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ILSNOW

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    February 17th, 2019 Snow Event

    in Lakes/Ohio Valley

    Posted

    LOT update 

    .UPDATE...
909 AM CST

Snowfall rates have been impressive within pockets of moderate to
briefly heavy snow this morning across the NW half of the CWA.
Various reports indicate 1 to locally 3 inches has already fallen
within an axis from Lee to Lake(IL) counties. Snow ratios under the
heaviest snow bands appear to be 20:1 or even slightly higher.

Much of this is being forced by a combo of low-level WAA/isentropic
lift with some assistance from an upper trough axis wrapping around
the upper low over NW IA. A notable back edge to the heavier precip
rates tied to the upper trough axis is currently across the southern
CWA counties and should clear the northern CWA by noon.

Given the heavier snow rates across the northern Chicago Metro, had
considered expanding the winter weather advisory to Kane/DuPage/Cook
counties. However, with a majority of the accumulation having
already occurred, higher impacts remaining confined to mainly the
morning hours, and the expectation for only light snow to continue
this afternoon and evening, felt an SPS was a better option to
message the ongoing heavier snow.

    Winter Storm - February 11-13, 2019

    in Lakes/Ohio Valley

    Posted 

    URGENT - WINTER WEATHER MESSAGE
National Weather Service Chicago IL
959 PM CST Mon Feb 11 2019

...Messy Winter Weather Tonight into Tuesday...

ILZ010>012-019-121200-
/O.UPG.KLOT.WW.Y.0012.000000T0000Z-190212T1500Z/
/O.NEW.KLOT.IS.W.0002.190212T0359Z-190212T1500Z/
Lee-DeKalb-Kane-La Salle-
Including the cities of Dixon, DeKalb, Aurora, Elgin, and Ottawa
959 PM CST Mon Feb 11 2019

...ICE STORM WARNING IN EFFECT UNTIL 9 AM CST TUESDAY...

* WHAT...Significant icing. Total snow accumulations of 2 to 5
  inches and ice accumulations of up to three tenths of an inch.

* WHERE...Lee, DeKalb, Kane and La Salle Counties.

* WHEN...Until 9 AM CST Tuesday.

    Winter Storm - February 11-13, 2019

    in Lakes/Ohio Valley

    Posted 

    National Weather Service Milwaukee/Sullivan WI
826 PM CST Mon Feb 11 2019

.UPDATE...Will be upgrading Milwaukee, Waukesha, Jefferson and
Rock counties to a Winter Storm Warning shortly. Too much evidence
from Model Certainty and Probablistic snow totals for amounts in
these areas from 6 to 8 inches. In addition, am concerned about a
period of freezing rain/drizzle Tuesday morning as ice stops
falling into the cloud layer for several hours. Concerns over
persistent lift from lake enhancement affecting parts of
Milwaukee county later tonight into Tuesday morning. All guidance
also takes 850H circulation across far northeast Il late tonight,
which places parts of Rock and Jefferson counties in favorable
track for heavier snow. Will leave Walworth, Kenosha and Racine in
advisory for now.

Widespread snow continues to spread northeast across the CWA this
evening. Strong coupled jet structure and aloft and a period of
impressive sloping frontogenetical forcing in the low levels
overnight. One troubling piece is that the best low level forcing
from the frontogenesis does not intersect the better dendritic
growth zone.

    Winter Storm - February 11-13, 2019

    in Lakes/Ohio Valley

    Posted

    LOT update 

    National Weather Service Chicago/Romeoville, IL
737 PM CST Mon Feb 11 2019

.SHORT TERM...
734 PM CST

Through tonight...

Only modest changes to going forecast so far this evening.
Guidance is struggling with the northern extent of the warm nose
this evening. Several raob soundings over the area indicate
temperatures within the warm nose over Chicago to be between 0.5
and 1.5C, sufficient for partial if not full melting aloft.
Surface obs indicate some sort of wintry mix that includes liquid
precip over all but the northern tier of counties in the CWA.
Adjusting temperatures aloft accordingly brings the wintry mix
farther north across the CWA and nets slightly lower snow totals
and higher ice totals between the I-80 and I-90 corridors, and
tightens the snow gradient in that area. Little change was made
farther north which is expected to remain primarily snow.

    Winter Storm - February 11-13, 2019

    in Lakes/Ohio Valley

    Posted

    LOT UPDATE

      

    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.
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