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Area forecast discussion
National Weather Service Chicago/Romeoville, Illinois
1135 am CST Mon Dec 9 2019

Short term...
314 am CST

Through tonight...

The next 18-24 hours will feature a fairly harsh temperature
change and gusty winds, especially late this afternoon into this
evening. But in terms of high impact weather the period looks
mainly free of that. Of note are some wind gusts that could exceed
35 mph between 4-9 p.M. Because of these winds being onshore for
Porter County, Indiana, conditions overnight will be near
Lakeshore flood advisory.

Early morning surface analysis indicates the entire County Warning Area is within
the warm sector of a broad 999 mb surface low over the Central
Plains into central Iowa. The system warm and nearly stationary
front is draped basically over the Wisconsin Stateline. Along
this boundary, low-level moisture convergence is maximized and
areas of less than one mile visibility in fog/drizzle have
developed across Iowa into southern Wisconsin, with some patchy
dense fog within that. This surface warm frontal zone is somewhat
broad, so do have some concerns some of that low visibility could
still build back, or just the low stratus could settle to the
ground, into Winnebago, Boone, and possibly Ogle, McHenry, and
lake counties this morning. Will continue to monitor observational
trends and webcams closely for any need of a dense fog advisory
in far northern Illinois.

The warm sector over the County Warning Area early this morning is characterized
by mid to upper 40s, which is ten degrees warmer than normal
highs this time of year. The warm sector will remain over the
entire area through the early afternoon and eastern locations
through the entire daytime period with only a slight climb, though
that will result in a few lower 50s. A sheared short wave evident
on water vapor imagery early this morning has provided a 40 kt
moisture transport low-level jet corridor into our eastern County Warning Area
which has been responsible for areas of showers, while elsewhere
much shallower and weaker isentropic ascent is resulting in areas
of drizzle from lowering stratus. Expect after daybreak this
morning that it will be mainly drizzle with only scattered
showers, with the showers primarily along/east of I-55 or north
into Wisconsin.

The cold air mass is well-defined upstream under a digging upper
trough, and this trough will be responsible for maturing of the
aforementioned surface low. As that occurs this afternoon into
tonight, the process will usher that cold air into the area.
Model guidance the past 12-24 hours has steered the cold front
through the area quicker and see no reason to go against that,
with an arrival time of 3-4 PM toward I-39, 5-6 PM into the
Chicago Metro, and 6-8 PM for Northwest Indiana. This cold front
will support a drop from mid to upper 40s right before passage to
20s areawide by midnight to mid teens to lower 20s by daybreak
Tuesday. Those temperatures are certainly not that bad for
December, but this will be accompanied by Stout winds, thus
providing wind chills 30-35 degrees cooler than air temperatures
this morning. These winds will be especially gusty in the
immediate several hours after fropa, with the combination of 1.)
Cold advection and steepened low-level lapse rates, 2.)
Unidirectional west-northwest winds within the deepening boundary
layer, and 3.) A strong isallobaric component with the surface
low deepening and quickly pulling away. This is why some gusts
could approach 40 mph in that 4-9 p.M. Window.

With the system still maturing, there is limited wraparound
moisture and deformation at our latitude, so precipitation
chances in the cold air look brief with the cold front and in the
immediate wake of it. If materializing, these should be rain or
maybe a poor quality snow shower or two. While there does continue
to be a signal through the evening for stratocumulus (matching
upstream satellite conditions) the depth looks shallow, and thus
the potential for any cold advection- driven freezing drizzle
looks quite low. Even in the low chance it were to briefly occur,
pavement temperatures will lag the falling air temperatures. Also
on that note, any "flash freeze" type concern will be minimized
too due to lack of any appreciable rainfall today and wind speeds
sufficient for drying conditions.

Otherwise, some lake effect snow showers are likely to move into
northern parts of Porter County overnight. Some of these could be
moderate in intensity at first but with quickly strengthening
subsidence, these should taper in intensity into the pre-dawn
hours of Tuesday. This Lakeshore area too is likely to see some
waves of 9+ ft for a period late evening into overnight hence some
concern for minor Lakeshore flooding.

Mtf

&&

Long term...
338 am CST

Tuesday night through Sunday...

Cold weather Tuesday and Wednesday remains the main concern in
the extended with temps around 10 degrees below normal for early
December.

