Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 021625
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1025 AM CST Fri Dec 2 2016

.UPDATE...
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
PoPs again.

Castro

&&

.SHORT TERM...
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
CWA 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 CWA. 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.

Rodriguez

&&

.LONG TERM...
320 AM CST

Sunday through Thursday...

The start of the period on Sunday will likely see precip
spreading northeast across the region, with the first snow
accumulation for the area appearing possible. Guidance in pretty
good agreement with handling this next system, with upper level
trough anticipated to swing through the central Conus Saturday
night into Sunday. As this approaches, expect precip to really
blossom as it spreads into northern Illinois early Sunday morning.
All guidance this morning has been more aggressive with the QPF
output on Sunday and this makes sense given the anticipated large
scale ascent ahead of this trough and WAA. Guidance has also come
into agreement with the extent of the cold air, with the column
now appearing to support all snow for a longer duration Sunday
morning into early Sunday afternoon. Did increase pops with cat
pops now over much of northern Illinois but if trends continue,
could see bringing cat pops over other areas in the CWA. Although
there is a possibility for this light snow to arrive after
midnight Saturday night, don`t anticipate areas in north central
Illinois to see snow until after sunrise on Sunday. This will then
spread east/northeast through the morning, with once again, all
snow appearing likely. It`s during this time that a window of
moderate intense snow could occur. Did lower temps on Sunday with
precip throughout the day likely limiting any rises but do think I
may have not lowered them enough as its possible for all snow to
continue into much of the afternoon. However, with some lower
confidence how the thermal profiles will evolve Sunday afternoon,
did leave some mention of liquid. Given this shift towards more
snow Sunday, did increase totals with one to two inches of
snowfall possible with the higher amounts expected to be over
north central Illinois at this time.

Still keeping an eye on a couple of systems which will bring
additional chances for snow this next week. After a dry period
Sunday night into Monday, turn attention towards a potentially
energetic wave lifting northeast out of the southern Plains and
into the mid Mississippi Valley Monday night and then into the
Ohio Valley on Tuesday. Thermal profiles still in question with
this system as precip is expected to move across the area, with
confidence lower with precip type. Would think that all rain would
be the main precip type but could see the stronger forcing
supporting a cooler column, especially with CAA quickly becoming
possible early Tuesday morning. A little too early to have too
much confidence with precip type, and so have made mention of both
rain and snow. Pattern remains active after this second system,
with potential continuing for another system by midweek.
Confidence is low with exact timing/placement of this system with
the confidence on the details also low. However, current guidance
is indicating that colder air which would support mainly all snow
could be in place among all the varying guidance. Definitely a
period to continue to monitor, if not for the snow potential but
for cold temps as the coldest air of the season so far arrives on
the backside of this system.

Rodriguez

&&

.AVIATION...
For the 12Z TAFs...

Fairly uniform conditions will rule the terminals today.

Occasional patchy light rain/drizzle will fall out of the MVFR
cloud deck mainly through the morning period as a sheared out
vorticity center slowly slides through. The forcing is not very
strong thus any drizzle would be patchy and short lived but could
occasionally drop cigs down to low end MVFR while the predominant
ceilings have been more mid range MVFR or low end VFR. Observed
and forecast soundings suggest the stratus will remain below an
inversion and largly hover in the 2000-3500 foot range and this
is confirmed by observations upstream through the upper Midwest.

A weak surface trough will pass through the terminals this
morning, and this will shift winds to more of NW direction,
possibly briefly more NW than the 310 degree direction indicated
in the TAFS, and be occasionally gusty but below 20 kt and largely
just with the wind shift where the gusts will be most common. High
pressure will arrive later tonight and Saturday morning which will
weaken winds, but quite possibly keeping the stratus at least
broken into Saturday.

KMD

&&

.MARINE...
320 AM CST

A Weak surface trough will pass across the lake early this
morning resulting in a minor wind shift to northwest. As this
occurs there may be a brief uptick in winds. Otherwise, the
surface pressure pattern involves a departing surface low across
Quebec and high pressure across the northern plains/upper Midwest.
A decent gradient exists to maintain near to slightly below small
craft advisory level winds today, possibly briefly gusting to 25
kt through the early afternoon across NW Indiana. Otherwise the
low will slowly pull off to the Atlantic shore off New England
through Friday night while the high moves closer. This will act to
diminish the west-northwest winds. The high will graze the
southern tip of the lake Saturday night. Another weaker surface
pressure trough will move across Lake Michigan Sunday afternoon
and evening.

More impactful weather may arrive mid to later in the week as a
series of low pressure systems will move northeast from the
gulf/southern plains through the Ohio Valley and the eastern Great
Lakes Tuesday and again Wednesday evening. The first low will
bring a cold front through, but a stronger cold front will arrive
with the second low as this low is forecast to strengthen as it
moves northeast. West- northwest gales would be certainly possible
in this pattern later Wednesday night through Friday if this plays
out as current guidance indicates.

KMD

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...NONE.
IN...NONE.
LM...NONE.
&&

$$

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