Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Chicago/Romeoville Illinois
411 PM CST Tuesday Jan 27 2015
304 PM CST
Primary concern in the near term revolves around the stratus deck
currently over the southwestern 2/3rds of the County Warning Area. Would anticipate
a continued very slow clearing from the east for a couple more hours
before low level flow diminishes this evening before backing to
southerly overnight...which may well send stratus sloshing back
north into areas that had previously cleared out. Tried to reflect a
trend like this in the hourly sky grids and resultant temperature
grids...but given the poor model handling of this cloudiness it
pretty much guaranteed that our current sky grids will be wrong
somewhere and need updating with accompanying changes needed in
temperatures tonight. Where skies become/stay clear tonight could see
freezing ground fog develop given the light winds...but will let
evening shift watch trends and add freezing fog as needed.
For Wednesday have opted to be optimistic in pushing stratus north
and breaking it up in the morning but this very well may end up
being too optimistic. Assuming there is some sunshine tomorrow prior
to the arrival of extensive middle and high level cloudiness then highs
could push well into the 30s...particularly in the less snow covered
and more urbanized areas. Developing low pressure to our west
should result in tightening pressure gradient and strengthening
southerly winds Wednesday.
304 PM CST
Thursday night through Tuesday...
Brisk south winds...warm air advection...and cloud cover in advance
of the system Wednesday night should result in nearly steady temperatures
in the evening and probably rising a bit overnight. Could see a bit
of light rain or drizzle in the warm sector Wednesday night followed
by some potential for a bit of wrap around rain or snow showers
Thursday on the back side of the system. Still looks like pretty
limited moisture with the system and highest quantitative precipitation forecast focused to our
north with light amounts in our County Warning Area and only minimal threat of any
accumulated frozen/freezing precipitation given the forecast track and
Shot of seasonably cold air will knock temperatures to just a hair below
average Friday. Vigorous Alberta clipper is forecast to result in
an amplifying long wave trough over the central Continental U.S. This weekend.
Still some large spread between models with respect to the details
this weekend...but there has been some agreement the past couple
days on the general idea of a large swath of accumulating snow
potentially affecting the region Sat night into Sunday. Arctic air
is forecast to surge south into the area in the wake of this system
early next week...but the duration and magnitude of this Arctic
intrusion are dependent on which model you look at meaning fairly
low forecast confidence at this point.
//Ord and mdw concerns...updated 22z...
* gusty south-southeasterly winds tomorrow.
* Chance for MVFR ceilings or visible late tonight.
Broad MVFR stratus deck blankets the region today but has been
steadily eroding from the northeast...with clearing skies making
progress Down Lake Michigan this morning and into the Chicago area
early this afternoon. Cloud bearing flow out of the northeast this
morning will back to the north this afternoon and evening so expect
the progress of the clearing to slow some...and may even reverse
tonight as a surface ridge builds across Indiana with flow becoming
more southerly. Surface winds will generally decrease from middle
afternoon on as the surface high nears...and go light and
variable for a period this evening and overnight...before turning
south/south-southeast tomorrow. Gusty winds pushing 20 knots are
expected tomorrow afternoon.
//Ord and mdw confidence...updated 22z...
* low-medium confidence in ceiling/visible trends overnight.
//outlook for Ord/mdw for 00z Thursday-12z Tuesday...updated 12z...
Wednesday night...chance of rain/drizzle. MVFR/IFR possible.
Thursday...chance of rain/snow. MVFR likely/IFR possible.
Friday and Saturday...primarily VFR. Chance of snow Saturday
Sunday...chance of light snow. IFR possible.
Monday...MVFR ceilings possible.
318 PM CST
Lighter northerly winds in place this afternoon as high pressure
spans the region...will further diminish this evening as this high
moves overhead. This high will then continue southeast towards the
Ohio Valley tonight as a trough of low pressure moves east across
the Central Plains. This will help the winds to turn back more
southerly and then begin to slowly increase later tonight into
Wednesday. This increasing trend will continue throughout the day
Wednesday...with southerly winds really increasing during the
afternoon time frame. This will likely create winds hazardous for
small craft...and so have issued a Small Craft Advisory for
Wednesday afternoon through late Wednesday night when winds will
diminish with the approaching low pressure. This will likely be
brief as winds will quickly increase once again on the backside of
this system Thursday. Also...southerly winds across the southern
half of the lake could hover around 30 knots a for a brief window
Wednesday night. Will need to monitor trends during this
period...but will make mention of occasional gale force gusts
during this time. Expect strong winds to be in place on
Thursday...really increasing into the afternoon and evening hours.
Gales are appearing likely for the entire lake during this
time...even for the nearshore waters...but with speeds diminishing
once again late Thursday night into Friday.
Lm...Small Craft Advisory...lmz740-lmz741-lmz742-lmz743-lmz744-
lmz745...noon Wednesday to 4 am Thursday.
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