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Area forecast discussion 
National Weather Service Chicago/Romeoville Illinois
1123 am CDT Monday may 4 2015

Update...
1111 am CDT

A somewhat disorganized cold front draped across the area from
southwest to northeast will drop toward central Illinois this
afternoon. Scattered to isolated showers persisted south of this
feature late this morning...but weak convergence along the
boundary along with pooling of moisture may support increased
coverage this afternoon during peak diurnal heating. More
effective moisture convergence develops late this evening into
the overnight hours as the front begins to lift back north.

Despite increasing instability near the front this afternoon and
tonight...deep layer shear is not favorable for supporting
widespread organized convection...but isolated stronger storms
could produce some small hail.

Lenning

&&

Short term...
331 am CDT

Through Tuesday...

Convection associated with an approaching cold front is moving from
northwest to southeast through northern Illinois this morning. Its well
defined gust front stretches from Southern Lake Michigan through Pontiac
Illinois with scattered storms forming along the gust front. A broader
area of showers lags behind the gust front and latest radar imagery
suggests the area of showers is quickly dissipating. Two additional
convective complexes are further west over northeast Iowa and
northwest Missouri and the Iowa complex may play a part in what
happens this morning.

Thinking we will see additional scattered shower and thunderstorm
develop along the gust front as it plows through the rest of the County Warning Area
early this morning. Much less confident on how long the showers will
persist over north central Illinois and if they will even reach North East
Illinois and Northwest Indiana. The ongoing convection over northeast Iowa may
shift east producing showers along the Illinois/WI state line...or it may
dissipate and we see a dry morning outside of the convection along
the gust front. Since confidence is very limited...decided to keep a
low end chance to slight chance of rain through middle morning.

The cold front which currently stretches from northeast WI through
south central Iowa will slide through the County Warning Area this morning and then
stall over LaSalle County Illinois through Benton County Indiana late this
afternoon. Short term guidance suggests cape will increase this
afternoon so have likely probability of precipitation spreading across the County Warning Area this afternoon.
Have medium confidence in how far north the convection will reach.
Currently have likelies over the I-88 corridor but the northern
extent may remain south of I-88.

A middle level wave pushes north into Iowa this evening and expecting
it to force the next round of storms. Expecting showers and and a
few embedded thunderstorms to cover the County Warning Area by late this evening.
Storms should remain tied to the stalled front Tuesday and have
medium-low confidence in how convection will behave. Thinking the
nocturnal wave of storms will dissipate as they lift north away from
the front...while additional scattered convection will fester along
the front. Kept at least a low end chance over the whole County Warning Area even
though areas north of I-80 may be dry Tuesday afternoon.

For temperatures...today should be a bit cooler with the cold front moving
through so have highs around 70 along the Illinois/WI state line and then
in the upper 70s south of I-80. Winds should turn onshore along the
lake keeping temperatures in the middle 60s to around 70. Tuesday looks even
cooler if you are north of the front with highs in the 60s...and
only in the 50s along the lake. South of the front...locations
should be in the upper 70s to low 80s. Breaks in cloud cover could
lead to a few sites getting into the middle 80s.

Jee

&&

Long term...
331 am CDT

Tuesday night through Sunday...

The stationary front begins to shift north as a warm front Tuesday
night and could see isolated to scattered showers and perhaps
thunderstorm development north of the front. Cape values drop off
quickly overnight so not expecting widespread thunderstorm
development.

We are in the warm sector Wednesday with soundings showing no cap and
a few hundreds of j/kg of cape. Isolated to scattered thunderstorm
development is possible especially Wednesday afternoon north of a Rensselaer
to rfd line. Temperatures will be warmer away from the lake with highs in
the low to middle 80s. It will be in the middle to upper 60s along the
lake. Guidance features a few lobes of vorticity moving along the
upper level ridge...but they should remain west of the County Warning Area. This
would lead to a dry night Wednesday night...but if the vorts shift
east...could see scattered shower development.

