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Area forecast discussion 
National Weather Service Chicago/Romeoville Illinois
403 am CDT Tuesday Mar 31 2015

Short term...
338 am CDT

Through Wednesday...

Low pressure across the southern tip of Lake Michigan at daybreak
will continue to race eastward this morning. A trailing cold
front...partially helped by the cooler air over the lake...will drop
over the entire forecast area by middle morning. At least temporary
overcast low clouds are likely in northeast Illinois within the
marine modified cool sector of the system during this
morning...possibly even into afternoon. Synoptically the winds
will want to become more northwest by late morning into
afternoon...however the speeds will also be diminishing as the
advection behind the low eases. This should allow for a lake
breeze to develop...or at least for continued onshore flow...into
parts of northeast Illinois and especially Northwest Indiana. All
this leads to some temperature undulations in Lakeside
counties...with highs forecast in the middle 40s to low-middle 50s from
near to further from the lake in those counties. If clouds persist
longer those immediate Lakeside temperatures may end up being in
the upper 30s. Far outlying areas from the I-39 corridor and
DeKalb to Kankakee and southwest from there should moderate to
the upper 50s to lower 60s.

Increasing upper heights will be seen across the area during the
day Wednesday. Strengthening southerly flow especially by
afternoon will bring an impinging low-level thermal nose into the
area. With 925mb temperatures around 10c this should allow for
upper 60s to lower or even middle 70s across much of the forecast
area. One major challenge though is that as pressure falls first
start advancing in...the winds look a little more backed to the
southeast near the Illinois shore...and the gradient could be
light enough to allow a lake breeze to creep inland. This may not
make much progress inward and probably retreat later in the
day...but could present a significant difference in temperatures
for much of the day from what a southwest wind would yield. Have
leaned toward the lake breeze...albeit not as chilly as it could
be...and gone more 50s in northeast Illinois within 10-20 miles
of the shore...but confidence on specifics 36 hours out with this
phenomena is low.

Mtf

&&

Long term...
338 am CDT

Wednesday night through Monday...

A northern stream trough will move over the international border of
the northern plains Wednesday night to the Great Lakes by Thursday
evening. A sub 990mb low is expected with this system across
Ontario with moisture advected northward ahead of its cold front.
The southerly winds will make for a mild night Wednesday night with
lows in the 50s. Convection is likely to trigger across the Missouri
River valley into southern Minnesota Wednesday evening and evolve
east within increasing upper forcing and replenishing moisture
transport. Showers and likely some thunderstorms should translate
into our region by Thursday morning. Have continued to focus the
probability of precipitation temporally...guiding closer to the slower 00z ec and NCEP NAM
and sref models.

The morning clouds and showers and storms will make for a challenge
with temperatures and ability to destabilize or revamp any
ongoing convection Thursday afternoon...in addition to some
discrepencies in timing already from guidance. Boundary layer dew
points are expected to be into the middle-upper 50s using the better
initialized ec across the current high dew point air of the
Southern Plains. So if scattering and some re-warming can
occur...this will likely result in enough instability for
convection to redevelop near the frontal zone. Given middle-level
winds of 45-60 knots this could support some organized stronger
storms...for our area mainly along/south of I-80 given the current
ec and NAM.

Have not made any major changes beyond this time period. Friday
is forecast by guidance to have a short wave...possibly
strong...translate along the frontal boundary to our south
resulting in some rain immediately north of its path. But again
some shifts in this are certainly plausible. The boundary looks
to return as a warm front later this weekend and the GFS is the
quickest with this. The more defined this boundary becomes the
greater the challenge with both temperatures and possibly earlier
shower/storm chances. Sunday especially could have highs areawide
in the 60s/lower 70s...or 40s especially in northeast Illinois
with a more easterly wind.

Mtf

&&

Aviation...

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

* wind directions today...with potential for north-northeast
(350-020 deg) winds early shifting to northwest and then
possible lake breeze in the afternoon especially at mdw.

* Potential for lower MVFR/higher end IFR ceilings this morning.

Rc

//discussion...updated 09z...

IFR ceilings are pushing southward in Wisconsin...quickest along the
southeast Wisconsin shore. A period of lower ceilings is becoming more
likely for Ord/mdw/gyy and possibly as far west as dpa but how
low is in question due to dry air mass over the terminals at this
time. Will go with a low ceiling in the Ord/mdw/gyy tafs and monitor
observational trends.

Rc

..from 06z...

A clipper low pressure system is centered over southern Wisconsin
early this morning...with strong pressure falls immediately ahead
of it resulting in strong southwest winds gusting up to 25 to even
30 knots over northeast Illinois. Winds will shift to northwest and
then north behind the low and lake influence could bring a brief
period of north-northeast winds to Ord/mdw between about 12z and
14z. Low ceilings over central Wisconsin may also spread
southward...aided by the lake influence...but confidence is
low...so have added sct010 in Ord/mdw/gyy tafs from 13z to 15z.

Wind direction forecast remains tricky through the day today...but
current thinking is that winds will back to west-northwest
everywhere except gyy...which will hold onto northerly winds given
proximity to the lake. Pressure gradient will diminish later in
the afternoon...with lake breeze expected to make some headway
inland...though it is uncertain how far. Have indicated a wind
shift in the mdw taf during the late afternoon but not brought the
shift to Ord. Light northeast winds could then redevelop late
tonight into early Wednesday.

Rc

//Ord and mdw confidence...updated 09z...

* high in wind forecast through about 9z-10z today...low
thereafter.

* Medium in low ceiling potential this morning...low in duration.

Rc

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

Wednesday...VFR.

Thursday...shra/some thunderstorms and rain likely. Periodic IFR possible.

Friday...chance of MVFR ceilings. NE winds 10kt or greater possible.

Saturday and Sunday...primarily VFR.

Izzi

&&

Marine...
403 am CDT

A fast moving clipper system will quickly move east of the
southern tip of the lake this morning. Given strong southwest
winds that occurred ahead of the low overnight...a brief burst of
15 to 25 knots north winds appears likely on the southern tip of the
early this morning. Have not issued a Small Craft Advisory but
will monitor trends and issue a short duration advisory if
necessary. Lighter winds tonight into Wednesday morning will
shift south and increase during the day Wednesday and especially
Wednesday night as a frontal trough approaches the lake. Warm air
mass ahead of the trough will limit mixing over the Cold
Lake...but expecting speeds to 30 knots given high sustained winds.
Will likely need a Small Craft Advisory for the nearshore during
this time. A period of stronger north winds is possible on Friday
as low pressure moves up the Ohio Valley. Speeds will depend on
the track and strength of the low pressure area.

Rc

&&

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

$$

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