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Area forecast discussion 
National Weather Service Chicago/Romeoville Illinois
219 am CST Monday Nov 30 2015

935 PM CST

Main concern is with light precipitation chances overnight into early
Monday...primarily along/west of I-39. 00z dvn radiosonde observation and recent
aircraft soundings featured a not very deep but very dry layer
centered from about 900 to 850 mb. Forecast soundings generally
maintain this dry layer as ascent increases from shortwave lifting
northeast from Iowa. Observation from west central Illinois and southeast
Iowa this evening have shown light rain falling from a middle cloud
deck...with isolated probable sleet (up) observation. Ice pellets would be
caused from wet bulbing of very dry air aloft.

Do not have high confidence in measurable precipitation occurring in
north central Illinois overnight given the very dry air
aloft...and some guidance such as 00z NAM keeping things dry
through the night. Generally kept slight to low chance probability of precipitation from
previous could see some light precipitation making it
through the dry layer. If precipitation does occur...favoring
primarily light rain or sprinkles...though did indicate a slight
chance for sleet based on reasoning in first paragraph. Not
overly concerned about any freezing rain or drizzle since
temperatures have actually been rising in many spots this evening.
Dewpoints have also been slowly rising across most of the there will not be much room for surface wet bulbing.
Raised min temperatures a bit given trends...with lows for most locations
where precipitation may occur likely above freezing.



Short term...
305 PM CST

Through Monday...

Main forecast concerns/challenges are with precipitation chances late
tonight with a slight possibility for mixed precipitation...and then with
increasing chances for rain throughout the day Monday.

In the near term...generally quiet conditions across the County Warning Area this
afternoon but while middle/high level moisture continues to push
across much of the region. Satellite imagery showing more dense
cloud cover currently pushing north...with any remaining areas
across far northern Illinois which are still observing partly
cloudy to quickly transition to mostly cloudy skies. This is in
response to a more vigorous shortwave lifting northeast through
Missouri and which will continue draw closer to the County Warning Area
through this evening. As this occurs...veering low level wind
field will be supportive of helping shift warm air advection axis to the
northeast into northwest Illinois after midnight and into the
early morning hours on Monday. Guidance varies to overall
northward progression of the current precipitation shield still situated
well west/southwest of the County Warning Area. Likely complicating factor is the
lower level dry air still in place...which could possibly even
limit much of any precipitation for areas along and west of i39 late
tonight/Monday morning. Despite the varying guidance with respect
to precipitation potential...a more probable solution at this time would
be for brief/isolated light precipitation to develop and reach the County Warning Area in
the 8-9z time frame...similar to what is currently occurring over
west central Illinois. Once again...dont expect much of any
widespread development outside of isolated to widely scattered
precipitation from around 8-9z into 12z...and have limited probability of precipitation to slight
chance/low chance.

If precipitation does occur...precipitation type is a little concerning given
the thermal profiles and possible surface temperatures. By the time
precipitation becomes more possible...thermal profiles would support rain
to occur. With my night time lows expected to be around freezing
in these locations...there will be a slight chance for freezing
rain. Current forecast temperature grids would indicate temperatures
falling to around these lows when the precipitation potential arrives.
However...with the warm air advection occurring....hourly temperature trends may
be another complicated factor. Did trend the hourly temperature grids
tonight to reflect this expected warm air advection...but did not trend
completely toward a steady or an even slightly warming solution.
It is possible that low temperatures could occur later this evening and
then remain steady or even rise...possibly above freezing. With
these scenarios in place...did make mention of rain and freezing
rain in the grids for tonight for areas in north central Illinois. not overly concerned for any widespread issues
resulting for this possibility of mixed precipitation...given the
marginal surface temperatures and weak nature of the precipitation.

It does look like slowly progressing upper level low over the
central Continental U.S. Will continue to inch closer on Monday...with a
vigorous vorticity lobe expected to swing through in the afternoon.
Blossoming precipitation shield will work its way east into much of the
County Warning Area on Monday...with most locations observing rain by middle to late



