Forecast Discussions mentioning any of "ACARS" "MDCRS" "AIRCRAFT" "TAMDAR" "AMDAR" "WVSS" received at GSD on 02/11/19

Area Forecast Discussion

National Weather Service Chicago/Romeoville, IL

 847 PM CST Sun Feb 10 2019

.UPDATE...
829 PM CST

Main update this evening was to extend the mention of light snow
and freezing drizzle through late this evening across much of the
area. Latest radar imagery depicting persistent scattered precip
development across northern IL and northwest IN. The stronger
forcing has departed to the east, but isentropic ascent remains
focused across the area. This should allow for an additional 2-3
hours of precip, before this shifts to the east overnight. The
trend over the last couple of hours has been for all light snow
north of I-80, and mainly freezing drizzle for locations along and
south of I-80. A fine mist has been observed here at the office in
Romeoville, with a light glaze noted on the cars. However, at
this time, there are some small snow flakes mixing in with the
mist. AMDAR soundings from this evening did note some moisture
right around -8c, which is right on the lower threshold to support
snow development. This has been just enough to support snow in
some locations this evening, but also why we are likely seeing
light snow and freezing drizzle. So, for the remainder of the
evening, think most locations will see this mix before the precip
departs. Have issued graphicasts and an SPS to message this
potential along with some possible slippery conditions and given
the light nature of the precip along with the short window this
evening, will hold off on any headlines at this time. Will
continue to monitor trends/obs though, especially for locations
between I-88 and I-80 where this mix will likely be more
prevalent.

Rodriguez

&&

.SHORT TERM...
230 PM CST

Near term discussion through tonight...

Snow came down quickly as we finally saturated the low layers
across portions of northeast Illinois this afternoon. The dry slot
of this wave is already reaching the Illinois border, thus the
window for snow is not very long, only a few hours of the deeper
saturation, thus accumulations will not be all that significant,
around a half inch up to 1 inch in most locations. The snow should
be winding down in the coming hours.

There is some weak ascent in the low levels that will continue
behind this wave once we lose deeper saturation. Observations in
the dry slot across Iowa has indicated a transition from -SN to
either unknown precip (likely FZDZ) or freezing drizzle with some
snow. Low cloud bases are not widespread upstream which is
indicative of the freezing drizzle likely not being long lived,
but we will lose the deeper moisture while some lingering forcing
with this wave and low level isentropic ascent continues into the
evening. Given the aforementioned, just need to monitor the
situation to see if any light accums are possible.

Late this evening into the early overnight we do appear to be
in between waves, so in spite of there being lower clouds, the
freezing drizzle potential appears lower after mid evening
tonight. The low level frontal zone will setup just to the south
across central Illinois. Isentropic ascent into this frontal zone
on SW flow ahead of our next wave may increase FZDZ potential
late.

KMD

Short Term Discussion about the Monday into Tuesday event...

Quite a messy set-up is unfolding across the CWA Monday afternoon
through Tuesday as the system responsible for active weather across
the Pacific NW affects the western Great Lakes.

Monday: A shallow stratus layer will likely reside under a deep
layer of dry air aloft in the morning. Other than possibly some very
week isentropic lift, forcing will lack during the morning hours on
Monday for all but the far southern CWA. The layer of isentropic
ascent will quickly deepen in the afternoon in concert with mid-
level saturation, so a shield of a mix of light snow and freezing
drizzle may shift north across the north half of the CWA mid to late
Monday afternoon. Confidence on occurrence is somewhat low at this
time.

Monday night: A combo of WAA, the approaching mid-level trough, and
the left-exit of an upper jet should allow for a widespread swath of
moderate precip to shift NE across the CWA during the night. The
increased low-level WAA should push the surface freezing line to
along the I-80 corridor, thus ending the threat of freezing rain
south. Between the I-80 and I-88 corridors, including Chicago, the
dominate precip type looks to be freezing rain, whereas precip
should remain snow generally north of I-88. Given thermo profiles
only slightly below freezing below 700 hPa to the north, expectation
is for any snow to be fairly wet, with potentially <10:1 SLRs.
Lastly, east winds may gust as high as 30 mph through the night
across the north half of the CWA.

Tuesday: The surface low will track NE across the southern CWA into
NW Indiana during the morning. CAA will ensure, changing rain to
snow from NW to SE across the CWA through the day. Snow
accumulations during the day will range from 1-2" north to around a
half inch south. Meanwhile, NW winds will quickly increase with
gusts to 35 mph by sunset Tuesday across much of the area. Given the
wet snow potential, blowing snow does not appear to be a significant
issue at this time.

