Scientific Forecaster Discussion

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Area forecast discussion 
National Weather Service Chicago/Romeoville Illinois
453 PM CDT Thursday Apr 17 2014

Discussion...
309 PM CDT

A relatively benign weather period is in store for the next few
days...with the only chance of any precipitation through Sunday being
tonight. The main forecast concerns over the next few days will be
temperatures...as there will likely be some decent temperature
gradients across the area...due in large part to the onshore flow
expected across northeastern Illinois on Friday and Saturday.

Currently there is a good amount of middle and higher level cloudiness
streaming northeastward across the area ahead of a sheared out short
wave...currently in place across the middle and lower Missouri Valley.
Model guidance continues to spark off an area of showers across
portions of the region this evening and overnight as this short wave
disturbance approaches and induces some isentropic ascent across the
area. Overall...I followed close to the inherited forecast...which
places the chances for this activity mainly from middle evening through
late tonight for areas along and south of a line from Chicago to
Mendota.

A 1030 mb surface high is expected to build in across the upper
Great Lakes region on Friday. This will set up a north-northeasterly
onshore flow across the area during the day. As such...expect chilly
conditions on Friday across northeastern Illinois and northwestern
Indiana...where a enhanced lake breeze will likely keep temperatures
in the 40s fore areas near the lake. Farther
inland...however...temperatures should warm into the 50s...with even
some low 60s possible well inland.

Temperatures begin to moderate area-wide on Saturday as the lower
level flow turns out of the southeast in response to low pressure
moving across the northern plains and southern Canada. Temperatures
look to climb into the 60s for most areas...with the exception of
far northeast Illinois...where southeasterly onshore flow looks to
keep things cooler. Sunday definitely looks to be the best day of
the weekend. South-southwesterly flow is expected to transport an
even warmer air mass in across the area. Model guidance is in very
good agreement in advecting in 15 (9) degree celsius 925 mb (850 mb)
temperatures. Local climatology for these values this time of year
indicate high temperatures getting into the low to possibly middle 70s
across the area.

The next chance for precipitation looks to be on Monday as another
weak frontal system and middle level disturbance pushes across the
area. Following in the wake of this frontal Monday...looks to be
another day or two period of northeasterly onshore cooling
flow...which will again could set up a decent thermal gradient
between the Lake Shore and areas farther inland.

Way out...towards the end of the current forecast period...guidance
is in good agreement in developing a good storm system across the
plains..they just differ significantly with the timing. This system
looks to produce a period of much warmer weather and also another
threat for showers and thunderstorms at the end of the period.

Kjb

&&

Aviation...

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

* winds become north late this evening and then east around 10 knots by
late Friday morning.

Krein

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

Low pressure is slowly weakening and shifting northeast over
Wisconsin. Its cold front is moving through the eastern terminals
now and expecting winds to turn west behind the front. The MVFR to
IFR ceilings to our north and west appear to be dissipating and not
moving into northern Illinois. Should see the MVFR ceilings over rfd scatter
and lift over the next couple of hours...and have medium
confidence on the timing. The ceilings may linger a bit longer over
rfd than expected.

High pressure spreads over the region this evening with winds
diminishing and also becoming north by late this evening. The
upper level trough swings through this evening as well and
guidance continues to show scattered rain showers along and east
of I-55 this evening. Gyy would have the best chance of seeing any
precipitation...but that chance is very low. Therefore kept all of the
tafs dry for now.

Winds continue to veer to the east tomorrow with winds increasing
to around 10 knots by the late morning.

Jee

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

* high confidence in wind shifts...medium confidence in timing.

* High confidence in all other forecast elements.

Krein

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

Friday night...VFR.
Saturday...VFR.
Sunday...VFR.
Monday...chance rain. MVFR/IFR possible.
Tuesday...VFR.
Wednesday...slt chance rain.

Bmd

&&

Marine...
216 PM CDT

Weakening low pressure over northeast Wisconsin this afternoon
will continue to lift northeast and weaken...reaching northeast
Ontario by daybreak Friday. This low has a frontal boundary that
will continue to slide east across the lake this afternoon...with
winds slowly turning westerly to northwesterly. The gradient will
be continue to weaken this evening...as an area of high pressure
located across central Canada will slide east and elongate
covering the western Great Lakes region by midday Friday.

The next weather system will begin to develop Friday night across
the Central Plains...allowing a tightening gradient to develop
across the southern half of the lake early Saturday. It does not
appear at this time any substantical increase in the winds will
occur. Otherwise the next chance for moderate to strong winds will
be from early next week on as a stronger low pressure system could
impact the region.

Beachler

&&

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

$$

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