Area Forecast Discussion
Issued by NWS Chicago, IL

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194
FXUS63 KLOT 280202
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
902 PM CDT SAT AUG 27 2016

.UPDATE...
902 PM CDT

For evening update:

Only minor tweaks to going forecast this evening, primarily to add
some detail for an axis of higher pops roughly Quad Cities to the
north suburbs of Chicago through late evening. Weak surface trough
along this axis had been a focus for some weak TCU development
since late afternoon, with isolated-scattered showers and
thunderstorms developing around sunset. This is likely in response
to large scale ascent associated with a mid-level short wave
trough propagating across the upper Midwest per GOES vapor
imagery. Various convective-allowing guidance indicates this
activity should weaken over the next couple of hours, though some
additional showers or isolated thunderstorms may fester across the
area into the overnight hours in this region of weak but persistent
warm/moist advection. Would expect coverage to be fairly low,
with slight chance pops sufficient overnight.

Otherwise, mild and muggy overnight with some patchy fog likely
with weak surface gradient. Some dense fog possible, though as
previous forecast indicated, this is mainly expected in typical
low spots and more fog-prone locations.

Ratzer

&&

.SHORT TERM...
237 PM CDT

Through Tonight...

We continue to maintain the chance for isolated storms later this
afternoon and evening though confidence in that occurring remains
low. With the very humid conditions coupled with light winds
later tonight, had added the potential for some overnight fog as
well.

The surface warm front continues to ease northward and as of 230
pm is located near the Illinois/Wisconsin border. Clouds remain
sluggish to scatter south of this, though southerly winds have
still ushered in mid 80s temperatures and mid 70s dew points up
to I-80. Streamline analysis by high-res models indicates modest
flow and moisture convergence along the boundary though any
cumulus in that area has looked mushy. The weak short wave
disturbance earlier over Iowa is now more sheared from southwest
Wisconsin southward over the Mississippi River Valley and still
may instigate isolated showers or storms over northwest Illinois
into north central Illinois over these next several hours as
convective temperatures are reached under subtle upper level
cooling and lift. With 700mb winds around 40 kt, effective shear
values are enough to support a couple stronger updrafts and
temporary storm organization, again highly conditional on actually
getting low-level instability to be realized (i.e. difficult with
broken lower clouds) and having some storms to develop.

Further south and east in the warm sector, some congestus has
been seen over the south Chicago metro into northwest Indiana. If
any showers or storms were to develop here these next few hours,
they would be very widely scattered (probably 10% or less
coverage) and bring temporary very heavy rainfall.

If shower activity forms it should gradually wane through the
mid-late evening into overnight. Patches of clouds should
continue regardless of whether or not there are storms, though some
thinning or clearing is expected. With lows expected to dip at
least a few degrees below current dew points, there is a fog
threat. Similar to humid days over the past two months, would
expect if fog can develop for it to be more patchy and only dense
in favored areas.

MTF

&&

.LONG TERM...
311 PM CDT

Sunday through Saturday...

Potential remains for continued chances for showers and
thunderstorms Sunday into early next week, with guidance still
advertising scattered development on Sunday. Fairly moist boundary
layer conditions will still be in place and while instability
increases throughout the day, still think model depiction of
precip development seems reasonable. However, limiting factors
such as rising heights, retreating upper level jet, and building
surface high will be in place. So don`t think these showers and
thunderstorms will be as widespread as guidance is showing, with
more of a focus likely across the southern CWA along fringe of
building high pressure. Still see a possibility for an isolated
stronger storm on Sunday but weaker shear in place should help to
limit this potential. Brief heavy downpours would still be
possible, but it also appears that the current high PWATs over the
region will slide away from the area Sunday into early next week
and would think that any flooding potential would lower.
Maintained high temps in the low to mid 80s and with the high
dewpoint air still in place, humid conditions will continue.

Guidance still being aggressive and continuing precip development
overnight Sunday but think its being a bit too aggressive. Don t
see any real surface focus and with lacking energy aloft and the
LLJ focused will west of the area, lowered pops Sunday night. If
any precip occurs, it will likely be early in the evening and stem
from the days convection. Monday is still appearing to be a
similar day to Sunday, and have made little changes during this
time. Also have similar thoughts with regard to precip potential
Monday night, thinking guidance is a bit overdone. However, it
does appear that there could be some shortwave energy pushing
through the region during this time, that could help with
development. So, did leave pops with the higher potential across
north central Illinois. Still expecting system to push through the
region Tuesday into Tuesday night with additional precip chances,
before high pressure begins to spread across the region and brings
an end to the active/wet conditions.

Rodriguez

&&

.AVIATION...
For the 00Z TAFs...

Primary concern this TAF cycle is with potential for IFR or lower
cigs and some fog late tonight into early Sunday morning. Isold
SHRA between RFD and ORD should dissipate early this evening
evening with a tendency for a gradual decrease in cloudiness this
evening. Some partial clearing, winds likely going light/variable,
and ample low level moisture could allow for stratus and fog to
develop late tonight. Always tricky predicting fog/stratus,
particularly this time of year, so confidence isn`t particularly
high. Widespread LIFR cigs and fog developed over IA last night,
so a repeat is certainly possible tonight into northern IL,
particularly outside of urban heat island area of Chicago. Have
brought RFD/DPA down the lowest, but not out of question that
conditions could end up lower than 00Z TAFs reflect at ORD and
MDW. Conditions should quickly improve by mid morning to VFR by
late morning.

Izzi

&&

.MARINE...
212 PM CDT

Winds variable across the lake this afternoon as a weak trough of
low pressure and frontal boundary move through the region. Despite
this system on the weaker side, it is helping for slightly higher
speeds of 10 to 20 KT across the lake. This system will continue
to slide east through tonight, with high pressure then expected to
build back across the lake Sunday. As this high remains in place
into Monday, expect lighter winds to be observed. Southwest winds
will then increase Monday night into Tuesday out ahead of low
pressure moving through the region.

Rodriguez

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...NONE.
IN...NONE.
LM...NONE.
&&

$$

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