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Area forecast discussion 
National Weather Service Chicago/Romeoville, Illinois
527 PM CST Fri Dec 2 2016

Short term...
302 PM CST

Through Sunday...

Main concern is the very likely first accumulating snowfall of the
season for much of the area on Sunday. Leading up to this minor
anticipated snowfall, it appears the area will largely continue to
be locked in the clouds. Visible satellite continues to show the
stratus deck extending all the way back to the northern High
Plains. Forecast soundings suggest that top of inversion just
above 900 mb will hold tight, trapping moisture below it, and no
deviation of the northwesterly flow at the cloud bearing level to
allow for any substantial scouring of the clouds. Thus have
collaborated sky cover forecast to cloudy/mostly cloudy through
Saturday night. A loss of the cyclonic mid-level flow and brief
short-wave ridging will finally cut off the isolated
shower/sprinkle drizzle regime that has shifted to along/south of
I-80 this afternoon. Any lingering light precip should end by
early evening.

With the cloud cover expected to persist, raised low temperatures
well above guidance tonight. Low level dry advection of lower dew
points filtering into the area from the north will likely enable
slightly cooler lows than this morning, but still mainly low 30s
outside Chicago and mid 30s in the city. Saturday will feature
mostly cloudy skies, with *maybe* a few breaks in the overcast,
and highs in the mid to upper 30s. Once again bumped up low
temperatures Saturday night with expectation of overcast skies.

Attention then turns to Sunday. Guidance is in good agreement in
an energetic northern stream shortwave approaching by the early
morning. Warm air advection/isentropic ascent will increase, with additional
large scale lift from the short-wave trough. Some light snow may
break out in the pre-dawn hours, with a solid area of steady light
to perhaps occasionally moderate snow then spreading eastward
through the morning and into the afternoon. Temperatures will rise
into the mid 30s after starting in the lower 30s, so snow ratios
are favored to be 10:1 or less. Guidance again bumped up quantitative precipitation forecast
amounts, though the official forecast blend with wpc is a bit
below a straight NAM/GFS/ECMWF/Canadian blend. The lower ratios
and antecedent mild Road and ground temperatures are the main
limiting factors from higher snow amounts as well as impacts.
Forecast snowfall amounts are generally under 1" east/southeast of
I-55 and 1-2" to the northwest, closest to 2" along/west of I-39.

The city of Chicago reported pavement temps in the upper 30s to
around 40 this morning. These are likely lower outside of Chicago,
though still above freezing, and clouds will also likely prevent
a sharp drop in pavement temps up until snow onset. For this
reason, suspect that the main accumulations will be on
grassy and colder surfaces, especially in the urban and suburban
areas. Cannot rule out some slushy accumulations on roads, again
with a better chance on rural less traveled roads and elevated
surfaces. Reduced visibility could end up being primary travel
related issue.

With forecast soundings largely supportive of mainly snow as
precipitation type even well south of I-80, have trended grids
that way, though rain may mix in before the precipitation ends in
the afternoon.



Long term...

Sunday night through Friday...

143 PM...forecast concerns include precip timing Monday night/
Tuesday and again Wednesday/Wednesday night...along with precip
type. Much colder air...and below normal...expected Thursday and
Friday next week.

High pressure will be moving across the Ohio Valley Sunday night
into Monday. Any lingering precip Sunday evening will likely be
just flurries/drizzle and at that time surface temps appear to
remain above freezing.

The models have come into better agreement with each other for the
12z runs but there remains uncertainty with one surface low
moving northeast across the plains Monday night into Tuesday and a
second low lifting across the Ohio Valley/mid Atlantic Monday
night through Tuesday night...and whether these systems eventually
merge into a larger system as previous runs have suggested. GFS
has been consistent with the current trend for the past day or so
with the Gem/ECMWF now trending that direction. As a result...low
temps Monday night are expected to be slightly cooler...perhaps
lower 30s but precip may only reach into the far southern cwa
through Tuesday morning. Thus there appears to be more potential
for mixed precip or at least a rain/snow mix but now affecting
much less of the cwa. Temps should warm enough Tuesday to change
any precip over to mainly light rain but location/timing is
uncertain enough that much of the area could end up being dry and
confidence for dry conditions does look better for Tuesday night
into Wednesday morning.

Colder air now spreads into the area Wednesday as a weak wave
quickly lifts northeast along the transition to much colder air.
ECMWF develops a weak low and both the ECMWF and Gem show light
qpf into Wednesday evening. Given current trends...looks cold
enough then for any precip to be mainly snow but confidence in
this solution is low. As colder air spreads across the region
Wednesday night through the end of the week...highs may stay in
the 20s with lows in the teens. Add strong/gusty westerly
winds...min wind chills Thursday and Friday mornings may fall to
around zero. Cms


for the 00z tafs...

Limited impacts in the tafs with just some MVFR likelihood later
tonight and first thing Saturday morning.

Cyclonic flow continues across the region and will into early
Saturday. Cloud depths are about 2,000 to 2,500 ft thick on
recent aircraft soundings in the region and almost all sites are
reporting one overcast layer, using a good sign it is solid and
not going anywhere without any notable advection. Keep the overcast
clouds forecast through the taf period. Any cloud bases below
2,000 ft are a low likelihood and probably only possible at rfd
late tonight. There could be a few holes in the cloud cover late
Saturday into Saturday evening, but that is low confidence.

Beyond the current taf period...there is likely to be snow during
the day Sunday at the airports with IFR visibility at times.
Temperatures look to be 33 to 36 during the snow. Around an inch
is forecast at Ord, mdw, and gyy, with one to two at dpa and rfd,
though accumulation on pavement probably will be difficult.




143 PM...low pressure over southern Quebec will slowly move east
this evening as it merges with another low over the North
Atlantic. High pressure over the plains will slowly build east
across the lakes overnight into Saturday. The gradient between
these two systems has allowed prevailing northwest winds 15-25kt
over the lake and these will slowly be diminishing as the high
builds east. Low pressure will move across the northern plains and
southern Canada Sunday as weak high pressure builds across the
Ohio Valley Sunday night into Monday. This will result in an
increasing west/southwest flow Sunday which will turn back to
light southerly Monday night.

Confidence during the Tuesday/Thursday time period next week is
rather low as the models show one low lifting northeast from the
northern plains/upper Midwest into Canada Tuesday and a second low
moving across the Ohio Valley/mid Atlantic Tuesday/Tuesday night.
While there does currently appear to be consensus with this
idea...there remains uncertainty as to where these lows will track
and if they may merge into a stronger low near the Great Lakes
region. Confidence is high that by Wednesday night/Thursday...
winds will turn northwest and much colder air will spread across
the region with a period of northwest gales possible Thursday. Cms


Lot watches/warnings/advisories...


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