NWS Chicago Area Forecast Discussion 1054 PM CST Thu Nov 30 2017

1200 PM CST

Through Friday...Weather is quiet through Friday. Occasionally gusty west-northwest winds behind this morning`s cold front will ease late this afternoon and even more tonight as high pressure across the plains expands across the southern Great Lakes. With the light winds and clear skies, expect a colder night tonight, with lows in the 20s away from downtown Chicago. The high will shift east of the area Friday afternoon, allowing generally light southwest winds to develop. Temperatures will therefore climb back close to today`s readings in the upper 40s to low 50s. Expect some passing high clouds given the progressive flow aloft.




Friday night through Thursday...

Main concern for the longer term forecast period remains temperature trends for next week.

The longer term guidance remains on track in suggesting a significant pattern shift with a quick transition from above normal  temperatures to below normal. While the longer range models show some slight timing differences on the exact timing of the frontal passage that will usher in much colder air early next week, the signal remains clear. The highest temperatures will be on Monday as strong warm advection peaks in advance of the cold front, with highs ranging from the upper 50s to lower 60s. The ECMWF is trending a little faster than the GFS, but these timing differences will likely have little impact on the overall forecast trends, with the main idea being that Monday will be unseasonably warm, Tuesday will be a transitional day with near normal temperatures and Wednesday will be the first day of the colder trend. By Wednesday, highs will drop into the low to middle 30s. The main change to the forecast for the longer term is that the models are likely to be trending a bit too high with temperatures toward the end of next week as strong cold advection would suggest lower temperatures, with the longer range guidance likely being impacted a bit too much by climatology. So, by late next week, max temps for some areas could only be in the upper 20s. The only significant chance for pcpn for the long term period will be associated with the frontal passage, with some rain likely along and just in advance of the front in the strong warm advection and then transitioning to a rain/snow mix or some light snow as the colder air filters in behind the front. At this point, do not anticipate any measurable accumulation.


.AVIATION... For the 06Z TAFs...

VFR conditions and light winds expected through the TAF cycle.

- Izzi



The very progressive pattern will continue to impact Lake Michigan for the next several days.  A series of low pressure systems will continue to generate periods of strong south winds, followed by a brief lull before the next system generates another round of strong south winds. The next period of strong south winds will be Friday night and Saturday. However, this system is expected to be a bit weaker than previous systems and gales are not expected. The next system will be quite a bit stronger and south gales are expected over the lake from late Sunday night into early Monday evening. There should be a short lull in winds as the frontal trough crosses the lake, but then strong northwest to north winds should develop in the strong cold advection behind the front, with gales possible late Monday night or tuesday, depending on the timing of the frontal passage.



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