Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 150808

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
208 AM CST Fri Dec 15 2017

208 AM CST

Through tonight...

Yet another shortwave pivoting around the longwave trough that has
been established over the region will clip the CWA today as the
parent trough retreats. At the surface, a weak cold front
trailing south from low pressure over the Upper Great Lakes will
move across the area this morning. Before cooler air has much
chance to filter in, winds will likely begin to back in advance
of the next system with warm air advection beginning this evening.

While there could be a few breaks, overall it appears that post
frontal stratus should be extensive today. Stratus could begin to
clear out tonight as low level southwesterly flow advects in drier
low level air mass, however mid-high level cloudiness should have
already moved in resulting in mainly cloudy skies tonight. Cannot
rule out some flurries from the stratus deck today into this
evening, particularly early this evening as mid level deck arrives
potentially seeding the stratus. Not expecting accumulations, but
could see some flakes flying at times today into this evening.
Cloud cover tonight is expected to keep temps from falling off and
have raised forecast lows generally above guidance.

- Izzi


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


208 AM CST

Fresh west winds today will gradually subside tonight as low
pressure over Lake Superior moves east tonight. Next bout of
significant winds looks to arrive Monday night with increasing
southwest winds initially, possibly nearing gale force, then west
to northwest gales appear to be fairly likely behind the front
Tuesday through early Wednesday. Active pattern looks to persist
late next week with another storm system bringing next bout of
high winds to the lake late in the week.

- Izzi





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