Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Chicago/Romeoville Illinois
544 am CST Friday Nov 28 2014
340 am CST
Main forecast concerns/challenges are with light snow this morning
with light accumulations for the northern third of the
County Warning Area...slight possibility for a brief period of freezing rain or
sleet early this evening for far northern Illinois...and a warm up
for this weekend.
Latest radar imagery showing some echoes across the western half
of the County Warning Area...but with this not amounting to anything at the
surface at this time. Approaching middle level impulse with strong
warm air advection ongoing across the region is allowing for persistent lift over
the County Warning Area. Latest observation indicating still only middle level cloud cover in
place as a good amount of low level dry air in place is limiting
saturation early this morning. Latest guidance still varies to the
extent of snowfall this morning...varying from none to some
development across northern Illinois through the 8-9 am time
frame. Have maintained previous forecast of likely probability of precipitation as I do
think it will snow this morning with areas across northwest
Illinois likely observing snow within the next hour. I do wonder
the extent of any moderate snow...once again owing to the drier
air. Nonetheless...developing snow for northern Illinois early
this morning will progress east over the next several hours with
the best chances for more accumulating snow around of around one
half inch or less likely occurring closer to the
Illinois/Wisconsin border. This precipitation will depart to the
northeast by middle morning...leaving dry conditions and cloudy skies
in place through early afternoon.
Although guidance not really indicating it...current strong warm air advection in
place this morning will likely continue this afternoon. While this
occurs...another upstream impulse will drop southeast towards the
County Warning Area late this afternoon into early evening. I did introduce slight
chance probability of precipitation during this period mainly for the far northern tier of
counties...as if it were to precipitate...column would be
supportive of a wintry mix. With decent warming in the low levels
and surface temperatures still hanging around freezing or slightly
below...precipitation could be in the form of light freezing rain. Once
again...am not confident with this scenario but with persistent
forcing throughout today...dont think it would take much for any
precipitation to fall out the clouds later today. Will need to continue
monitor this time frame...mainly through the 1-2z time frame
before entire precipitation shield fully lifts north of the County Warning Area tonight.
This will leave dry conditions in place through most of the
weekend before slight chance for rain across the southeast County Warning Area
Sunday with the approach another wave. Made minor changes to highs
Saturday and Sunday warmer temperatures still anticipated...but lingering
cloud cover could limit the potential to reach going forecast
340 am CST
The start of next week will be marked with a return to a colder
air mass...as high temperatures on Monday are in the 20s. Conditions will
generally be dry through middle week...with the possibility of light
snow on Tuesday. Guidance then begins to vary going to
Thursday...with the approach another system. At this time...it
appears to be timing issues as chances for precipitation return once
again late in the work week...with some warming likely during this
//Ord and mdw concerns...updated 12z...
* patchy high end MVFR through 13z with light snow.
* South winds around 5-7kt will increase with gusts nearing 20kt
* Possible patchy fog developing after midnight tonight.
Considerable high based clouds continue to slide southeast across
northern Illinois/northwest in airfields early this morning. Regional
radar indicates the back edge of the light precipitation nearing
krfd...and should be slowly ending around 13-14z for Ord/mdw/gyy. Most
of the ceilings with the light snow have remainined above 6kft
above ground level...however there may be brief dip to 3kft above ground level for Ord/rfd
before lifting back up. The light snow should only reduce visibilities to
5-7sm...otherwise not expecting any further reductions to visibilities
this morning. Better mixing will develop midday and allow
southerly winds to slowly increase...and likely produce gusts near
20kt. Then after 02z gusts will quickly dissipate. High based clouds
will linger all aftn/eve...then slide northeast. There is some
indication from guidance that patchy fog may try to develop late
tonight as warm air aloft pushes north over northern Illinois/northwest
in airfields. This may be able to produce some patchy low clouds
and reduce visibilities. Guidance also indicates the light precipitation to remain
north of the airfields.
//Ord and mdw confidence...updated 12z...
* high confidence in light snow ending shortly after 13z.
* High confidence in ceilings remaining VFR this morning with the light
snow. Then remaining VFR for the rest of today.
* High confidence in wind dir/speed/gusts.
* Medium confidence in patchy fog developing late tonight.
//outlook for Ord/mdw for 00z Sunday-12z Friday...updated 12z...
Saturday night...mainly VFR conds.
Sunday...mainly VFR. Possible light rain in the morning.
Tuesday...slight chance of light snow early with patchy MVFR ceilings.
Thursday...mainly VFR. Patchy light rain or snow with patchy MVFR
218 am CST
Weakening ridge of high pressure centered over the Tennessee
Valley this morning...will slide east across the middle-Atlantic
region this evening. This will allow a tightening gradient to
develop over the lake...allowing southerly winds to steadily
increase. Across the northern plains...an area of low pressure is
expected to develop and slowly push east towards the upper Midwest
Saturday morning. There may be a period this evening that small
craft conditions could develop in the Northwest Indiana nearshore
waters. Otherwise wind speeds should remain just under criteria. This
will maintain a relatively tight gradient over the lake through
Saturday...then as the low moves northeast towards James Bay early
Sunday the gradient should briefly diminish. Then the focus turns
towards Sunday afternoon/evening as a strong ridge of high
pressure is expected to slide south from the Canadian rockies into
the norhtern plains. This is expected to redevelop the tight
gradient over the lake and may push wind gusts to around 30 knots. At
this time guidance has not suggested anything stronger for Monday.
High pressure then quickly slides across the lake Monday
night/early Tuesday. Then later on Tuesday as high pressure moves
east...a tight gradient returns and could be strong enough to
produce gales across the central/northern portions of the lake
Tuesday evening. Winds should slowly diminish Wednesday night into
early Thursday...although guidance then indicates another increase
to the winds may occur later in the week.
Visit US at http://weather.Gov/Chicago (all lowercase)
Follow US on facebook...twitter...and youtube at: