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Area forecast discussion
National Weather Service Chicago/Romeoville, Illinois
534 PM CST sun Dec 15 2019

Short term...
307 PM CST

Through Monday night...

An impressive east-west oriented snow band currently extends from
northeast Kansas through central MO and into south central Illinois this
afternoon. Light snow continues to spread northeastward across
central Illinois which is expected to reach the area later this evening
(7-10pm timeframe). The heavier snow bands are expected to remain
south of our area.

This lead wave pushes through overnight, with sufficient
isentropic lift overspreading the warm front for accumulating snow
across the area. Currently expect mostly a light coating to half
inch for areas north of I-80 and totals gradually increasing to
1-3 inches as you head south. Uncertainty remains concerning
possible mesoscale banding resulting in locally higher snowfall
totals up to an inch for some areas. The latest hrrr is hinting
that a heavier band could set up just south of I-80, so it will be
Worth watching over the next few hours if amounts need to be
adjusted a tad higher for some areas. Winds will thankfully be
light, so visibility reductions would mainly be a concern under
the heavier bands.

Expect this first wave to push out of the area by 6-7am with some
lingering flurries possible into the morning hours. A brief lull
is expected through the day on Monday before a secondary (and more
southerly) wave grazes our southern counties that evening. There
is a possibility that this stays south of our area, but a few more
tenths of an inch of snow are possible across our far
southeastern counties.

Petr

&&

Long term...
157 PM CST

Tuesday through Sunday...

Trough digging into the northeast U.S. At the start of the long
range period will provide for a glancing blow of some cold air
mid week. Medium range models are in good agreement in lifting the
upper trough out late in the week with broad upper ridging/zonal
flow developing over much of the country late in the week into
the weekend. This should result in temps moderating to above
average late in the week through the weekend and into early next
week. Given the expected lack of snow cover and recent cool bias
in the nbm, could see temps warmer than forecast over the weekend
and into early next week. Pattern looks dry through period with
little meaningful risk of any precipitation.

- Izzi

&&

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

Aviation forecast concerns:

- period of light snow tonight, and associated vis/cig impacts.

Regional radar mosaic depicts an area of light snow spreading into
northern Illinois from the southwest late this afternoon. The bulk of
this is expected to set up from the I-80 corridor southward, in
association with strongest forcing for ascent which will be
focused downstate. Lighter snow is expected to develop farther
north across the terminals this evening however, once our dry low
level airmass is saturated from the top down. Gyy, farthest south
of the taf sites, will likely see a period of IFR visibility and
lower MVFR ceilings. Impacts are likely to be less significant
farther north, though a period of light snow appears likely from
mid-evening through the pre-dawn hours (roughly 03-04z through
about 10z). Minor accumulations of a few tenths of an inch are
possible at the taf sites, with amounts decreasing northward from
gyy and the I-80 corridor. Passage of a mid-level disturbance
prior to sunrise should end precip by that time. Forecast
soundings do indicate the potential for lingering MVFR ceilings
through the morning hours, then scattering to VFR by early
afternoon.

Weak surface pressure gradient, in the vicinity of a high pressure
ridge axis just northeast of the area, will keep winds light and
somewhat variable in direction across the area this evening. Winds
should become light northerly later tonight, in response to weak
surface low pressure wave tracking through the Ohio Valley.
Several models support winds turning north-northeast (010-020 deg) by midday
or early afternoon Monday, with speeds generally less than 10 kts.

Ratzer

&&

Lot watches/warnings/advisories...
Illinois...none.
In...none.
Lm...none.
&&

$$

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