Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Chicago/Romeoville, Illinois
627 am CDT Wednesday Aug 31 2016
216 am CDT
For those of you eager for an end to the muggy and wet period, you
are in Luck. High pressure continues to build southeast across
the upper Midwest and into the Great Lakes over the next few days.
The leading cold front is moving through northeast Illinois early
this morning. Dewpoints are creeping down behind the front.
After daybreak the cold front will be south of the area across central
Illinois, but dewpoints will be slower to fall. There are some
lower clouds across northwest Michigan that rap guidance suggests
will bleed south Down Lake Michigan this morning and impact
northeast Illinois and northwest in for a time, just not clear how much
coverage will be. Model guidance is in pretty good agreement in
breaking out some scattered showers/isolated storms along and
ahead of a slightly more pronounced dewpoint gradient later this
morning closer to and southeast of Chicago, and then continuing
southeast across east central Illinois and Northwest Indiana into the
early afternoon. Hrrr/rap forecast soundings, capped overnight
with the nighttime frontal passage depict a narrow window of
uncapped though very weakly unstable conditions near Chicago/Gary
and slightly more-so farther south, such that combined height
falls and density gradient will support this activity. Dewpoints
fall more rapidly from mid afternoon on and soundings dry out
quickly. It will be a breezy day, especially across the eastern
half of the area.
A secondary surge of cooler air comes in as a shortwave trough on
the backside of the eastward moving Canadian upper low digs over
Lake Michigan. Significant thermal differences between this air
mass and the Warm Lake will lead at least broken lake induced
Clouds. Lake induced equilibrium levels are higher than depicted
yesterday, about 8-11k feet depending on your model of choice, but
the moisture still appears shallow enough preclude much more than
clouds or some sprinkles in spite of some hints of some weak qpf
(actually over 0.1 of quantitative precipitation forecast on the Gem, higher on the hiresarw/nmm)
and the GFS/NAM depicting lake induced cape of about 200-300 j/kg.
Therefore have introduced a non-measurable mention of
precipitation at this point as the thermodynamic/unidirectional
wind profiles are marginally supportive. Nonetheless, it will
feel autumn- like Thursday morning.
216 am CDT
Thursday night through Tuesday...
High pressure builds over eastern Canada and southward over Lake
Michigan for dry and sunny conditions with highs in the 70s. The
high retreats east Saturday as a tropical/subtropical system
slides northeast along the Atlantic coast. Southeast flow will
allow some temperature recoveries as the large upper ridge
At this point model guidance suggests the upper heights will
remain high enough due to high amplitude blocking across the
northern Atlantic, which implies that most of the Holiday weekend
will remain dry. The amplified southwest flow pattern across the
region will bring the return of warmth and humidity Sunday into
the early half of next week. Highs look to return to the mid 80s
with dewpoints back in the upper 60s to possibly low 70s.
Precipitation chances will eventually increase as a cold front
for the 12z tafs...
A cold front has pushed across the region overnight. Early morning
fog channel infrared imagery reveals a narrow band of IFR to low end
MVFR stratus pushing southwest across northern Illinois. Think
that dpa and rfd will be in and out of this stratus within the
next couple hours; however, additional stratus has developed over
Southern Lake Michigan and is makings its way towards mdw/gyy. Ord
appears will be right on the edge of this second wave of stratus
so opted to extend tempo MVFR conditions a couple more hours there
while going prevailing at mdw/gyy. Rap low level moisture fields
picks up on this well, and does indicate improvement between 14z
and 16z. Once status lifts this morning, expect VFR conditions to
prevail through the remainder of the day with breezy north-northeast to NE
winds. Winds should diminish some overnight. On Thursday, cooler
air will continue to move into the region and could result in some
light lake effect showers or sprinkles for the Chicago area
terminals, along with the possibility for MVFR clouds.
246 am CDT
A cold front has pushed south across Lake Michigan with a strong
north to northeast breeze developing across the lake. These strong
winds will continue into Thursday. A Small Craft Advisory remains
in place for the Indiana nearshore waters and will expand to
include all of the Illinois nearshore waters for early this
afternoon through Thursday evening. Conditions are expected to
improve Thursday night into Friday as high pressure builds across
the region, with southerly flow overspreading the lake for the
weekend and continuing into early next week.
Illinois...beach hazards statement...ilz006-ilz014...1 PM Wednesday to 10
In...beach hazards statement...inz001-inz002...1 PM Wednesday to 10
Lm...Small Craft Advisory...lmz740-lmz741-lmz742-lmz743-lmz744-
lmz745...3 PM Wednesday to 10 PM Thursday.
Visit US at http://weather.Gov/Chicago (all lowercase)
Follow US on facebook...twitter...and youtube at: