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fxus63 klot 072325 
afdlot

Area forecast discussion
National Weather Service Chicago/Romeoville, Illinois
525 PM CST Sat Dec 7 2019

Short term...
209 PM CST

Through Sunday night...

Quiet, dry weather persists tonight into Sunday, before light
rain chances increase going into Sunday night. Milder temperatures
will finish out the weekend.

Surface high pressure has moved east of the region this
afternoon, with modest south winds gradually increasing in
response to surface pressure falls across the upper Midwest. After
a cold start to the day, temperature rise has been slow, thanks
to a persistent patch of mid-level clouds which continues to work
east across the forecast area. Most areas should see at least
partly sunny skies this afternoon, with the trailing edge of the
mid-deck moving across the Chicago Metro and Northwest Indiana
counties over the next couple of hours.

South winds will persist tonight, as an area of surface low
pressure passes across/north of the northern Great Lakes through
Sunday morning. This will keep temps from falling off too much
overnight, and will also gradually begin to pull higher dew point
air into the region into Sunday. Various guidance hints at
development of low clouds into especially southern parts of the
forecast area by morning, which along with increasing high level
cloud cover from the northwest should result in mainly cloudy
skies by sunrise. Cloud cover will thicken during the day Sunday,
as deep forcing for ascent increases, thanks to a weak lead mid-
level short wave and upper level divergence aided by a coupled
left-exit/right-entrance region of upper jet streaks across the
plains and northern Great Lakes. This looks to increase the
potential for drizzle/light rain by Sunday evening, as well as an
expansion of low clouds and likely some fog Sunday night. Large
scale ascent will continue to ramp up Sunday night, as an upper
trough approaches from the plains, with associated surface low
pressure moving into the lower Missouri Valley overnight. Rainfall
amounts look to be fairly light Sunday night, ranging from a few
hundredths northwest, to perhaps one to two tenths in our eastern
counties. Despite clouds, and Sunday night precip, south-southwest
low level winds gusting to 25 mph at times and associated low
level warm advection will boost temps into the low-mid 40s Sunday
and will likely only fall a few degrees Sunday night.

Ratzer

&&

Long term...
308 PM CST

Monday through Saturday...

The main forecast concerns through the period will continue to
focus on:
-chances of light rain/drizzle and fog Sunday night and on Monday.
-Potential of a short period of light snow or freezing drizzle
Monday evening.
-A period of very cold weather/wind chills centered around
Wednesday.

A weak surface low is expected to track northward over northern Illinois
Monday morning in response to a fast moving lead mid-level
impulse emanating from the southwestern Continental U.S.. because this
southern stream system will lead the main trough (and deep cold
airmass) digging over central Continental U.S. For midweek, mild air in place
will result in much of the precipitation remaining in the form of
liquid through the day Monday. The expectation is that light
rain or just some drizzle will be ongoing at the start of the
period Monday morning. It still appears that the main corridor
more favorable for continuous rainfall will be over my
southeastern counties over in and east central Illinois and points south
and east into the Ohio River valley. This region should be in a
better location to take advantage of upper level jet support as
the exit region of a 130+ kt upper level jet streak noses into the
mid- Mississippi and Ohio River valleys on Monday. Elsewhere
across northern IL, drier mid-level air will try to work in over
the area, in spite of very moist low- levels remaining in place.
This is thus likely to support low clouds, fog and areas of
drizzle and/or some light rain showers at times on Monday. The fog
in northern Illinois could become dense in some areas of northern Illinois
late Sunday night through Monday morning as the surface low tracks
along a surface frontal boundary right over northern Illinois. So this
will be a concern that will need to continue to be monitored.

Later Monday afternoon into the evening the surface low will
quickly track northeastward into southeastern Ontario and into
southwestern Quebec. As this occurs, an associated cold front will
drop southward across the area. Winds are likely to turn north-
northwesterly and become gusty in the wake of this frontal passage
early Monday evening, and this will help usher in a colder airmass.
Because temperatures will begin to fall off to around freezing
across portions of northern Illinois by early Monday evening, the
forecast challenge shifts to precipitation type. While typically a
rain to snow precipitation type would be favored in such a
situation following a cold frontal passage, there is a
complicating factor. As the airmass overhead becomes cold enough
to support snow Monday evening it appears that the mid-levels
will remain dry, resulting in cloud top temperatures remaining
warmer than -10c. This suggests a lower potential for
heterogeneous nucleation in the precipitation generation layer,
which is necessary to produce ice crystals and ultimately snow in
the clouds. For this reason, any lingering light precipitation
Monday evening could fall as some drizzle or freezing drizzle.
Fortunately it looks like any freezing drizzle would be short
lived, so I do not think it will cause major issues. In spite of
this, I felt it prudent to at least add in a mention of some
freezing drizzle with the light snow for portions of northern Illinois
Monday evening.

The main weather story then shifts to the arrival of a very
cold airmass for midweek. The Heart of this much colder airmass
will be in the process of shifting over the area during the day on
Tuesday. For this reason, temperatures on Tuesday will likely be
in the 20s, possible even dropping through the day as The Heart
of the cold airmass approaches. The coldest period will be
Tuesday night and Wednesday, when morning lows will drop into the
lower signal digits and only top out in the teens Wednesday
afternoon. Unfortunately, it will feel even colder than this,
as northwest winds of 10 to 15 mph continue Tuesday night and into
Wednesday ahead of an approaching surface Arctic high. This will
result in wind chills down around -15 to -10 over much of northern
IL, and values in the signal digits below elsewhere. Wind speeds
should abate during the day Wednesday as the center of a strong
~1040 mb surface high shifts overhead. This will help ease wind
chill readings during the day.

This Arctic airmass will remain progressive. For this reason,
after a very cold day Wednesday, temperatures should begin to
moderate back into the 20s on Thursday and well into the 30s for
Friday and Saturday. For now, the chances for precipitation are
on the low side for Friday and Saturday, though we will have to
keep an eye on the potential for more precipitation next weekend
as it does appear that the upper level pattern across the country
could remain on the active side.

Kjb

&&

Aviation...
for the 00z tafs...

Winds just off the deck will increase to 40-45kt by late evening
and continue overnight resulting in a marginal low level wind shear set-up. The
strong winds will likely draw a developing slug of IFR cigs north
into eastern Illinois and northwest in, reaching the immediate Chicago
area terminals near sunrise. For now, only brought cigs down to
low end MVFR at Ord/mdw due to low confidence, but later forecasts
will probably need to nudge those cigs down to IFR if forecast
trends persist.

Low confidence in cigs forecast persists Sunday morning into the
afternoon. Very real possibility exists that the IFR/low end MVFR
cigs will push east of the terminals with VFR conditions
developing. Mixed signals in the guidance with still a number of
models suggesting continued MVFR, so opted to defer to previous
tafs and hang onto MVFR cigs for now.

If low cloud clear out, look for MVFR cigs to redevelop later
Sunday afternoon with continued lowering to IFR Sunday evening
with some light rain, drizzle, and light fog expected to develop.

- Izzi

&&

Lot watches/warnings/advisories...
Illinois...none.
In...none.
Lm...Small Craft Advisory...lmz740-lmz741-lmz742-lmz743-lmz744-
lmz745...9 PM Saturday to 3 PM Sunday.

&&

$$

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