Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Chicago/Romeoville Illinois
640 am CDT Tuesday Sep 30 2014
332 am CDT
Main forecast concerns/challenges are with cloud cover and temperatures
today...and then increasing chances for showers and thunderstorms
late Wednesday night into Thursday and Thursday night.
Cold front now south of the County Warning Area early this morning as low stratus
has moved over much of the region...while temperatures continue to fall
into the 40s and 50s under a steady northeast wind. With isolated
showers now east of the County Warning Area expect dry conditions today but with
this stratus likely hanging around for some time this morning.
Guidance appearing to be too quick scattering out these clouds
this morning and trying to indicate there will be holes by 12z.
With this not a likely scenario have gone more pessimistic with
trends this morning and dont have any real clearing until late
morning/midday. Most areas will likely observe a scattering trend
into the afternoon...but with areas near the lake holding onto
these clouds for much of the day. Did lower high temperatures as well as
slowed temperature trend this morning...with it possible for locations in
far northeast Illinois and Northwest Indiana not making it out of
the 50s today.
Quiet but cool night tonight as boundary layer flow veers and any
lingering cloud cover coming off the lake pushes north into
Wisconsin later this evening...and most locations observing clear
to partly cloudy skies. Fairly energetic vorticity maximum currently
lifting north through the Central Plains will continue north into
the northern plains today into tonight. As this occurs...upper
level ridging will be brief tonight as ridge axis shifts to the
east and we quickly transition towards a southwest flow pattern.
This will allow for a period of warm air advection going into Wednesday with
temperatures rebounding back into the 70s for most locations. Made
minimal changes to going forecast highs on Wednesday as
uncertainty with approaching middle/high level cloud cover could play
into the extent of warming that occurs.
The best warm air advection/moist advection will remain just to the west for most
of the day Wednesday but this will change Wednesday night as
middle/upper level trough digs across the Central Plains...and
strengthening low level jet becomes oriented towards northern Illinois. This
coinciding with fairly energetic middle level flow...developing
precipitation shield should begin reaching most locations in northern
Illinois after the midnight time frame. Despite increasing
instability Wednesday night...instability axis does appear to stay
just to the west/southwest of the County Warning Area during this time. Although
this may be the case...strengthening low level jet could offset any lacking
instability and so have maintained chance thunder wording over the
entire County Warning Area...with the far west and southwest areas likely
observing best chances for thunder. Moisture will continue to
increase during this time and do feel there could be some periods
of more moderate rainfall...but this wont likely occur until very
late in the night and towards early Thursday morning.
Better agreement observed with varying models this morning
regarding approaching middle/upper level trough and associated
surface feature Thursday and Thursday night. Surface low should
lift right along the Mississippi River area into southwest
Wisconsin Thursday and then into the Great Lakes region Thursday
night. Continued large scale support during this period will allow
for a wet day Thursday into Thursday night as periods of showers
and thunderstorms are likely. A slower departing trend will occur
Thursday night...and did keep likely wording longer into the
night. Although moisture and instability will continue to increase
during this time...it does look like the best instability will
remain south of the County Warning Area. So at this time...am not overly concerned
with any severe potential but am more concerned with heavy
rainfall and possible localized flooding...especially as the best
overall moist advection occurs into Thursday night.
//Ord and mdw concerns...updated 12z...
* periodic IFR ceilings in lake effect cloud/drizzle band for the next
* MVFR ceilings gradually lifting late morning into the afternoon but
remaining MVFR...possibly through the evening. Period of
scattering possible this afternoon.
* North-northwest-north winds with lake effect band next few hours but shift to
north-northeast expected by middle morning...going further northeast into the
afternoon. Speeds around 10 knots.
Large area of 1000-1500 feet ceilings is spread across the region this
morning. Radar mosaic shows a narrow band of lake effect light
rain/drizzle offshore of kmke southward to Ord/mdw. This is
producing IFR ceilings and MVFR visibility. Winds on the west side of this
band have turned north-northwest with north-northeast winds on the east side of the band.
Expect this band to slowly pivot eastward into middle morning but in
the meantime Ord/mdw will likely be in and out of IFR ceilings and
MVFR visibility in drizzle. Once the band shifts east or even dissipates
winds should turn north-northeast then further northeast into the afternoon.
Speeds look to remain around 10 knots.
Once the IFR improves MVFR will persist but should see scattering
towards rfd/dpa this afternoon with flow off the lake likely
keeping MVFR ceilings at Ord/mdw/gyy through the day...though
scattering is possible. Low level winds will turn southeasterly
this evening which will push the low level moisture and associated
cloud cover to the northwest. There may be some expansion of
coverage as this occurs so rfd/dpa may get back into MVFR but will
keep coverage scattered for now. Do not have a great deal of
confidence in timing of ceiling evolution tonight and may be holding
onto them too long. Skies do look to have scattered or less
coverage Wednesday morning.
//Ord and mdw confidence...updated 12z...
* high confidence in at least periodic IFR next few hours. Medium
confidence in end time of IFR.
* High confidence in MVFR ceilings through the day with bases rising
through afternoon. Medium confidence in MVFR ceilings this
evening/tonight...but low confidence in how long they may last.
Low confidence in potential for a period of scattering this
* High confidence in wind trends. Medium confidence that speeds
will be 10 knots or less this afternoon.
//outlook for Ord/mdw for 00z Thursday-12z Tuesday...updated 12z...
Wednesday night...increasing chance for shra/tsra. MVFR possible.
Thursday...thunderstorms and rain and MVFR likely. South winds.
Friday...low chance rain showers. MVFR possible early. Gusty west winds.
Saturday...VFR. West winds.
Sunday...VFR. Slight chance rain showers. Southwest winds.
Monday...VFR. West winds.
257 am CDT
Strong northerly winds will gradually decrease in speed into the
afternoon as high pressure ridging moves across Lake Michigan. The
pressure gradient will tighten up somewhat Wednesday as the ridge
shifts east and low pressure over the Central High plains tracks
north to Manitoba by Wednesday afternoon. This will allow winds to
shift to the southeast tonight. Will continue with the Small Craft
Advisory through this afternoon. Steady south to southeast winds
will continue into Thursday when low pressure develops over the
Texas/Oklahoma Panhandle and tracks northeast to the u.P. Of
Michigan Thursday night. Forecast models have struggled with the
track and intensity of this system though there has been very
general agreement in the low tracking northwest of the lake and
deepening while doing so. While specifics are still undetermined
the stage will be set for a period of stronger winds across the
lake...starting out southerly then turning westerly as a cold
front passes Thursday night. Several other more subtle surface
troughs may cross Friday into Saturday allowing winds to fluctuate
between west and northwest. A period of gales still looks
possible...primarily after the passage of the main front as colder
air spreads in. Wind speeds will gradually ease going into Sunday.
Lm...Small Craft Advisory...nearshore waters
until 7 PM Tuesday.
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