I can’t believe we are closing out on another month! Time being a “constant” is hogwash if you ask me. Seemingly, Mother Nature has had some different timelines, too.
Cold air hit a bit early for the season, and by mid-month we were setting record cold temperatures. Yet, at the end we are flirting with record highs. Saturday was the warmest day of the month -- tying the 1st for a high of 72-degrees.
Big ups-and-downs are clearly indicated in Denver’s temperature record for the month. It’s interesting to note at this point that Denver will record a colder than average November due to the extreme cold in the second week, and not because the majority of the month was cold. Take away that one week, or make that cold snap a bit more traditional, and the month would be ending much warmer than average.
Still, the month ends colder than average by a few degrees, and also wetter than average by a fraction of an inch. Not from snow ironically, but from rain that pushed us above average precipitation (on the 7th, DIA had 0.21” of rain).
The big picture hasn’t changed much in recent weeks. Throughout the month the sea surface temperatures have changed little in the Northern Pacific and Northern Atlantic. However, the El Niño signal has increased a bit. I’ll leave that discussion to Brian Bledsoe as it could impose season-scale changes into the spring months.
In the interim, there is very little trigger to activate a major pattern change for us. Western US ridging and Eastern US troughing continue. A persistent blocking pattern to our east allows for our wide temperature ranges; this flow allows for deeper troughs, and higher ridges, and slows the progression of the entire pattern. Without a significant change in the oceans or the Arctic (the AO shows nothing new), we will not see much change for December. I foresee half to more than half of the days near average and/or above. This is supported in our recent post on the seasonal outlook from the Japanese climate modelers.
What remains to be watched is the influence of the warming South Pacific, and how rapidly it should warm. Also, how the north Pacific temperatures change could help, or continue to hurt, our moisture potential for the month. All indications, at this time, are for a drier than normal December, unless we can get things to flip during the second half of the month.
Today’s system may be colder, but it can’t be called cold
That’s exactly how the month will start off. Yes, a cold front is moving through today. Yes, it will be colder than we have been by 25-35 degrees; this is not a pattern changer though. For today and Monday, our actual highs (mid-40s) will be near-normal, despite this being a strong cold front.
There is a bit of upslope coming in with this front. However, our depth of moisture is so shallow that we can’t expect much more than clouds to form. What little moisture there is will be squeezed out on the central and northern mountains and perhaps in Larimer County on the Cheyenne Ridge.
Since this isn’t a downsloping system for the Cheyenne Ridge -- ( it’s really more of a lateral flow) -- I could be convinced of the potential for light snowfall in Larimer and Weld Counties Sunday night and Monday morning. I would listen to anyone that would argue Jefferson and Douglas County might also see a bit snow, too, based on simple orographics; this system is quite moisture-starved, however, so I wouldn’t count on any accumulation. To see snow falling is not out of the question at all. To see it stick, though, I’d be the first to say “I’ll be darned”.
This system is like so many recent others where the coldest air will miss Colorado to the north and east. The issue is with the depth of the eastern trough and the height of the western ridge favoring the coldest air to sink east of us. Until that trough-ridge pattern changes, why would would anyone call for an extended cold snap? We won’t.
Following the cold front will be temperatures warming back to above average by Tuesday. Near average and above temperatures will continue through the week into the weekend. This warming will take place despite a weak mid-level trough that will pass over the state from California on Wednesday, which is too westerly to bring any cold air from the Arctic. A Pacific air mass from an unusually warm Pacific will keep us… well… warm. Thankfully it will send snow to our mountains, which we’ll provide a post on soon.
I think the ECM has slipped up the past week. It hasn’t handled the surface features well enough for me to trust it. The deterministic output has had quite a difference from the ensemble mean. GFS has had greater run-to-run consistency. Although both are too cool at the surface with the recent warm temperatures, the closer relation to GFS’s deterministic and ensembles give me greater confidence going through the next 3 to 5 days. The H500 pattern has been and continues to be similar for both, and includes the CMC in their camp too (although CMC amplifies the troughs and ridges a bit more). All three show a tame H500 pattern through the week. We may hope for a more active second week of December if we only look at the deterministics, but ensembles leave me feeling more realistic.