Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Chicago/Romeoville Illinois
458 PM CST Thursday Dec 5 2013
250 PM...forecast concerns include snow potential across the
southeast County Warning Area tonight into Friday...potential for an accumulating
snow across much of the area Sunday into Sunday night and below
normal temperatures through the period.
The southeastern County Warning Area will remain on the far northern edge of the a
winter storm that will affect a large area of the country tonight
into Friday. It appears the best chance of snow will be mainly on
Friday but maintained chance probability of precipitation tonight. Fairly low confidence
on just how far north the light snow will extend on Friday with
the potential for flurries/very light snow as far northwest as a
pnt/gyy line and if this does occur...then some minor accumulation
perhaps up to an inch would be possible southeast of a Gibson City
to Rensselaer line and adjustments to the current forecast may be
needed as trends emerge overnight. With a large area of high/middle
clouds spreading across the region...temperatures will likely be slow to
fall tonight and bumped lows up a few degrees across the eastern
half/third or so.
Clouds will begin to clear Friday afternoon into Friday evening
with high pressure spreading east from the plains Friday night
through Saturday night allowing temperatures to fall into the single
digits most areas. But with the center of the high still west of
the area through sunrise Saturday...a northwest wind of 10 miles per hour
will push wind chills down into the -5 to -15 range.
As the high shifts east Sunday...broad weak warm air advection
begins to develop ahead of weak low pressure that will move across
the region Sunday night. This will allow a fairly long duration of
periods of light snow from Sunday midday or afternoon into Sunday
night with good agreement among the GFS/Gem/ECMWF...though some
timing differences still remain. But current trends would suggest
the best time for accumulating snow will be centered on Sunday
evening and while its a little early for specific amounts...a few
inches of snow accumulation will be possible...with the highest
amounts across northern Illinois...with lowering amounts south of I-80.
There is some potential for the snow to lingering into Monday
morning and have maintained low chance probability of precipitation during this time...but
confidence decreases with the time.
Lows Saturday night will likely be reached in the evening with
increasing cloud cover and steady or slowly rising temperatures by Sunday
morning. Highs in the middle/upper 20s look on track Sunday and
should remain fairly steady into Sunday evening. As winds shift
westerly and colder air spreads back into the region...temperatures will
begin to fall overnight into Monday morning. Temperature trends
Monday night through middle week become a bit complicated as a lobe
of very cold air moves across the area with a weak ridge Monday
evening...then warm air advection develops by Tuesday morning
ahead of a fast moving clipper like system. At this point have
maintained current low temperatures...which could still occur in the
evening. Too much uncertainty from this distance to make any large
changes. Also the possibility of some light snow or flurries with
this system. Cms
//Ord and mdw concerns...updated 00z...
Surface high pressure will continue to build across the plains and
into the middle Mississippi Valley through Friday as low pressure
continue to lift northeastward across northern Quebec. Overall this
will support a northwesterly flow across the terminals later
tonight through Friday. It appears that VFR conditions will prevail
at the main terminals across northern Illinois as ceilings look to
remain at or above 10,000 feet above ground level. However...a storm system to our
south will try to spread some lower clouds and light snow in across
portions of central Illinois and into portions of northern Indiana
Friday afternoon. In addition...it appears there could be some lake
effect cloud cover developing late Friday afternoon...and some of
this activity could approach kgyy. In spite of this...I have
remained optimistic and kept this mention out of the tafs.
//Ord and mdw confidence...updated 00z...
* high for all elements.
//outlook for Ord/mdw for 12z Saturday-12z Thursday...updated 00z...
Sunday...chance of snow...especially late. IFR conditions
Monday...chance of snow with MVFR or lower early.
Tuesday and Wednesday...mainly VFR.
214 PM CST
Strong area of low pressure just northeast of Lake
Superior...continues to strengthen and lift northeast this evening
towards James Bay. A trailing cold front has pushed southeast across
Lake Michigan earlier today...with winds generally from the
west/northwest. A tight gradient remains across the northern half of
the lake...where frequent gales to 35 knots continue. At this time
these gales are expected to persist through this evening...then as
the low pressure continues to lift northeast away from the
area...the gradient will diminish with winds gales coming to an end.
Freezing spray looks to be a distinct possibility for the northern
half of the lake through tonight.
Further south this evening a continued west to northwest flow will
keep waves elevated along the Indiana nearshore waters. This will
keep the small craft conditions going for the Indiana nearshore through
Friday morning. With much colder air flowing across the lake...while
the winds are expected to remain below small craft conditions...it
will not take much wind to keep waves elevated. So could see the
need for another extension of the Small Craft Advisory for Indiana.
Northwest flow will likely persist with a much lighter gradient
through the first half of the weekend...with high pressure sliding
overhead sun. Then another system is forecast to develop across the
Southern Plains sun...arriving across the Great Lakes region Monday.
This system deepens quickly to 29.6 inches Monday evening just north of
The Straits of Mackinaw. This will result in an increasing gradient
and possible gales for Monday night through daybreak Tuesday. Then a
brief lull may occur with a weak low pressure pivoting east across
the western Great Lakes Tuesday morning...then a tight gradient returns
lmz868 until 9 PM Thursday.
Small Craft Advisory...nearshore Gary to Michigan City in until 10 am Friday.
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