Forecast Discussions mentioning any of "ACARS" "MDCRS" "AIRCRAFT" "TAMDAR" "AMDAR" "WVSS" received at GSD on 01/30/20

Area Forecast Discussion

National Weather Service Chicago/Romeoville, IL

 800 PM CST Wed Jan 29 2020

.UPDATE...
800 PM CST

A persistent weak meso-low has been noted on regional radar mosaic
off of the southeast Wisconsin lakeshore this evening. Lake effect
snow showers associated with the southwest flank of the meso-low
had come ashore in the north shore area of Cook County early this
evening. The activity has since weakened a bit, with the better
radar echoes off-shore, as it has drifted south-southeast over
portions of the north side of Chicago. Have added in PoPs for
scattered snow shower activity over the Chicago shore and a few
miles inland over the next 1-2 hours. With the light intensity and
temps right around freezing, wouldn`t expect much in way of
impacts, with a dusting possible on colder/grassy surfaces and
untreated paved surfaces.

Further inland, scattered flurries will remain possible through
the evening. Midway Airport has been reporting flurries with
unrestricted visibility since mid day. Also seeing a few
automated sites with minor (4-5 mile) visibility restrictions that
may be indicative of flurries/very light snow. Cloud bases
(2000-3500 feet) appear to be too high to support freezing
drizzle. Will update PoPs as needed for the light lake effect snow
showers this evening. The rest of the forecast is on track, with
ever-persistent stratus over the region limiting the temperature
fall tonight.

Castro

&&

.SHORT TERM...
239 PM CST

Through Thursday night...

The stagnant weather pattern will hold strong into the latter half
of the week. Continued intermittent flurries will be seen into
this evening and cannot rule out a dusting for some near lake
locations and in the southwest forecast area as well. Temperature
trends continue near-flatline.

A widespread blanket of stratus clouds remain intact over the area
this afternoon with no changes in immediate sight due to an
inversion and trapped low-level moisture of 1,500-2,000 ft thick
per recent aircraft soundings. Light winds off the lake are
abating and so lake-generated flurries are becoming more
scattered. A fairly well-defined upper trough nearing the
Mississippi River will continue to shift east and dampen into
tonight. Ascent- induced saturation at 10,000 ft or higher
associated with this is feeding some light snow into the existing
stratus across eastern Iowa and western Illinois. Some of this is
likely to clip the far southwest and southern forecast area early
this evening, but a spotty dusting as most is all that is
expected. Otherwise just intermittent flurries area wide. Some
lower cloud bases are forecast to move into north central
Illinois, dragged in at least in part by the trough. While some
guidance hints at a marginal freezing drizzle profile, this does
not have many of the ideal ingredients and even if it were to
materialize it would be light and probably not impacting, so none
noted in the current forecast.

After a cloudy Thursday, an approach upper wave will bring height
falls and synoptic ascent into the area later Thursday evening and
more so overnight. While the system`s warm advection area aloft is
somewhat ill-defined, saturation depth is better with this system
than the past couple waves. Have added a slight chance of light
snow overnight with this and again a non-zero chance of some
freezing drizzle, though not mentioned in the forecast at this
time.

Guidance again seems too cool with lows tonight in this pattern
and have inched up and keep the strongly muted diurnal range
going forward.

MTF

&&

.LONG TERM...

Friday through Wednesday...

201 PM...Forecast concerns include light snow/minor accumulation
possible Friday night...brief warmup Sunday/Monday...possible
storm system Monday night through Tuesday night.

Light snow or flurries may be ongoing Friday morning...especially
across the northeast cwa and flurries may persist at times
Friday. But the next wave and still looking a tad stronger will
move across the area Friday night. This feature has been fairly
consistent on the models and will have the potential to produce
some minor snow accumulations...likely up to an inch particularly
across the north half of the cwa. Bumped up pops some with this
forecast and added a little more qpf. It would appear that the
main precip type will be light snow but as it tapers off Saturday
morning...there could be some freezing drizzle potential but did
not include with this forecast.

Warm air rapidly spreads across the area aloft Saturday night and
it does appear the low clouds will clear out by Sunday morning. If
sunny conditions do materialize Sunday...highs should easily reach
the 40s with snow cover across the north the only limiting factor.
Highs in the south could potentially reach 50.

There still remains quite a bit of uncertainty for highs on
Monday. The gfs/gem are in good agreement turning winds northeast
by midday Monday through the I-80 corridor. While the ecmwf is
still southerly on Monday...it now shifts winds northeast by early
Monday evening. There is likely going to be a rather sharp temp
gradient in or near the cwa Monday with temps near 50 still
possible in the south. Blended temps did drop a few degrees for
highs Monday and given the latest trends...this seems reasonable.

There has been a general shift a bit to the south with the system
for Monday night through Tuesday night...especially from the gfs
with qpf amounts lower than 24 hours ago. Still too early for
details with this system but the idea of some kind of rain mixing
with then changing to snow along with a possible wintry mix is
still reasonable with the potential for some snow accumulation.
Though how fast this change/mix were to happen and where it may
occur is clearly uncertain. Pops are considerably higher now from
the blended guidance...but still in the chance range. cms

&&

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

Main aviation forecast concerns include persistence of MVFR
ceilings, and occasional flurries.

Weak surface high pressure remains in place across the upper
Midwest early this evening, with light variable winds and a large
area of generally MVFR stratus across the forecast area. Local and
regional radar continue to depict weak returns indicative of
flurries or very light snow, which may affect the terminals at
times especially this evening. This light precip was generally
being driven by weakening convergent onshore flow off Lake
Michigan, as well as the approach of a mid-level disturbance
crossing the Mississippi into IL. This larger scale forcing, and
mid-level cloud deck which may seed the lower stratus layer from
above to create flurries/very light snow, will pass east of the
area around midnight, decreasing the precip potential. Flurries
off the lake will also likely become less prevalent this evening
as well, as low level winds above the surface continue to weaken.

There has been an upward trend in MVFR ceilings across parts of
northern IN and southwest MI since this afternoon, with some
locations around the forecast area seeing a scattering of the
lower MVFR layer and becoming VFR with ceilings 3500-4000 ft.
High-res guidance would suggest this may continue for a time this
evening, before settling back to a more solidly MVFR height
overnight and much of Thursday. Not much change in surface winds
is expected through the night, with light/variable or calm
conditions likely. Light winds may acquire a more southwesterly
direction by Thursday afternoon, though generally remaining less
than 10 kts.

Ratzer

&&

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

$$

Visit us at weather.gov/chicago

Follow us on Facebook, Twitter, and YouTube at:
www.facebook.com/NWSChicago
www.twitter.com/NWSChicago
www.youtube.com/NWSChicago