Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Chicago/Romeoville Illinois
405 am CDT sun Oct 4 2015
314 am CDT
Yesterday at this time we were discussing how our County Warning Area was in the dry
airmass to the west of the deeper moisture off the Atlantic coast
continues to stream northwestward. Things have changed and that dry
axis now lingers just to our west with a much drier kdvn sounding
this morning...while local amdar soundings depict a very moist low
level airmass with a deep inversion in place and a fairly deep moist
Lower level cold air advection on Saturday will be replaced by
modest low to middle level warm air advection. While this would
normally suggest a warm response at the surface...this will
initially act to increase the inversion and keep the stratus locked
in place. Modest low level ascent in the stratus...depicted well on
rap 295k isentropic surfaces... will support very light rain/drizzle
through the morning...before ascent weakens. Forecast soundings
support a weakening of the inversion in Northwest Indiana in the
afternoon...though it looks to hold pretty firm in NE Illinois.
Forcing features are subtle Sunday...but cannot rule out some
drizzle throughout the day with low confidence on timing... though
the chance should be lower than in the morning. Because of the
continued cloudiness...expect temperatures to remain cool...with
areas only in the south to hit the 60s.
On Sunday night...the upper flow turns more westerly. Weak low level
ascent continues in the stratus layer supporting some slight chance
probability of precipitation as a weak surface trough lingers over Lake Michigan and 925/850
baroclinic zone remains draped across the northern tier of our area.
This would most likely be a light rain/drizzle that could linger as
drizzle into the morning hours. Then as Monday GOES on...weak
surface high pressure will sink south from Wisconsin. We maintain a
northeast flow but it will be much lighter than in days past. Again
in spite of the warmer lower level temperatures...it looks like
lower clouds will stick around for a while if not remain locked in
place again....with the best chance for clearing and a decent warm
up along and south of I-80.
314 am CDT
Monday night through Saturday...
By Tuesday the surface high is overhead. Surface flow will be in a
transition period from onshore...north-northeasterly Monday
night...to more of a westerly direction Wednesday. Stratus may
continue to feed into far NE Illinois and Northwest Indiana in the
morning...possibly hanging near the lake in the afternoon as
weaker flow may preclude weak lake breeze development. Weak cold
air advection ensues but decreased cloud cover should support a
warm up to the 70s for many areas away from the lake.
The surface ridge axis shifts east Wednesday...opening the door for
warming southwest flow. It should be a quiet day with widespread
70s...with the exception again away from the lake where onshore flow
will keep readings cooler in the upper 60s. The wildcard is how
dense high clouds will be.
Northern stream shortwave energy...still way out in the Pacific...is
prognosticated by medium range guidance to move through the northern tier
of the country Wednesday...then southeast through our local area
Thursday into Friday. Model guidance has slowed a bit...but still
pegging the best chances for rain/isolated thunder Thursday
afternoon/evening...lingering in Northwest Indiana into early Friday
as the trough axis amplifies. Northwest flow behind this continues but weak
surface features suggest dry/slight chance probability of precipitation beyond this time
//Ord and mdw concerns...updated 09z...
* ceilings trending to IFR early this morning before improving to low
end MVFR again later today
Strong northeast winds have continued to ease and gustiness
subsided and not anticipating winds to increase again through the
taf cycle remaining northeast around 10kt. Low end MVFR ceilings have
overspread the terminals and will likely build down to IFR
overnight. Lower sun angle and sharp inversion likely to keep
stratus locked in Sunday into Sunday night with just a slight
diurnal rise in ceilings to MVFR expected either late Sunday morning
or early Sunday afternoon. Some periodic -dz or very light rain
could occur from time to time...but with impacts on visibility expected
to be negligible...duration likely brief...and having no impact on
Ops have opted to keep tafs dry. Ceilings look to build back down to
IFR Sunday night after sunset.
//Ord and mdw confidence...updated 09z...
* medium-high confidence in over ceilings trends through taf
cycle...low to medium confidence on precise timing
* high confidence in remaining forecast elements
//outlook for Ord/mdw for 12z Monday-12z Saturday...updated 00z...
Monday...MVFR possible. NE wind.
Tuesday...VFR. Vrb wind.
Wednesday...VFR. NE wind.
Thursday...rain showers likely. MVFR/IFR possible. S wind.
Friday...slt chance rain showers. MVFR/IFR possible. NE wind.
157 am CDT
The unseasonably strong high pressure over eastern Canada that has
been responsible for day after day of strong northeast winds has
begun weakening. Models suggest this weakening will continue at an
accelerating pace today which should allow winds to weaken
substantially today. The high will then settle south across the
Great Lakes and allow for a couple days of light winds. A weak
cold front will move across the lake by middle weak but gradient and
push of cooler air behind the front look fairly weak with no
substantial winds/waves expected after things die down this
Lm...Small Craft Advisory...lmz743-lmz744 until 4 PM Sunday.
Small Craft Advisory...nearshore Burns Harbor to Michigan City in until 10 am Sunday.
Small Craft Advisory...Illinois nearshore waters until 9 PM Sunday.
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