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Area forecast discussion 
National Weather Service Chicago/Romeoville Illinois
1234 am CDT Friday Apr 25 2014

Discussion...
905 PM CDT

Low pressure has moved into west central Illinois with bands of
showers continuing to lift northeastward ahead of it. Shower
coverage has increased across the eastern County Warning Area in the last hour or
two but intensity remains light with steadier shower activity
across the western County Warning Area. Radar mosaic shows a pretty well defined
back edge to the precipitation moving eastward across eastern Iowa. This
will work into the western County Warning Area early tonight with all precipitation
expected to be to the east by 12z.

Mdb

//prev discussion...
311 PM CDT

Synopsis...showers and possibly a couple of thunderstorms move
through this afternoon with additional showers expected overnight.
Friday will be dry and warm with rain possible Friday night. Then
chances of rain increase for Saturday night through most of next
week. In addition temperatures begin a slow decline from late this weekend
through next week with below normal temperatures expected for most
of next week.

Through tonight...
the upper level ridge has pushed east of the County Warning Area while a trough is
over the plains. The trough will move overhead tonight. At the
surface low pressure is over southwest MO and it will move over
central Illinois tonight. A warm front lies west and south of the County Warning Area
with showers stretching from WI through MO. The best cape values
remain south of the County Warning Area where more lighting is being seen...but have
seen a few strikes in the scattered convection moving into the far
western County Warning Area. Given the weak instability...went with a slight chance
of thunder west of a Harvard to Pontiac line this afternoon. As the
low passes to our south this evening...thinking the slight chance of
thunder will shift east and cover most of northern Illinois.
However...with only minor amounts of instability to work with...only
expecting isolated claps of thunder at best.

Showers on the other hand are very likely throughout the night with
storm total quantitative precipitation forecast values ranging from around 0.75 inches across north
central Illinois to 0.3-0.5 inches over Northwest Indiana. Winds turn northwest behind
the low and overnight min temperatures will be in the middle 40s to around 50.

Friday and Saturday...
the upper level closed low associated with the trough lags behind
the trough a bit and passes over Lake Superior and far Northern Lake
Michigan tomorrow. The surface low shifts east into the Ohio River valley
tomorrow with rain ending early in the morning. A ridge then builds
in during the afternoon and overnight. As such looking at skies
clearing and a good mixing day with northwest to west winds gusting 25 to 30
miles per hour in the afternoon. Still thinking we could have drier mix down
to the surface so kept dewpoints below guidance.

Maximum temperatures will be in the upper 60s to low 70s and then lows will be
in the 40s with increasing cloud cover. A weak shortwave may produce
a few showers Friday night and have a slight chance of probability of precipitation in for
that. Confidence is low in the showers overnight.

The high shifts east on Saturday with sky cover increasing in the
afternoon and overnight. A backdoor cold front slides down the lake
with winds turning off of the lake. This will lead to high temperatures
along the lake only getting into the upper 40s to low 50s. Outlying
areas should warm nicely into the 60s and maybe even low 70s. Kept
Saturday dry since the ridge is moving overhead. However guidance
continues to feature an upper level wave moving in with precipitation
forming Saturday evening and night. Confidence is medium in the
timing and location of precipitation so capped probability of precipitation at chance.

Sunday through next Wednesday...
next week still looks rainy and cool with a cut off low hanging out
over the region. The upper level low begins over the SW US Saturday
afternoon and then cuts off over the plains Sunday aftn/eve. It then
moves over the Midwest Tuesday evening and remains over the region
through Thursday before finally shifting east. At the surface...a
low forms over the plains Saturday night and remains over the plains
into Monday before moving over the region Monday night. The low then
stalls over the region through most of next week. Guidance has the
low finally shifting east late on Thursday or Friday.

Cut off lows are notorious rain producers with lobes of vorticity
circling around them but not helping to advance the pattern or
system. As such...have a chance or better of rain from Sunday
through Wednesday. The 7 day total quantitative precipitation forecast outlook from wpc has 2-3
inches of rain. Note this does include the rain we receive this
afternoon and tonight...but it still is a substantial amount of rain
nonetheless.

Not overly impressed with our thunder chances...but think the best
chance for thunder is Monday. However as the lows warm front hangs
out south of the County Warning Area Sunday...have a slight chance of thunder in
along and south of I-80. Then have a chance of thunder in for Monday
along and south of I-80. Thinking the threat of thunder will then
slowly diminish Monday night into Tuesday as the low moves across
central Illinois.

The jet still swings well south which allows colder air to wrap
around the low. There will be a cooling trend through next week
with highs on Sunday in the middle 50s along the lake to around 70
across the southern County Warning Area. By Wednesday we are looking at highs in
the 50s across the County Warning Area and lows possibly dipping into the upper 30s
overnight.

Jee

&&

Aviation...

//Ord and mdw concerns...updated 06z...

* showers through the predawn hours.

* Periods of MVFR ceilings into early Friday morning.

* Wind shift to the northwest overnight.

* Strong/gusty west/northwest winds Friday.

Krein

//discussion...updated 06z...

A band of rain associated with low pressure moving through central
Illinois will persist until around 07-08z at rfd and 09-10z for the
Chicago area terminals. Latest radar imagery shows a sharp cutoff
along the back edge of the precipitation as drier air is drawn around the
western periphery of the low. There may also be a period of MVFR ceilings
in a convergence band along an inverted surface trough axis extending
north from the surface low.

The surface low is expected to push into northern Indiana through the
morning hours...with winds backing through northerly to northwesterly by
late morning. There could be a short period of light and variable
winds during the pre-dawn hours as the inverted surface trough axis passes
over the terminal areas. By late morning wind will become stronger
and more gusty out of the northwest as cold advection sets up
being the passing low pressure system. The latest high resolution
short range model guidance suggest that a weak surface ridge will
build in behind the exiting system...allowing wind to back to more
due westerly by early afternoon...with speeds remaining around
10kt...with a few higher gusts possible through the remainder of
the afternoon.

//Ord and mdw confidence...updated 06z...

* high confidence in periods of rain through the predawn hours.

* Medium confidence in MVFR ceilings through the predawn hours.

* Medium confidence in wind trends through the period.

* Medium to high confidence in all other forecast elements after
the predawn hours.

Krein

//outlook for Ord/mdw for 12z Saturday-12z Thursday...updated 00z...

* Saturday...dry/VFR.
* Saturday night and Sunday...chance of rain showers.
* Sunday night through Monday night...rain showers likely. MVFR possible.
* Tuesday through Wednesday...chance of rain showers.

&&

Marine...
158 PM CDT

Two weak low pressure centers were located west of Lake Michigan.
One was steadily weakening over central Iowa...while a second was
beginning to strengthen across central Missouri. The two low
pressures are expected to consolidate this evening...then steadily
lift northeast/east across central Illinois overnight. In the
meantime...winds will generally remain easterly to southeasterly.
A tight gradient does exist...which has resulted in winds at times
gusting to 30kt this afternoon. Guidance has been consistent in
depicting winds diminishing tonight...then winds will turn
west/northwest. This will allow the waves to subside across the
north/northwest portions of the lake...while waves begin to build
once again for the Indiana nearshore waters Friday midday through
Friday night.

High pressure will slide east across Manitoba to north of Lake
Superior early Saturday. This will allow winds to turn northerly
early Saturday then northeast to easterly across the lake by
Saturday afternoon. Then a prolonged period of generally easterly
winds is expected across the lake Sunday through Tuesday.

Beachler

&&

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

$$

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