Headlines: First, have decided to hold off on any Winter Storm
Watches at this time. Had considered one for the northern tier of
counties plus Ogle county. Warning criteria of 6"/12hr or 8"/24hr
will likely not be met given the low snow ratios. However, impacts
from the very wet snow may ultimately justify the need for a warning
if guidance continues to indicate higher QPF over 0.5". However,
confidence is quite high that solid advisory-level snow will be met
across the northern tier of counties Monday evening into Tuesday
afternoon, especially given the potential of rather wet snow.
Farther south, confidence on ice amounts is low to medium. This
results in a conundrum of whether or not to issue an advisory for
lower-confidence icing that starts earlier than higher confidence
advisory-level snow that starts later to the north. Ultimately
decided to issue an advisory across the northern two tiers of
counties. While confidence is medium for icing across the Chicago
metro and west, a potentially colder solution of more snow than ice
would still warrant an advisory for moderate wet snow instead. In
other words, while precip type (snow vs. freezing rain) remains
unclear, there will likely be impacts either way. Farther south
along the I-80 corridor, there are some concerns that slightly
higher surface temps may significantly reduce icing amounts, so
have opted to not issue an advisory there at this time, but a
colder solution require an expansion to the south.

Kluber

&&

.LONG TERM...
235 PM CST

Tuesday night through Saturday...

The main upper trough axis will shift across the area Tuesday
night, while the surface low will deepen further and spread to the
eastern Great Lakes. This pattern would suggest precip will taper,
but increased low level lapse rates and deep enough saturation for
dendritic snow suggest snow showers will continue for some areas
into the evening hours. Modest pressure rises suggest some wind-
whipped snow showers are possible with some light accumulation of
a few tenths of an inch.

Wednesday may be the quietest day of the week in the weather
department. Weak height rises in wake of the trough will allow a
surface ridge to slid over the area, making for cool but dry
conditions.

Our time in the quiet weather zone will be short lived as we key
in on the track of another Pacific Northwest upper low which
appears like it could take a bit more northern track across the
country before reaching us. This trough will induce another decent
surface low in lee of the Rockies on Thursday. Southerly flow
ahead of the low will push the warm frontal boundary north through
a good portion of the area and allow temperatures to get above
freezing nearly area wide ahead of this next fairly potent system.
There is fair agreement that the low will spread from the central
Plains Thursday northeastward toward the eastern Great Lakes into
Friday. This could well be another messy system with the
baroclinic zone (which includes the 0C isotherm) draped somewhere
across the area. This pattern would generally suggest a rain/mix
to snow event. While models are in decent agreement on this
general pattern, subtle differences in the low track would support
anything from a rain/wintry mix turning to snow event, or all
snow for some areas, and whether we get significant snow or get
dry slotted and miss the better QPF. Needless to say confidence on
a messy system is fairly high with several inches of snow a decent
possibility, but the details are of low confidence.

Weak through transient ridging will make for dry, cold, and breezy
conditions on Saturday.

KMD

&&

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

Another tricky forecast ahead with precip type concerns both at
the beginning and end of the 30-hr TAFs. Early evening surface
analysis reveals a surface trough draped west to east across
central Illinois. This trough axis is progged to inch northward
through the evening, then shift south again overnight as weak
surface wave propagates east along the trough.

PRECIPITATION: Overrunning precipitation continues to wane in
intensity north of the quasi-stationary front this evening.
Anticipate this trend to continue as isentropic ascent weakens
over the next 2-4 hours with precip shutting off thereafter for
the remainder of the overnight hours. While the terminals continue
to report mainly snow early this evening, loss of ice nuclei is
expected as moisture becomes more shallow. This may result in a
transition to patchy freezing drizzle through mid evening, then
anticipate precip to end altogether. Confidence in freezing
drizzle is low, especially at ORD and RFD which are farthest from
the surface trough and low ceilings. There is a slightly better
chance, but still not great, farther south by MDW and GYY which
are closer to IFR ceilings.

Most of the day Monday should be dry, but attention will turn to
the southwest as low pressure lifts from the southern Great
Plains towards the local area by late Monday night. Precipitation
with this next low is expected to begin overspreading the
terminals Monday evening. There remains low confidence in what the
prevailing p-type will be given ongoing model differences.
Leaning towards NAM thermal profiles which have slightly faster
warming aloft as compared to GFS soundings, but at this point
couldn`t rule out periods of freezing drizzle, snow, rain, and
sleet at various points in the evening and overnight. Accumulating
snow is more favored at the northern terminals, including ORD and
RFD which could see several inches of snow. While GYY and MDW
will also likely see light snow accumulations, the more southern
terminals are more favored to see modest ice accumulations as
well.

WINDS: East winds this evening will back to the northeast late
this evening and overnight. Winds will become increasingly gusty
through the latter half of the day Monday as the next surface low
approaches the region. Anticipate gusts into the low to mid 20 kt
range by Monday evening.

CEILINGS AND VISIBILITY: IFR/LIFR ceilings are in place near the
surface trough axis over central Illinois north to roughly the
I-80 corridor. These lower cigs will trend slightly farther
north overnight. For the TAFs, do bring IFR cigs into ORD
overnight, though there is a small chance they remain MVFR
overnight. Anticipate IFR or near IFR ceilings to persist through
much of the day Monday. Finally, misty conditions may result in
visibilities between 2-5SM overnight.

Deubelbeiss

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...Winter Weather Advisory...ILZ010-ILZ011-ILZ012-ILZ013-ILZ014...6
     PM Monday to 9 AM Tuesday.

     Winter Weather Advisory...ILZ003-ILZ004-ILZ005-ILZ006-ILZ008...9
     PM Monday to 4 PM Tuesday.

IN...None.
LM...None.
&&

$$

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