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afdlot

Area forecast discussion
National Weather Service Chicago/Romeoville, Illinois
239 PM CST Wed Dec 11 2019

Short term...
238 PM CST

Through Thursday...

Surface ridge continues to build across the region this
afternoon, with remaining stratus south of I-80 slowly eroding.
Additional erosion this afternoon will allow clear skies going
into this evening. With clear skies and surface ridge in place,
expect temps to steadily fall into the evening with the night time
lows occurring during this time. Upstream warm air advection will spread across
the area tonight ahead of an approaching system, with steady or
rising temps likely during the overnight hours. Continued warm air advection and
gusty southerly winds on Thursday will support above normal temps
in the 40s. Have kept temps in the low 40s, as returning mid/high
clouds later tonight will hold in place on Thursday. Approaching
mid level vort and surface low are expected to move just north of
the cwa, with the bulk of the associated precip likely also
staying to the north. Surface front on the backside of this system
does work its way toward northwest Illinois late in the day and could
be a source for light precip development Thursday afternoon. If
precip were to develop, highest chances would be for locations
near the Illinois/WI border and would expect it to be liquid.

Rodriguez

&&

Long term...
320 am CST

Thursday night through Tuesday...

In the extended, there continues to be an unsettled and at least
likely cloudy pattern Friday and Saturday. Right now would be a
little surprised to get through Saturday with no snow in the cwa,
even if just light and brief,given the primary global models and
their ensemble solutions the past few runs. A more impacting snow
on Saturday is a non-favored outcome at this time, and a
lessening one in recent runs, but some guidance (11.00 gfs) does
still indicate that and a couple ensembles so Worth mentioning
here.

Another amplifying trough from Ontario into the Great Lakes is
forecast to evolve late Friday into the weekend, though
definitely not as deep or cold as the current one. This will be
driven by a split flow regime and a stronger subtropical jet, with
some potential phasing likely to the east of the area. With a
the split flow on Friday and Friday night, it's a bit nebulous
for precipitation forcing throughout that time. Had thought about
removing or lowering some of the chances provided by the blend as
it looks like a low quantitative precipitation forecast and spotty if it were to materialize.
Profiles get marginal Friday night for both ice and surface
temperatures if south winds are light enough. It will be
something to watch but low confidence in any light freezing precip
at this juncture.

For Saturday, the left exit region of the upper jet has been
consistently forecast to overspread some of the area, and as such
a gradually deepening low is forecast to be taking shape in or
just east of the region. If this occurs at a strong enough
magnitude and in-sync with the northern stream, enough cold air
should wrap in for some snow and possibly accumulating snow. The
11.00 gefs members have about a third that indicate a tenth of an
inch plus of quantitative precipitation forecast during Saturday in the eastern half of the cwa,
with the GFS deterministic solution much higher than that. This is
why we lean more toward the ec and Gem solutions right now. Did
boost pops some on Saturday afternoon as that remains a consistent
overlap in those two global solutions for some precip
accumulation. Again it may just be a light rain to light snow and
departing so not for much duration and of nil impact, tough to
say yet at this juncture.

No changes made to the blended solution for early next week, with
still an active westerly jet over the middle of the country. The
Monday period has been favored to have a southern stream system
moving northeastward in the middle or eastern part of the
country, and with a potential large quantitative precipitation forecast swatch. We right now are
favored on the northern to beyond the far northern part of that.
Overall the pattern looks to yield slightly below normal
temperatures through early next week.

Mtf

&&

Aviation...
for the 18z tafs...

Stratus and flurries will continue to drift south of all of the
terminals over the next hour, with VFR and dry conditions then
expected for the remainder of the day. These VFR/dry conditions
are then expected to persist into Thursday, however, an upstream
system and light precip may approach the terminals very late in
the period. At this time, it appears that this precip should stay
just to the north, but with VFR ceilings likely returning Thursday
afternoon. Steady westerly winds will become light and variable
this afternoon, and then increase again out of the south southwest
later tonight into Thursday morning. Gusty southerly winds and
increasing flow aloft likely on Thursday, with the potential for
low level wind shear also increasing.

Rodriguez

&&

Lot watches/warnings/advisories...
Illinois...none.
In...none.
Lm...Small Craft Advisory...lmz743-lmz744-lmz745...6 am Thursday to
midnight Friday.

Small Craft Advisory...lmz740-lmz741-lmz742...6 am Thursday to 9
PM Thursday.

&&

$$

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