the TAF sites is very small. There could be isolated showers
around during the mid to late afternoon but expect these tops to
be shallow if they were to develop.

Light or even variable winds will be seen early this evening
before a light northwest wind. Any IFR fog is expected to be
south of the TAF sites. Winds should flop northeast during Friday



227 PM CDT

A cold front associated with low pressure over Ontario and Quebec
has pushed through the lake this afternoon. Winds had generally
become a west northwest direction behind this boundary, but a
weaker surface pattern overhead has allowed for the winds to
become more variable across the lake this afternoon. The trough of
low pressure to the north still moving through the region will
allow the winds over the northern half of the lake to be slightly
more organized and stronger tonight, until a return to a northwest
wind occurs late tonight into Friday for all of the lake. This
will also be due to high pressure moving through the western lakes
Friday, and will help for diminishing winds into the afternoon.
Lighter winds will be in place for most of Friday night until
winds increase out of the south southeast Saturday into Saturday
night. However, at this time, speeds should remain in the 10 to 20
KT range.






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