Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Chicago/Romeoville, Illinois
143 PM CST Fri Dec 2 2016
1025 am CST
Updated forecast to include a mention of rain and snow showers.
Have been tracking a narrow band of enhanced reflectivity moving
southeast across northern Illinois. This band roughly denotes
leading edge of a wind shift from westerly to northwest winds.
Forecast soundings suggest that there would not be enough ice
present in the clouds for any snow. However, observations suggest
that cloud depths are just deep enough that the enhanced band
actually contains a brief burst of snow, with Rockford Airport
briefly dropping to 1 mile visibility at around 9am. This band
could similarly be bringing a brief burst of snow to the western
Chicago suburbs as of this writing. Above freezing air and Road
temps will prevent any Road impacts. Farther south, expect that
precipitation will mainly be in the form of rain. High-res
guidance indicates that this scattered shower activity could
persist into the afternoon south of I-80, so may need to update
320 am CST
Through Saturday night...
Main forecast concerns/challenges are with periodic sprinkles or
drizzle today, a period of light snow expected across the entire
County Warning Area Sunday, and then two other systems providing additional
chances for snow later in the week.
Latest radar imagery depicting widely scattered sprinkles or
drizzle persisting over a good portion of the cwa, in response to
several sheared out vorticity maxima traversing the cyclonic flow
still in place. Do think these isolated to widely scattered
sprinkles/drizzle will likely continue over the next several
hours. While steeper low level lapse rates remain in place, brief
periods of higher intensity precip may still occur. Deeper
moisture/crystals still nonexistent early this morning and expect
this to persist. However, short term guidance hinting deeper
moisture may briefly return early this morning, which can be noted
on satellite imagery. This may allow some flurries to briefly mix
in there but given lower confidence on this possibility, have
excluded any mention in the forecast. Additional precip today in
the form of continued scattered sprinkles/drizzle may continue
through midday/early afternoon, especially as surface trough axis
swings through the County Warning Area. Conditions then anticipated to be dry into
the middle part of the weekend, while near normal temps are
expected. Have continued more pessimistic sky cover into tonight
and Saturday as it's appearing that the stratus will remain in
place. Some possible clearing may occur on Saturday but if it were
to occur, it would be brief as mid/high clouds return in advance
of the next system to affect the area.
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 18z tafs...
A large bank of stratus continues to persist from the northern
plains to the Great Lakes and Ohio Valley. Though some breaks may
develop on Saturday, expecting the stratus to largely remain over
the terminals. Cigs will range from lower end VFR to high end
MVFR. West-northwest winds around 10 kt this afternoon will
diminish this evening, with lighter west winds continuing through
the rest of the period.
Gyy and mdw may get clipped by a southeastward tracking band of
showers, though it may pass just south, so carried a vcsh mention
through early this afternoon.
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
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