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Area forecast discussion 
National Weather Service Chicago/Romeoville Illinois
1124 am CDT sun Mar 29 2015

Update...
652 am CDT

Keeping a close eye on surface temperatures as an area of
precipitation advances eastward across Iowa towards northwest
Illinois. There have been multiple reports of freezing rain and
icing in Iowa into southwest Wisconsin early this morning. These
areas were more vulnerable for this precipitation type given it began
before daybreak allowing temperatures not to moderate. For our
forecast area...surface temperatures are likely to warm to above
freezing before precipitation begins near Rockford and the I-39
corridor from 9-10 am...however wet bulb cooling will likely
bring the temperature down a few degrees at onset if the precipitation is
moderate enough. This could result in brief freezing rain across
north central Illinois...which already did have a brief chance in
the forecast for...but day shift may need to address with separate
statement.

Mtf

&&

Short term...
338 am CDT

Through tonight...

A well-defined upper circulation is seen on early morning water
vapor imagery moving southeastward into southern Minnesota early this
morning. This is a quick-moving Pacific system and despite much of
the Gulf being cut off...does have an axis of moisture/saturation
within its warm advection Wing with showers from southern Minnesota across
Iowa. As this elevated warm and moist advection translates over our
area this morning...it will be the primary focus for
precipitation...bringing a 3-4 hour period from west-to-east.

Recent lightning strikes around 330 am near the Twin Cities indicate
the presence of elevated instability and rap soundings do translate
steep lapse rates above 750mb into the area within this zone of
precipitation. With this instability and the compact area of strong
qg forcing of the upper wave...there may be enough
intensity/dynamics of precipitation at first for a brief but
moderate intensity mix with sleet or snow. This chance...which has
increased...looks to be north of I-80 and prior to noon. High-
resolution convective allowing models do show higher simulated
reflectivity and hydrometeors/precipitation type favoring this brief
mix...though the specifics vary from model to model. Am not
expecting this to be a big deal in the areas it briefly occurs due
to warming low-level temperatures...though visibility could sharply come
down for a narrow window of time.

Rain showers should end from west to east for most area by early-middle
afternoon. The coverage of showers behind the leading edge may not
be as widespread as thought earlier...but still feel a few hour
window of categorical probability of precipitation covers this situation well. The upper low
in tandem with the low-level cold front may help provide some
regenerating showers across far northern and northeast Illinois late
this afternoon. Most areas should finish under 0.15 of an inch.

The other concern for today is the strong winds. While the surface
low with this system is well north in Ontario today...the rapid
progression of the system will bring pressure falls of 4-6mb per
3hr moving directly over the area this morning. This isallobaric
component as well as some shallow but likely realized mixing of
Stout gusts will bring regular 20-27 miles per hour winds with 35-40 miles per hour
gusts from middle-morning through early afternoon...basically in
tandem with the precipitation window. This is a little below Wind
Advisory criteria but something still to note so will continue
Special Weather Statement.

Temperatures should climb from sunrise through precipitation start
time...and then hold steady or slightly dip in the first 1-2 hours
due to wet bulb cooling. Temperatures will warm this afternoon as
downslope warm advection is dragged this far east within the Pacific
system warm sector. Highs in the Lower-Middle 40s north to around 50
toward Pontiac should be reached late in the afternoon/early evening
just ahead of the cold front

Mtf

&&

Long term...
338 am CDT

Monday through Saturday...

In general a milder weather pattern will be in place on Monday
through Thursday...though with some exceptions in the day to day
details. While area will be under northwest flow at the middle and
upper levels behind sunday's departing wave on Monday...it will be
a downsloping/warm advective regime. This will especially be the
case during the afternoon as warm air advection ramps up in advance of a clipper
system dropping southeast across the upper Midwest. 850 mb temperatures
will warm to the middle single digits celsius and 925 mb temperatures to
around 10 celsius by early evening...which will yield highs a bit
above normal in the middle 50s to lower 60s. Will need to watch for a
lake breeze during the late afternoon as gradient will weaken with
approach of clipper.

Similar to previous shift...there is decent agreement between the
GFS and European model (ecmwf) on track of the system to the north of the County Warning Area
Monday evening and night. This keep the brunt of the precipitation to our
north...with just a glancing blow of low probability of precipitation in the northeast
third of the County Warning Area. In the wake of this system...the air mass will
not undergo much cooling...but guidance has come into good
agreement on a favorable setup for a Strong Lake breeze to push
inland on Tuesday. Highs will reach the upper 50s to middle 60s
inland and lower to middle 50s along the shore...followed by rapid
cooling into the upper 30s to lower 40s closer to the lake during
the middle to late afternoon.

