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Area forecast discussion 
National Weather Service Chicago/Romeoville Illinois
645 am CDT Friday Oct 9 2015

Short term...

Through tonight...

326 am...forecast concerns include much cooler temperatures today
along with a chance for lake effect rain showers and then low
temperatures tonight.

A cold front is moving south across the County Warning Area early this morning
with temperatures slowly falling into the 50s. Low clouds are spreading
in behind the front. As colder air moves across the lake...the
hrrr continues to develop some lake effect showers by middle morning
and spreading those into mainly Northwest Indiana by early
afternoon. Confidence is fairly low regarding how widespread these
may become but enough of a signal to include slight chance probability of precipitation
for a few hours midday. Cloud cover will then begin to thin/
scatter into the middle/late afternoon as drier air works into the
area. Assuming there is some sunshine later this afternoon...
temperatures may drift back to around 60...but temperatures will generally be in
the middle/upper 50s for much of the day.

High pressure quickly builds across the area this evening with
winds diminishing slowly through the afternoon...then likely light
and variable this evening. With clearing skies...temperatures will
quickly fall into the 40s and its possible some of the usual cool
spots could dip into the upper 30s by Saturday morning. Cms


Long term...

Saturday through Thursday...

326 am...forecast concerns include high temperatures and winds both
Sunday and Monday.

The high will move southeast of the area Saturday night. Winds
will turn back southwesterly Saturday and may be a bit breezy
across northern Illinois where the gradient will be stronger. As warmer
air spreads back into the region...highs Saturday will reach the
middle 60s most areas. Warm air advection continues Saturday night
into Sunday with 850mb temperatures into the +15c to +17c range Sunday
afternoon. With no significant cloud cover expected and the low
levels mixing to near or just under 850mb...current highs for
Sunday in the upper 70s to near 80 look on track and could be
several locations that tag 80. Southwesterly winds will become
quite breezy with gusts to 30 miles per hour possible.

Sunday night likely to be very warm for middle October. With normal
low temperatures in the middle 40s...low temperatures Sunday night probably near 60
with some upper 50s in more rural/outlying areas. This should help
temperatures jump quickly Monday morning. However...a cold front will be
approaching the area and may arrive around or shortly after
sunrise. Cooler air spreading in behind this front makes for a
tricky temperature forecast. Regardless...temperatures likely will be falling by
mid/afternoon. In addition...winds will be shifting to the west/
northwest and could become quite strong as forecast soundings show
30-35kts at the top of the mixed layer. A bit early to go this
strong but windy conditions are possible Monday.

A progressive but generally dry pattern is then expected into late
next week as a series of troughs and ridges move across the
region. Cms



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

* MVFR ceilings lingering through late afternoon...trend VFR with mostly
clear skies near 00z.

* Northwest winds turning north and increase...speeds between
10-14kt and gusts up to 24kt at times late morning.

* Winds turn northeast 010-030 degree direction around 16z or shortly
after. Wind speeds/gusts diminish after 22z.


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

Considerable stratus layer has developed and continues to slide
south/southeast across northern Illinois/northwest in airfields.
Guidance continues to indicate that MVFR ceilings will hover around
1800-2500ft above ground level through mid-morning...then begin to trend towards
3kft above ground level by midday and then start thinning towards VFR and mostly
clear skies by this evening. Mixing and a tight pressure gradient will
supply increasing northerly winds with speeds by early morning
reaching 10-14kt and a few gusts in the upper teens...then by
middle-morning speeds will hover between 12-15kt and gusts may
increase to around 20-24kt. With high pressure building in...winds
will quickly diminish around 00z and may become light and variable
or less than 5kt overnight into daybreak Sat.


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

* high confidence in wind dir/speeds. High confidence in gusts
developing and in timing of gusts.

* High confidence in MVFR ceilings...and in duration.


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

Saturday night and Sunday... VFR. Breezy SW wind.
Monday...VFR. SW winds turning breezy northwest.
Tuesday...VFR. West wind.
Wednesday...VFR. Winds quickly turning breezy northwest.
Thursday...VFR. West/northwest winds.



228 am CDT

Low pressure has lifted northeast across Lake Huron and will
continue to slide east into the northeastern United States by late
tonight. Meanwhile a trailing cold front has cleared the southern
tip of Lake Michigan...bringing a tight gradient across the lake
and winds turning northwest to north. A broad ridge of high
pressure will quickly arrive later today...however between the two
systems winds are expected to increase and become gusty at times.
Current guidance is indicating gusts to approach 30kt at times for
the open waters and also portions of the Illinois/in nearshore...mainly
focused after daybreak through early to mid-afternoon. Wind
direction may linger more from a 340-350 direction...which would
likely allow waves to build more for the Indiana nearshore and
southern half of Lake Michigan open waters. But if winds can turn
to a 360-020 direction...then waves would also build for the
Illinois shore. Waves may approach 8-10ft in the southern half and
nearshore waters later today...and despite high pressure building
overhead...the cooler air may help maintain larger waves a little
longer into the overnight hours before subsiding early Saturday.

High pressure then slides east Saturday afternoon with winds
turning southwest. A tight pressure gradient returns again late in
the weekend...which should allow waves to build for the
central/northern half of Lake Michigan into Monday. There may be a
period of gusty winds to 30kt Saturday afternoon and a few gales
may occur as well. Periods of strong southerly winds look to
continue over the lake through early Monday. Thereafter...a cold
front...associated with the strong low moving from central Canada
into the Hudson Bay...will shift over the Lake. A west-
northwesterly wind will likely set up in the wake of this frontal
boundary by Monday evening.



Lot watches/warnings/advisories...
Lm...Small Craft Advisory...nearshore waters
until 4 am Saturday.



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