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Area forecast discussion 
National Weather Service Chicago/Romeoville, Illinois
348 PM CDT Wed Oct 26 2016

Short term...
315 PM CDT

Through tonight...

Main concern is ongoing rain showers and areas of drizzle
associated with weak surface low pressure area with center
currently crossing MS river just south of mli. Additional concern
with potential for patchy fog this evening and part of the
overnight. Projected path of the surface low will keep warm front
along and south of I-80. Behind back edge of main area of showers
in northern Illinois associated with shortwave ahead of surface
low, cool and saturated profiles with weak lift continuing has
resulted in areas of drizzle blossoming. Saturated column area
wide and lowering cloud bases will support patchy/areas of drizzle
in the midst of the lingering showers that will gradually exit
east. For the next few hours, scattered moderate/briefly heavy
downpours are favored in NE Illinois and northwest in. Temperatures will remain
nearly steady or slowly rise as the surface low approaches this
evening and then fall in the wake of it as modest low level cool
advection occurs. Extensive low stratus deck will remain overnight
even after wind shift and near term guidance hints at potential
for a few light showers on back side of upper trough after
midnight toward daybreak. Will need to monitor trends for pop
adjustments. As mentioned earlier, have some concern for patchy
fog development, but wind shift behind low and already existing
stratus deck casts doubt on how widespread it might become, with
low confidence in fog trends.



Long term...
233 PM CDT

Thursday through Wednesday...

A handful of interesting items in the extended forecast, the least
of which is a couple very mild spells, as a progressive weather
pattern continues. In terms of high impact weather, at this time
there is potential to see notable gusty winds later Friday
afternoon into the evening and then again on Monday afternoon and
night (halloween).

In the wake of tonight's low pressure, a weakening but still cold
advection pattern will sustain stratus into Thursday morning with
high confidence. As the surface high moves overhead in the
afternoon, the cloud-bearing flow will become light and we should
be left with a very slowly weakening inversion. Given that
satellite imagery this afternoon indicates clouds all the way to
north of the international border, have went down the cloudy
forecast Route. The 26.12z nmm WRF column condensate forecast,
which tends to work fairly well in cold air advection stratus/stratocu
situations, indicates the County Warning Area cloudy all day, supporting this

Temperatures should only rebound to near or just above 50 in the
afternoon. Thursday evening temperatures may dip fairly sharply if
clearing occurs near Sundown, before gradual warm advection
overnight halts the drop.

An active northern stream jet, primarily anchored over the
Canadian border, will continue to develop progressive lows over
the upper Midwest, with the next one on Friday. Strong warm
advection associated with this is still forecast to bring 925mb
temperatures to around 15c by mid-afternoon with accompanying
surface highs forecast in the mid 60s to lower 70s. These
temperatures will likely hold steady through much of the evening
as the maximum nose of the thermal tandem with the low-
level jet...spreads atop the area. Concern remains of some 35+
mph gusts during the late afternoon into the evening given the 40
mph winds forecast at 1000-1500 ft on NAM/GFS soundings and the
favored low-level mixing. Just back on October 17th, a somewhat
similar regime provided a very windy evening (though a deeper
surface low), so something to keep an eye on.

Chances for precipitation remain low Friday night, though an
uptick in clouds is likely and maybe some virga/sprinkles, though
that presently is more likely just east/northeast of the area.
The cold frontal passage timing on Saturday continues to have
some uncertainty, though it does look to be at some point during
the daytime hours. Depending on how early will of course influence
temperatures. Southern parts of the County Warning Area on Saturday could get well
into the 70s if a later time, such as indicated by the 26.12z ec
pans out.

A brief shot of near normal temperatures will move into the area
later Saturday and into Sunday, with onshore flow expected into
Chicago and adjacent lake areas. A disturbance is expected to move
along the slowly moving cold frontal boundary Saturday
evening/night, and guidance has had an upward trend in some light
rain potential north of the surface front with this (along the
elevated baroclinic zone). Have the higher pops later in the
night, but there are chances in the evening.

Finally, the final day of October into the 1st of November looks
to be potentially quite mild, with another low tapping into the
already warm thermal ridge to our south. The timing of the thermal
ridge will again play into just how warm we get on Monday, with
again 70+ possible given the magnitude of anomalous
heights/temperatures indicated. If anything confidence is
increasing greatest in a mild and breezy Halloween evening. When
the cold front does progress through, whether it be late Monday
night or Tuesday, the column again looks dry for any widespread
rain chances.



for the 18z tafs...

* IFR cigs and IFR to low MVFR visibility this afternoon through
evening, with low cigs lingering into Thursday.
* East-southeast winds shifting north-northwest tonight.
* Periods of light rain/showers and drizzle through this evening.

Weak low pressure currently across southeast Iowa will track
across northern Illinois this evening. Rain showers and drizzle
north of the warm front extending east from the low will continue
this afternoon and transition to the light rain/drizzle being
observed across northwest Illinois for the rest of the area. Rain cooled air is
aiding in quickly lowering cigs and now anticipate IFR to occur at
all terminals this afternoon. Cannot rule out LIFR, particularly
at rfd. The low clouds should stick around on Thursday, with
bases rising to MVFR. Confidence in overall ceiling trends and timing
is medium. Will amend as needed to refine timing for late
afternoon/evening rush.

Of additional concern is the low visibility with the showers and
especially in the drizzly regime in northwest Illinois that should
also translate east-southeast. Some of the guidance is hitting
low visibility fairly hard tonight. Tafs include IFR visibility at rfd, with
low MVFR at other sites, but may also need to make adjustments
with only low-medium confidence. Southeast winds 10-15 kt with
higher gusts will trend to more east-southeast 10 kt or so north
of the low path this evening. Then after some possible variability
in immediate vicinity of low center, winds will shift to northwest/NNW,
with northwest winds continuing on Thursday. This wind shift should also
scour out the lower visibility overnight. Have medium-high confidence in
easterly wind component lingering at Ord/mdw through the evening



304 PM CDT

Low pressure will move over or near the far southern tip of Lake
Michigan this evening. As this approaches, the east-southeast
winds will gradually ease. The winds will turn northeast and then
north overnight with the low passage. Over the Illinois and
Indiana nearshore, winds will drop fairly quickly in speed this
evening, however continued wave swell especially along the
Illinois nearshore should support Small Craft Advisory conditions.
With the northerly winds behind the low on Thursday morning, waves
are likely to re-build some along the Indiana shore and may
support another short Small Craft Advisory for 3-6 ft waves.

The active pattern will continue with the next low pressure
forecast across the upper Midwest on Friday, tracking to near Lake
Superior Friday evening. Gales look probable over the open water,
especially north and central given the low trajectory. Strong
offshore winds should again provide Small Craft Advisory winds
across the Illinois and Indiana nearshore areas Friday afternoon
and evening. Winds then turn north by midday Saturday though waves
are expected to remain just below criteria.



Lot watches/warnings/advisories...
Lm...Gale Warning...lmz777-lmz779 until 10 PM Wednesday.

Small Craft Advisory...Illinois nearshore waters until 10 am Thursday.

Small Craft Advisory...Indiana nearshore waters until 10 PM



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