A WINTRY START TO 2019...
After getting off to a blazing start in November, we haven't seen much of winter the past 4 weeks. The primary reason is the split flow that's been evident in the polar and sub-tropical jets. Without significant phasing (coupling) cold air intrusions have been limited and brief in nature when they occur.
In the short term, it does look like the two jets will try and co-exist as they attempt some degree of phasing this weekend. Below you can see the two distinct streams separate but being drawn to one another Sunday night.
By New Years Eve morning the southern stream energy is captured on the east side of the mean trough. The absorption of the energy is the phasing process that leads to a storm over the mid-Mississippi Valley.
At the surface the NAM shows a low in central Illinois at noon New Years Eve day. There is likely to be some snow or mixed rain/snow/freezing rain on the NW fringe of the precipitation shield.
What we don't know is how strong the phasing will be and how soon it happens. That's a big deal because it impacts the track and the thermal profiles which dictate where freezing precipitation occurs. Right now most models including the NAM. 3k NAM, EURO, and GEM show at least some light snow accumulations from my area (eastern Iowa) northeast into Wisconsin. East of the Mississippi much of the event is comprised of rain. The GFS is the odd man out further SE and weaker with light QPF.
This is the type of storm that could give forecasters problems right up until the event as models are known to have difficulty handling phasing issues. So for now I just have to go with the general idea that a weather system is on the table New Year's Eve day...winding down in the evening. Precipitation type is far from certain as are any snow accumulations. If the GFS is right there won't be any precip. at all.
Whatever happens with the precipitation, it does appear temperatures are going to turn sharply colder New Year's Eve night and New Year's day. Readings will fall to the teens by New Year's day and hold steady or even fall during the afternoon. Wednesday morning it will feel plenty wintry with lows in the single digits.
Wind chills in the range of 5 to 10 below are possible.
As it stands now I don't look for the frigid readings to hang on long as temperatures return to seasonal levels by the end of next week. We'll get a better handle on the storm in the next 24-48 hours. Until then, roll weather...TS