Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Chicago/Romeoville Illinois
906 PM CST Thursday Dec 18 2014
Update...846 PM CST
Weak lake effect snow showers remain largely offshore...but have
had reports of flurries or a light mist in lake and Cook counties of
Illinois. Expect flurries/patchy freezing drizzle to remain
possible for the evening...diminishing quickly overnight. No other
changes to the going forecast for the evening.
249 PM CST
Mainly cloudy conditions expected to persist through Friday as
dirty high pressure moves across the region. High res guidance is
showing hints of lake effect precipitation developing tonight...though
forecast soundings suggest that low inversion heights would keep
the threat of any measurable lake effect precipitation very low. Still
plausible that there could be some snow grains/flurries into
northeast Porter County tonight as low level flow veers more
249 PM CST
Going into the weekend fairly quiet conditions expected. Weak
northern stream shortwave will pass mainly just north of the region
Saturday...but moisture/forcing look too limited for any
precipitation...though warm air advection cloudiness looks to be a good bet. Another weak
shortwave passes just to our north on Sunday with GFS squeezing out
some light quantitative precipitation forecast. European model (ecmwf)/WRF-NAM and parallel GFS keep light precipitation to
our north...so have opted to hang onto a dry forecast for now for
Sunday but with plenty of cloudiness of course.
Next week a significant pattern change is forecast to take place
across the northwestern hemisphere with significant long wave trough
setting up over the upper Mississippi Valley and eventually the
eastern Continental U.S.. the system that will eventually carve out the deep
trough over the region is actually the same significant storm that
just affected Japan...so it has a long fairly and data sparse region
of the Pacific to traverse before arriving in our area. As is
typical with large scale pattern changes...there have been
significant run to run model variations in the handling of the
details of this cyclone/developing trough and would anticipate
further changes in later model runs.
At this point...fairly high confidence that a stormier and
eventually colder pattern will settle in next week. Initially looks
to be more of a rain/shower event early in the week transitioning to
more flurries/snow showers as the upper low settles in over the
region. Latest runs really arent looking too foreboding for
significant snowfall...but at this point would really caution about
latching onto any one solution as there will likely be more model
variability in the coming days. Certainly some potential exists for
travel impacts heading into the middle of next week.
//Ord and mdw concerns...updated 03z...
* MVFR ceilings will likely persist through the overnight hours into
Friday morning. Possible dip to ceilings around 1400ft above ground level overnight.
With high pressure overhead and the lack of mixing...it is likely
that the MVFR ceilings that have been persistent across the region
will continue overnight. To the north of the taf sites ceilings have
dipped in a few locations to around 1400ft above ground level. Some guidance is
beginning to indicate that ceilings could dip further at Ord/mdw
closer to 1400-1600ftagl overnight. Fortunately winds will remain
northwesterly to northerly with speeds generally between 4-7kt
through at least the next 18 hours possibly 24 hours. Ceilings could
eventually push back to VFR late Friday morning...but this is a low
confidence based on the recent past.
//Ord and mdw confidence...updated 03z...
* high confidence in all forecast elements.
//outlook for Ord/mdw for 12z Saturday-12z Thursday...updated 00z...
* Saturday and Sunday...dry/VFR. Light south winds.
* Monday...chance of rain/snow. MVFR likely. Gusty southerly winds.
* Tuesday...chance of rain/snow. IFR likely. Southwest winds.
* Wednesday...chance of snow. MVFR likely. Strong/gust west winds.
230 PM...deep low pressure is moving northeast through Nova Scotia
while a broad area of high pressure extending from Ontario to the
central Gulf Coast slowly moves east and will extend from Quebec
through the lower Mississippi Valley by Saturday. As the high
pressure continues to very slow spread to the east across the
lakes...winds will become light and variable for a period Friday
afternoon and Friday night. A weak trough of low pressure
developing over the northern plains as the high builds east will
set up light southerly flow over the lake. This initial trough
will move little through Saturday and then a low pressure system
is expected to drop out of the southern Canadian rockies by Sunday
evening...helping to deepen the trough into a closed low. This low
is then expected to track east to Wisconsin early next week and as
the low gets closer to the lake...the southerly gradient will
strengthen to 20 to 25 knots as the low deepens as it approaches the
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