Forecast Discussions mentioning any of "ACARS" "MDCRS" "AIRCRAFT" "TAMDAR" "AMDAR" "WVSS" received at GSD on 01/27/21

Area Forecast Discussion

National Weather Service Chicago/Romeoville, IL

 935 PM CST Tue Jan 26 2021

.UPDATE...
930 PM CST

The main focus is on lake effect snow for parts of northeast
Illinois and northwest Indiana on Wednesday. An SPS was just
issued to alert of the potential for travel impacts due to rapid
changes in conditions over short distances, including periods of
poor visibility and snow and/or slush on some roads. Some of the
00z CAM guidance is rather concerning tomorrow morning, but given
typically lower confidence in these scenarios on the west and far
south side of the lake, opted for SPS issuance as opposed to
Winter Weather Advisory. If it becomes apparent in analysis
overnight that the lake effect snow will become single banded with
moderate to heavy rates AND stall out for 2-3 hours+, then needing
a WWA is a good bet. Just wasn`t confident enough to make that
call this evening.

A dual banded structure of light lake effect snw showers have
drifted into Lake and eastern Porter Counties this evening. The
appearance on radar is not too impressive and a look at soundings
indicates that lift is on the weaker side, which should be the
case through the night. This will change on Wednesday morning as a
clipper-type mid-level short-wave will approach the region and
induce some cold advection down the length of Lake Michigan, with
850 mb temps cooling down to -11 to -13C. given the unusually mild
lake surface temps, this will bring 850 mb to lake delta Ts of up
to 15C and lake induced CAPE of 100-150 j/kg. As a con to the
setup that has been noted over the past few days, lake induced
equilibrium levels (inversion heights) are not all that
impressive, topping out at 5kft-6.5kft, depending on the model
sounding you look at (which means DGZ will only be partially
saturated). Also, while the delta Ts are more supportive than
would typically be the case with that air mass over the lake in
late January, still not extremely favorable.

On the other hand, what we will likely have in place to possibly
compensate for the somewhat negating factors of the setup is very
good lake induced low level convergence and strong lower level
omega (lift) due to the convective processes and large ascent
from the short-wave trough. Have noted that in the past at times
setups on the southwest to extreme southern tip of the lake can
perform better than expected from more marginal ELs in the
presence of very favorable convergence and low level lift. Adding
to this concern from past events is the 00z CAM guidance,
including the high-res WRFs (locally 6"+ on ARW!) and the HRRR
(though HRRR has been mostly overdone with LES this season).

All the above being said, the big question as always comes down
to residence time of the band over given locations. Feel fairly
confident that at least moderate to briefly heavy rates will
accompany the activity, but lower confidence on the all importance
residence time question. It appears the convergence will
retrograde the activity from current location into portions of
eastern Cook County and affecting downwind areas of eastern Will
and possibly eastern DuPage and northeast Will as well. The lake
effect band(s) would then eventually migrate back into Lake IN and
then Porter later Wednesday PM. With the above concerns, hit the
messaging harder in already issued SPS and plan to issue a graphic
shortly highlighting these concerns. Will brief midnight shift on
the potential and depending how trends emerge will dictate whether
a WWA is needed in the early morning.

Castro

&&

.SHORT TERM...
259 PM CST

Through Wednesday night...

Forecast challenges continue to focus on additional snowfall
accumulations from the lake effect snow showers tonight and on
Wednesday. While there is not much of a focus for these showers,
the expectation is that we will see these snow showers become
better focused into portions of Cook county and Lake county IN
tonight as a land breeze convergence zone sets up over southern
Lake Michigan. This land breeze convergence zone may keep the main
focus for these snow showers into central and southern Cook
county (and adjacent areas of Lake IN) through much of Wednesday
morning before this focus gradually shifts eastward into Lake and
Porter Counties in IN Wednesday afternoon and evening.

