Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 090530

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1130 PM CST Sun Dec 8 2019

203 PM CST

Through Monday night...

Main short term forecast concerns revolve around development of
light drizzle/rain tonight, likely resulting in areas of fog as
well. Cooling of the column behind a cold front late Monday also
presents some minor p-type concerns, but without significant
impact as it looks to occur primarily as precip ends.

Early afternoon surface analysis shows weak low pressure passing
north of the Great Lakes in association with a minor-amplitude
short wave transiting the region. A cold front trails southwest
into the Plains, with south-southwesterly low level flow across
the forecast area ahead of the frontal trough axis. A deeper upper
trough across the western CONUS will eject eastward tonight, with
a new surface low developing along the southern end of the cold
frontal trough over the Plains. This low will deepen as it lifts
northeast across far northern IL on Monday, pulling the cold front
through the area during the afternoon and early evening hours.

In the near term, shallow low level moisture continues to slowly
increase across the forecast area this afternoon, seen this
morning in upstream soundings at ILX, DVN and SGF. Patchy low
clouds have developed at times across the cwa during the day in
response to this shallow moist advection. Low level moisture is
progged to deepen across the area tonight, as high pressure ridge
across the Gulf coast states drifts east, and richer moisture
makes its way north into the Midwest. This will present itself
locally as a blossoming of low clouds and eventual development of
drizzle and light rain later this evening/overnight. Saturated low
levels, combined with light precip and diminishing winds in the
weakening gradient ahead of the approaching surface low, will also
like result in fog development overnight. Guidance doesn`t
suggest widespread dense fog (vis less than 1/2SM), though would
expect a considerable amount of 1-3SM type fog into Monday
morning. The axis of strongest moist advection sets of from
central IL into northern IN tonight into Monday, resulting in
generally light (<0.10 inch) rain amounts across much of the cwa,
with highest amounts of 0.10-0.20 inches generally east of the
I-57 corridor across our southeastern cwa. No issues with p-type
through Monday morning however, as surface temps will remain in
the upper 30s and 40s thanks to the persistent south flow, high-
dew points and cloud cover in the warm sector ahead of the low.

Drier low level air spreads in behind the cold front Monday
afternoon, with guidance generally trending toward less precip
lingering late in the day/evening across the forecast area.
Surface temps dip below freezing from the west after dark, though
shallow nature of low-level moist/cloud bearing layer noted in
forecast soundings is not cold enough to support ice nucleation
for the most part, thus potential for precip to mix/change over to
snow before ending appears minimal. If anything, there could be
some brief patchy light freezing drizzle as surface temps fall
below freezing, but it appears this would be as precip ends and
would be of very short duration if it occurs at all with
decreasing cloudiness late. Perhaps more noticeably, winds become
blustery from the west-northwest behind the front Monday night,
with temperatures falling into the teens and low 20s in most areas
for overnight lows. The combination of blustery winds and colder
temps will support wind chills in the single digits especially
across IL parts of the forecast area by Tuesday morning.



203 PM CST

Tuesday through Saturday...

The main weather story during the period continues to be the quick
hit of cold weather Tuesday through Thursday night.

Forecast guidance has been slowly cutting back on the magnitude
of cold expected on Wednesday. For example, guidance was
suggesting that highs could be held down into the middle teens
across much of the area Wednesday, while now guidance is several
degrees warmer for Wednesday, with highs near 20 north and into
the mid to upper 20s south. This correction may be due in part to
the models catching onto the fact that snow cover will not be
obtained over the area Monday evening. So, with this in mind, our
updated forecast will have a bit warmer (but still cold)
temperatures mentioned for Wednesday.

Aside from the changes to back off on the magnitude of the cold a
bit Wendesday, the weather story/message has not changed. Expect
breezy and cold conditions to set up over the area beginning on
Tuesday, when highs will only be in the mid to upper 20s. The
heart of the this cold airmass should move overhead late on
Wednesday, then conditions will begin to moderate back to around
40 later in the week.

While much of this period will remain precipitation free, it does
appear that the area could be in for a short period of light snow
Wednesday morning. During this period, a fast moving weather
disturbance is expected to track southeastward across the western
Great Lakes region. While this disturbance is only expected to
induce a modest lower-level wind response early Wednesday, the
presence of a strong low-level baroclinic zone over the area could
allow for a more favorable interaction supportive of a short
period of light snow during the morning. For this reason, I have
continued the mention of a chance for snow during this period.
Some minor accumulations could occur with this period of snow,
but the short duration should keep amounts around, or less than, a
half inch.

Precipitation chances over the weekend will be in the low end
chance category at this time. While guidance this far out is not
in agreement on the specifics of any precipitation chances for our
area, the larger scale weather pattern over the CONUS certainly
looks to favor some periods of more inclement weather at times
over the region this weekend. Temperatures should remain near, or
just above seasonal averages during the later portion of the



For the 06Z TAFs...

CIGS: MVFR CIGS expected to envelop over the terminals early in
this TAF cycle with CIGS likely to lower IFR soon thereafter. CIGS
will likely to continue to build down as boundary layer moistens
with LIFR CIGS expected early Monday morning, likely lasting
through midday before slow improvement. More pronounced
improvement is likely behind cold front late Monday afternoon with
continue lifting to high end MVFR with skies likely clearing late
Monday night.

VSBY: Light fog and eventually some drizzle will accompany the
lowering CIGS and result in lowering VSBY. For now continued to
err a bit on the optimistic side only bringing VSBY down to IFR,
through potential certainly exists for a period of denser fog and
LIFR VSBY for a time Monday morning, particularly at DPA and RFD.
VSBYs expected to improve midday and then go unrestricted behind
the cold front late in the afternoon.

PRECIPITATION: Forecast soundings would still suggest a
respectable set-up for drizzle later tonight and especially Monday
morning. Will probably be some scattered showers/pockets of light
rain, especially late tonight through mid-morning Monday, though
it looks like any rain should be fairly insignificant in both
duration and intensity.

WINDS: Behind the cold front late Monday afternoon look for winds
to shift to W to WNW along with a rapid increase in speed/gusts.
Forecast soundings support gusts solidly in the mid-upper 20 knot
range, though given the magnitude of cold air surging in, wouldn`t
be surprising to see some gusts to, or perhaps a bit above, 30 kt
through mid evening. Magnitude of the wind speeds and gusts will
likely ease slowly late in the evening into the overnight, in
particular after skies clear out.

- Izzi


LM...Small Craft Advisory...LMZ743-LMZ744-LMZ745...4 PM Monday to 3
     PM Tuesday.

     Small Craft Advisory...LMZ740-LMZ741-LMZ742...4 PM Monday to 9
     AM Tuesday.




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