Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 150541

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1141 PM CST Thu Dec 14 2017

135 PM CST

Through Friday...

Seasonal colder conditions remain in place this afternoon with
passing higher clouds. Northwest flow aloft will continue tonight,
and will steer another weak disturbance through the region. There
have been some flurries/light snow upstream in Minnesota with this
feature. We see very little if any model guidance painting any
accumulation with this feature, other than some flurries after
midnight into early Friday. Can`t argue against a brief isolated
dusting given the cold temperatures and secondary cold surge with
the wave. This forcing will exit east mid morning. Warm advection
will ensue ahead of another clipper on track for Friday night,
with this one again largely to remain north of the Wisconsin
border. We would just expect some increase in higher clouds
during the afternoon. With the warm advection and least sun breaks
in the middle of the day, we could tack on a few degrees from
today. Temperatures will still be below normal.



211 PM CST

Friday night through Thursday...

Main concern for the long term forecast period will be precipitation
type Sunday morning.

The upper level pattern is trending toward a high amplitude,
slowly progressive longwave pattern. By Friday night, broad upper
ridging will build over the ern 2/3 CONUS with deep troughing over
the west. This will shift the pattern from cooler nwly flow aloft
to more moderate swly flow. Southern stream shortwave energy will
be the primary weather driving systems for the long term. In
general, temperatures for the long term forecast period should
trend slightly above normal at least into early next week as the
area remains under upper ridging, with highs in the upper 30s to
middle 40s with more seasonable lows in the mid 20s to mid 30s
through the period. The main concern for the period will focus on
pcpn spreading into the region from the southwest. The longer
range models are coming into a little better agreement on the
timing of an upper closed low expected to located over the
southern end of the Sea of Cortez lift newd and evolves into an
open wave. The longer range models still have some timing
differences, which is to be expected 72-84 hrs out, but these
differences are only on the order of +/- 6 hrs, so confidence is
increasing regarding the impacts and pcpn associated with this
system. This is the type of setup that would be conducive to at
least a period of freezing rain, changing over to rain.
Temperatures early Sunday morning are likely to be sub-freezing
and then increasing through the day as broad warm advection sets
up in advance of the mid level shortwave and an associated sfc
trough/warm front. Timing of freezing pcpn is always tricky, and
more so at this time range, but there is increasing confidence
that there may be a short period of freezing rain as the pcpn
begins to move into an environment of sub-freezing temperatures at
the surface with a strong warm intrusion aloft. Latest guidance
would suggest that temperatures should rise above freezing at the
surface relatively quickly, setting up a relatively short period
freezing rain potential. However, given the timing differences
among the models at this time, there are still chances that either
the pcpn could be all liquid, should the warmer air at the
surface outrun the pcpn shield, or the period of freezing rain
could be a little longer should the pcpn begin earlier, while the
sfc is still below freezing. This will need to be monitored
closely in forecast updates.


For the 06Z TAFs...

Primary aviation forecast challenge is timing/occurrence of MVFR
cigs through the period. Area of MVFR cigs over north central IL
should move across the terminals overnight, though upstream
observations indicate variability in cigs with areas of VFR. Cold
front will move across the terminals after sunrise, with upstream
observations showing a fair amount of MVFR in the wake of the
front, have opted to include prevailing MVFR Friday, though it is
probable that there will be periods of VFR, especially in the
afternoon. Confidence in cigs decreases Friday evening, potential
exists for lower MVFR to hang on Friday evening, but for now
playing slightly more optimistic forecast showing improvement to
VFR. Only other forecast concern is potential gustiness Friday.
While cloud cover may tend to mitigate the wind gust frequency
and magnitude some, mixing should be sufficient for at least
several hours of fairly regular gustiness.

- Izzi


211 PM CST

A relatively quiet period of weather is setting up for Lake
Michigan for the next several days as a series of weak clipper-
type lows track across the upper Great lakes region. The next
chance for impactful weather and Gale Force winds will not be
until early next week when the passage of a cold front. There is
still some uncertainty as to the timing of the cold front passage,
but the latest model guidance is suggesting Monday night or early
Tuesday for the frontal passage, with west to northwest gales for
Tuesday into Wednesday.





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