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

Update...
842 PM CDT

For the evening update...I have tweaked probability of precipitation a bit through the
evening to go with low end chances for mainly isolated
showers/storms along and northwest of I-55. It appears the main
focus for higher coverage showers and storms over the next couple
hours will be east of I-55. Later tonight the main focus will be
with the likely development of additional rain and some possible
storms in advance of an approaching cold front...now shifting across
central Iowa. The 00 UTC soundings at both kilx and kdvn indicate
poor middle-level lapse rates over the area...and hence minimal
MUCAPE. Therefore...it appears the overall thunder threat will
remain on the low side with this activity later tonight.

No real changes have been made beyond tonight. However...there are
some concerns for some heavy rainfall potential on Saturday across
the area as Stout middle-level disturbance over western Arkansas
induces a lower level wave along the approaching cold front on
Saturday. This could result in a band of frontogenetically induced
rain across northern Illinois...and showers and thunderstorms across
my southern counties. Given the precipitable water values are
expected to be at or even above the 00utc radiosonde observation value of 1.69 inches
at kilx...heavy rainfall is certainly a concern. However...it
appears that areas along and north of Interstate 80 will remain
relatively stable...so the potential of very heavy rainfall rates
appears to be lower given the lack of better convective potential.
Areas farther south across northwestern Indiana and my east central
Illinois counties look to remain closer to the warm sector and hence
within the area of better convective potential. Therefore...it
appears that these areas could have a higher threat for some heavy
rain rates given the abundance of atmospheric moisture in place.
Overall...it looks like most of the area will see some good amounts
of rain in excess of a half inch. However...some areas could
experience much higher amounts...especially south of Interstate 80.
With this in mind...quantitative precipitation forecast amounts may need to be raised if the 00 UTC
guidance continues to support heavy rain.

Otherwise...it will be turning much colder and windy during the day
across northern Illinois.

Kjb

&&

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 00z...

* scattered rain showers and a few isolated thunderstorms and rain mainly north and south of
Ord/mdw this evening. Additional rain showers/isolated thunderstorms and rain developing
late tonight/early Saturday 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...improving late in the day.

Ratzer

//discussion...updated 00z...

Humid southwest flow will be in place across the terminals through
tonight...with clusters of scattered showers and isolated
thunderstorms at times. Through the evening hours...it appears
that the greatest concentration of rain showers/thunderstorms and rain will be off to the
south and southeast of Ord/mdw with a disturbance tracking from
central Illinois northeast into northern Indiana. Another smaller area
of less numerous rain showers currently near krfd will shift northeast of
Chicago area. While isolated rain showers may occur at just about any time
tonight...it appears that more organized rain showers and isolated thunderstorms and rain
coverage will increase again after midnight/early Saturday morning
as another middle-level disturbance approaches. Middle-level lapse rates
are not very impressive...and while some isolated or embedded thunderstorms and rain
are possible early Saturday morning...coverage is expected to be
too low to include in point taf forecasts. Rain is anticipated to
continue through much of Saturday as surface cold front moves
through and upper trough crosses the region. Majority of guidance
indicates ceilings deteriorating to IFR or perhaps even LIFR Saturday
morning as rain persists in cooler air behind cold front...with
improvement to MVFR/VFR not expected until very late afternoon or
toward evening.

Southwest winds ahead of cold front tonight will gust 15-20 knots
through sunset before diminishing. Strong cold frontal passage
Saturday morning will produce sharp wind shift to northeast...with
winds becoming strong/gusty during the day. Gusts near 30 knots
possible...gradually diminishing Saturday evening.

Ratzer

//Ord and mdw confidence...updated 00z...

* high confidence in any rain showers being isolated and mainly
south/southeast of immediate Ord/mdw terminals.

* Low in timing of rain showers trends overnight...but high in periods of
rain through much of Saturday.

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

* High in wind shift to strong north-northeast...medium in timing.

Ratzer

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

Sunday and Monday...VFR. Northeast winds.

Tuesday...VFR. East winds.

Wednesday...VFR. East-southeast winds.

Thursday...VFR. South winds.

Ratzer

&&

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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