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Area forecast discussion
National Weather Service Chicago/Romeoville, Illinois
1059 PM CST sun Dec 15 2019

835 PM CST

Have issued a Winter Weather Advisory for Livingston, Ford,
Iroquois and Benton counties in collaboration with weather forecast office ilx and
dvn. Expecting snow amounts through the overnight of up to 2-4" in
these counties. The most persistent frontogenetically driven
banding will set up in the advisory counties, yielding steady
light to moderate and perhaps briefly heavy snowfall rates, with
visibility down to 1/2 to 1 mile at times.

For areas to the north of the advisory, have been monitoring
observational and radar trends off to the west and now within the
County Warning Area. Top down saturation of very dry antecedent air is occurring,
with light snow now occurring here at the National Weather Service office in
Romeoville. It appears overall that light accums up to one inch
near and south of I-88 corridor and up to 1-3" I-80 and south are
more probable. Transient mesoscale banding and weaker overall
forcing, especially with northward extent, as well as drier air
fighting the forcing, will lead to the south to north gradient.
The forecast snowfall amounts have been increased slightly from
the previous forecast, with a nod toward aforementioned
observational trends and recent rap/hrrr runs.

The several hour period of snow through about 3-4 am will cause
snow/slush covered roadways given temperatures down in the 20s.
The most pronounced travel impacts are expected where localized
amounts up to 3-4" are possible in a fairly short duration, which
is within the advisory counties. Have issued a Special Weather Statement for locations
north of the advisory, with lesser snow amounts. However, some
travel impacts are likely due to well below freezing temps
supporting accums on untreated roads, especially those less
heavily traveled. The snow will end before the morning commute, so
have gone with a 6 am end time of the Winter Weather Advisory. It
is possible though that some snow/slush cover will remain on less
traveled roads, something to keep in mind for the morning drive.



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.



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


for the 06z tafs...

Aviation forecast concerns:

- period of light snow and IFR vis/MVFR ceilings through around

- Potential for lingering high-end MVFR ceilings Monday morning.

Regional radar mosaic depicts a broad area of light snow from
eastern Iowa across much of northern/central Illinois and in at 0430z.
Dry low levels have taken a little while to saturate across the
Chicago Metro, though snow finally now reaching the surface. While
heavier banded snowfall was occurring south of the terminals
across central Illinois/in, surface observations indicate widespread IFR
visibilities in the 1-2sm range across much of northern IL, and
this is expected to affect the terminals for the next few hours.
Ceilings were generally MVFR, though with some IFR across the
southern Chicago Metro area, such as gyy. Snow should taper/end
from west to east, by about 08z for rfd and 10-11z for Chicago
sites. High-res guidance indicates higher MVFR ceilings may linger
into Monday morning, especially from Chicago eastward. Improvement
to VFR ceilings is expected by midday/early afternoon, with mid
and high clouds remaining across the region.

Weak surface high pressure ridge remains just north and northeast
of the forecast area, resulting in light/variable winds at this
time. Winds should gradually settle to a northerly direction by
morning, as low pressure moves through the lower Ohio Valley.
Winds may turn 010-020 for a time Monday afternoon, though speeds
should generally remain less than 10 kts.



Lot watches/warnings/advisories...
Illinois...Winter Weather Advisory...ilz032-ilz033-ilz039 until 6 am Monday.

In...Winter Weather Advisory...inz019 until 6 am Monday.



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