Scientific Forecaster Discussion
fxus63 klot 132348
Area forecast discussion
National Weather Service Chicago/Romeoville, Illinois
548 PM CST Wed Nov 13 2019
225 PM CST
Through Thursday night...
The chances and timing of light snow over far northern Illinois remains
the primary forecast concern for this evening.
GOES 16 water vapor channels indicate the main mid-level impulse
of interest for this evenings light snow chances is now entering
western Iowa. This feature is expected to track across northern Illinois
later this evening. A lower level wind southwesterly wind response
to this approaching disturbance has been resulting in some
impressive isentropic upglide across the area through the day.
However, very dry conditions in the lower atmosphere has prevented
much more than some mid and high level cloud cover to this point,
and this looks to continue to be the case for at least the next
While moisture and forced ascent will remain rather lackluster, it
appears that some light snow will develop this evening, primarily
over far northern Illinois (north of I-88) and southern WI as some
aforementioned mid-level disturbance approaches. The snow should
be of rather short duration, lasting only a few hours, before
ending from west to east after midnight. Some minor accumulations
remain possible, with amounts of around a half inch or so possible
near the WI state line. However, farther south, into The Heart of
the Chicago Metro area, no more than a dusting is expected, and
little to no snow south of I-80.
Temperatures tonight will not drop off much from this afternoons
readings, so expect lows to remain in the 20s for most areas. On
Thursday dry conditions are expected. Cloudy skies will continue
over the area through Thursday morning, but some decreasing clouds
are expected later in the day. Temperatures will remain cold, but
with The Heart of the early season Arctic airmass retreating out
of the area, expect temperatures to be on a slow warming trend.
As a result, expect highs Thursday to reach the low to mid 30s,
or about 5 to 8 degrees warmer than todays readings.
149 PM CST
Friday through Wednesday...
Dry but still colder than average weather will take hold for the
end of the week and into the weekend, as broad, flat upper ridging
spreads across the area in the wake of thursday's departing wave.
Guidance continues to indicate a weak cold front will sag into
the area during the day Friday, with which shallow moisture and
winds turning northeast off the Lake May provide some cloud cover
by afternoon. Neutral to weak cold advection will maintain cool
conditions, with highs Friday only in the 30s.
Medium-range global runs remain consistent in advancing another
Pacific northwest upper trough toward the region later in the
weekend. Low level winds gradually veer south Saturday night into
Sunday, with mid-level moisture returning and increasing cloud
cover. The trough is progged to amplify across the Mississippi
Valley Sunday-Monday, with the main short wave energy digging
south of the area. While precip potential returns, it appears
large scale forcing will be somewhat disjointed during the period
with southern and northern stream waves embedded in the flow. For
now, Sunday into Sunday night appear to have a slightly better
potential for some light rain/snow, though the pattern lends lower
than average confidence for timing at this distance. Weak but
persistent warm advection in advance of the large scale trough
axis should allow a slow moderation in temps, but still below
for the 00z tafs...
The main aviation weather focus is on a brief period of mainly
light snow associated with a quick-moving weather disturbance this
evening, mostly for locales near and north of the I-88 corridor.
Light snow has developed in vicinity of rfd at this hour which will
continue to spread eastward through the evening hours. A very dry
airmass remains entrenched across northeast Illinois, with
dewpoints only now just rising out of the single digits and into
the lower teens. As a result, it should take a bit longer for the
lower-levels to saturate sufficiently to support -sn closer to
Ord/dpa/mdw. The weather disturbance responsible for this
precipitation is quite moisture-starved, so overall not expecting
much in the way of substantial vsby/cig reductions as snow moves
in. Based on current trajectories, it looks like the main focus
for some light accumulations of up to 1 inch will mainly end up
just north of Ord closer to the state line, with chances for
wintry precipitation dropping off quickly with southward extent.
Can't rule out some light snow/flurries into gyy later, but
chances/impacts still appear too low for an explicit mention in
the current tafs.
Snow should shift east of the region late this evening--after
06-09z--as surface winds begin to swing more out of the west to
northwest. MVFR cigs look to prevail through the rest of the night
and may linger through the morning/early afternoon as there isn't
a notably drier airmass upstream of this disturbance.
Lm...Small Craft Advisory...nearshore waters
until 9 PM Wednesday.
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