Scientific Forecaster Discussion
fxus63 klot 121107
Area forecast discussion
National Weather Service Chicago/Romeoville, Illinois
507 am CST Wed Dec 12 2018
330 am CST
Have decided to cancel the Winter Weather Advisory that had been
in effect for this morning. The previous concern for mixed
precipitation and an icy glaze coinciding with the commute is
becoming increasingly unlikely to materialize. Overnight aircraft
soundings from rfd have shown a very dry profile, matching up with
current observation of overcast skies with a cloud base height of
15kft. Looking along/just west of the MS river, minimum ceiling
heights are 5-7kft. The rap and hrrr have been consistent since
Tuesday afternoon in indicating that there will not be sufficient
saturation for the brief narrow warm advection band of precip to
form and move across northern Illinois. The 3km namnest and a few
other cams still hold onto the possibility of a narrow band of
precip right ahead of the system dry slot. Therefore, greatly
backed off on pops but still held onto chance pops along/north of
I-88 through mid morning, highest near WI border, with slight
chances down to around I-80.
Looking at current surface obs, most temperatures are at to a few
degrees above freezing. Thus, if any brief precip occurs, can't
rule out a wintry mix with the marginal thermal profile, but with
temps likely to remain at or above freezing through mid morning,
do not anticipate roadway impacts for most of the area that had
been in an advisory, with *maybe* an outside chance for a very
brief period of light rain with temps around 32. A perhaps
*slightly* better chance of frozen precipitation may arrive in the
mid to late morning in portions of north central and northwest
As the cold pocket of the vigorous clipper/hybrid overspreads the
area, temps may dip to near or locally just below freezing.
Surface obs do show <1000 ft ceilings back across Iowa under the
system cold pocket, so if any drizzle falls out of this deck in
mid- late am across northwest cwa, can't rule out some very
spotty/patchy freezing drizzle. Confidence in occurrence and
coverage is low and with temps at worst being at to 1-2 degrees
below 32f after spending the earlier morning above freezing, Road
temps may not respond in time to enable any localized icing.
Finally, as the clipper low tracks across southern WI, in response
west-southwest winds will pick up and gust to around 25 mph,
which also is not a favorable set-up for ice accretion. For
reference, the freezing drizzle event this past January over parts
the Chicago area had 500ft or less ceilings and light/calm winds
along with temps in the 20s where the worst impacts occurred.
Given all the above considerations, opted to make the difficult
decision of cancelling the headline before any precip arrived.
Early afternoon highs will be in the upper 30s to lower 40s from
Chicago to LaSalle County and points southeast, with earlier highs
northwest of there in the mid 30s cooling to the low 30s as the
cold pocket arrives. Some partial clearing for a time tonight and
very weak pressure gradient should enable temps to dip into the
mid to upper 20s to around 30 in Chicago.
223 am CST
Thursday through Tuesday...
Our next weather maker is across the Pacific northwest early this
morning. The energy associated with this system will be split
across a broad trough axis spanning the country, with a more open
wave across the northern Great Lakes, while strong upper jet on
the back side of the trough will allow the southern stream to
develop a closed low. Model agreement with the track of this low
was not great to this point, but the 0z global runs are in a bit
better agreement with digging this low well south of the region,
reaching the arklatex region Thursday evening. Regardless,
warm/moist southwest flow advection will feed 1"+ precipitable
water values into the approaching system. While those values may
not get this far north, a decent inverted surface trough will
setup. Therefore, expect a blossoming shield of precipitation
Thursday night. Model thermal progs suggests the column will be
warm enough for all liquid precipitation.
As mentioned above, mid range models now are in better agreement
with the track the European model (ecmwf) has been showing for a few days, where
the bulk of the system will continue east of the area but along
and south of the Ohio Valley. The low will be close enough that
we will need to watch how quickly it exits our region and if any
rain bands will continue to pinwheel around the low, therefore
some chances for rain continue into Friday, and maybe into
Saturday with the GFS ensemble/NAM hanging the low a bit farther
west. Still, confidence has increased maybe a little bit,
suggesting a farther south track and less impacts from this system
after Thursday night into Friday and is reflected in lower precip
chances after the initial surge and no snow.
One area of concern will be with the back edge of the
precipitationshield across north central Illinois where some
guidance does try to advect some colder low level temperatures in.
This would bring some icing potential. Have low confidence in
this occurring given the model discrepancies, and given the lack
of a significant cooling behind this system.
Outside of a cold front on Sunday, upper level heights will be on
the rise Saturday and again early next week, suggesting a
transition to a milder pattern, and one that at least for now is
still largely dry.
for the 12z tafs...
Concerns center around lowering ceilings later this morning and
afternoon, and whether there will be any drizzle this afternoon.
Wind gusts to 25 kt are also possible this afternoon.
Other than some virga, most areas will remain dry through mid
morning. As the center of low pressure shifts in mid to late
morning, there could still be a brief window of light
rain/drizzle. Only period of concern for freezing drizzle may be
toward rfd in the afternoon. Expect MVFR ceilings to reach rfd
mid morning, then toward the Chicago terminals toward 18z, with
ceilings to lift mid to late evening. Could get close to IFR at
Low pressure will track from northwest Illinois and then on across
southern Wisconsin. Winds will start off the period
southeasterly, but then shift to southwesterly as the continues
toward Sheboygan Wisconsin late in the day. Gusts may approach 25
kt this afternoon after winds have shifted to southwest. Winds
will become more light and variable tonight as they back toward
southeast for Thursday.
Lm...Small Craft Advisory...Winthrop Harbor to northerly is. Until noon Wednesday.
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