Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Chicago/Romeoville Illinois
549 am CST sun Jan 25 2015
329 am CST
Have furthered the diminishing snowfall trend in the forecast for
today...mainly north of Interstate 80. Across the entire area this
looks to be a mainly low impact...nuisance type snow...with some
communities in the north quite possibly not seeing any
accumulation. Have continued a Special Weather Statement for areas
along and south of I-80.
Satellite imagery this morning depicts the closed upper circulation
across western Iowa with an area of cooling cloud tops in advance of
this moving into northwest Illinois. This is correlating with mainly light
rain or rain/snow mix in the small window of deeper moist
advection ahead of the low. Simple extrapolation of the current
area of precipitation in Iowa and the developing deformation on the
backside clearly indicate the southern County Warning Area is more supported for
precipitation than the north. At the surface...temperatures are
around or just above freezing across much of the forecast area. A
developing cold front is advancing southward into northern Illinois with
dew points in the middle to upper 20s north of this feature...another
hindrance of appreciable precipitation in northern areas of the
Short range guidance like its long range counterparts the past
several days show a fair amount of discrepancy...namely on coverage
of precipitation today and precipitation type in the southern County Warning Area this
morning...but they lend support to the observational trends. As
the area of rain associated with elevated isentropic ascent moves
into the southern forecast area...profiles support a period of
rain or mix initially...and some locations might have air
temperatures right at freezing. Would expect any freezing rain
threat to be short lived as profiles trend toward snow. Mesoscale
convergence and baroclinic forcing for induced lift would become
more favored just to the northeast of the closed 850mb
circulation as it pivots more eastward. That is favored across
the southern tier or two of counties later this morning into
early afternoon. Profiles are mild for ideal snow growth...and
maybe even for snow at all down toward Gibson City for much of the
morning. Have snow totals around one to two inches from Peru to
Rensselaer and southward for today...with snow departing near or
soon after Sundown.
As for areas north of Interstate 80...should see a gradual increase
in light snow areawide by mid-morning. Dry corridor near 850mb
seen on early morning amdar soundings over Chicago should limit
intensity of any snow in the northern forecast area...as will just
less forcing. Areas of northeast Illinois still look to have
onshore flow behind the cold front...but forecast inversion
heights as well as adapted ones from the 00z grb radiosonde observation last
evening...indicate very marginal depth of convective clouds. Thus not
expecting much if any enhancement to already minimal synoptic
forcing over northeast Illinois. Could see some disorganized light snow
or flurries late this afternoon into this evening as profiles of
the lake induced stratocu cool into the ice initiation layer...but
this should not be a big deal.
Areawide the northeast winds will increase to 30 miles per hour and maybe even
a little higher just of the lake. With minimal snow north not
expecting any blowing snow issues...and south the low snow-to-
liquid ratios should suppress much for blowing issues...though
could lead to more reduced visibility during the time of falling
snow this afternoon.
329 am CST
Monday through Saturday...
The disturbed flow pattern will turn more northwesterly Monday and
Tuesday as a major system is expected to develop off the northeast
coast. Given this possibly leading to a slower pattern...as well
as the trend of todays system...it is quite conceivable that the
next disturbance for Monday afternoon and night trends west as
well...which the ec has had in its solution for three consecutive
runs. This system though is not as strong as todays as it is more
of a sheared area of vorticity along the lingering baroclinic
zone. Forcing for precipitation looks pretty light once again Monday
afternoon and night but enough to go a little higher on probability of precipitation for
northern parts of the forecast area. Right now guidance is in
decent agreement with saturating deep enough for precipitation type to be
light snow if it can occur...but need to keep an eye out for some
freezing drizzle potential in these light ascent situations.
A slow moderation looks to occur Tuesday into Wednesday and this
could be very minimal depending on cloud cover which remains a big
question mark. Have leaned more cloudy Tuesday but still think
with a westerly push Wednesday would have sun potential. Long
range guidance indicates a Pacific system to be the next one to
possibly bring precipitation to the area Wednesday night into
Thursday. Continue probability of precipitation during this time frame and at this point
profiles are near the border for rain or snow...so did not get too
fancy with the forecast precipitation type. Behind this system it
seems favored for temperatures to cool some during the latter
half of the week...to slightly below normal values. Obviously any
fresh snow cover would help lean that more toward below normal.
