Forecast Discussions mentioning any of "ACARS" "MDCRS" "AIRCRAFT" "TAMDAR" "AMDAR" "WVSS" received at GSD on 02/13/19

Area Forecast Discussion...UPDATED

National Weather Service Chicago/Romeoville, IL

 825 PM CST Tue Feb 12 2019

.UPDATE...
820 PM CST

Had earlier extended the duration of the snow in the forecast,
with an end from west-to-east during the overnight hours. Some
flurries may linger after daybreak in Chicago and northwest
Indiana.

The classic mid-latitude cyclone has its large comma head
centered over the western Great Lakes, with impressive 170dm
height falls in the past 12 hrs on the 00Z GRB sounding. A well-
defined moist and warm conveyor belt extends into the commahead
and deformation area curled into northern Illinois and northwest
Indiana. Regional 00Z soundings and recent aircraft soundings show
a moist inversion of several thousand feet in or closing in on
the dendritic growth zone, and we have observed a couple periods
of better dendrites mixing in here at the office. This profile
with some lift will help support snow longer into the night even
while the stronger forcing inches away from the area. Rates are
light to occasionally moderate, and enough for additional light
accumulation and cannot rule out an additional inch in northeast
Illinois from this point forward.

Westerly winds continue to solidly blow. It should be noted there
are a handful of AWOSs that are likely dealing with frozen
anemometers and thus not capturing the peak or even any gusts at
all. Peak gusts of 38-45 mph have been measured at most
functioning ASOS and AWOSs with sustained winds peaking in the
upper 20s. These winds have been causing shallow blowing snow
especially in north central Illinois, where multiple social media
reports from the I-39 and U.S. Highway 251 corridor capture the
extent of that. We as on office decided earlier not to confuse
things with a headline switch and note the additional winter
impacts with the Wind Advisory. These other impacts continue to
include at least scattered power outages which have been reported
and these will continue to be a threat through the night. In fact
not far from the office there was a transformer to blow just about
an hour ago.

Going temperatures and wind chills for early Wednesday morning
look good, with wind chills bottoming out -1 to 5.

MTF

&&

.SHORT TERM...
1254 PM CST

Through Wednesday...

Near term concerns center around snow this afternoon, scattered snow
showers this evening, and impacts from gusty winds on visibility
along with the threat of downed trees and power lines where
significant ice accumulations exist.

Dense fog earlier today that occurred while we were under the center
of low pressure is lifting as the low will continue toward the
eastern Great Lakes tonight. Shortwave energy ahead of an
approaching upper trough will drive an area of snow over a decent
portion of the area early this afternoon. Accumulations should
remain fairly modest. The main upper trough axis will shift
overhead and will drive modestly steep low level lapse rates later
this afternoon and more so tonight. In addition, cold air will
surge back in behind the low through this evening which will drive
the low level thermal profile to one supportive of decent
snowflake production. Therefore expect snow to transition to at
least scattered showers through the evening.

Winds are picking up from the west early this afternoon. The
upstream high pressure system is not quite as high as what we saw
last week when we had fairly widespread 45-55 mph wind gusts, but
the low level wind profile does suggest gusts of 40-45 mph are
certainly plausible, locally higher south. Nonetheless, these
winds will lead to significantly reduced visibility in any
stronger snow showers. A few tenths of an inch, maybe locally up
to an inch remain possible through tonight. Some areas of also had
up to 1/2" of ice which will weigh down branches and power lines.
These winds will add stress that could lead to falling ice, which
has been reported in downtown Chicago, along with additional tree
damage and power outages. We have continued the Wind Advisory
through tonight with the mention of these additional concerns, and
have converted (or left in place) the northern tier of counties to
a Winter Weather Advisory for these concerns where the winds may
be a bit lighter but there stands a longer chance of snow showers.

This system will slowly loosen its grip on the region on Wednesday.
We do maintain some breezy winds in the morning, which should give
way to some easing in the afternoon. We will even get some partial
clearing, at least for a little while Wednesday. Highs will remain a
tad below normal, with readings in the mid 20s to lower 30s.

KMD

&&

.LONG TERM...
146 PM CST

Wednesday night through Tuesday...

