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Area forecast discussion 
National Weather Service Chicago/Romeoville Illinois
825 am CST Friday Dec 19 2014

Short term...
259 am CST

Through Sunday...

The main concerns during the short term will be some periodic
chances of very light snow or drizzle through the weekend as several
weak...and fast moving middle level disturbances shift across the area.

Surface high pressure is centered across much of the middle and upper
Mississippi River valley early this morning. This feature is expected
to remain in control across northern Illinois during the day today.
Overall...which will result in a very light and variable wind
field across the area today. An area of lake effect flurries
continues across portions of the Chicago area...and into
northwestern Indiana near Lake Michigan. Model soundings indicate
just deep enough inversion heights to support ice in the
clouds...and given mdw has been reporting only -sn for most of the
night it appears the threat of freezing drizzle will be low. All
of this activity should wane across the area during the morning as
the wind field continues to weaken over South Lake Michigan in
response to the approaching surface ridge of high pressure.

Surface winds will begin to shift out of the south by early Saturday
morning as this surface high begins to shift east of the region. As
this occurs...guidance is insistent with bringing a weak upper
disturbance across northern Illinois Saturday morning. This weak
wave is noted in the water vapor imagery currently across
southeastern Alberta. Moisture appears to remain limited with this
wave. However...I think this wave...and some weak isentropic ascent
could squeeze out a few flurries Saturday I have
included a slight chance for such in the forecast.

Warm air advection/isentropic ascent will continue to increase
across much of the Midwest and Great Lakes region into Sunday in
response to an increasing southerly wind field ahead of a storm
system...which is expected to develop across the plains. This could
again result in a period of light precipitation across mainly far northern
Illinois very late Saturday night and into Sunday.
Overall...confidence is low as to how far south this light precipitation
will be able to develop...but given the potential that if this
precipitation does develop as far south as northern Illinois...there could
be a period of freezing drizzle into early Sunday morning before
temperatures jump above freezing...I have opted to include a slight
mention late Saturday night into early Sunday...mainly for areas
north of Interstate 90.



Long term...
259 am CST

Sunday night through Thursday...

The long term period will be a period of a significant weather
pattern shift across North America and as appears likely
that the central Continental U.S. Will become much more active into the
Holiday week. The main concerns locally with this pattern
shift...will be precipitation types...and the actual evolution
of...potentially a significant storm system across portions of the
Great Lakes region Christmas evening.

Unfortunately...the details of this storm system this far out are
still largely large scale pattern shifts such as this
typical result in lower predictability...especially with regards to
the track and actual strength of a developing storm system. What
appears to be for a period of rain across the
area late Monday and Monday night as what appears to be the initial
storm system...begins to shift a warm front across northern
Illinois. Given the warm sector placement of my area...precipitation type
looks to be liquid...with surface temperatures likely to remain
above freezing Monday night with continued southern flow.

The main uncertainties come into play by Christmas model
guidance continues to advertise a significant amplification to the
upper level pattern across the eastern Pacific and North America. A
strong Pacific jet (near 40 degrees n)...initialized at over 180 knots
in this evenings model output is forecast to result in the building
of a potent upper ridge just off the California coast by early next
week. Teleconnections related to such pattern shift certainly support
the digging of a significant trough...and a good possibility of an
storm system across the central Continental U.S. During the upcoming Holiday
week. previously mentioned confidence is low with the
details of how and where exactly this storm may track. This is a
period Worth watching at least some snow and possibly
strong northwest winds could be in store.




//Ord and mdw concerns...updated 14z...

* MVFR ceilings to persist much of period...lifting near VFR or
possibly scattering out for a time middle-morning through this

* Light winds 5 knots or less. Likely variable or calm later
today/this evening.


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

Diffuse surface high pressure ridge over the area will drift very
slowly to the east today and tonight...resulting in light winds
and persistent MVFR cloud layer trapped beneath shallow inversion.
Area of clearing south of the terminals from mli-dnv-ind not
moving much...though there has been some erosion of ceilings
southwest of Chicago. Forecast soundings generally poor at
handling moisture beneath this shallow inversion...but there is
some indication of some raising of ceilings or even scattering
out for a time later this morning into this afternoon. MVFR
ceilings likely to redevelop and/or lower later tonight however as
surface ridge passes and light south-southwest flow develops and
leads to moistening of low levels.

As for winds...light northerly surface flow around 5 knots expected to
become variable at less than 5 knots as surface ridge moves through
today and this evening. Winds then become south-southwest around 5
knots tonight into Saturday morning.


//Ord and mdw confidence...updated 14z...

* medium-low in ceiling heights...though medium in general trend.

* High in light wind trends.


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

Saturday night...MVFR likely. Light south winds.

Sunday...slight chance flurries/freezing drizzle early. MVFR likely.
South-southeast winds.

Monday and Tuesday...chance of rain...especially Monday night. MVFR
likely. Southeast winds Monday becoming southwest Tuesday.

Wednesday...chance of snow. MVFR possible. Gusty west winds.

Thursday...dry. VFR.



156 am CST

Main marine concerns this morning are a period of strengthening
south-southeast winds late Sunday through Monday night...then the
potential for west gales late Wednesday into Thursday.

Fairly quiet stretch of weather across Lake Michigan today through
Saturday as a large area of weak high pressure drifts slowly east
across the region. Light north winds today will become somewhat
light/variable...before becoming light south on Saturday as the
surface ridge axis passes to the east. Southerly winds will then
begin to increase Saturday night and more so low
pressure develops in the Lee of the northern rockies and the
gradient begins to tighten across the plains and upper Midwest.
Medium range forecast models are in general agreement in developing
a sub-1000 mb surface low across Iowa or southern Minnesota by
midday Monday...which would lead to south-southeast winds in the
20-25 knots range through Monday before occluding and weakening as the
low moves across Wisconsin and Lake Michigan on Tuesday.

However...various model and model ensemble members then develop a
second low south of the lakes along the trailing cold front Tuesday
night...which would lift north and combine with the filling Great
Lakes low...deepening rapidly Wednesday and Wednesday night over the
eastern lakes. This scenario would lead to quickly increasing
westerly winds across Lake Michigan...quite possibly west gales...
late Wednesday into Thursday/Christmas day as the low pulls away to
the northeast.



Lot watches/warnings/advisories...


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