Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Chicago/Romeoville Illinois
204 PM CST sun Nov 29 2015
1003 am CST
Main update this morning was to increase cloud cover across the
entire County Warning Area for today...and even into this evening as an abundance
of middle/high level moisture continues to lift northeast across the
region. Fairly energetic flow with several Stout waves is noted on
satellite imagery...and this will aid in a further push of more
dense cloud cover to the southwest today. Latest radar imagery
depicting some weak returns across west central Illinois this
morning while stronger returns remain across northern Missouri and
southern Iowa. These returns near the County Warning Area are not amounting to
anything with rather dry air still in place this morning...and
while observational data indicating cloud bases above 10kft. While
large scale ascent will be present...better focus/warm air advection noted well
to the west/southwest will remain this way for much of today and
allow for continued dry conditions across all of the County Warning Area.
220 am CST
After a quiet and seasonably cool Sunday...short term focus is on
rain chances and timing for Monday/Monday night.
Our West Coast upper low continues to spin out in the west...and
with ridging across the Gulf of Mexico...a continued SW feed of
moisture continues to stream across the Central Plains east into the
middle-Mississippi and Ohio valleys. Surface high pressure has built
into the area allowing drier air to advect in downstairs and allow
for an erosion of cloud cover. Therefore it will be a colder start
to the day. Weak shortwaves continue to move through the SW
flow...one ongoing in Iowa bringing some light precipitation. Models
in decent agreement that as this wave moves through later today it
all we will see is some increased middle to high cloud as we maintain a
cool dry NE low level flow. With more sunshine in spite being
contaminated somewhat...should see most places in the 40s today.
In spite of some differences in the upper low evolution...models are
in much better agreement on the track of the surface low Monday
night into Tuesday. A strong upper jet on the south side of the low
will allow the upper low to eject east/northeastward. The
corresponding surface low moves to around Omaha by Monday
evening...then northeast through the Twin Cities Tuesday morning and
off to Lake Superior Tuesday night. Lower level warm advective southeast
flow increases overnight into Monday over the lingering
baroclinic zone. There is some disagreement late Sunday night and
Monday morning as the warm advection ramps up as to how much
precipitation occurs Monday morning. This could provide some
concern with a surface NE wind in place that a few areas in north
central Illinois could be near/slightly below freezing in an area
of 295-300k isentropic lift...similar to what is ongoing this
morning in Iowa. The eastern extent of precipitation should be
limited due to an initial feed of dry southeast flow...but we could see
some rain Monday morning especially south and west. Better chances
for more widespread rain are in the afternoon/early evening as the
warm front lifts north. With a pretty potent warm advection push
coupled with a dry slot approaching...not unheard of to see some
convective elements with the rain shield or some isolated thunder.
Better height falls occur over the area Monday night...with the
best upper dynamics just to our west.
220 am CST
Tuesday through Saturday...
The leading edge of cold air will push through Tuesday morning.
Meanwhile dry slot moves overhead for much of Tuesday...so in
spite of continued height falls not expecting much precipitation
Tuesday...maybe some isolated flurries/drizzle. More noticeable
will be the cold southwest winds.With such an expansive upper low
and thus corresponding warm/cold conveyor belt structure...a large
area of wrap around moisture in the deformation region of the
system will rotate through our area later Tuesday into early
Wednesday. Thermal profiles appear supportive of at least a rain-
snow mix initially before secondary batch of cold advection should
suggest all snow. Quantitative precipitation forecast numbers still modest...but some light
accumulations/dustings are quite plausible overnight into the
Wednesday morning commute.
Much quieter conditions on tap for the latter half of the week.
The massive upper low will open up and lift east on Wednesday
which should end precipitation fairly quickly. A surface ridge
will set up as a weak disturbance clips the area Thursday with no
real impacts. The upper ridge builds Friday/Saturday along the
eastern U.S for milder conditions Friday and Saturday.
//Ord and mdw concerns...updated 20z...
* a rogue bkn018 ceiling off the lake seemed to be short
lived...so will continue to handle this with mention of few-sct.
* Northeast flow will turn more easterly tonight but remain light
in the 6-8kt range with occasional 9-10kt also possible.
* Sprinkles or virga likely early Monday...rain developing in the
middle to late afternoon.
A plume of lower ceilings that developed over the lake has been
lifting out of the gyy area and across the mdw terminal late
this morning. This plume is just about 15 miles wide and is not
expected to linger long over any one location...but tafs include
scattered-broken at mdw and few-scattered at Ord...which already is panning out
in the 1-minute data.
Surface and low level winds will remain out of the east...and are
expected to maintain a very dry layer centered around 5000 feet
into the morning and early afternoon tomorrow. Above this level
there is considerable moisture streaming into the area from the
southwest. Forecast models are wanting to develop some light rain
starting tomorrow morning. This may work out for rfd but at the
mdw/Ord locations this should mostly be evaporating until the
lower levels can saturate later tomorrow afternoon or early
//Ord and mdw confidence...updated 20z...
* medium to high confidence regarding behavior of narrow band of
bkn018 clouds this afternoon and lack of MVFR ceilings.
* High confidence for other elements.
//outlook for Ord/mdw for 00z Tuesday-12z Sunday...updated 12z...
Monday night...rain with MVFR lowering to IFR. Southeast winds.
Tuesday...IFR possible early lifting to MVFR. Slight chance of
rain during the day and light rain/snow possible
during the night. Gusty southwest winds.
Wednesday...MVFR. Slight chance of light snow in the morning.
Gusty west winds.
Thursday-Saturday...VFR. Southwest winds.
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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