Rockford Scanner™: WINTER STORM WARNING IN EFFECT FROM 9 PM THIS EVENING TO 3 AM CST SUNDAY

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Through Saturday night...
 
It still appears that the highest potential for significant 
freezing rain and sleet accumulations (~0.25") will be in the 
warning area across far north central IL tonight. Forecast 
guidance has trended a bit slower with the arrival of the 
subfreezing airmass farther south and east towards the Chicago 
suburbs tonight. For this reason, it continues to appear that most
areas outside of the current warned area could remain above 
freezing through the night, resulting in little threat of 
freezing, at least through daybreak Saturday. 

While areas outside the currently Winter Storm Warning should 
remain mainly ice free tonight, the threat of heavy rain possible
flooding will be the main concern. The possibility is there to 
have 1 to 3 inches of rain tonight right into the Chicago metro 
area, and this could result in flooding concerns across the area.

The freezing line should gradually sag southward into parts of the
western and northern suburbs of Chicago around, or shortly after 
daybreak. This should occur after the heaviest rainfall occurs, so
only light freezing precipitation is possible over parts of the 
area on Saturday (mainly north and west of the city of Chicago).

Precipitation is expected to redevelop over the area later in the
afternoon and evening Saturday. This will occur as strong large 
scale forcing for ascent associated with the primary mid/upper
level impulse shifts over the area. Precipitation type with this 
second wave of heavier precipitation will change to snow over much
of my northern IL counties early in the evening as the column 
dynamically cools, but will only gradually change to snow over 
northwestern Indiana before ending. The snow is likely to be 
moderate to heavy over northern IL Saturday evening. Given the 
likelihood that the snow could fall heavy at times due to some 
strong mesoscale forced ascent within the low to mid level 
deformation zone, a quick 2 to 6 inches are possible before the 
snow ends shortly after midnight. The heaviest amounts are likely 
to be north of I-80, with little accumulation expected over east 
central IL and northwestern IN.

 

 

 


URGENT - WINTER WEATHER MESSAGE
National Weather Service Chicago IL
403 AM CST Fri Jan 10 2020

...Periods of Wintry Precipitation, Possibly Significant, Tonight
Through Saturday Night...

...WINTER STORM WARNING IN EFFECT FROM 9 PM THIS EVENING TO 3 AM CST SUNDAY...

* WHAT...A mix of freezing rain and sleet is likely by mid to late
  evening, possibly mixed with snow at times overnight and
  Saturday morning. Total ice accumulations in excess of two
  tenths of an inch are likely tonight into Saturday morning, with
  sleet accumulation also possible. While precipitation may
  lighten in intensity Saturday late morning through mid
  afternoon, a period of heavy snow is likely late Saturday
  afternoon and night. Total snow accumulations of 3 to 7 inches
  are expected. Northeast winds will gust as high as 40 mph
  overnight into Saturday and combined with ice and snow may cause
  further impacts.

* WHERE...Portions of north central and northeast Illinois.

* WHEN...From 9 PM CST this evening to 3 AM CST Sunday.

* IMPACTS...Travel will be significantly impacted at times,
  especially mid to late evening through early Saturday morning,
  and then again later Saturday afternoon into Saturday night.
  Some power outages and tree damage are possible due to the
  combination of ice and wind. Patchy blowing snow could
  significantly reduce visibility later Saturday afternoon and
  night.

PRECAUTIONARY/PREPAREDNESS ACTIONS...

If you must travel, keep an extra flashlight, food, and water in
your vehicle in case of an emergency.

 


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Through Saturday night...

The well-advertised complex winter storm is still on track across 
the area tonight through Saturday night. Heavy rain, mixed precip 
including freezing rain and snow/sleet, and strong winds will 
produce significant hazards across at least portions of the area 
during this time.


Synopsis: A well-defined trough near the Four Corners region early 
this morning will shift east into the Texas panhandle by this 
evening, then turn northeast, crossing over or just northwest of the 
CWA early Saturday evening. Surface cyclogenesis is ongoing across 
west Texas at this time with a trough axis extending NE to another 
low between Lake Superior and James Bay. The trough represents a 
cold front that is drifting southeast into the northwest CWA, 
located roughly from Dixon to near Rockford as of 3am.

The front will continue to edge southeast today, then stall roughly 
along the I-55 corridor late this afternoon into this evening as the 
developing low takes control of the mass field in the region and 
induces stronger south winds ahead of the cold front. Guidance 
continues to show mixed signals on exactly where, and in what form 
(concentric vs. elongated) the surface low will track across the 
region, but a passage generally just southeast of the CWA late 
Saturday afternoon is appearing most likely.


