Forecast Discussions mentioning any of "ACARS" "MDCRS" "AIRCRAFT" "TAMDAR" "AMDAR" "WVSS" received at GSD on 01/03/24

Area Forecast Discussion

National Weather Service Chicago/Romeoville, IL

 604 PM CST Tue Jan 2 2024

.UPDATE...
Issued at 604 PM CST Tue Jan 2 2024

An earlier perusal of observations and AMDAR soundings suggest that
cloud depths are, at least initially too shallow to support too much
freezing drizzle just yet. And really that is the case overall.
However, we did get some reports of flurries in the echoes that show
up on the Lacrosse (KARX) radar, and in recent scans of the
precipitation echoes that have shown up on the Milwaukee (KMKX)
radar. We did hear of some reports south and east of Madison, WI
of pockets of freezing drizzle under these light echoes.

The echoes in Milwaukee are more a mix of mist/patchy/short lived
freezing drizzle, and associated with some broader forcing, but
given observations will hang onto some patchy freezing drizzle
mention this evening. This will be favored after 8-9PM around
Rockford and after 10-11 PM in the Chicago area. Temperatures at
Rockford are already at freezing, and for Chicago with this first
batch of drizzle, temperatures may hover closer to 32-33 in
Chicago, but will be falling below freezing with high confidence
after midnight.

Closer to La Crosse the echoes appear to be tied more so to the
surface trough axis where one would conceptually expect an increase
in forcing in the lower level cloud deck. This main forcing window
would be favored overnight into tomorrow morning. While we have seen
some -SN reports farther upstream in WI, temperatures in the cloud
deck appear to get as cold as -8C, which probabilistically would
only support a low chance of ice nuclei (favoring drizzle/freezing
drizzle instead of snow) but still cold enough to at least support
the potential of flurries mixed in as was observed.

Temperatures overnight do show some spread, but the 90th percentile
minimum temperatures still drop below freezing, adding decent
confidence in if precip occurs, it would be with sub-freezing
temperatures. Freezing drizzle is notoriously difficult to forecast,
but could be highly impactful. Model QPF is at least suggesting an
uptick overnight into the morning, so this will be a period to
watch, and will add some freezing drizzle to the forecast for this
time frame as well. Confidence admittedly is still low.

KMD

&&

.SHORT TERM...
Issued at 242 PM CST Tue Jan 2 2024

Through Thursday...

Early this afternoon, a blanket of low stratus is filling in over
the CWA from the northwest. This stratus deck is attributed to a low
level moisture plume that`s being pushed across the region ahead of
a pretty potent vort lobe aloft. At the surface, this is translating
to a subtle frontal feature that`s currently pushing through central
Iowa. Ahead of the boundary across central and eastern IA and up into
western WI, we`ve seen a number of reports of reduced visibilities.
For the most part, these reports have read as low as 2-3 miles but a
couple of isolated sub one mile reports have snuck in there as well.
However, reports of lower visibility have become even more scarce
since the start of the afternoon. The saturated layer just off the
surface out west appears just a bit too shallow for bonafide precip.
However, it certainly looks capable of producing some very light
drizzle or a mist, especially given boundary layer lapse rates
supportive of saturated ascent and a little extra oomf in the
presence of some modest PVA.

This environment will progress across the CWA later this afternoon
and through this evening. As it does, we`ll too see saturation build
down and deepen overall, steepening boundary layer lapse rates, and
improved synoptic forcing within a marginal PVA regime. However,
looking at upstream obs and late model guidance, a fine mist is
looking more likely than drizzle, especially during the late
afternoon and early evening. This would favor less impacts to
visibilities. Light drizzle is looking a bit more likely during the
middle and late evening when the main low level vort lobe drops
through the CWA, forcing improves a tad, and we`re progged to see
the most favorable saturation. Overnight, a little bit of drying
just off the surface and less ascent convinced me to leave the
forecast dry, although some mist or very light drizzle may want to
continue into the morning.

Temperatures this afternoon are sitting the middle 30s to around 40
degrees. Conditions will begin to fall below freezing early this
evening with widespread sub-freezing temps expected by mid-evening.
That said, it`s possible that any drizzle that materializes could
freeze to the surface. This raises some concern for patches of slick
travel late this evening and overnight. And if drizzle does decide
to stick around through the night, it could be a slippery Wednesday
morning commute for some.

Deepening, albeit still shallow, saturation could favor some
additional light precip tomorrow, particularly in the afternoon. For
the first few hours of the afternoon, forecast soundings are unsure
whether we`ll saturate deep enough to see cloud ice with the top of
the saturated layer floating right around -9 or -10C. Signs are
pointing toward yes, so I included a mention of flurries possible
tomorrow afternoon and evening. Late in the afternoon, namely in the
21-00Z timeframe, there is fair agreement on a quick hit of mid
level moisture pulling the saturated layer into the lower bounds of
the DGZ and marginally steep lapse rates up through around 850mb.
This could result in a round of snow showers, mainly north of I-80
where elsewhere may not saturate deep enough for full-on snow
showers.

