Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 250829

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

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 06Z TAFs...

A period or two of thunderstorms are still expected overnight,
though it appears the better chances will be over the southern
terminals and points south.

An area of thunderstorms is currently moving towards the Chicago
area terminals. However, the most intense portion of this storm is
moving east along the I80 corridor. Therefore, it appears the KORD
may only get a brief shower with this activity hear over the next
hour. KMDW and KGYY have a better chance of getting a short period
of heavier rain and thunder with this initial line of storms.

Following this first line of storms, additional showers and
thunderstorms to the southwest over west central Illinois are
expected to track east-northeastward into portions of northern
Illinois. These storms will likely impact the terminals after 8Z
through around or shortly after daybreak today. After this second
wave of activity moves out of the area by 13z it appears that only
some widely scattered showers/storms could develop along the cold
front later this morning into the early afternoon. However,
confidence on these storms impacting the terminals is too low to
include more than a VCSH at this time.



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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