Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Chicago/Romeoville Illinois
554 am CST Sat Nov 28 2015
435 am CST
An update to cloud cover and lake effect parameters today since
that was not discussed too much earlier. There is a convergent
axis along the western half of Lake Michigan. Observations in
Racine/Kenosha show some very light snow with limited visibility
reductions. Forecast soundings indicate that the low level
unstable layer is very shallow...supportive mostly just continued
cloud cover through the day. Tmke radar is showing some minimal
low dbz echoes. This is low impact...but could see some flurries
into far northeast Illinois this morning.
200 am CST
Still seeing some light precipitation echoes on klot radar early
this morning. The earlier batch of light rain/drizzle along the I-90
corridor lies in an axis of 700 mb frontogenesis...which will weaken
quickly early this morning. Surface temperatures are marginally at
or below freezing...but this activity is moving quick and no reports
of accumulation...just some mping reports of sleet and ASOS reports
of unknown precipitation. Farther south...weak echoes fall in a
warmer environment along a lower level thermal gradient where some
continued light rain/drizzle remains possible through the
morning...with accumulating rain just along our southern tier of
counties and south into central Illinois.
The main baroclinic zone has been shoved southward by a fairly
strong area of Canadian high pressure with northerly streamlines
down through The Heart of the country into southern Texas.
Additional shortwave energy will ride over the boundary today and
Sunday...but the focus will be central Illinois south to the Ohio
Valley. Meanwhile...the local area will see benign zonal flow aloft
today shift to weak ridging on Sunday. This is due to northern
stream low pressure shifting to eastern Canada while the deep upper
low in the west amplifies. At the surface high pressure in the
plains will linger to our west today...then slide by to our north
Sunday. For sensible weather...this means dry and seasonable
conditions...though may feel a touch cool given the brisk north-
northeast winds. With shortwaves still propagating east to west
across the area...there will still be lingering cloudiness
today...especially south of Interstate 80...with less coverage on
200 am CST
Sunday night through Friday...
On Sunday night...isentropic upglide will commence as low level flow
begins to back to the southeast in response to the ejecting western
upper low. Surface cyclogenesis will commence east of The Rockies.
As the surface low moves northwest through the plains and into the
middle Missouri Valley...leading shortwave energy will allow a warm
front to lift northward through the day. Drier air will limit the
northward extent of rain in the morning with many areas dry...but
rain chances increase...especially in the afternoon and evening
hours as upper level height falls increase significantly across the
Details become a bit sketchier Monday night into Tuesday morning as
uncertainties in the track of the strengthening surface low create
problems in terms of being able to predict how cold the temperatures
will be and how quickly the main precipitation axis moves through.
Lower level temperatures coming crashing down Monday night...but
have carried rain over the area through this period as the precipitation
shield will be along and ahead of the cold/occluded front. The upper
level dry slot moves overhead which may also lead to a period of
drizzle/light precipitation overnight as well as the main rain axis shifts
north into Wisconsin.
The upper low passes overhead Tuesday into Wednesday. The eventual
low track will determine how much deformation precipitation
occurs...along with whether it will fall as rain/snow or a mix of
the two. There still appears a decent bet that some wintry precipitation
will occur...but during this time precipitation quantitative precipitation forecast numbers appear somewhat
low. It also looks at least breezy Tuesday night into Wednesday as
the low moves northeast. The latter half of the week appears on the
quieter side as a sprawling surface high in the plains will shift
east over the Midwest.
//Ord and mdw concerns...updated 12z...
* north-northwest winds at Ord shifting to north-northeast during
the early-middle morning with speeds around 10 knots and occasional
* Mainly MVFR ceilings prevailing this morning and likely into early
afternoon...with a chance these ceilings could persist longer.
Northerly winds continue to ever so gradually steer drier air into
the region...however a layer of low-level moisture remains trapped
below an inversion. This has continued MVFR ceilings and should
support them through this morning. Some added moisture and weak
instability off of Lake Michigan will help far northeast Illinois
to see the MVFR ceilings the longest...and its possible it could feed
MVFR clouds through the entire afternoon. There has been sporadic
light echoes across far southeast WI and far northeast Illinois with
light precipitation briefly observed associated with these. The
chances for any of that to reach Ord or mdw today remains below 10
Low-level convergence from Lake Michigan has helped to keep winds
at Ord slightly west of north in direction early this
morning...while mdw remains locked into a north-northeast
direction. North-northeast should be the prevailing and think it
wont be too long before Ord sees that. Speeds are forecast to drop
below 10 knots an hour or so after Sundown.
//Ord and mdw confidence...updated 12z...
* high in winds being mainly north-northeast for much of today.
Low-medium in 14z timing of shift.
* High in MVFR ceilings this morning and bases remaining above 1500
feet. Low-medium in specific scattering time.
//Outlook for Ord/mdw for 00z Monday-12z Saturday...updated 12z...
Sunday night...VFR. East winds.
Monday...rain and MVFR/IFR likely late in the day and especially
during the night. East winds.
Tuesday...MVFR. Chance of rain during the day and rain or snow
during the night. Strong southwest winds.
Wednesday...MVFR. Slight chance of rain or snow. West winds.
Thursday-Friday...VFR. Southwest winds.
144 am CST
A high pressure ridge oriented west-to-east will expand across
the central part of the lake today. This will continue to
gradually ease the winds that have been so gusty across the
southern part of the lake since yesterday. Waves will be even
slower to fall...but are still forecast to drop below 5 feet in the
far southern part of the lake by sunset.
The next system of interest will be a low pressure developing
from the Central Plains Monday evening to near the Minnesota/WI border
by Tuesday morning. The stiff easterly winds ahead of this will
encompass the lake by late Monday into Monday night. This system
is forecast to occlude during Tuesday...with southwest winds and
colder air wrapping around the bottom of the system and over the
lake. The system depth indicated by computer models at this point
does not necessarily scream gales...but the pattern is
conceptually one conducive for some temporary gales or near gale
force gusts during that Tuesday into Tuesday night time frame.
Lm...Small Craft Advisory...nearshore Gary to Michigan City in until 10 am Saturday.
Small Craft Advisory...nearshore Winthrop Harbor to Gary in until 3 PM
Visit US at http://weather.Gov/Chicago (all lowercase)
Follow US on facebook...twitter...and youtube at: