Forecast Discussions mentioning any of
"ACARS" "MDCRS" "AIRCRAFT" "TAMDAR" "AMDAR" "WVSS" received at GSD on 11/30/20
Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
803 PM CST Sun Nov 29 2020
800 PM CST
No significant changes to the forecast message this evening,
there is still a chance for some snow showers into NE Illinois
late tonight into tomorrow morning, with the better chance for
travel impacts still focused into northwest Indiana.
Evening AMDAR soundings and the 00z DVN sounding depict dry air
in the mid levels, with a shallow lower level cloud deck along and
just behind the cold front this evening. The 0z ILX sounding is
showing at least a bit more saturation, but still not much
precipitation is reaching the ground to our south as well
associated with the compact low pressure area across eastern IL
into Indiana. With some subtle moisture increase from the SW, we
could see some light precipitation begin in NW Indiana late this
evening or overnight. Forecast soundings are marginal for snow
before midnight. What will be noticeable this evening is the
colder and drier air that will advect in from the northwest behind
the leading cold front. We are still fairly mixed in the cold
advection regime thus expect gusts to 30-35 mph even increase some
The secondary cold front will approach overnight into early
Monday. This will be the impetus for lake effect snow showers. No
significant changes to the forecast are planned at this point.
Surface-925 mb winds do suggest that the main snow shower axis
will point into NW Indiana, but some subtle backing to the 850 mb
(around 5kft) may steer some of the activity into NE Illinois
into early Monday (including the morning commute) also as the
moisture player deepens and gets into the better snow growth
region. Of note, the NBM (ensemble) produces a ~30-40 chance of a
dusting of snow at MDW and points eastward to the lake, and points
southward in IL, which could be in play briefly Monday morning,
but some high- resolution ensemble guidance has increased the
chances in recent runs, but have shown a slight downward trend in
234 PM CST
Tonight through Monday night...
Focus during this period continues to be on lake effect snow for
primarily northwest Indiana and lakeshore flooding from persistent
strong north winds.
A cold front stretching from Evanston to Aurora to Mendota as of
230pm will continue southeastward through the remainder of the CWA
through early evening. Meanwhile, weak phasing of a sharp upper
trough centered over Minnesota and an upper low over southeast
Oklahoma will begin by early this evening, with a resultant sharper
and stronger trough axis crossing the CWA tonight into Monday
An axis of post-frontal light precip may develop southeast of I-55
mid-evening into the early overnight hours as the right entrance of
an upper jet streak interacts with a marginal increase in moisture
ahead of the upper-low. Moisture profiles still appear to be too dry
to support any appreciable precip, but a light rain/snow mix
transitioning to all snow with little to no accumulation is possible.
The arrival of colder 925-850 hPa air concurrent with the passing
upper trough late tonight will quickly raise inversion heights and
resultant lake-induced equilibrium levels to around 8kft while
850hPa-SST delta Ts increase to around 15C. Adding in an extensive
fetch owing to low-level cyclonic curvature following the long axis
of Lake Michigan, LES parameters support a potentially impactful LES
episode for portions of northwest Indiana. However, there are mixed
signals as to whether the LES structure will be in the form of one
dominant band or a mix of several bands of varying intensity. Most
near-term high-res guidance has been indicating the latter, which is
not entirely surprising given the extensive generally uniform field
lacking any substantial land-breeze/offshore wind component from WI
or MI. This will ultimately result in much more variability in
snowfall amounts, with amounts hinging highly on if one of those
bands becomes dominant and if individual band motion is transient or
stationary. A multi-banded structure and little onshore flow does
favor a shoreline convergent band along the Illinois shore, edging a
few miles inland across Chicago, Monday morning before winds begin
to slightly veer.
The overall message of the forecast has changed very little over the
past couple days, but the variable nature of LES always lowers
confidence in snow amounts. The latest forecast is for totals
ranging from around one half inch on the immediate Chicago shoreline
to 3-5 inches for portions of Porter County and northeast Jasper
County. Locally higher and lower amounts are likely, especially if
LES bands remain more stationary. Additionally, marginal low-level
temp profiles with surface temps at or above freezing (especially
near the lake) and the DGZ residing above the layer of maximum omega
in the convective PBL support SLRs as low as 10:1. More pronounced
bands may tap into the DGZ, but also have increased riming in the
convective column, further complicating SLRs. This may be a limiting
factor in snow totals, particularly near the lakeshore.
