Forecast Discussions mentioning any of
"ACARS" "MDCRS" "AIRCRAFT" "TAMDAR" "AMDAR" "WVSS" received at GSD on 11/24/20
Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
905 PM CST Mon Nov 23 2020
905 PM CST
The overall forecast looks on track this evening. 00Z ILX/DVN
RAOBs and recent MDW AMDAR soundings reveal a substantial amount
of dry air in the 750-900 hPa layer across northern Illinois. This
instills some concern of the behavior of the initial precip band
at the surface, with the potential for pockets of heavier precip
instead of a solid line shifting northeast across the area during
the overnight hours. With that said, latest hi-res guidance still
supports moderately to briefly heavy snowfall rates for a 1-2
hour period at onset with the initial band. Have therefore opted
to make very little change other than to tighten the PoP gradient
on the northeast side of the precip band through daybreak
Tuesday. Additionally, have moved up the start time of the
advisory for Ogle and Lee counties to midnight as moderate to
briefly heavy snow may be ongoing as early as around 1 am.
310 PM CST
Through Tuesday night...
The first Winter Weather Advisory of the season has been issued
for parts of the far west and northwest Chicago suburbs, out to
north central and northwest Illinois portion of the CWA. Advisory
was issued in collaboration with WFO DVN and MKX to emphasize
impacts to travel during the morning commute, including
slick/slushy spots on some roads and visibility as low as ~1/2
mile at times. Most of the CWA except parts of the far south
should have a period of all snow as precipitation type and low
visibility from this event. However, with more marginal >32F
surface temps and overall lower snow totals forecast, impacts
should be mitigated.
A potent mid/upper level impulse over the Great Basin region will
be the primary driver of the weather system set to impact our
local weather through Wednesday. A developing surface low across
the front range of the Rockies is occurring in response to the
mid/upper level short-wave digging into the Four Corners region of
the southwest CONUS. The associated downstream southerly low-
level mass response will drive a southerly 50+ kt low-level jet
from the Plains east-northeastward into the Upper Midwest tonight
into Tuesday, resulting in a rapidly northward shifting plume of
deep Gulf Moisture into the region. This will set the stage for
rapid precipitation development into the area later tonight,
despite antecedent very dry air aloft.
The area of precipitation will blossom and/or expand northward
across the CWA overnight as warm advection and isentropic ascent
become quite robust. As alluded to in intro, precipitation type is
expected to be mainly in the form of snow for most of the area for
at least a few hours after midnight through the early morning.
With baroclinic zone oriented from northwest to southeast and
continued strengthening warm advection will gradually transition
p-type to rain from southwest to northeast. The warming aloft will
gradually offset dynamic cooling processes along with an easing of
forcing as a lead positive tilt wave shifts northeast of the area.
While it can`t be ruled out that some sleet may briefly mix in
during the transition, the increasing southeast surface winds
should lead the surface temps to rise safely above freezing as the
>0C air aloft rushes in. Thus, think that p-type transition for
most locations will be one of snow to rain.
While the window for steady snow with this system will generally
only be for about 3 to 6 hours for most locations, there is
increasing concern that the rates could be moderate, to even
briefly heavy into early Tuesday morning, thus accumulations
appear likely for many areas of northern IL inland from the lake,
and this would impact the Tuesday morning commute. Forecast
soundings, especially from the NAM/WRF guidance and RAP have an
approximate 2-4 hour window of good co-location of very strong
omega through the DGZ, which is a favorable look for moderate to
briefly heavy snowfall rates.
Looking more in depth, model guidance continues to suggest that a
stout, but transient band of 700-500 MB frontogenesis will
develop northward into the area very late tonight into early
Tuesday morning. The associated ageostrophic response should force
good ascent right through the favored dendritic growth zone, as
noted above. This in combination with very steep lapse rates
through this layer from around 600 mb through 400 mb suggest that
the snow may fall at a heavy rate for a couple hour period early
Tuesday morning. There is even support for larger snow flakes due
to aggregation processes in the lower-levels, resulting from a
deep moist isothermal layer just below 0C from the surface to just
above 700 mb. This could thus support accumulations on some area
roads, in spite of marginal surface temperatures, so impacts to
the Tuesday morning commute look probable, especially outside the
main urban center of Chicago. Exact response of road temps to
heavier snow rates is uncertain, but with signal for strong
dynamic cooling evident, think that surface temps will be kept
around 32F. This should enable for slushy accums on some roads,
especially lesser traveled secondary roads and bridges.
Total snow accumulations look to be in the 1-3" range I-80 and
north in Illinois and roughly along/west of I-294, with little/no
accumulation along the lakefront in Chicago. Can`t rule out some
isolated >3" amounts closer to the Wisconsin border within
advisory area where change to rain will be the latest. Any
accumulation in the city stands to predominantly be on grassy/colder
surfaces and main travel impact would be from the lower visibility.
Speaking of visibility, favorable factors listed above will also
be favorable for periods of visibility below 1 mile during the
heaviest snow, possibly as low as 1/2 mile at times. All in all,
with the greater impacts to the commute likely focused into
interior northern Illinois, opted to issue the Winter Weather
Advisory for McHenry, Kane, DeKalb, Winnebago, Boone, Lee, and
Ogle Counties, in effect from 2AM to 10AM.
