Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 272013

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
313 PM CDT Thu Oct 27 2016

146 PM CDT

Through Friday...

The morning RAOBs from ILX and DVN indicate a staunch inversion
above 900 mb with significant dry air above that level. Abundant
moisture below the inversion along with visible satellite only
showing a small thinning in the status will combine to keep cloudy
conditions present through the evening. The inversion has also
precluded any efficient mixing, so winds will remain at or under 12
mph. High pressure will enter from the southwest overnight, allowing
clouds to start scattering out and causing winds to become light and
variable. The last of the H85 thermal trough will be slow to exit
tonight, so we will experience one last cool night before the noted
warming trend through the weekend.

On Friday, 500 mb heights will be on the rise ahead of mid-level
ridging. This is in concert with a warming southwest 30KT low level
jet and return flow from surface high pressure moving east of the
forecast area. The warm advection regime will allow 850 mb temps to
rise to 17C which climatologically should correspond with surface
temps close to 70F. Have opted to keep the forecast on the higher
side of guidance given the warm air aloft and quality of mixing. The
main limiting factor to any higher temperatures is the decreased
insolation this time of year. Early day cloud cover will also play a
small role.

Pressure gradient between high pressure in the OH river valley and a
low developing in northern MN poses a challenge to the Friday
evening wind forecast. Hourly pressure drops near 1 mb/hr will
persist from 15 to 00z. The isallobaric effects should allow for
winds over 20mph for much of that time period and gusts in the 30-35
mph range. Forecast soundings show decoupling in the 21-00z
timeframe, although with 1,000 ft winds at 30-40 KT, it is still
possible to see some significant SSW wind gusts into Friday evening.
Have continued to hedge winds on the high end of guidance due to
uncertainty in models.



313 PM CDT

Friday night through Thursday...

The main forecast concerns during the period will continue to
focus on:
1) The unseasonable warmth continuing Friday night and Saturday.
2) Windy southwest winds Friday night.
3) Timing of a cold frontal passage and any associated potential
for showers and storms late Saturday into Saturday night.

The pressure gradient across the Great Lakes region will compress
Friday and Friday night as a weak, but very fast, eastward moving
area of low pressure over the Upper Midwest impinges upon high
pressure over the eastern CONUS. This will result in a period of
stronger winds into Friday evening, when the strongest isallobaric
pressure falls in excess of 8 MB/6HR is expected to overspread
the Upper Great Lakes. With winds in the 925 to 950 MB layer
expected to increase to 35 to 40 KT during the evening...some
strong wind gusts of this magnitude could occur over portions of
northern Illinois, though confidence on getting gusts above 35 KT
is low at this time. Given the strong WAA regime it is unlikely
that temperatures will drop much, if at all, with sunset. So, this
may help keep the lowest levels well mixed and allow the winds to
remain gusty.

These breezy southwesterly winds could continue into at least a
portion of the day Saturday, but this will be dependent on the
timing of an approaching cold frontal boundary, which the model
guidance has been struggling on lately. Current forecast guidance
now suggests a slower southward progression across the area
Saturday evening and night. If this slower progression does pan
out, it will likely result in a very mild day across the area on
Saturday (Highs in the low to mid 70s), as 925 MB temperatures are
expected to be around 16-19C (~90th percentile for the time of
year). The record high on Saturday at O`Hare (Rockford) is 78
(81) degrees. So, while it will be unseasonably mild across the
area Saturday, it may be hard to match these record values, as it
does appear there could be a decent amount of higher level
cloudiness around.

As the cold front sags southward into northerly Illinois late
Saturday afternoon or Saturday evening, it appears that some
showers and potentially some thunderstorms may develop over the
area. Guidance is indicating that moisture return ahead of the
front could be enough to result in some modest destabilization
supportive of some thunderstorms. However, uncertainty remains
as to how much rain may fall across northern Illinois Saturday
night. This will ultimately be determined on the actual strength
of the frontogenesis along the southward sagging baroclinic zone.
If the frontogenesis does remains strong into Saturday night, as
some guidance is indicating, then there will be the potential for
some decent rain amounts over portions of northern
Illinois/northwestern Indiana Saturday night, especially
considering precipitable water values could be near record values
(~1.4") for this time of year. At the present, we have remained
conservative, with only chance POPs and rather low QPF. However,
these may need to be bumped up with future forecast updates if
the current trends continue.