Long term...
252 PM CST

Monday night through Wednesday...surface low pressure will
continue to deepen and track northeast reaching southwestern Iowa
by Monday evening and the Twin Cities Tuesday morning. The primary
push of rain is expected to cross the local area from southwest to
northeast Monday evening as isentropic ascent is maximized. The
upper trough will strengthen over western Iowa bringing a very
pronounced dry slot northeastward into north-central and northeast
Illinois late Monday night and into Northwest Indiana early
Tuesday morning. This will signal the end of this round of
rainfall and its not out of the question that some thunder occurs
at the tail end. Some ascent does remain as the deeper moisture
departs to the north and east which may result in some drizzle
very early Tuesday but ascent diminishes after daybreak so much of
Tuesday morning may remain dry. Colder air begins to filter in
through the day Tuesday with deeper moisture expanding into
northwest and north central Illinois into the afternoon. With the
upper low moving into central Wisconsin there may be enough weak
lift to generate some very light precipitation in the northwestern County Warning Area
but this looks minimal if it occurs at all. With colder air
spreading in precipitation could be a mix of rain/snow or even some
sleet. Chances for precipitation increase somewhat Tuesday night
through Wednesday morning as the deformation zone associated with
the primary trough axis moves across the area. With increasingly
colder air moving in mixed precipitation may occur for a short time
Tuesday evening but the column will likely cool enough for all
snow by Tuesday night. Precipitation amounts will be light with a coating
to a few tenths of snow possible in a few spots...mainly north.
The trough axis clears Northwest Indiana early Wednesday afternoon
so precipitation will end from west to east from middle morning
Onward. For temperatures...lows Tuesday morning will be in the middle
to upper 30s with highs only warming into the upper 30s and lower
40s Tuesday afternoon thanks to the influx of cool advection. Lows
around 30 are expected Tuesday night with highs near 40 Wednesday.

Thursday through next weekend...modest upper ridging spreads into
the region behind the departing trough while surface high pressure
spreads across the central and Southern Plains. Warming will occur
aloft with 850 mb temperatures warming back into the single digits above zero
with a southwesterly wind developing at the surface. An upper trough
and associated surface low swing eastward across Ontario Thursday
night. Any impact on temperatures aloft will be determined how far south
this trough digs as it passes...though it is still expected to
remain north of the area. Highs Thursday should be in the low to middle
40s and middle to upper 40s Friday provided the trough stays well
north. A more amplified upper ridge arrives behind the Ontario
trough with surface high pressure shifting across the Ohio Valley
and into the middle-Atlantic and New England Saturday. This setup could
allow for more notable warming potential for Saturday and/or Sunday
with breezy southwest winds and middle level warm axis potentially
overhead. Guidance continues to struggle with the magnitude of middle
level warming aloft so its tough to say how temperatures will play out.
Based on the range of middle level temperatures displayed by recent model
runs...surface highs could range anywhere from the middle 40s to even
upper 50s based on 850/925 mb temperature climatology. Will be something to
keep an eye on the next few days.




//Ord and mdw concerns...updated 06z...

* east winds gradually increasing in speed through Monday.
* Gradually increasing chances of rain showers Monday afternoon
becoming likely by early Monday evening.
* MVFR ceilings early Monday evening falling to IFR during the
evening...and probably to LIFR by daybreak Tuesday.
* Chance for IFR visibility during rain and drizzle Monday evening
and in drizzle/fog through daybreak Tuesday.

//Discussion...updated 06z...

Moisture aloft is slowly return northward across the region
overnight into the morning. Very dry low-level air across
northeast Illinois and Northwest Indiana is expected to keep
Chicago area taf sites free of any precipitation through that time
with just virga...though cannot rule out sprinkles. For
rfd...spotty very light rain or even some brief ice pellets are
possible overnight per radar trends. The dry air should keep any
precipitation light.

Stronger moisture return during middle-late Monday afternoon is
favored to increase rain shower coverage across parts of the
region...especially north central Illinois impacting rfd. The best
time for rain across the area looks to be Monday evening. The
lower atmosphere is expected to saturate quickly with the help of
the precipitation and winds turning a little more south of due
east. IFR ceilings with potential LIFR by overnight into early Tuesday
morning are forecast. Some drizzle may occur apart from showers
and linger into overnight...which could keep visibility into the
IFR category.

//Ord and mdw confidence...updated 06z...

* high in winds.
* Medium-high in rain showers Monday evening but low in specific
start time.
* Medium-high in ceilings falling quickly Monday evening and medium in
LIFR developing overnight.
* Low in visibility Monday night.

//Outlook for Ord/mdw for 12z Tuesday-12z Sunday...updated 00z...

Tuesday...IFR/LIFR ceilings improving slights to high end IFR/low end

Tuesday night and Wednesday morning...snow showers or flurries
likely. MVFR likely...IFR possible.

Wednesday afternoon...MVFR ceilings likely.

Thursday through Saturday...primarily VFR.



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 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 the 15 to 25 knots range through
Thursday...before dropping on Friday as ridging moves across.



Lot watches/warnings/advisories...
Lm...Small Craft Advisory...lmz740-lmz741-lmz742...3 PM Monday to 4
am Tuesday.



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