Area Forecast Discussion
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FXUS63 KLOT 280854

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
354 AM CDT Fri Oct 28 2016

354 AM CDT

Today through Sunday night...

Main short term forecast concerns are breezy and unseasonably mild
conditions today into Saturday, before shifting to a cold frontal
passage Saturday afternoon signaling a shift to more typical
seasonable temperatures. Rain is expected to develop post-
frontally Saturday evening, with the highest potential overnight
Saturday night into early Sunday.

Early morning surface analysis depicts high pressure centered
across the Midwest, just off to the east of the forecast area. Low
pressure was developing to our northwest across the Dakotas in
response to a 115 kt upper jet streak and associated mid-level
short wave propagating eastward from the northern Rockies. The
surface low is progged to develop quickly eastward across the
northern Great Lakes through tonight, with surface pressure falls
and a tightening surface pressure gradient inducing strengthening
south winds across the area. Given the relative lack of low
cloudiness upstream this morning, suspect that model soundings mix
the boundary layer a little too shallowly this afternoon this
evening/overnight. Thus have continued to lean 2-3 degrees warmer
than mos guidance for temps today, closer to what 925 mb
climatology would support, with afternoon highs from the low 70`s
west/south to the upper 60`s in the northeast. With models likely
decoupling boundary layer too strongly tonight, expect gusty
conditions to continue with strongest winds likely occurring this
evening in the 35 mph range. Temps will likely dip a little with
sunset, but should then remain fairly steady in the low 60`s
overnight in the persistent south-southwest winds.

As the surface low continues to track away from the northern Great
Lakes Saturday, a trailing cold front will sag across the region.
Guidance has wavered a little with the timing of the front into
the cwa over the past couple of days, though there is now pretty
good agreement that the boundary will settle into northern IL and
to the south end of Lake Michigan during the afternoon. Southwest
winds and dry thermodynamic profiles in the warm sector ahead of
the front will once again support unseasonably mild temperatures
in the mid-upper 70`s in many area, though cold fropa and a shift
to northeast winds will temper things a bit across far
northern/northeast IL. By evening, another short-wave is progged
to be approaching from the west, with increasing forcing for deep
ascent developing and inducing frontogenesis within the baroclinic
zone north of the surface cold front, leading to saturation and
precipitation development across the region. Most of this appears
to be initially off to the north or across the far northern
portion of the cwa near the IL/WI border area into early evening,
eventually spreading south across the area overnight. Passage of
the short wave trough axis Sunday morning, and development of
subsidence should bring an end to precip by midday, with high
pressure spreading in Sunday afternoon and night. While cooler air
spreads in behind the front Saturday night and Sunday,
temperatures are expected to be near normal for late October, in
the mid-upper 50`s north and low-mid 60`s south.



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 06Z TAFs...

- Brief MVFR cigs overnight and patchy fog
- Strong gusty south/southwest winds Friday afternoon and Friday
- Transition to low level wind shear late

MVFR cigs continue to erode on the leading edge as they translate
eastward early this morning. Latest short term guidance suggests
that the length of time for MVFR may be short overnight, but there
is some additional redevelopment of lower clouds and fog, but with
the high clouds passing throuight these are not likely to fill in
quite as solidly as the RAP/HRRR suggest later this morning,
though confidence on this is low/medium the NARRE, one of the go-
tos in the near term suppors the RAP/HRRR in the
redevelopment/re-expansion overnight into daybreak that MOS
guidance does not suggest. If it does occur, it will be closer to
daybreak, and will need to monitor.

Concern shifts to increasing south and eventually southwest winds
this afternoon and this evening. Low pressure will quickly move
east across the upper midwest today and north of Lake Michigan
tonight. South winds will largely be the rule today, maybe
acquiring a slight sw component. At this point critical crosswind
thresholds do not appear to be met today. Winds will then veer to
southwesterly as the low shifts east. This does appear to be the
period of strongest winds in the evening, with mixing continuing
during this time, before transitioning to a continued low level
wind shear concern later in the evening and overnight hours. Winds
will be more of a SW than S during the peak winds.



356 PM CDT

A very active period of weather is forecast over the Great Lakes
over the next several days. There are two storm systems of
interest, which will likely have a large impact on the winds over
the lake.

The first storm system will be a fairly weak, though very fast
eastward moving area of low pressure, which is expected to track
from the Northern Plains late tonight to southeastern Ontario by
early Saturday morning. Given the fast movement of this system,
and the fact that high pressure will be dominate over the eastern
CONUS, the pressure gradient will become strong over the lake late
Friday and into Friday night. This should support higher end
southerly gales of 40 to 45 KT, especially over the northern half
of the lake late Friday afternoon through the evening. Farther
south southerly gales of 35 to 40 KT are still expected over the
open waters of the lake. Because of this, we upgraded the gale
watch to a warning. We decided to leave the near shore waters out
of the gale headline at this time. In spite of this, 30 KT
southerly winds are expected, with some gale force gusts still
possible, especially Friday evening. To cover this threat a small
craft advisory has been issued.

A cold front will begin to shift southward down the lake during
the day on Saturday, with winds becoming northerly, albeit
lighter, in its wake. A surface ridge of high pressure will then
build over the lake on Sunday. As this low passes to the east on
Monday of next week, forecast guidance is forecasting the
development of a much stronger low over the Northern Plains. This
low then looks to take a similar track to our Friday and Friday
night low. Because of this, it looks as though there will be good
chances for another high end southerly gale event Monday and
Monday night.



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

     Small Craft Advisory...LMZ740-LMZ741-LMZ742-LMZ743-LMZ744-
     LMZ745...3 PM Friday TO 7 AM Saturday.




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