Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 261937

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
237 PM CDT Wed Oct 26 2016


1110 AM CDT

No major changes to going forecast thoughts, though did tweak
hourly PoP and weather resolution. Lead shortwave coupled with
strong low level jet, despite minimal instability, resulted in
embedded thunder reaching Chicago. General trend should be for
less coverage of thunder this afternoon and gradual sinking
southward of any isolated cloud to ground/in cloud strikes with
instability axis remaining off to our southwest. Respectable
rainfall rates have been occurring with the showers given above
normal PWAT values for late October, but not anticipating any
flooding issues other than minor ponding on some roadways.

Trailing shortwave just ahead of weak surface low pressure and
noted by drying on W/V imagery has resulted in showers filling
back in behind primary rain shield and should see this continue
eastward. Some embedded heavier echoes will result in downpours,
though as mentioned above, expect any lightning to be

Other big story today is the gusty east-southeast to southeast
winds as maximum pressure falls transit the area. This is particularly
along the Illinois shore where coastal platforms have been showing
persistent gusts to around 40 mph. Winds will diminish this
afternoon as gradient weakens with approach of surface low, though
strong winds hanging on longest along immediate IL shore.
Temperatures will be steady to slowly rising through the 40s north
of the low track and exhibit more appreciable rises to the south.
Overall tweaked high temperatures downward some accounting for



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 CAA stratus/stratocu
situations, indicates the CWA 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 CWA 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 VSBY 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 NW IL 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 CIG trends and timing
is medium. Will amend as needed to refine timing for late
afternoon/evening rush.

Of additional concern is the low VSBY 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 VSBY fairly hard tonight. TAFs include IFR VSBY 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 NW/NNW,
with NW winds continuing on Thursday. This wind shift should also
scour out the lower VSBY overnight. Have medium-high confidence in
easterly wind component lingering at ORD/MDW through the evening



353 AM CDT

Main concerns are with the expected gales across much of the lake
this morning through the afternoon and early evening. Low pressure
over Nebraska and Iowa will continue to push northeast towards the
southern end of the lake today into tonight and as this occurs,
expect southeast winds to quickly increase this morning. Gales to
35-40 KT are likely this morning through the afternoon for the
nearshore waters and south half of the lake, while gales continue
to be a possibility over the north half. Have not issued any
headlines over the north half, but will need to continue to
monitor for the possibility of gales in this location later this
morning. Gales will then likely be focused only over the open
waters later this evening, before diminishing with the eastward
shift of this low pressure system. With gales diminishing, higher
waves hazardous to small craft will likely linger for a period
tonight into Thursday. Next focus of concern will be Friday
afternoon into Friday night as another area of low pressure moves
through the region. Windy conditions are appearing likely, with
gales becoming more probable over much of the lake during that



LM...Gale Warning...LMZ777-LMZ779 UNTIL 10 PM Wednesday.

     Gale Warning...NEARSHORE WATERS UNTIL 7
     PM Wednesday.




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