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Area forecast discussion
National Weather Service Chicago/Romeoville, Illinois
324 PM CST Sat Jan 21 2017

Short term...
308 PM CST

Through Sunday...

After a sunny and unseasonably warm day by January standards,
forecast concerns shift quickly back to the redevelopment of fog
and low stratus across the forecast area tonight.

Afternoon surface analysis depicts low pressure lifting northeast
across southern Minnesota, with a warm front extending east across
southern Wisconsin and Southern Lake Michigan. South of the warm
front, skies were generally sunny with temperatures in the mid
50's to lower 60's across the cwa. The surface low is progged to
fill quickly tonight across the upper Midwest as the parent mid
level short wave lifts north, with the pressure gradient weakening
and becoming baggy across the area later this evening. Given
generally clear skies and quickly diminishing winds, radiative
cooling and surface dew points in the 40's are expected to allow
for relatively quick fog/low stratus development in the hours
after sunset. Low clouds and light fog were already trying to
creep east across northwest Illinois this afternoon, slowed by better
mixing in the sunny/breezy warm sector across the area. Would
expect this stratus fog to expand east and southeastward this
evening as we lose the gradient and mixing, along with new
development in cooling with loss of diurnal mixing. Will very well
need fog headlines sometime later this evening, though confidence
in speed of development/expansion is low at this point. Temps
to remain above freezing through the night, so at least no
freezing issues expected.

Next upper level wave across the Southern Plains is progged to
lift out across the lower Ohio and lower Mississippi River valleys
through Sunday, with surface low pressure tracking well south of
the forecast area. Guidance is in good agreement in depicting an
inverted surface trough developing northeast into northern
Indiana later tonight, which becomes the focus for some light
precipitation across our far southeastern tier of counties late
tonight into Sunday. Best chances remain south of the cwa, though
have included some low pops across that area. Forecast soundings
depict some weak elevated cape as well from late evening into the
overnight hours, thus an isolated thunderstorm can't be ruled.
Out. Low level winds which go light/variable tonight become
northeasterly Sunday, pulling cooler air into the area and likely
slowing any erosion of fog/stratus. Even so, temps appear to
remain well above climo in the 45-50 degree range through the day
Sunday, a little cooler along the Lake Michigan shore.

Ratzer

&&

Long term...
231 PM CST

Sunday night through Saturday...

The main challenges during the period will be with the timing, and
associated local impacts, of a storm system expected to impact
the lower Great Lakes region by midweek.

Prior to the midweek storm system, a potent storm system will
move over the Ozarks on Sunday, than to the central Atlantic coast
on Monday. While this storm system will be too far to our south to
produce much precipitation over the area, a few rain showers will
still be possible on Monday over far eastern Illinois and into
northwestern Indiana. Otherwise, this system will result in a
period of breezy cooler northerly winds Sunday night and Monday.
Expect high temperatures for Monday and Tuesday to be several
degrees cooler (low to mid 40s) than this weekend temperatures.

Attention then turns to what appears to be a quick larger scale
pattern shift towards a +pna pattern (upper ridge in the west,
upper trough in the east) over North America by mid to late week.
As this pattern shift occurs, the eastern Pacific storm system
will be driven eastward over the central Continental U.S. And into the lower
Great Lakes region sometime around midweek. While model and
ensemble forecast guidance is in fair agreement with the
evolution of the larger scale pattern supportive of this storm
system, there are still some rather large differences in the
timing and strengthen of an associated surface low. Most notable
has been the faster trend in the GFS solutions over the past 24 to
36 hours. The 12z GFS now appears to be on the fast side of all
the guidance, with the forecast surface low already shifting
towards Lake Huron by 12z Wednesday, which even outpaces the gefs
mean. Meanwhile, on the other end of the envelop is the 12z
Canadian, which has the surface low all the way back over
northwest Missouri at 12z Wednesday. Finally there's the 12z
ECMWF, which seems to fall right in the middle of the pack, with
the low over northwest Illinois during this time.

All in all these large differences in timing still result in low
confidence with the specifics of this system, especially in terms
of timing a change from rain to snow over the area. Because of
this, we have not made any major changes to the forecast during
this period. If the track of the low does go over far northern
Illinois or southern Wisconsin, the p-type would likely be mainly
rain Tuesday night. The question is than when and how much of the
precipitation could change to snow on the back side of the storm
system late Wednesday or Wednesday night. Also, in addition to
the precipitation, if the surface low ends up on the stronger
side, and tracks just north of the cwa, it could support a period
of warm temperatures followed by strong westerly winds over the
area.

Colder weather and some lingering periods of snow showers or
flurries look to continue during the later portion of the week.

Kjb

&&

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

Main aviation concerns are for the potential redevelopment of
IFR/LIFR conditions tonight and which could linger through Sunday
morning.

Surface low pressure was lifting northeast along the
Iowa/Minnesota border at midday, with a warm front extending east
along the Illinois/Wisconsin border per surface analysis. Low
clouds and fog have given way to VFR conditions with the warm
front now north of the terminals, with breezy south winds and
mainly clear skies to continue through early this evening. The
exception to this will be at krfd however, as an area of IFR/MVFR
stratus is slowly moving east across eastern Iowa and far
northwest Illinois. This is expected to move back into the krfd
area by late afternoon with development of MVFR cigs initially on
the eastern periphery of the stratus deck.

Concerns increase this evening as the low over Minnesota fills and allows
the winds to weaken significantly with sunset, with moist low-
levels likely allowing for redevelopment of fog/low stratus and/or
expansion from Iowa/WI/NW Illinois overnight in the weak gradient.
Forecast soundings suggest the potential for LIFR cigs in 200-500
ft range late tonight/early Sunday, with only gradual improvement
likely as surface winds shift to the north-northeast in response
to another low pressure system moving through the Ohio Valley.

Confidence in overall trend is medium-high, though timing of how
quickly low stuff develops this evening/tonight is low as is
timing of improvement Sunday.

Ratzer

&&

Marine...
323 PM CST

The main weather concerns over the lake continue to be the dense
fog over the open waters of Lake Michigan. Given this fog will
likely not be going anywhere anytime soon, we have extended the
fog advisory for most of the open waters through 3 PM Sunday.
Northerly winds are expected to set up over the lake by Sunday
night into Monday as a strong area of low pressure tracks well
south of the lakes region. While the low will be well south of the
area, northerly winds up to around 25 knots will still be possible
over Southern Lake Michigan Sunday night and Monday. These winds
will likely result in Small Craft Advisory level waves of 4 to 7
feet over the Southern Lake Michigan near shore waters during this
period.

Another storm system is expect to develop over the plains by
Tuesday morning, then track east-northward over the southern
lakes region by midweek. This system could result in a period of
enhanced winds over the lake around midweek.

Kjb

&&

Lot watches/warnings/advisories...
Illinois...none.
In...none.
Lm...none.
&&

$$

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