In the middle to late week period...operational and ensemble
guidance is in fair agreement in the synoptic pattern...however
there are significant timing differences. Midlevel ridging will
focus into the Midwest on Wednesday...with good southerly low
level warm and moist advection (850 mb and 925 mb temperatures warming to
10-15 celsius by late weds) resulting in highs at least in the
60s...and then dewpoints surging into the 50s Wednesday night. A
fairly strong surface low looks to ride along the Canadian
border...with showers and possibly thunderstorms developing
along/ahead of trailing cold front given presence of fairly moist
airmass. Timing discrepancy between the GFS/gefs and European model (ecmwf) in this
frontal system is fairly substantial...with GFS favoring much
quicker Wednesday night into Thursday timing and European model (ecmwf) favoring
Thursday afternoon into Thursday night. This will ultimately
placement of probability of precipitation and how fast cooling to near or below normal
levels will arrive later in the week. Of note...00z European model (ecmwf) did
abandon idea from past few runs of significant synoptic system
over the region on Friday...but period will need to be watched as
it did have good ensemble support prior to 00 UTC cycle.

Rc

&&

Aviation...

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

* rain exiting toward 1730z. Brief MVFR visibility possible until then.
Additional showers possible late afternoon.

* South to south-southwest winds gusting 30-35 knots through the
afternoon becoming west then northwest with cold frontal
passage 00 or 01z.

* Ceilings may lower to near 030 this afternoon.

Mdb

//discussion...updated 16z...

Precipitation is moving across the Chicago/northwest in terminals and exiting
the rfd area. Upstream observation behind the rainfall show lower gusts
but would expect this to be short lived with mixing recovering.
Will need to keep an eye on this though. Cloud cover may may
ultimately temper the frequency of gusts...especially prior to
18z. Observation show a small area of MVFR ceilings across northeast Iowa and
may skirt rfd into midday but will keep this north of the other
terminals for now. Some spotty showers or sprinkles may linger
through the afternoon but will dry the tafs out until late
afternoon when the cold front may bring some showers ahead of it.
May also need to speed up the cold frontal passage timing by about
an hour or so with upcoming tafs.

Mdb

From 12z...

Southerly winds are already increasing early this morning with
gusts over 20 knots being noted across northern Illinois...and much
stronger winds within a couple thousand feet above ground level able to be mixed
down by mid-morning. Expecting the wind direction to veer ever so
slightly this morning...but still be 190-210 when gusts reach
around 35 knots at taf sites by middle-late morning.

A 3-4 hour period of rain showers will move from west-to-east
across the area from middle morning through early afternoon. A
temporary period of mixed precipitation is likely at
onset...especially at rfd...but even briefly at the chicagoland
taf sites. This is most favored to be sleet or snow if it
occurs...though for rfd it could also be freezing rain based on
upstream observations. Will monitor trends closely to see if that
needs to be added into the rfd taf. A warming atmosphere will
likely allow shower activity to be rain by early afternoon.

As a cold front approaches...winds will turn southwest this
afternoon and then westerly behind the frontal passage which will
be near or slightly after Sundown. Wind speeds will also decrease
some though still with some gusts during the evening. In
addition...it appears there will be some light rain showers along
this front as well.

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

* high confidence in precipitation timing. And that MVFR visibility in rain
would be brief.

* High confidence in 30-35 knots gusts and direction through the
afternoon. There could be a brief lull in gusts immediately
following precipitation.

* Medium confidence in ceiling trends.

Mdb

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

Monday night...slight chance of rain and MVFR. Southwest winds.
Tuesday...VFR. Northeast winds.
Wednesday...chance of thunderstorms and rain during the night. South winds.
Thursday...chance of rain. West winds.
Friday...chance of rain. Northeast winds.
Saturday...VFR. West winds.

Mtf

&&

Marine...
358 am CDT

An active marine weather period will be seen over Lake Michigan
the next 24 hours as a quick-moving and deepening low pressure
system cruises eastward across Ontario. The rapid movement of this
system will drive strong southerly gales today...with even storm
force winds expected across the north late this morning through
middle afternoon. The south half is most favored to have 40
knots...possibly at times 45 knots gales. In the Illinois and Indiana
nearshore the south gales are expected by middle morning and will
last through the day.

The system cold front will move across the lake from west to east
very late today into early this evening. This will bring a
diminishing and turning to the west-northwest of the winds.

A clipper system will drop over the lake on Monday night. There
remains uncertainty on specific timing of this...but do think a
period of Small Craft Advisory criteria winds are possible on both
the Illinois and Indiana nearshores.

Mtf

&&

Lot watches/warnings/advisories...
Illinois...none.
In...none.
Lm...Gale Warning...lmz740-lmz741-lmz742-lmz743-lmz744-lmz745-lmz777-
lmz779 until 7 PM Sunday.

&&

$$

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