Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Chicago/Romeoville Illinois
538 am CST Monday Nov 30 2015
325 am CST
The short term will be the more active period with some mixed precipitation
in north central Illinois early this morning. Next on the docket
will be a round of rain later today/tonight followed by the
potential for some light snow Tuesday night into Wednesday.
Right out of The Gate...there has been blossoming area of
precipitation across eastern Iowa and western Illinois that has
expanded into north central Illinois. Dual-pol data of ice pellets
was confirmed by observations of rain turning to a brief period of
sleet as the lower level dry layer was overcome and wet-bulbing
lowered the layer temperature to produce sleet. There is an
impressive shortwave moving through Iowa in an area of focused
isentropic ascent in western Illinois extending into the north
central Illinois. This ascent exists farther eastward...but drier
conditions and less forcing from the upper wave are limiting
clouds to 9000-10000 feet. Height falls are weak initially
today...thus expect that precipitation will be slow to progress
eastward...and this is confirmed by much of the short term
guidance. Therefore this initial precipitation shield should lift into
Wisconsin. Warm advection is turning any lingering precipitation over to
Model guidance has trended a bit farther west with the surface low
associated with the massive upper low in the western U.S. In terms
of where the low will strengthen and eventually occlude. What this
means for our area is that it will take until the main system
approaches and the associated cold front nears to get in on more
organized rain. Plus...rain totals look a touch lower. Better
moisture return is expected later today and into this evening as
lower level flow turns more southerly. Will likely see echoes on
radar streaming in across the area through the day...this will
likely be virga or sprinkles initially. The main rain band will move
through tonight. As the dry slot with the system moves
through...loss like rain will transition to a drizzle for a brief
time before the cold front arrives early Tuesday.
Tuesday will be a blustery day as strong SW winds remain gusty and
usher in colder air. Lower level moisture will likely remain trapped
meaning mostly cloudy conditions but not much precipitation. A more
significant shot of cold advection occurs Tuesday night as the upper
low passes overhead with its cold core. A large deformation region
associated with the cyclone will support some light snow showers
Tuesday night into Wednesday morning. Some light dustings/accums
would be possible during this time.
325 am CST
Thursday through Sunday...
The extended period looks relatively quiet. Following the departure
of the cool upper trough...ridging aloft settles in for Thursday...
resulting in near normal conditions with increased chances for
sunshine...though their could still be some lingering low clouds
Thursday morning in addition to some lake effect clouds in Northwest
Indiana. Upper level heights generally rise Friday into the
weekend...and as this occurs a very strong surface high will
builds over the eastern half of the country. Increased southerly
flow will result temperatures increasing through the 40s with
continued sunshine. Maybe a few more clouds find their way into
the region toward Sunday...but still quiet and relatively mild for
//Ord and mdw concerns...updated 12z...
* east winds gradually increasing in speed today.
* Isolated rain showers this afternoon becoming likely by early
this evening and persisting with possible drizzle into
* MVFR ceilings arriving near sunset falling to IFR through the
evening and maybe quite quickly. LIFR ceilings late tonight
possibly into the beginning of the Tuesday morning rush.
* Chance for IFR visibility later this evening into overnight.
Returning moisture aloft will keep radar echoes...some light
showers while some virga...streaming north across north central
Illinois this morning. While there was temporary sleet earlier at
the rfd Airport...the potential for that looks to end after 15z.
Some of these radar echoes are likely to creep over northeast
Illinois by early this afternoon. Low-level dry air in place is
quite marked over Chicago right now and should keep echoes mainly
virga...though cannot rule out temporary VFR rain this afternoon.
The better chance for rain showers will be after dark with an axis
or two of showers expected to be progressing northeast across the
In-between showers this evening and after them into the
overnight...drizzle looks like a possibility. This tends to often
result in IFR visibility...but some uncertainty remains on how
widespread and long-lasting the drizzle will be...so have not gone
too low in visibility at this time. Confidence is quite a bit
higher in low ceilings occurring tonight. Upstream observations show
locations in MO and Iowa having dropped quickly and expect a similar
trend for Rockford early this evening. For Chicago sites that drop
may be slightly delayed and take longer to fall once it starts due
to the aforementioned dry air...but envision LIFR or near LIFR
clouds to certainly be in place at daybreak Tuesday.
Easterly winds will turn southeast tonight and then veer southwest
by daybreak Tuesday and increase quickly in speed. Gusts in
excess of 25 knots are possible for a period Tuesday morning.
//Ord and mdw confidence...updated 12z...
* high in winds.
* Low in showers this afternoon while medium-high this evening.
* High in ceiling trend and ceilings dropping to 700ft or lower
tonight...but low-medium in timing.
* Low-medium in visibility tonight.
//Outlook for Ord/mdw for 00z Wednesday-12z Monday...updated 12z...
Tuesday night...MVFR with IFR possible. Chance of light rain
changing to snow. Southwest winds.
Wednesday...MVFR with early morning IFR possible. Chance of light
snow. West winds.
Thursday-Sunday...primarily VFR. Southwest winds.
215 am CST
It will be an active first half of the week across Lake Michigan as
ridging departs eastward today and low pressure develops to the
southwest. Easterly winds in advance of this low will encompass
the lake today and strengthen this afternoon. Small craft criteria
winds and waves will be seen by middle-afternoon across the Illinois
nearshore...especially from downtown Chicago northward. Winds will
turn southeast later tonight with waves building over northern
parts of the lake and a temporary easing over the far south.
With the low occluding over southern Minnesota...the system will
spiral a cold front northeastward over the southern and central
part of the lake Tuesday morning. The setup favors southwest winds
to quickly increase early Tuesday morning with gusts likely to
temporarily exceed 30 knots. Cips analog guidance for similar events
as to that forecast indicate 30 to 40 percent of them produce
gale force gusts near the lake...so will need to monitor for a
brief gale period over the south Tuesday morning. Winds will turn
westerly behind a secondary cold front on Wednesday morning. Wind
speeds will remain elevated...in the 15 to 25 knots range through
Thursday...before dropping on Friday as ridging moves across.
Lm...Small Craft Advisory...lmz740-lmz741-lmz742...3 PM Monday to 4
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