Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Chicago/Romeoville Illinois
112 am CST sun Nov 29 2015
925 PM CST
High pressure is centered over the upper Mississippi Valley this
evening...resulting in calm to light northerly winds. A bit of a
tricky sky cover forecast...with upper jet and middle level moisture
over the southern 1/2 or so of the County Warning Area resulting in persistent middle
clouds...while there is a lingering lake effect stratus plume near
the Illinois/northwest in shore. Middle clouds over Iowa not expected to
arrive in areas across far northern Illinois that are currently
mostly clear until later in the overnight. Therefore...lowered
mins in these areas a few degrees to the lower to middle 20s/21 to 24
range. Elsewhere...going forecast lows of upper 20s to lower 30s
outside Chicago and middle 30s downtown appear to be on track.
321 PM CST
A dome of high pressure sprawled from the upper Midwest across the
Great Lakes has pushed an axis of moisture and precipitation south
of the area into the Ohio and Tennessee valleys today. This high
and the corresponding ridging in the upper flow should support
continued dry weather through the weekend...although there will be
plenty of moisture streaming through the area aloft...and models
are hinting at the possibility of trying to spin up enough
shortwave energy in the ridge to support some light showers
tonight into Sunday. Latest radar images show a pocket of returns
moving out of southeast Iowa...but forecast soundings and recent
acars profiles both indicate the presence of very dry air in a
layer around 850- 900mb locally. Even under the radar returns the
most that is being reported is light drizzle. Evidence would
suggest keeping the forecast dry and favoring virga or sprinkles
over measurable rainfall tonight and tomorrow.
By Sunday night into Monday models remain in good agreement in
kicking a large upper low out of the southwest Continental U.S. And into the
Midwest. The surface reflection of this low lifts out of the
western Gulf into Missouri and Iowa on Monday. Models are not
agreeing as well in regard to the onset of precipitation during the day
Monday. Have continued with the idea of focusing the best chances
during the afternoon...but probability of precipitation may need to be boosted in the late
morning if the faster NAM and GFS solutions become the favored
trends. Forecast soundings also support the possibility of thunder
Monday evening into Monday night with strong warm advection
around the 850mb level and decent middle level lapse rates.
321 PM CST
Monday night through Saturday...
On Tuesday and Wednesday the surface low lifts through Lake Superior
into Ontario while the upper low spins across Wisconsin. Cooler and
drier air filters in around the back side of these...raising the
possibility of a rain and snow mix or just some light snow toward
midweek before the upper trough finally moves farther away to the
The last half of next week looks clear and dry under the
influence of broad high pressure across the central Continental U.S..
especially by Friday and Saturday as the ridge slides east and
puts the local area into milder southwest flow...daytime highs may
end up being a few degrees warmer than the forecast presently
//Ord and mdw concerns...updated 06z...
* patchy lake effect MVFR ceilings through the morning.
* Northeast winds possibly reaching 9-10 knots briefly this
Nearby high pressure across Wisconsin will keep wind speeds
lighter today. A northerly wind this morning will become more
northeast by noon with speeds coming up a few knots. The flow from
Lake Michigan this morning will continue to support some patchy
lake-induced 1800-3000 feet clouds. If these go broken it should be
//Ord and mdw confidence...updated 06z...
* low in MVFR clouds actually going broken...but high that bases
will remain above 1500 feet.
* High in wind speeds remaining below 10 knots for most of the taf
//Outlook for Ord/mdw for 12z Monday-12z Saturday...updated 00z...
Monday night and Tuesday...MVFR possibly dropping to IFR. Occasional
rain showers and dz.
Wednesday...MVFR ceilings likely. Chance of flurries.
Thursday and Friday...primarily VFR.
111 am CST
General high pressure is prevailing across the western and northern
Great Lakes this morning and will move eastward today...allowing for
wind direction to be variable at times over the center part of the
lake. Easterly winds will increase throughout Monday and Monday
evening in response to low pressure deepening and moving northward
across the Midwest. Small craft criteria conditions are likely over
at least a part of the Illinois shore for a period late in the day
Monday and Monday evening. Winds will shift southerly overnight
Monday night and then turn sharply southwest as a cold front wraps
around the low pressure. This type of weather pattern regularly
result in at least occasional gusts to or near gale
force...although the system center will be slowing and occluding
throughout Tuesday and multiple models keep gusts more 25-30 knots.
With 30-40 percent of the cips analog guidance members indicating
gusts of 35kt plus near the lake in similar pattern...have just
enough confidence to mention occasional gale gusts over the
southern part of the lake Tuesday at this time. Winds look to
turn westerly Wednesday and gradually ease in speed going into
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