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Area forecast discussion 
National Weather Service Chicago/Romeoville Illinois
516 PM CDT Friday may 29 2015

Short term...
248 PM CDT

Through Saturday evening...

Messy weather pattern in place rest of today and tonight as the
region falls in the warm sector ahead of a cold front dropping
across the upper Midwest and plains this afternoon. Expect
uncapped modest instability to be in place through late this
evening tonight though forcing is fairly weak resulting in
unfocused isolated to widely scattered shower activity. Middle level
lapse rates continue to be weak in the 5-6 c/km range overnight so
dont see a significant increase in the thunder chances through
early overnight...and severe threat is fairly minimal with an
isolated wet microburst being the main concern. The combination of
poor lapse rates...increasing moisture and relatively high melting
levels will greatly limit the hail threat despite deep layer shear
increasing through the evening.

Synoptically...we are not under a favorable area of forcing most
of the evening and early overnight hours however models do
continue to develop convection over the area. Undercut model probability of precipitation
through the early overnight hours leaving about a 30-40 pop County Warning Area-
wide...thinking a lot of the model vorticity that is generated is
a result of convective feedback. A more robust upper level wave is
prognosticated to lift across the middle-Mississippi Valley early Saturday
morning and the cold front approaches from the northwest...so do
gradually increase probability of precipitation through the predawn hours. The cold front
should be near the Illinois/WI state-line around sunrise tomorrow
morning. Bumped up lows for the northern half of the County Warning Area with a
slower than expected arrival of the front.

Cold front should clear the southern counties of the County Warning Area by early
to middle afternoon. Northern counties will have early morning
highs then temperatures falling through the day as an unseasonably
strong cold front pushes through. 30 knots winds coming down behind
the front over a mostly stable Lake Michigan are expected to
increase in the warmer nearshore waters. Winds gusting to around
40 miles per hour are expected to bleed inland...with gusts of 30 to 35 miles per hour
expected farther inland. Strong winds and the onshore component
will drive waves into the 8 to 12 foot range so there is the
possibility of flooding along the bike path in Chicago and other
typically flood prone areas along the south end of the lake
mainly early afternoon through the evening.

Instability does have a little time to build in the southern two
thirds to half of the County Warning Area ahead of the front by middle morning...so
there may be better chances for thunderstorms during the day
tomorrow ahead of the front. Instability quickly diminishes
behind the front and some models /especially GFS/ show a stable
wave developing along the front with rain continuing behind the
front as an f-general band develops. Expect a lot of the forecast area
to fall into the upper 40s by middle to late tomorrow afternoon...and
sites along the lake experiencing wind chills in the middle to upper
30s...only two days before meteorological Summer begins.

Deubelbeiss

&&

Long term...
303 PM CDT

Saturday night through Friday...

A prolonged period of dry and seasonably cool weather is expected
with high pressure setting up shop over the Great Lakes region
into the first half of next week. Chilliest conditions will likely
be Sunday with gradual warming trend through remainder of the
extended period as high slowly shifts eastward allowing for
gradually more southerly flow to develop as upper ridge builds
eastward into the eastern half of the Continental U.S. Again by end of next
week allowing temperatures to return to above average levels.
Prominent/Strong Lake enhanced northeast winds Sunday will be
followed by more typical lake breezes next week...with lake
influence lessening by the day as gradient turns more southernly
with time.

Izzi

&&

Aviation...

//Ord and mdw concerns...updated 22z...

* isolated rain showers through 23-24z...scattered thunderstorms and rain possible this evening
though mainly south/southeast of Ord and perhaps mdw. Increasing
rain showers/thunderstorms and rain potential late tonight/early Sat am.

* Wind shift to strong/gusty north-northeast on Saturday morning
and continuing through the day.

* IFR ceilings/possibly LIFR/developing Saturday morning.

* Periods of rain and visibility restrictions Saturday morning through
the afternoon.

Rc/ratzer

//discussion...updated 18z...

Latest radar shows isolated-scattered rain showers activity over the
region and trends suggest this general regime will continue
through at least middle afternoon. Due to generally weak
instability...none of the showers has been able to produce
lightning over Illinois and in. There may be a window this afternoon
where enough instability could build to allow for some isolated
thunderstorms and rain...but confidence in direct terminal impacts remains low.
An upper disturbance is expected to clip northwest/north central
Illinois this evening...with some potential for isolated-scattered
shra/tsra. Winds will be occasionally gusty out of the south-
southwest through the afternoon then lighter SW this evening.

A better chance for more widespread showers looks to
arrive with the approach of a strong cold front toward daybreak.
However given unfavorable timing would not expect anything more
than very isolated thunderstorms and rain so have pulled thunder mention. As wind
shift from light southwest to strong/gusty north- northeast winds
occurs middle- Saturday morning...marine airmass driven southward
will be conducive for IFR to possibly LIFR ceilings...especially near
Lake Michigan. In addition...a Stout upper disturbance moving over
the area will generate steady/occasionally moderate to even heavy
rain that will taper off late in the afternoon. Winds will be
sustained in the 15-20 knots range after the wind shift...with gusts
up to about 30 knots for the eastern terminals.

Rc

//Ord and mdw confidence...updated 22z...

* high in isolated to scattered rain showers over region remainder of
late afternoon...medium in direct airfield impacts...low in any
thunderstorms and rain.

* Low in rain showers/thunderstorms and rain trends this evening and overnight.

* High in wind shift on Saturday to strong north-northeast
winds...medium in timing and speeds.

* High in IFR ceilings developing Saturday morning...medium in
timing...low in LIFR.

* High in periods of rain on Saturday...low-medium in specific
visibility impacts.

Rc/ratzer

//outlook for Ord/mdw for 00z Sunday-12z Friday...updated 12z...

* Saturday night and Sunday...dry/VFR. Gusty northeast winds.
* Monday and Tuesday...dry/VFR. East winds.
* Wednesday...slight chance of thunderstorms and rain. Southeast winds.
* Thursday...chance of thunderstorms and rain. South winds.

&&

Marine...
253 PM CDT

Strong cold front will move down the lake late tonight and
Saturday morning. Impressive shot of cold air for this time of
year will come surging southward down the lake. Gradient supports
winds increasing to around 25kt behind the front. Thermally stable
conditions over much of the open waters will likely result in very
shallow mixing and probably no significant gusts...though some of
the anemometers on the taller SHIPS could see gales for a time
Saturday. Far southern portions of the lake and in particular the
near shore waters are substantially warmer with lake surface temperatures
in the middle 50s to lower 60s. Water temperatures are still in the upper
30s over the Open Lake and that will help modify the air temperatures
down into the 40s...which then will get advected over top the
relatively warm near shore and far Southern Lake water and result
in better mixing and much greater chance of gale force winds. Have
hoisted a Gale Warning for areas with warmer water
temperatures...including nearshore and far southern Open Lake zones.
Winds slowly subside Sat night through Sunday though remain
moderately brisk.

Izzi

&&

Lot watches/warnings/advisories...
Illinois...Lakeshore Flood Advisory...ilz006-ilz014...1 PM Saturday to 4 am
Sunday.

In...Lakeshore Flood Advisory...inz001-inz002...1 PM Saturday to 4 am
Sunday.

Lm...Gale Warning...lmz740-lmz741-lmz742-lmz743-lmz744-lmz745-lmz777-
lmz779...9 am Saturday to 10 PM Saturday.

&&

$$

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