While the snow amounts alone would not be too significant for an LES
event, strong north winds will lead to potentially hazardous
conditions late tonight through the day Monday. North winds are
expected to gust to 35 mph inland and to 45 mph near the lake. The
combination of the wind and snow warrants the issuance of a Winter
Weather Advisory for Lake, Porter and Jasper counties, especially
with rapidly varying conditions affecting the major interstate
transportation corridor through northwest Indiana.
Regarding the strong north winds, the combination of large waves to
15 feet along the IN shore and surge up to a foot will lead to a
potentially significant lakeshore flood event. A Lakeshore Flood
Warning remains in effect for Lake and Porter Indiana. Impacts will
likely also be noted into the Chicago shoreline, with a Lakeshore
Flood Advisory for waves to 10 feet in effect.
230 PM CST
Tuesday through Sunday...
On Tuesday, lake effect snow over northwest Indiana should be
winding down as the 994 mb low moves northeast into Quebec, allowing
surface winds to turn more northwesterly. The pressure gradient also
relaxes throughout Tuesday as high pressure ridges up from the
southwest. Northwest flow will keep daytime highs just a bit below
normal for the first day of December, despite the clearing as clouds
exit to the east with the low pressure system. Mostly clear skies
and light westerly winds Tuesday night should make this the coldest
night of the week, with lows in the lower 20s most locations except
for mid 20s toward downtown Chicago.
A northwest to southeast oriented ridge over the area Wednesday will
support continued clear skies and slightly above normal highs in
the lower 40s most locations.
Another low pressure system approaches from the southwest Thursday,
increasing our cloud cover, but continued west to southwesterly
winds will keep temperatures slightly above seasonal norms Thursday
and Friday. The low will slip south our of CWA sometime between
Thursday afternoon through Friday, bringing a slight chance of
rain/snow mix Friday afternoon through Saturday night, mainly
impacting the southern half of the CWA. The low moves to our east on
Saturday, taking the clouds and weather with it, with northwesterly
winds in its wake as a tightening of the gradient brings breezy
conditions on Sunday.
For the 00Z TAFs...
The primary aviation concerns through the next 24-30 hours are:
* Breezy northwest winds gusting 20-30 kts through much of the TAF
* MVFR cloud deck and lake effect snow showers likely to impact
GYY but also possibly MDW/ORD from early Monday morning onward
Breezy northwest winds will prevail through much of the TAF
period in the wake of a strong cold front pushing through the
terminals at press time. Generally speaking, VFR cloud cover is
expected through the TAF period between various cloud decks
through Monday morning and a scattered (diurnal) cumulus deck
Monday afternoon. The exception will be at GYY and possibly
MDW/ORD as an MVFR cloud deck surges south off Lake Michigan
behind a secondary cold front, perhaps as early as 6-7Z.
Numerous lake effect snow showers will develop behind the
secondary front and stream into northwestern Indiana and perhaps a
sliver of northeastern Illinois from roughly 8Z onward. At this
point, GYY stands the highest chance of being impacted by numerous
showers with associated brief drops in visibility. A lower chance
exists for a few showers to sneak toward MDW (and even lesser so
ORD) between 11Z and 16Z during which low-level steering winds
will have a modest component off Lake Michigan into the Illinois
Lake Michigan shore. It`s worth noting the NBM (ensemble) produces
a ~30-40 chance of a dusting of snow at MDW Monday morning. At
this point, it seems the chances for meaningful snow accumulation
or associated drops in visibility are too low (or will occur for
too brief of a period of time) to warrant a formal mention in the
outgoing TAF package...though again such chances are non-zero.
Current forecast on track with a period of gale force winds
late tonight through late Monday evening. A cold front is
currently moving across southern Lake Michigan, shifting winds
northwest. North gusts will increase to 30kt during the evening.
The gradient will tighten overnight into Monday as a ridge of high
pressure builds across the central U.S. and a storm system moves
northeast across the Appalachians. This storm system will reach
the eastern Great Lakes Monday evening and then stall before
slowly lifting north midweek. The gradient will slowly weaken
Tuesday into Tuesday night with winds turning more northwesterly
over southern Lake Michigan.
IL...Lakeshore Flood Advisory...ILZ103-ILZ104...midnight Monday to 6
IN...Lakeshore Flood Warning...INZ001-INZ002...midnight Monday to
Winter Weather Advisory...INZ001...3 AM Monday to 9 PM Monday.
Winter Weather Advisory...INZ002-INZ011...3 AM Monday to 6 AM
LMZ745...midnight Monday to midnight Tuesday.
Small Craft Advisory...nearshore waters
until midnight Monday.
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