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Area forecast discussion 
National Weather Service Chicago/Romeoville Illinois
623 am CST sun Dec 21 2014

Short term...
620 am CST

The main concerns during this period will mainly be focused on
the timing of rainfall expected Monday and Monday night.

Warm air advection/isentropic ascent has produced some drizzle and
even some occasional light snow/flurries across portions of north
central Illinois early this morning. Temperatures across the area
continue to hold steady either right at freezing or a degree or two
below in some areas. Therefore...there will continue to be threat
for some minor glazing on untreated surfaces across portions of
north central Illinois through around daybreak.
Thereafter...temperatures should warm above freezing...essentially
ending this threat. Aside from some lingering patchy drizzle this
morning most areas will be cloudy again...but dry as temperatures
warm into the upper 30s this afternoon.

Model guidance continues to be in fairly good agreement through
Tuesday...with an initial area of low pressure tracking eastward
from the northern High Plains later today...to the upper Mississippi
Valley by Tuesday. This still would keep all of northern
Illinois/northwestern Indiana in the warm sector through Tuesday. It
this system looks to be a decent rainfall producer for much of the
area...with the best timing being Monday afternoon through late
Monday night/early Tuesday morning. Several upper level short wave
disturbances rotating through the larger scale upper low over the
plains...should result in what appears to be two rounds of
rainfall...with the first around the Monday afternoon
timeframe...and the second Monday night. There is a small chance
that as the precipitation onsets late Monday morning...that it could
start as a mix of sleet and light snow...given the initial cool
atmospheric profile. However...warm air advection will quickly warm
things enough to result in all rain area-wide by early Monday
afternoon.

During the day on Tuesday it appears that we may experience a middle
level dry punch...which if does occur would result in low clouds
and perhaps some lingering drizzle and lower chances of much
additional rain. Given this...I have lowered probability of precipitation a bit during the day
on Tuesday. Temperatures across the area Monday night and Tuesday
as we remain in the warm sector look to be mild for this time of
year...with 40s expected.

&&

Long term...
620 am CST

Tuesday night through Saturday...

Forecast model solutions concerning the evolution of the surface and
upper level pattern across the central Continental U.S. From Tuesday night
through Christmas have become more divergent over the past couple
runs...with several solutions even now indicating the possibility
of a much weaker surface system over the Great Lakes into
Christmas evening due to the failure of the upper level trough
becoming negatively tilted over the Mississippi Valley...as many
models had been previously advertising. However...given this
system is still 3.5 to 4 days away...I am hesitant to make any
major changes to the going forecast for this period...as it
appears the latest deterministic model solutions are really just
showing US the inherent high amount of chaos in the system with
such a rapidly changing pattern. As such...I really have no
confidence in any of the latest 00 UTC deterministic guidance and
would prefer to see additional runs over the next day or two show
similar trends before beginning to making any big changes. My
hunch is we will continue to see adjustments in the deterministic
runs over the next couple of days as the models attempt to resolve
the evolution of the upper level pattern over the central Continental U.S..

Following this system...model guidance is indicating another
disturbance digging across the plains state by Thursday...and this
could result in another period of precipitation across portions of
the area by Friday. However...uncertainty with this storm is
extremely high...given that the actual evolution of the first storm
system will need to be ironed out first.

Kjb

&&

Aviation...

//Ord and mdw concerns...updated 12z...

* very light snizzle/MVFR haze/br early this morning.

* VFR ceilings likely lowering to MVFR by afternoon and continuing
tonight.

* South-southeast winds increasing 10-15 knots by midday...becoming southeast later
tonight. A few gusts over 15 knots possible this afternoon.

Ratzer

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

Southwest flow just above the surface bringing increased low level
moisture into the area. Extensive MVFR/IFR and LIFR ceilings to the
west and southwest...though lower ceilings more patchy across northern
Illinois/in terminals. Weak middle-level disturbance passing early this am
producing some very light --sn/--dz with some MVFR visible in haze/br
but should be of relatively little impact. Otherwise...immediate
upstream surface observation mainly VFR ceilings in 3000-4000 feet range. MVFR
ceilings in 2000-2500 feet range across Missouri and western Illinois then
expected to move in during the late morning/midday hours. Low
pressure over the northern plains tonight will back winds a
bit...with model trends supporting improvement to VFR at some
point...though confidence lowers with extensive upstream lower
clouds.

South winds near 10 knots expected to increase into 10-15 knots range by
midday...with some sporadic gusts above 15 knots possible this
afternoon though not frequent. Winds back more southeast later
tonight into Monday morning...increasing after sunrise.

Ratzer

//Ord and mdw confidence...updated 12z...

* medium in light precipitation and ceiling trends.

* Medium-high in winds.

Ratzer

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

Monday night...rain. MVFR/IFR likely. S-southeast winds.

Tuesday...chance of rain and MVFR/IFR especially early. S-SW winds.

Wednesday...chance of snow...possibly mixed with rain early.
MVFR/IFR likely. Northwest winds becoming west.

Thursday...dry. MVFR ceilings possible early. West winds becoming SW.

Friday...chance of rain or snow. MVFR possible. S winds becoming west.

Saturday...dry. Mainly VFR. West winds.

Ratzer

&&

Marine...
240 am CST

Main marine concerns continue to focus on strengthening
south-southeast winds today through late Monday...and potential for
gusty northwest-west winds...possibly gales...from late Wednesday
into Thursday.

Surface high pressure ridge moved east of the lake on Saturday
allowing winds to become southerly. The surface pressure gradient
will slowly tighten across the upper Midwest and western lakes today
as an area of low pressure moves southeast from the Lee of the
Canadian rockies...with south-southeast winds increasing into the
25-30 knots range from later this afternoon into Monday as the low
approaches from the northern plains. The is expected to occlude and
being to fill late Monday night...allowing the gradient to weaken
and winds to diminish early Tuesday. The remnant surface trough will
cross the lake late Tuesday night or early Wednesday...with light
winds shifting to the west-northwest.

Forecast model solutions for the surface pattern from Wednesday
afternoon through Thursday have become more divergent over the past
few runs...with quite a spread in the location track and depth of a
second surface low which is expected to develop somewhere across the
Ohio or Tennessee River valleys. Overall...guidance indicating a
weaker low during this period...though models are obviously
struggling with development of digging southern stream jet streak
and the resulting upper trough and downstream surface low
development. With such divergent solutions have avoided making large
swings in the forecast...and have held on to the general scenario of
previous forecast...though with slightly weaker wind field during
the late Wednesday-Thursday period. Adjustments will undoubtedly be
required as suite of models develop better convergence/agreement in
their solutions.

Ratzer

&&

Lot watches/warnings/advisories...
Illinois...none.
In...none.
Lm...Small Craft Advisory...lmz740...10 PM Sunday to 3 am Tuesday.

&&

$$

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