Quasi-zonal upper flow across the central CONUS begins to amplify
into an upper trough Wednesday night, as short wave energy
emanating from the Pacific Northwest phases with a trough across
Manitoba. This in turn induces a broad area of surface pressure
falls across the Plains, backing winds to the south and lifting a
warm front across the forecast area by Thursday morning. Overnight
min temps in the low-mid 20s will come during the evening, before
warming into the 30s overnight as the warm front lifts north.
Forecast soundings depict a dry low-level air mass below 700 hpa
initially, with a dry warm frontal passage expected. Large scale
ascent continues Thursday as the trough amplifies further,
eventually producing top-down saturation sufficient to produce
light precipitation especially across the northern half of the
cwa. This precip will fall primarily in the form of rain, with
temperatures continuing to warm into the upper 30s and lower 40s
during the day as the surface low passes north of the area across
Wisconsin. The trailing cold front will push east across the cwa
during the afternoon and early evening hours, with colder air
spreading in and eventually turning any lingering light precip
over to light snow before ending during the evening. Looks like
little or no accumulation however, by that time.

After precip ends Thursday evening, blustery and colder
conditions are anticipated as the low continues to deepen while
pulling away to the northeast. West-northwest winds gusting 25-30
mph and temperatures falling into teens across much of the area
will result in single digit (both above and below zero) wind
chills by Friday morning. Winds will gradually diminish during the
day Friday, though it will remain chilly as the forecast area
remains along the southeastern periphery of surface high pressure
ridge building across the Northern Plains. The next quick-moving
mid-level short wave is progged to approach the area late
Friday/Friday night, with guidance generally tracking this
disturbance across central and southern parts of Illinois/Indiana.
Forecast soundings depict cold, dry low-level air mass across
most of the cwa, and only the CMC edges the northern edge of the
light precip shield into the far southern counties overnight. With
GFS/ECMWF dry, and a tight gradient in snow amounts expected into
ILX and IND`s forecast areas, have limited chance of accumulating
snow to slight chance across the far southern tier of counties
Friday night.

Weak high pressure spreads across the area Saturday, before
another slightly more amplified mid-level short wave trough
approaches rapidly from the west Saturday night. This wave looks
to have a little better potential for producing light snow across
the southern parts of the forecast area, though the GFS and CMC
focus the axis of greatest accumulation (perhaps 2-3") just to our
south. The ECMWF is farthest north of the available guidance, and
would support at least chance pops area-wide. Guidance does give
some indication of a second short wave phasing into the trough
across the Midwest on Sunday, which may help light snow linger
into Sunday evening. In addition, northeast low-level winds may
result in some light lake effect snow showers persisting overnight
or even into early Monday over northeast Illinois and far
northwest Indiana.

The upper pattern then begins to evolve toward a deeper west
coast upper trough into early next week, with broad upper ridging
developing downstream across the Midwest. After several days of
below average temperatures from Friday through Monday, some
moderation back toward normal is expected beneath the building
ridge. The western trough is progged to be rather progressive,
approaching the region and bringing the next chance of organized
precipitation just beyond the end of the forecast by the middle of
next week.

Ratzer

&&

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

Low pressure over Michigan will continue to lift away from the
terminals, but lingering moisture/snow on the back side of the
system and strong gusty winds will continue to impact the
terminals through the remainder of the evening and likely through
the majority of the overnight hours. Regional radar shows light
snow stretching west into portions of eastern Iowa this evening
and will produce light additional accumulations this evening and
tonight. Through midnight local time , the terminals may see up to
an additional inch of snow with an additional dusting possible
through daybreak. In addition to any falling snow, strong winds
gusting 30 to 35 kt will result in blowing snow that will further
reduce visibility. Expect visibility to generally bounce between
1SM and 4SM through tonight, though cannot rule out briefly
dipping to 3/4SM at times. Wind are expected to gradually diminish
Wednesday morning, though gusts to around 20 kt will be possible
well into the afternoon. Expect ceilings to generally remain MVFR
through tonight, eventually scattering to VFR at some point the
day Wednesday, though confidence is low on the specific timing.

BMD

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...Winter Weather Advisory...ILZ003-ILZ004-ILZ005-ILZ006 until 3 AM
     Wednesday.

     Wind Advisory...ILZ008-ILZ010-ILZ011-ILZ012-ILZ013-ILZ014-ILZ019-
     ILZ020-ILZ021-ILZ022-ILZ023-ILZ032-ILZ033-ILZ039 until 3 AM
     Wednesday.

IN...Wind Advisory...INZ001-INZ002-INZ010-INZ011-INZ019 until 3 AM
     Wednesday.

LM...Gale Warning...nearshore waters until 9
     AM Wednesday.

&&

$$

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