Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Chicago/Romeoville Illinois
541 PM CST Sat Nov 28 2015
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 00z...
* lingering lake effect MVFR clouds this evening
Light flow off the lake continues to drive a lake effect MVFR deck
into the Chicago terminals beneath the middle level overcast. Hard to get
a good handle on how the lake effect clouds are trending since
they are obscured by higher clouds...but model guidance generally
suggests conditions growing less favorable for lake clouds during
the night so have conditions going VFR middle-late evening in tafs
but with low confidence.
//Ord and mdw confidence...updated 00z...
* low confidence in duration of lake effect MVFR ceilings
* high confidence in everything else
//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.
218 PM CST
Although speeds are relatively similar in the 10 to 20 knots range
across the entire lake this afternoon...surface high spread
through the middle part is allowing for a varying wind direction.
A westerly direction is in place across the north half while
northerly winds are persisting over the southern half. Despite
this persistent northerly wind...waves have shown a diminishing
trend today...and with the highest waves likely in the 3-5 foot
range across the far southern end of the lake at this time. With
speeds expected to further diminish tonight...waves will also
continue to diminish. Winds will return to all northerly over the
entire lake tonight into Sunday with speeds likely remaining in
the 10-20kt range. With high pressure shifting to the north of the
lake on Sunday...winds will turn more easterly later in the day
Sunday and Sunday night. This will continue Monday and Monday
night out ahead of the next approaching system from the Central
Plains...while speeds increase across the entire lake.
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