Heavy rain: Periods of light rain or drizzle are expected today, 
especially southeast. Then, an extremely unseasonably moist air mass 
with near +4 standard deviation PWAT air in the warm conveyor belt 
of the system will quickly advect into the region on strong 60-80 
knot low-level winds late this afternoon into this evening. Periods 
of moderate to heavy rain are expected across much of the CWA 
tonight through daybreak Sunday. With continued anomalously high 
soil moisture from a wet 2019, potentially some pockets of remaining 
ground frost, and high precip rates, flooding remains a concern 
across the area tonight. Brief rainfall rates of up to 1" per hour 
are possible. The area of most concern for flooding remains 
unchanged, around and southeast of I-55. However, heavy precip is 
still expected across the remainder of the CWA, but will begin to 
fall as mixed precip and limit some runoff overnight (more on that 
below). Finally, a 100-200 j/kg MUCAPE axis edging into the south 
half of the CWA tonight will support some embedded thunder, which 
would further enhance precip rates.


Winter Precipitation: A rather complex winter precip set-up is 
expected with this event, largely owing to the potentially slow 
movement of the surface cold front and northwest tilt of the low 
with height. For the most part, this forecast has trended to slightly 
more freezing rain/sleet than snow for portions of the area around 
and northwest of the cold front. Otherwise, no significant 
adjustments have been made.

The current expectation is for rain to transition to freezing rain 
across the northwest CWA by mid-evening, with the rain/freezing rain 
line eventually reaching from near Waukegan to Elgin to Mendota by 
daybreak Saturday. A ~2-3hr period of freezing rain will likely 
occur before the low-level cold layer deepens enough to allow for a 
transition to sleet. Indications are that an extensive period (up to 
6 hours) of primarily sleet is possible across the NW half of the 
CWA ahead of the surface low late tonight into Saturday as the NW 
tilted low with height continues to feed warm aloft well into the 
northwest CWA.

This southeastward advance of the freezing line will slow on 
Saturday in response to the approaching surface low, reaching only a 
line from near Gary to Morris to Pontiac by early evening. The line 
will then rapidly advance southeast, clearing the remainder of the 
CWA by midnight. By this point, the passage of the low below 700 hPa 
will allow deep CAA to take hold and quickly narrow the width of the 
rain/freezing rain/sleet/snow transition. In fact, locations 
southeast of I-57 may experience only a 1-2hr transitional period 
from rain to snow. Snow will quickly end from SW to NW overnight 
Saturday night.

Overall, an axis of ice accumulation around 0.25" with up to an 
inch of sleet is expected northwest of a line from Mendota to 
Elgin to Lake County (IL) away from Lake Michigan, with amounts 
quickly dropping to little to no accumulation southeast of I-55. 
Though QPF is expected to be high, ice accumulations will likely 
be hindered by warm hydrometeors, high precip rates resulting in 
run-off before freezing, and surface air temps only at or slightly
below freezing for a period. Regarding snow, amounts range from 
near 4-6" across the far north and northwest, to 2" or less 
southeast of I-55.


Winter Headlines: Counties in the northwest portion of the Winter
Storm Watch have been upgraded to a Winter Storm Watch due to
increasing confidence in warning criteria impacts from a
combination of ice, snow and wind, and wintry precip beginning as
early as this evening. Have continued the watch for Lake, Cook,
DuPage, Kane, Kendall and La Salle as impacts may be borderline
warning/advisory and due to a later onset of wintry precip (late
tonight into Saturday morning).

Kluber


Winds and Lakeshore Flooding...
This will be a noteworthy possibly even upper level impacts of 
this multi-faceted system, particularly lakeshore flooding for 
northeast Illinois and far northwest Indiana. Tonight as the first
low lifts northeast toward the Lower Great Lakes, the pressure 
gradient responds by tightening as pressures sharply fall (6-9 mb 
per 6 hrs). By midnight, these resulting northeast winds will 
become near unidirectional in the mixed boundary layer of a couple
thousand feet thick where cold advection will be occurring, and 
that will be north of the boundary including over the lake. By 
late tonight, gusts lakeside and at least several miles inland in 
Lake (IL), Cook, and Lake (IN) look to be 40-45 mph. This should 
result in a quick wave increase late evening into overnight. While
have moved this up some, still feel the midnight start time for 
the Lakeshore Flood Warning is adequate. 

Saturday will see the cumulative effects of wind-driven waves 
and the highest wave heights. A big factor with this will be
duration of the winds from the most ideal fetch length (010-020
degrees for NE IL). As the second low lifts northward, there 
should be some veering of the surface and boundary layer winds to
a more NE direction into Saturday afternoon. That would ease the 
fetch some. In total from late tonight through the day Saturday, 
winds look to gust 40-45 mph possibly even a tad higher into the 
lakefront for about 18 hours. The 10.00Z HREF supports this with a
mean of its high-res members providing 40-45 mph gusts along the 
Lake IL and much of the Cook shore.

We did some comparing the forecast for this event to what 
occurred with the Oct 31, 2014 event, that had substantial 
lakeside flooding in northeast Illinois including onto parts of 
Chicago's Lake Shore Drive. That event had more of a 360-010 wind 
direction for the entire event and 21 hours of 40+ mph gusts at
the Chicago Crib (including a period of 55-60 mph gusts). Feel 
that we will be a little shy of that wind direction and duration 
and certainly on those peak speeds. But the lake level is about 
three feet higher. So given that and with 12-18 ft waves forecast, 
certainly think this is well into warning level and there is a 
chance of higher level impacts such as Lake Shore Drive seeing 
some impacts. Also any northeast Illinois locations that 
experienced lakeside flooding with this past autumn's Halloween 
and Veterans Day storms will easily experience another level 
higher of impacts.