Blustery conditions continue into Tuesday as chilly northwest
winds remain quite gusty through much of the day. Little warming
should occur during the day due to continued cold air advecting
into the region, limiting highs to the 20s. Mostly clear skies
beginning Tuesday night should allow temps to drop quickly into
the teens and single digits across the area. Increasing cloud
cover from an approaching weak clipper system early Wednesday
morning could limit additional cooling. Despite this, northwest
winds will likely result in wind chills values in the single
digits to just below zero.

There is a brief period where we could see some flurries/light
snow across the area Wednesday morning as the aforementioned weak
clipper system moves through. The combination of strong 850
baroclinicity and associated isentropic lift/frontogenesis could
support a brief period of light snow, although uncertainty remains
on the overall intensity/coverage due to model variability and
being under a fairly dry airmass. Currently have a mention of
flurries across most of the area by Wednesday morning. If
confidence increases, it may be necessary to include some light
snow accumulations given that ground temperatures will be
sufficiently cold to limit melting. Highs Wednesday will again be
in the 20s and lows drop into the teens.

Temperatures begin to rebound Thursday as the surface high and
cold air shift to the north and east and southerly flow returns.
Highs on Thursday will be near normal in the mid 30s, and slightly
above normal by Friday and Saturday with highs in the upper 30s
to 40s. The overall pattern remains active during this period,
with slight precip chances Friday night through Sunday. Multiple
weak upper level disturbances track across the area and appear to
blend together in the forecast due to model variability. Will
continue to monitor trends as this gets closer to better determine
timing of rain/snow potential.

Petr

&&

Aviation...
for the 18z tafs...

Primary concerns for Chicago airports are:

- IFR ceilings and visibility early this afternoon in drizzle and
light fog. Generally 500-800 ft, though LIFR persists west of
Chicago early this afternoon.

- Strong west winds gusting as high as 35 kt this evening behind a
cold front which will move through 22-23z.

- Chance for scattered showers immediately along/behind the cold
front early this evening. Mainly rain expected, though a few wet
snow flakes may be possible, especially north/northwest of Ord.

Deepening low pressure was passing off to the northeast of the
forecast area at midday, with a sharp cold front trailing along
the Mississippi River. Ahead of the front, IFR/LIFR conditions
persist in the warm sector into early afternoon, with the lowest
conditions expected to remain generally west/northwest of Metro
Chicago terminals. A period of scattered rain or rain/snow showers
is expected along/immediately behind the cold front, though
coverage is expected to decrease south of Ord. Areas northwest of
Ord are more likely to see some brief wet snow showers, with
shallower moist layer further east into Chicago Metro terminals
less likely to support ice nucleation and snow. Showers should be
fairly short lived, with precipitation finished by early this
evening. Transition to MVFR ceilings/VFR visibility is then
expected quickly behind the cold front, with a gradual lifting of
ceilings and eventual scattering to VFR after midnight.

As for winds, will likely see some sporadic gusts 15-20 kts
developing ahead of the cold front this afternoon from the
southwest. Winds will turn westerly with the passage of the cold
front, with a period of strong winds gusting as high as 35 kt this
evening. Winds will generally remain in the 280-290 degree range
tonight into Tuesday, with speeds and gusts gradually diminishing
through the period.

Ratzer

&&

Marine...
500 am CST

A Gale Warning has been issued for a portion of the Northwest
Indiana nearshore. Behind a strong cold front early this evening,
winds will turn west-northwest and sharply increase. Gusts of
30-35 kt have high confidence. Gale force gusts are expected to be
more sporadic in the Illinois nearshore but that will have to be
re-evaluated through today as conditions will at least be close
for a Gale Warning there too. Winds will gradually ease late
tonight but high wave action will continue especially for parts
of Northwest Indiana.

Mtf

&&

Lot watches/warnings/advisories...
Illinois...none.
In...none.
Lm...Gale Warning...lmz744-lmz745...6 PM Monday to 2 am Tuesday.

Small Craft Advisory...lmz744-lmz745...2 am Tuesday to 3 PM
Tuesday.

Small Craft Advisory...lmz740-lmz741-lmz742-lmz743...4 PM Monday
to 9 am Tuesday.

&&

$$

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