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fxus63 klot 170308 

Area forecast discussion
National Weather Service Chicago/Romeoville, Illinois
908 PM CST Sat Nov 16 2019

908 PM CST

No major changes needed to the inherited forecast, but did nudge
low temperatures up a smidge and delayed the onset of
precipitation by a few hours into Sunday afternoon.

Mostly clear skies prevail late this evening across northeast
Illinois, but an expansive deck of mid and high cloud cover is
waiting in the wings off to our west. Temperatures haven't moved
all that much as they've begun to approach their dewpoints with a
light southeasterly breeze expected to persist through the
overnight hours. With clouds rapidly increasing in coverage after
midnight, we've nudged the low temperature forecast up a bit as a

Guidance continues to suggest the combination of a slightly slower
progression to the next incoming shortwave and potential for at
least some muted insolation through the morning hours may allow
temperatures to moderate a bit more--with some of the most
aggressive guidance even hinting at mid and upper 40 degree
readings south of I-80 Sunday afternoon. We've pushed our pop
timing back just a hair with this evening update, which, when
combined with the slightly warmer temperatures, has resulted in
even less snow in the gridded forecast, at least through early
Sunday evening. Could see things try to flip over to drizzle
through the evening hours as the atmosphere above 800 mb begins to
dry out, but not overly concerned with any -fzdz potential at this
point due to still rather warm surface temperatures (32-35
degrees), limited low-level warm advection, and convergence along
the weak incoming cold front.



Short term...
100 PM CST

Through Sunday...
high pressure across the eastern Great Lakes will slowly drift off
to the east tonight. Milder conditions, at least compared to our
recent stretch, and relatively lighter winds remain in place
through sunset with some passing high clouds.

As the high moves away, winds will shift to southerly late
tonight into Sunday. Temperatures will quickly fall initially but
then lows will level off as high clouds thicken and winds shift to

The upper level wave across the upper Midwest is a decent one, and
will be our next weather maker for Sunday/Sunday night. Our main
concern is precipitation type on Sunday. Expect some light
precipitation to be possible in the morning mainly west of the
Chicago area as warm advection ramps up fairly nicely ahead of the
upper trough. Most of the initial forcing will be rooted aloft
with precipitation generation out of a mid- high cloud deck.
Therefore expected lighter precip during this period. But this
precipitation could be a rain- snow mix or even all snow due to
the dry low levels, dewpoints below freezing, and near to slightly
above freezing surface temperatures.

Better chances for precipitation in the form of rain would be
Sunday afternoon, particularly later as lower level saturation
will occur with the approach of the trough axis. Expect decent
coverage of precipitation, again though it should be on the
lighter side.



Long term...
220 PM CST

Sunday night through Saturday...

Light precipitation will be rounding out the weekend. Besides a
chance for brief light snow late Monday night/early Tuesday
morning, the rest of the first half of the week looks mainly
quiet with the question of how long clouds persist. A consistent
signal remains for a late Wednesday night into Thursday period of
rain with potential for snow on the backside. High temperatures
briefly may reach normal Wednesday and early Thursday.

The aforementioned trough in the short term will still be digging
into the area on Sunday evening, lining up with a potential
vorticity anomaly and jet placement. There has been a slight
slowing solution in guidance the past 24 hours and that too makes
sense with this forecast evolution. Have extended the forecast
mention of light precipitation through Sunday evening. Surface
temperatures are likely to remain above freezing through midnight
Sunday night due to lingering southwest winds and cloud cover.
Wet bulb cooling within light rain will help the temperature
to cool, but forecast dew points look to increase immediately
ahead of the surface trough, so feel that precipitation should
remain as light rain possibly with a mix to inconsequential snow
before ending. As the depth of the moisture layer diminishes
through mid-level drying later in the evening, the precipitation
may shift over to drizzle. Temperatures still look to remain on
the positive side of freezing even with a buffer of a couple
degrees for uncertainty.

Cyclonic flow aloft will prevail through Monday and into at least
the day Tuesday. This means the potential for stratus /
stratocumulus with an inversion above a weak cold advection
boundary layer. Model guidance does show a sheared clipper
inducing some warm, moist advection aloft over part of the region
on Monday night, especially late Monday night. This could result
in a brief period of light snow if the lift and moisture coexists
enough. The European model (ecmwf) is slow enough with this in its solution that it
would actually overlap early Tuesday morning over Chicago, so
something we will keep one eye on.

Height rises are forecast to build into the area Wednesday ahead
of a positively tilted trough forecast to sweep into the middle of
the country on Thursday. After Wednesday being dry, chances for
warm advection-driven rain increase late Wednesday night into
Thursday. Guidance agrees on the surface low deepening throughout
Thursday as it moves northeast somewhere in the general Great
Lakes region. The current mean path and strength of this system
support cold advection enveloping the area amidst still
wraparound moisture and thus potential for a period of snow with
falling temperatures on gusty winds. There also could be some a
Lakeshore flooding threat in at least Northwest Indiana Thursday
night into Friday due to the strong onshore flow and elevated lake



for the 00z tafs...

VFR conditions are expected into Sunday morning as a surface ridge
extends southwestward into northern Illinois. East winds early this
evening will gradually become south-southeast (averaging 160 degrees) through
midnight. Confidence on precisely when the south-southeast winds will shift west
of 180 degrees is low, but timing is expected to be sometime mid to
late morning Sunday.

An upper-level disturbance will then bring unsettled weather through
the remainder of the period. Isolated -shra, possibly mixed with
some -sn, are possible late Sunday morning into the early afternoon,
though a rather dry layer under a cloud-deck of 8-10kft will likely
limit most, if not all, precip from reaching the ground. A higher
coverage of -ra along with MVFR ceilings will then spread across the
area by mid-afternoon and continue into the evening. The -ra may
transition to -dz with lower MVFR ceilings by late evening before
all precip ends with the passage of a surface front shortly after
the end of the current 30hr taf cycle at Ord/mdw.



Lot watches/warnings/advisories...


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