Scientific Forecaster Discussion
fxus63 klot 102040
Area forecast discussion
National Weather Service Chicago/Romeoville, Illinois
240 PM CST Tue Dec 10 2019
236 PM CST
Through Wednesday night...
Main forecast concern/challenge is with a period of light snow
Wednesday morning for parts of north central Illinois.
Early morning stratus and flurries have since diminished this
afternoon, with mostly sunny skies currently in place. Do think
skies will remain clear into this evening, however, it's possible
that locations south of I-80 in Illinois could get clipped with stratus
to the west. Slow moving weak wave dropping southeast across Iowa
will continue to dip into central Illinois this evening, with associated
stratus and even some flurries also likely continue to move
southeast. Current movement would keep the bulk of this moisture
just to the south, but once again, locations from Peru to Watseka
may get into a little more cloud cover later this evening.
Flurries would also be possible if this occurred.
Under most clear skies this evening, expect another chilly night
with temps in the teens. Decent gradient in place will support a
steady westerly wind tonight, that will help produce wind chills
in the single digits to just below zero. Expect another fast
moving wave to drop southeast across the region later tonight into
Wednesday morning, and initially support increasing cloud cover.
Forcing expected to further increase into Wednesday morning, as
warm air advection and some lower level fgen swing through, and should erode the
rather dry conditions in place. This will allow an area of light
snow to push across the area, especially across portions of north
central Illinois. Although confidence is growing for a period of light
snow Wednesday morning, do have some lower confidence with the
coverage and northward extent of this snow. Do think though that
locations along and south of a line from Rochelle to Kankakee, to
Rensselaer in will see either light snow or flurries. A more
focused area of snow is appearing to setup for locations along and
south of Mendota to Paxton in IL, with a few tenths up to a half
inch possible. Expect this snow to likely wind down by late
morning, with the remainder of the day into Wednesday night dry.
Surface ridge builds in behind this system Wednesday afternoon,
with a period of sunny skies likely. Temps look to dip into the
teens again Wednesday evening. Warm air advection ramps back up though during the
night ahead of an approaching system, with temps then expected to
rise into the low 20s.
236 PM CST
Thursday through Tuesday...
Thursday through friday: phasing of a compact northern mid-level
wave and a sheared southern mid-level trough will occur over the
Missouri Valley early Thursday, with an associated low tracking
northeast across Iowa. An area of isentropic ascent just ahead of
this wave and low will spread an area of precip across WI on
Thursday. This precip should remain north of the cwa, but there is a
low chance that a light mix of rain and snow may brush the WI state
line. Additional light snow may occur for the far northwest County Warning Area in
the evening as a decaying cold front also clips the area. Friday is
expected to be dry under mid to upper-level clouds ahead of an
approaching system for Saturday.
Friday night and Saturday night: guidance has been rather variable
on the handling of potential precip, including some snow, during
this period. A trio of shortwave troughs within deep broad cyclonic
flow across the north-central Continental U.S. Will shift across the region
Friday night through Saturday night. Latest guidance suggests one of
the waves will remain north of the County Warning Area. The other pair of waves will
likely phase to some degree over the upper Mississippi River valley
by Saturday morning. Meanwhile, the southern extension of the
broader trough will be shifting east-northeast across the Tennessee Valley.
Given this complicated set-up it is no surprise that guidance has
been rather variable. However, the previously more aggressive European model (ecmwf)
solution has fallen more in line with the GFS/gefs/CMC solutions
with its 12z run. Given this general model consensus, it appears the
phased trough will bring a chance for a rain/snow mix Friday night
into Saturday morning before a cold front and subsequent strong cold air advection
switch all precip to snow during the afternoon on Friday. In this
scenario, the heaviest precip in the form of mostly snow will occur
briefly for any given location Saturday afternoon. With a lot of
fine details that are impossible to determine this far out given the
phasing wave set-up, expect forecast changes as the weekend
Sunday through tuesday: generally zonal flow aloft over surface high
pressure across the upper Mississippi River valley will produce dry
and quiet conditions across the area during this time. The larger
scale flow will then break down by Monday as troughing over the
North Pacific digs into the western Continental U.S.. a prominent southern jet
will interact with this trough by late Monday, though guidance
differs on where this will initiate. The GFS depicts a stronger and
more potent mid-level trough producing a notable shield of precip
prior to its arrival across the area. This is an outlier with only a
few similar gefs members, so will hedge toward the rest of the model
suite that advertises light snow across the area Monday into Monday
night and keeps the bulk of precipitation to the south and east of
the forecast area.
for the 18z tafs...
Any lingering MVFR stratus and flurries just to the north of Ord
will continue to diminish this afternoon, with VFR and dry
conditions then expected into tonight. A fast moving system will
move across the region Wednesday morning, with VFR ceilings
returning across the terminals. Light snow and lower ceilings
associated with this system are expected to remain just to the
south of the terminals at this time. However, it's still possible
that the northern extent of this light snow could briefly clip the
terminals. If this were to occur, the window for this light snow
would be small with any snow accumulation still not likely. This
system quickly departs midday Wednesday, with clearing skies the
trend into the afternoon. A steady westerly wind, occasionally
gusting this afternoon, will persist this period. Winds will
likely quickly weaken Wednesday afternoon, as high pressure builds
across the region.
Lm...Small Craft Advisory...Gary to Michigan City in until 3 PM Wednesday.
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