Any rain with this frontal zone boundary should end across the
area by midday Sunday. Cooler weather, back near average in the
upper 50s to low 60s, is then expected for the afternoon on Sunday
as a surface high builds into the western lakes region. This cool
down will be short lived, however, as another, more potent,
surface low tracks eastward from the Northern Plains Monday into
southern Ontario by Tuesday. This low could become sub 990 MB and
fast moving, which may result in stronger isallobaric pressure
falls across the Upper Great Lakes. As such, expect another breezy
day developing Monday and Monday evening. Temperatures will also
warm significantly again back well into the 70s for Monday, and
possibly Tuesday as well.

Although temperatures by the middle to end of next week look to
be near to slightly above normal, there is considerable
uncertainty on precip potential and timing during this period. A
cold front, emanating from the aforementioned strong low over
Canada, is expected to push over the the area by around the middle
of next week, before possibly stalling out over or near the area.
This could set the stage for a period or two of showers and
thunderstorms, though confidence on timing is on the low side at
this time.



For the 18Z TAFs...

Stratus cloud shield across the Great Lakes is showing no sign of
breaking up except over small portions of the open waters of Lake
Michigan. Observations show widespread MVFR in the region, and the
morning soundings support the stratus staying put into the evening.
We may see some scattering and lifting of the clouds as high
pressure nudges into the region tonight. Do expect VFR ceilings
around 23z, but will not be surprised if high MVFR conditions
persist. Winds turn southerly on Friday as high pressure departs to
the east. As warm air advects into the area, expecting better mixing
to allow 20-25kt winds through the afternoon. There is potential for
higher gusts beyond the TAF period.



300 AM CDT

Active period on the lake over the next week. A brief period of
fresh northerly winds expected today in the wake of departing low
over southern portions of the lake. Winds should be enough to kick
of small craft advisory worthy waves for our NSH waters into at
least early this afternoon. Brief lull in active pattern tonight
as ridge moves by, but even by late tonight southerly winds will
be ramping back up. Low will track quickly from northern plain
into Quebec Fri into Sat. While low will not be particularly deep,
the fast movement will result in a period of southerly gales over
the lake Fri afternoon into at least the first half of Friday
night. Some concerns about stability issues keeping winds down
some, but even with some low level stability near the lake sfc due
to the warm air mass, forecast soundings still support a period of
high end gales over northern portions of the lake. Issued a gale
watch for open lake to cover this threat.

For the IL & IN nearshore waters, the potential for gales looks
briefer and confined mainly to Friday evening. Strongly considered
a gale watch for IL nearshore where potential looks greatest, but
after collaboration with MKX opted to hold off and let day shift
take another look. Southwest winds ease Saturday and shift north
as a front drops down the lake Saturday and could be a period of
moderate north winds behind the front late sat into Sat night,
which could push waves close to small craft advisory levels for
IL/IN NSH. Front will surge back north Monday with another period
of southerly gales likely Monday into Monday night. Low is
forecast to be even stronger and still fast moving, have
introduced gales over the open lake, but if low is as deep as
currently forecast then a period of storm force southerly winds
are possible, mainly northern portions of the lake.



LM...Gale Warning...LMZ777-LMZ779...4 PM Friday TO 3 AM Saturday.

     Small Craft Advisory...INDIANA NEARSHORE WATERS UNTIL 7 PM Thursday.

     Small Craft Advisory...LMZ740-LMZ741-LMZ742-LMZ743-LMZ744-
     LMZ745...4 PM Friday TO 3 AM Saturday.




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