The next surface low forms over the upper plains/Midwest Thursday
morning and reaches southern Ontario Friday morning. Its cold front
stalls over northern Illinois Sunday morning. Therefore looking at
another active period with multiple periods of showers and
thunderstorms late this week through the weekend. The forecast
looks oddly similar to what we will go through early this week.
Temperatures start above normal in the 80s away from the lake
Thursday and then cool to the 70s Saturday and Sunday. A lake breeze
will keep temperatures along the lake in the 60s. For what it is
Worth...wpc is forecasting upwards of 3 inches of rain over the next
7 days.

Jee

&&

Aviation...

//Ord and mdw concerns...updated 16z...

* west-southwest winds becoming northwest before turning north- northeast
this afternoon likely increasing to 10 knots or slightly higher
sometime through early afternoon before easing this evening.

* A brief period of MVFR ceilings around 2000 feet this morning.

* Small chance of additional rain showers 19z-21z through with chances of
rain showers/embedded ts sharply increasing by late evening into overnight.

* MVFR and possible IFR conditions with east winds Tuesday
morning.

Kmd

//discussion...updated 12z...

Storms and their outflow prior to daybreak have shifted the focus
for morning convective activity mainly south of the terminals.
The Post-convective winds have been variable but a northwest wind
should define itself by 14z or so at taf sites...and then the wind
is expected to veer to more north-northeast as a cool front drops
southward. The timing on this is challenging as earlier convection
has disturbed the wind field and disrupted the well-defined
nature of this front. It is possible that even with the wind shift
to north-northeast that 10+ knots speeds do not occur until a few
hours after the wind shift.

The surface front will stall to the south of the terminals and
keep the focus for most showers and storms just south of the sites
this afternoon. However...this evening will bring a couple of
upper disturbances from the west and a low-level jet steering
moisture atop and north of the boundary. This should Blossom
showers with at least embedded storms. The timing of this is
challenging but favoring late this evening and especially
overnight. While the most pronounced focus may be north of the taf
sites by Tuesday morning...lingering scattered showers and storms
are probable...which could cause additional issues as east winds
are expected at Ord and mdw.

Given the multiple areas of convection across the
Midwest...computer models have struggled over the past 12-24 hours
so specific timing in the tafs for convection is likely to remain
quite challenging.

//Ord and mdw confidence...updated 16z...

* medium-high in north-northeast winds this afternoon but low-
medium on timing and when speeds will increase over 10 knots.

* Medium-high that if a ceiling forms it will be around 1500-2000 feet
and will be short lived.

* Medium on timing of rain showers tonight and low-medium on thunder
coverage.

* High in east winds Tuesday morning and medium-high in MVFR
conditions. Low in whether IFR will occur and on timing of MVFR.

Kmd/mtf

//outlook for Ord/mdw for 00z Wednesday-12z Monday...updated 12z...

Tuesday night-Wednesday...chance of storms. Light east winds.
Thursday...chance of storms during the night. South winds.
Friday...chance of storms. South winds.
Saturday-Sunday...chance of storms. Winds becoming northeast.

Mtf

&&

Marine...
257 am CDT

A cold front will advance southward across the region this
morning reaching the far southern part of the lake by early
afternoon. This will turn winds to the north-northeast. While the
lake has warmed considerably in the past few weeks...the cooler
air behind the front combined with the high moisture along it may
generate fog namely across the southern part of the lake. Winds
will turn easterly later tonight and into Tuesday and then back to
the southeast and south Tuesday night into Wednesday as the front
returns northward as a warm front. A warm sector will remain over
the lake during Thursday and Friday meaning stability should keep
gusts mainly in the 20-25 knots ballpark during that time.

Mtf

&&

Lot watches/warnings/advisories...
Illinois...none.
In...none.
Lm...none.
&&

$$

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