Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Chicago/Romeoville Illinois
552 PM CST sun Nov 29 2015
305 PM CST
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 Iowa...one 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
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.
However...am 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
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 00z...
* gradually increasing chances of -shra Monday afternoon...-shra
likely by early evening
* VFR ceilings slowly falling to MVFR Monday evening...potentially to
IFR by end of 30 hour Ord taf
Large slow moving storm system will gradually begin to affect the
terminals Monday and particularly Monday night. Initially...dry
air in low level should result in primarily virga tonight into
Monday morning. Exception is that some very light precipitation in form
of -ra/-pl is possible tonight at rfd. Main batch of -shra likely
to move into the terminals late Monday afternoon and moreso Monday
evening. As rain showers moisten the low levels look for ceilings to build
downward Monday evening...potentially dropping to IFR prior to
06z. Light NE winds tonight will increase a bit and veer to closer
to due east Monday.
//Ord and mdw confidence...updated 00z...
* medium confidence in timing of precipitation Monday
* medium-high confidence in ceilings trends Monday evening
* high confidence in remaining elements
//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.
305 PM CST
Surface high pressure situated along and to the north of the lake
is allowing for relatively light northerly winds across the lake
this afternoon...with winds staying in the 10-15 knots range. As this
high exits the area tonight into Monday...developing low pressure
over the plains will lift north. This will allow for pressure
gradient to really strengthen across the lake...with easterly
winds expected to see a definite rise by Monday afternoon. Winds
do increase to the 15 to 25 knots and to 30 knots ranges by late Monday
afternoon and Monday evening. Winds over the lake should peak at
30 knots...with gales not expected at this time. These easterly winds
will create hazardous conditions for small craft late Monday and
Monday night...and so have issued an Small Craft Advisory for the Illinois shore.
Have not issued one for the Indiana shore at this time...but will
need to monitor this potential with later forecasts. As surface
low continues north...surface trough will lift across the lake and
help the winds turn more southerly late Monday night into Tuesday.
This will help bring a brief lull in the speeds during this
time...but with increasing winds expected once again into Tuesday
Lm...Small Craft Advisory...lmz740-lmz741-lmz742...3 PM Monday to 4
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