After the transition to rain clears the entire area to the north,
there will be continued periods of rain of varying intensity
through Tuesday night. As the surface low pressure area moves
closer after midnight Tuesday night, temperatures in the lower to
mid 40s and similar dew points (little/minimal T/Td spread) will
mean saturated low levels and a chance for fog development.
Especially where rain rates become light coupled with lighter
winds, will have to monitor the potential for patchy dense fog in
spots. There was a signal for this on some of the MOS guidance in
parts of northern Illinois.
158 PM CST
Wednesday through Monday...
Extended forecast highlights include mid-week rain, quiet and
relatively mild weather for the Thanksgiving holiday into the
weekend, then a low-confidence but potentially unsettled period
Sunday into Monday with a push of colder air early next week.
Guidance remains in good agreement in depicting a closed upper
level low near Kansas City early Wednesday, nearly stacked atop
its associated occluded surface low. Low level southerly winds
ahead of this circulation will continue to advect Gulf moisture
into the region during the day, maintaining rain/shower threat as
the low tracks northeast across the southeastern part of the WFO
LOT cwa by late afternoon. A couple of areas of better focus for
precipitation and higher rainfall amounts are noted, one across
our southeastern counties where unseasonably mild/moist (temps in
the mid-upper 50s and surface dew points around 50) in the
vicinity of the triple point of the occlusion will reside, and a
second farther north across northwest and far northern IL where
better baroclinicity exists along an inverted cold frontal trough
north/northeast of the low. Forecast soundings continue to depict
some weak MUCAPE across our southern counties Wednesday afternoon
near the triple point, worthy of at least some slight chance
thunder mention there. Rain should taper off and end from west to
east Wednesday evening as the upper trough and surface low move
off to the east of the forecast area.
Weak surface high pressure ridge moves in behind the departing
system late Wednesday night into Thanksgiving day, though given
the occluded nature of the departing low there is not much of a
push of cooler air, and no deep/strong subsidence to erode
lingering low level cloud cover. It will be dry and relatively
mild for the holiday however, with high temps around 50 degrees
expected. Quiet weather looks to persist through the end of the
week and into at least the first half of the weekend, as large
scale upper trough becomes positively-tilted across central and
western North America. Similarly dry and relatively mild
conditions appear in store for the region again Friday and
Models have shown some significant differences with the evolution
of the upper pattern later in the weekend into early next week.
Guidance is in generally good agreement in closing/cutting off an
upper low across the southwestern CONUS by the weekend, while the
northern portion of the aforementioned upper trough progresses
eastward with a dry pacific cold frontal passage across our area
Friday. Previous runs of the ECMWF and GEM then depicted another
upper trough phasing with the southern closed low and developing a
strongly amplifying upper trough and deep low surface low which
would affect the Midwest Sunday-Monday. 12Z runs of these models
have trended toward the GFS solution of a somewhat more
progressive southern stream low and no phasing/impressive
amplification of a deeper trough across the Midwest. Getting a
look at 12Z ECMWF ensemble mean does still spread some lighter
precip across especially our south/southeastern cwa Sunday/Sunday
night, so am a bit hesitant to pull pops completely during that
period so will maintain some slights/low chance focused on
Sunday/Sunday evening. Could be some rain/snow mix in colder
evening hours, though mainly rain expected with temps well above
freezing (40s) during the day.
For the 00Z TAFs...
548 PM...Forecast concerns include...
A period of moderate snow predawn hours thru mid morning.
Rain showers for the rest of the period.
Ifr cigs Tuesday morning...lifting to low mvfr in the afternoon.
Gusty southeast winds Tuesday.
An area of snow currently over southern IA and northern MO will
continue moving northeast and will reach the terminals in the
predawn hours. Current timing in the tafs looks on track. Vis/
cigs should quickly drop into ifr as the snow begins. There may be
a 1-2 hour period of vis in the 1/2sm range along with lifr cigs.
As the precip intensity begins to diminish by mid morning...the
precipitation will mix with then change to light rain. Medium
confidence on this timing. Ifr cigs are expected to lift to low
mvfr for the Chicago terminals...but ifr will be nearby to the
northwest. Ifr cigs are possible at ord/dpa...at least for a time
Tuesday afternoon. Rain showers will continue Tuesday afternoon
into Tuesday night. Eventually cigs will lower back to ifr Tuesday
Easterly winds will turn southeast later tonight with speeds
increasing to 10-12kts. Winds will remain southeast Tuesday with
some gusts into the 20kt range. Winds will slowly turn more
southerly Tuesday evening. cms
IL...Winter Weather Advisory...ILZ008-ILZ010...midnight Tuesday to 10
Winter Weather Advisory...ILZ003-ILZ004-ILZ005-ILZ011-ILZ012...2
AM Tuesday to 10 AM Tuesday.
LM...Small Craft Advisory...LMZ740-LMZ741-LMZ742-LMZ743-LMZ744-
LMZ745...9 AM Tuesday to 6 AM Wednesday.
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