NWS Chicago Forecast Discussion 323 AM CST Fri Dec 1 2017

320 AM CST

...Today through Monday night...

Meteorological winter will enter quite gently as dry and very mild
weather, with highs 10-20 degrees above seasonal normals,
persists through the weekend under the influence of zonal flow
aloft. Friday night and Saturday morning could see a period of
cloudiness as a weak shortwave passes aloft, but the more
significant weather holds off until later in the weekend or early
next week.

By Sunday night a strong low level jet develops over, or perhaps
just west of, the area ahead of a longwave trough digging deeply
into the western CONUS. The GFS/ECM solutions show a 50+ kt jet at
850mb bringing a warm front through Sunday night and PWATs in excess
of an inch from Sunday night through Monday night, which would be at
the upper end of the climatological range for this time of year.
Forcing from this warm and moist advection could even support
thunderstorm chances Monday night, but the more widespread chances
of rain and possible TSRA would come Monday evening as a strong cold
front swings through the area and brings an end to the mild pattern.

The warmest of these four days appears to be Monday behind the warm
front that pushes through Sunday night and ahead of the cold front
expected to arrive Monday night.



320 AM CST

Long Term...Tuesday through Friday...

A significant pattern change has been widely advertised for next
week. Behind a cold frontal passage Monday night, Tuesday highs
may only drop to near normal values around 40 degrees but the
brisk cold advection will be a shock after this mild spell. A
secondary cold front Tuesday night will help temperatures fall
further toward midweek with highs only reaching the lower 30s...or
less...Wednesday and Thursday. Things of course become less
certain at this time range, but Friday could be just a bit milder
as models suggest flow could turn back to the southwest for a
period. Then again, it could be just as cold as the previous two
days if the cold advection persists.

At this point it does not appear that any of these reinforcing
frontal passages will be associated with much moisture, so snowfall
does not look like a major concern with this first round of cold
winter temperatures next week.



For the 06Z TAFs...

VFR conditions and light winds expected through the TAF cycle.

- Izzi


320 AM CST

Mild and moderate southerly to westerly flow will be in place today
into Saturday before relaxing on Sunday, then the primary near-term
period of concern for the lake comes Sunday night with strong
southerly gales. These winds do not relax much before turning
westerly behind a cold front Monday night and persisting as gales
into Tuesday.




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