Lake effect parameters are not expected to get overly impressive
from a thermodyamics perspective. In fact, a recent AMDAR
sounding out of MDW suggests that lake induced equilibrium levels
are right at the -12C level, which is only about 5-6,000 feet
AGL. It appears unlikely that this will improve much, if at all,
overnight. This does add some concern to the quality of snow in
these lake effect bands. However, with a fairly consistent signal
for a good boundary layer focus along the lake breeze convergence
zone, this may act to compensate for the modest thermodynamic
parameters. For this reason, while this is unlikely to turn into a
major lake effect snow event for Cook or Lake county IN, we could
see enough of a focus to support a band of accumulating lake
effect snow showers into central and southern Cook/parts of Lake
IN late tonight through Wednesday morning. Our current forecast
mentions about 1 to 3 inches through Wednesday, with the highest
amounts near the lake in southern Cook and far northwestern IN.

As mentioned above, expect the focus for lake effect snow showers
to shift eastward into eastern Lake IN and Porter Wednesday
afternoon and evening. Some light accumulations will continue to
be possible in this area. Otherwise these lake effect snow
showers should come to an end altogether Wednesday night.

Outside of the lake effect snow, we look to see some decreasing
cloudiness tonight, especially over north central IL. With a
fresh snow pack on the ground, this could set the stage for a
cold night across this area, with temperatures falling into the
single digits in areas that see some decreasing clouds.
Temperatures on Wednesday then look to remain in the 20s over
north central IL, to around 30 southeast of I-55. Surface high
pressure looks to build eastward towards the area Wednesday night.
This could support an even colder night, with some subzero lows
and wind chills possible into early Thursday morning.

KJB

&&

.LONG TERM...
259 PM CST

Thursday through Tuesday...

Northerly flow will place our region under cold-air advection (CAA)
for approximately 36 hours by Thursday morning, and combined with
clear skies, we expect low temperatures Thursday morning to be cold,
with lower teens in the east, and single digits to below zero in the
west. High pressure will move over us to our east Thursday afternoon
into evening, keeping our skies clear, and flipping our winds to the
south. Although this wind shift will end the filtering of cold air
into our area, it will take some time for the the established cold
air to warm up, and temperatures will remain below freezing
throughout Thursday and into Friday morning. Eventually the sunshine
and warm-air advection (WAA) brings high temperatures to near
freezing, or even above freezing for southeastern counties Friday
during the day.

Our next weather-making system approaches Saturday. The pressure
gradient will tighten from south, and pump in warmer, moist air from
the south, with isentropic lift and associated precipitation well in
front of the surface low. With a couple of days of WAA, temperatures
at and just above the surface may get above freezing, so a wintry
mix scenario is initially possible with this system, but since the
system is still four days out, exact details are uncertain. Winds
will eventually turn northerly in the wake of this low pressure
system, allowing colder air to filter in Monday and Tuesday,
although lingering low clouds may modify our temperatures somewhat,
and keep us from bottoming out in the morning on those days.

BKL

&&

.AVIATION...
For the 00Z TAFs...

542 PM...Forecast concerns include...

Lake effect snow showers.
Northerly winds through the period.
Mvfr cigs through the period.

A band of lake effect snow showers has shifted southeast of mdw
and has also begun to weaken some but will affect gyy over the
next few hours. Flurries will also be possible for ord/mdw this
evening. Focus then shifts to another period of potential lake
effect snow around and just after daybreak Wednesday morning.
Medium confidence this will occur and likely affect gyy but low
confidence of how far west it may shift. Opted to carry flurries
for ord/mdw with a tempo for snow showers at mdw. If these were to
occur...vis could be considerably lower. This band may be near gyy
for several hours through midday before finally shifting east late
Wednesday afternoon.

Winds are expected to remain northerly through Wednesday afternoon
with a period of north/northwest possible tonight then going back
to north/northeast Wednesday morning then finally settling back to
northwest Wednesday afternoon/evening. Some gusts into the lower
20kt range will be possible this evening.

Mvfr cigs will likely continue across Lake Michigan and into
northwest Indiana through Wednesday afternoon. Some scattering is
possible at ord/mdw/dpa but confidence is low to try to time any
possible scattering. Another period of mvfr cigs would be likely
Wednesday morning with the above noted lake effect snow shower
potential. cms

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...Small Craft Advisory...Gary to Michigan City IN until 9 AM Thursday.

     Small Craft Advisory...Winthrop Harbor to Gary IN until 3 AM
     Thursday.

&&

$$

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