As for whether needing a Wind Advisory, I certainly think the high
gusts are a key message, but with the winter weather headline in
effect, feel that we may mention it there for right now, even
though the highest gusts may not overlap the wintry weather
perfectly. Will note in messaging/graphics as well.

MTF

&&

.LONG TERM...
404 AM CST

Sunday through Thursday...

The weather pattern will certainly remain progressive and active
into next week, with multiple Pacific systems. These are expected
to be so close to each other's heels that the column does not 
moderate much. So a chance of snow is presently forecast for 
roughly north of I-80 Sunday night and area wide on Monday night, 
before possibly a transition to liquid on Tuesday within the 
system warm advection aloft. For Sunday night's wave, this does 
have a negative tilt in guidance and a compact, progressive jet 
and vort max. This may be able to produce a few inches given that
forcing for lift and Pacific origin moisture, though right now
those magnitudes of any snow are more favored to the northwest of
the area, but within the envelope of uncertainty.

Only real change in the extended was to lower highs on Sunday a 
smidge more in north central and the northern Chicago suburbs 
where snow accumulation is expected Saturday. With the increase 
in clouds on Sunday evening already, temperatures are not likely 
to dip much that night and may even climb.

Looking ahead to midweek, Wednesday night could be a chilly night,
with both the GFS and EC bringing a strong cold front through.
Given the likelihood of snow Saturday in the northwestern half of
the CWA, and other chances after that, could see some lows near
zero if the guidance is not too cool on predicted 850-925 mb
temperatures going into Thursday morning.

MTF

&&

.AVIATION...
For the 06Z TAFs...

Forecast concerns include...

Strong south/southwest winds early this morning.
Wind shift to northeast Friday morning.
Strong north/northeast winds Friday night.
Lifr cigs tonight lifting to ifr Friday.
Moderate/heavy rain Friday night.
Freezing rain/sleet northwest IL Friday night.

South/southwest winds are still gusting into the mid 20kt range
late this evening and these will diminish over the next few hours
and turn more west/southwest. A cold front will move into
northwest IL overnight and then across the terminals later Friday
morning. Winds will shift to the north/northeast behind this
front. Speeds are expected to increase to near 10kts in the
afternoon and then steadily increase Friday evening with gusts to
30kts by late Friday evening and perhaps into the 30-35kt range by
early Saturday morning.

Mvfr cigs developed this evening and have been slowly lowering and
this trend is handled well by guidance which is showing cigs
lowering to lifr by morning and followed these trends which are
similar to the going forecasts. Cigs are expected to lift at least
back to ifr Friday but may remain in ifr as the front slows and
stalls across the area. Guidance is also in good agreement with
cigs improving to at least mvfr and possibly vfr at rfd for a time
Friday before mvfr cigs and then ifr cigs move back across
northwest IL Friday evening.

The models have backed off on rain chances tonight but showers are
still possible along/ahead of the front and as this front slows
and stalls...rain showers may persist through midday for the
Chicago terminals but there does now appear to be a lull in the
precip with the afternoon hours potentially mainly dry across much
of the area.

Moderate to heavy rain is then expected to spread across the
entire area early Friday evening and should continue through
Saturday morning. As colder air spreads across northwest IL...the
precipitation is expected to turn to freezing rain and then mix 
with sleet and snow. There is still some uncertainty as to how 
fast this occurs at rfd...but opted to go prevailing freezing rain
and sleet at rfd by late Friday evening. A wintry precip mix is 
possible at dpa...but likely after 06z Saturday...the end of the 
period. For the 30 hour ord taf...precip may also begin to mix by 
12z Saturday but confidence is too low to include any mixed precip
with this forecast. cms

&&

.MARINE...
404 AM CST

Strong high pressure will move from the Dakotas tonight to 
Ontario on Saturday as a trough of low pressure develops from 
Michigan south to the Arklatex. This will create a tight gradient 
over southern Lake Michigan with northeast winds quickly 
increasing this evening. Strong low pressure will then move 
northeast along this trough Saturday afternoon into Saturday 
evening, prolonging the strong northeast winds. Gales gusting to
40-45 kts are likely, and cannot completely rule out a few storm
force gusts in especially the Illinois nearshore. Waves are likely
to build to 12 to 18 ft late tonight into Saturday. The low will 
begin to weaken over the eastern Great Lakes Sunday morning with 
winds first turning northwest Saturday evening then quickly 
diminishing through early Sunday afternoon. 
nv-author-image

Rickie

I enjoy bringing the community the news on what is going on in the area. Be sure to check out my personal website as well! http://RickieTraeger.com RockfordScanner@Gmail.com