Low level winds will turn north-northwesterly behind the low level
trough axis tomorrow afternoon. The cooler flow over the relatively
warm lake is expected to throw up some lake effect snow, primarily
for parts of far northwest Indiana, beginning as early as the late
afternoon and continuing late into the night, if not early Thursday
morning. Some lighter snow showers are likely to bleed over into
Illinois as well with the greatest chances being during the evening
when the low level flow has a stronger northerly component than
later in the night. This doesn`t look like a big snow producer by
any means with forecast soundings along the Indiana shore showing
lake-induced ELs just tapping into the DGZ and marginally steep
lapse rates through the boundary layer. Nonetheless, a few tenths to
locally around an inch of snow over far northwest Indiana look
attainable.

Doom

&&

.LONG TERM...
Issued at 242 PM CST Tue Jan 2 2024

Thursday night through Monday...

Key Messages:

* Waves of flurries and light snow showers are expected Friday night
  through Sunday morning (20 to 40% chances), with little to no
  accumulation.

* There remains a signal for an impactful storm system somewhere in
  the region next week.

Discussion:

Thursday into Friday currently look "quiet" as the backside of a
surface high pressure system slowly works through the region.
Increasing low- mid-level WAA ahead of troughing across the southern
Plains will support increasing clouds, altogether leading to a
relatively muted diurnal temperature range with highs in the lower
30s and lows in the mid 20s.

Friday night through Sunday, the aforementioned upper-level trough
will move through the Great Lakes in "disaggregated" fashion,
leading to several opportunities for light rain and snow. However, a
rather pronounced (nearly 15kft) deep dry layer will be in place
across the Lower Great Lakes, suggesting there may not be much more
than flurries and periodic rain and snow showers as each wave passes
overhead. At this point, opted to maintain the inherited 20 to 40%
PoPs across the area particularly from Friday night through Sunday
morning, keeping in mind the pattern does not look overly supportive
of anything impactful. Overcast skies will continue to mute diurnal
temperature ranges, with overnight lows in the mid 20s and highs in
the low to mid 30s.

Early next week, ensemble model guidance remains steadfast in the
development and ejection of an anomalously deep western trough and
associated deepening surface low pressure system toward the Ohio
River Valley. Conceptually speaking, the pattern bears similarities
to those that bring impactful weather to our region, including but
not limited to strong winds, falling temperatures, and periods of
steady rain and/or accumulating snow. Exactly where and at what
magnitude each manifest will be an opportunity for refinement in
later forecasts. For now, our message is to keep an eye on the
forecast for next week.

Borchardt

&&

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

The main aviation weather concerns are:

* Threat for -DZ and/or -FZDZ this evening and tonight.

* IFR cigs this evening, lifting to MVFR which will persist
  through much of the day on Wednesday.

* Flurries or -SHSN develop Wednesday afternoon.

The main focus tonight remains on the potential for drizzle and
freezing drizzle development at the regional terminals. As is
typical in these scenarios, overall confidence is not particularly
high, but a few recent trends are helping aid the forecast a bit.
Cigs will continue to lower through low-MVFR and eventually IFR
this evening. As this occurs, cloud depths will deepen, and may
become just deep enough to support light precipitation. In the
last hour, saw a report of intermittent freezing drizzle near MSN
which is associated with a recent uptick in light radar returns.
Tracking this feature southward brings it into RFD towards 02z and
then the Chicago-area sites towards 04z. At RFD where temperatures
are cooler, have a mention of -FZDZ in a TEMPO group to cover this
threat. At ORD/MDW with temperatures currently running 2-4 degrees
above guidance, not super confident in temperatures falling under
32/33 as this first batch of precip arrives, so have precip
mentioned in a -DZ TEMPO for now.

After midnight as a weak surface boundary moves through,
conditions are forecast to become a bit more favorable for drizzle
production. Have added a second TEMPO group with -FZDZSN to cover
this second window. The addition of the SN mention was based on
the La Crosse, WI weather office reporting snow/flurries and IR
satellite showing slightly colder cloud top temperatures which may
support a bit more snow production as opposed to pure drizzle.

End users should expect changes to the TAFs tonight with AMDs to
refine timing and potentially prevail -DZ/-FZDZ as trends
establish. At ORD/MDW, air temperatures just under freezing
tonight (perhaps 30-32) and warm ground temperatures may help
blunt glazing issues on runways, but do expect aircraft icing on
colder surfaces.

Cigs will slowly lift through the day on Wednesday, but MVFR are
expected through much of the day. Another disturbance will bring a
a period of -SHSN during the afternoon and evening.

Carlaw

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...None.
&&

$$

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