//Ord and mdw concerns...updated 12z...
* IFR ceilings early with light snow developing this morning. Ceilings may
actually come up a bit with the snow...though visibilities MVFR/occasional
IFR are probable with some light accumulation. Snow expected to
taper off to light shsn/flurries by early afternoon.
* MVFR ceilings expected this afternoon and this evening.
* Strong gusty northeast winds around 30 knots at times. Winds slowly
diminish through this evening.
Weak cold frontal boundary has sagged into northeast Illinois
early this morning...with IFR/LIFR ceilings noted in the vicinity of
the frontal boundary. Expect the LIFR ceilings to be somewhat short-
lived however...as winds ramp up from the northeast.
Radar clearly shows circulation with Alberta clipper dropping
southeast across Iowa...with expansion of weak returns over northern
Illinois and a few reports of light snow beginning to reach the
ground. High-res hrrr has been fairly consistent in brining north
edge of more organized snow band across the terminals this morning
into midday/early afternoon...and is expected to produce a period
of light accumulating snow with occasional IFR visibilities. Snow should
taper off to light snow showers/flurries this afternoon with ceilings
rising to MVFR.
As indicated above...strong/gusty northeast winds are already
ramping up early this morning with gusts 20-30 knots Alread across
far northern Illinois. 040-060 degree winds near 20 knots with gusts near 30
knots are expected to contiune into the afternoon before slow decline
in wind speeds and a gradual backing to the north occurs. Winds
will diminish more quickly tonight...as a weak high pressure ridge
builds in from the northwest. This should allow lower clouds to
scatter out overnight.
//Ord and mdw confidence...updated 12z...
* high in wind trends.
* Medium-high in snow timing...medium in details of ceiling/visibility
* Medium-low in timing of scattering MVFR ceilings tonight.
//outlook for Ord/mdw for 00z Tuesday-12z Sunday...updated 12z...
Monday night...chance light snow/MVFR.
Tuesday...MVFR ceilings possible.
Wednesday...mainly VFR during the day then chance of -rasn and
MVFR/IFR at night.
Thursday...chance of snow. MVFR/IFR possible.
Friday...chance of MVFR ceilings.
Saturday...chance of light snow/MVFR.
137 am CST
Main marine forecast focus continues to be a period of strong
northeast winds across the southern 2/3 or so of Lake Michigan
through this evening.
A cold front has pushed south across much of Lake Michigan since
Saturday morning...with cold high pressure spreading across
Manitoba...western Ontario and the northern lakes. Early this
morning an area of low pressure was over Iowa...and this low will
move quickly southeast to the lower Ohio Valley by this evening.
This low and the high pressure to the north of the lakes will
combine to rapidly tighten the surface pressure gradient across the
lake this morning...with north-northeast winds increasing to 25-30
knots. Recent observation platforms along the West Shore of the lake
have been gusting 26-27 kts behind the cold front as far south as
Milwaukee. These stronger winds will continue to develop southward
down the remainder of the lake this morning. Forecast soundings
indicate winds to 30 knots will be prevalent through the day...with a
few gale force gusts to 35 knots possible. Winds will remain gusty into
the evening hours across the far southern portion of the lake...but
will diminish rapidly from the north as a weak ridge of high
pressure spreads in behind the quick moving low which move to the
Carolina coast by Monday morning.
The weak ridge will move quickly southeast of the lake Monday...
with winds briefly becoming southerly and increasing back around 20
knots or so ahead of another low pressure system which will drop
southeast across the lake Monday night...while filling. Weak
gradient conditions will then linger across the lake Tuesday and
Tuesday night as the trough fills and a weak ridge moves across the
region. Indications are that another low will develop across the
plains mid-week...and will approach the lake from the west late
Wednesday. This may result in a brief period of enhanced
east-southeast winds late Wednesday or Wednesday night...with a
period of gusty northerly winds possible Thursday as the low moves
east of the lake.
For the Illinois/Northwest Indiana nearshore area...Small Craft Advisory
is in effect from 6 am CST this morning as winds/waves quickly
increase and build. Advisory headline will remain in effect through
middle-morning Monday...as waves slowly subside below 4 feet.
Lm...Small Craft Advisory...Indiana nearshore waters until 9 am Monday.
Small Craft Advisory...Illinois nearshore waters until 9 am Monday.
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