After a bit of a hiatus, I’m back! Changing jobs and moving takes a toll after all. There is an interesting week of weather ahead, its had some pretty impressive signals in the global ensembles. Why all the activity all of a sudden? Let’s dive in.
SSWE – Sudden Stratospheric Warming Event
For a number of months now, a lot of talking heads have been screaming “sudden stratospheric warming event!!” rather misguidedly. Well, a couple of weeks ago, a more reliable signal entered the global ensemble model space, and it indeed has occurred.
This plot of 10 hPa plot shows the u winds (east-west) have indeed now reversed. When the stratosphere warms such suddenly, and the polar vortex weakens, chaos usually ensues. How that shakes out is far less predictable than folks would have you believe. It does not mean *everywhere* will get cold. But it usually means a big pattern shift, with amplified waves, so *somewhere* is going to get cold. Notice also a quick recovery by the end of February, then perhaps another event to start off March. That’s another story for a different day though.
Pattern Change / Amplification
The last week or so, the global models have come into incredible agreement on the tropospheric response to the SSWE. Going back to the 00Z cycle on Wednesday, February 15th, we can see remarkably tight clustering from the combined solutions of the 100 members of the GEFS, GEPS, and EPS. We’ll use the WPC Cluster Tool to look at this in the slideshow below.
As we go out in time in the forecast, we get generally more members favoring deep troughing in the west. I would hesitate to call this too much of a pattern change, because this has been fairly common this year (Western US troughing), but this round is a little different. Deeper and more inland (vs coastal) would indicate quite cold air west of the divide. Notice also that while most clusters had the brunt of the trough impacting the Western US, cluster 4 for Wednesday 23-Feb (5th image) does keep more of that trough north and to the east. Let’s fast forward to a more recent run to see how confidence has changed and what details are now emerging.
In this more recent cluster analysis, we see very very tight clusters, with very very minor difference between them for Wednesday, 22-Feb (24hrs ending 00Z 23-Feb). We have two very anomalous troughs, one over the Western US, and one over eastern Canada, with a strong ridge in the southeast US. This is a very potent setup for a couple reasons. 1) The interior trough in the west allows arctic air to spill much further south and west than is typically possible – we’ll talk a bit about that later. 2) The two troughs combined with the ridge yields a storm track for the plains with consistent access to both quite cold air AND rich gulf moisture. This is definitely a pattern for long-duration, heavy snow somewhere.
If we keep going forward in time, we see this Western US trough is not progressive, but might actually retrograde as it weakens and the Southeast US ridge amplifies. This is another recipe for a rarity in the southwest – low elevation snow.
So, what’s it all mean?
The meteorology is pretty cool, but itself, it has no value. So let’s start connecting to impacts. There are several tools available to do this. We’ll start out west first.
Front Range Whiplash
We’ll start with one of the teases from the title. Now that I live in the front range (and throughly enjoy it), time to give it some love. Let’s start by looking at EPS temperatures for Denver, Colorado.
The front range is a land of extremes which is easy to see here. 50s and 60s are pretty typical of front range winter weather (snow doesn’t last too long here thanks to compressed westerlies), but this is as good of an example as any. The front range, unlike areas further west such as the Wasatch Front is not closed off – therefore is more prone to extreme shifts in a short amount of time. Without mountains to the north, east, or south, inversions have a hard time setting up shop, and there is nothing to stop arctic air from diving down. Tuesday will be a bluebird day for the front range with temperatures flirting with the 60s, while Thursday morning could bring temperatures flirting with the -20s! By the way, I have published a python notebook that makes such graphics on my github. With the dip in temperatures, what about snow? We’ll use the NWS NBM to looks at some bookends for snow amounts.
In the slideshow above, we have 10th percentile (low end / 90% chance of exceeding), 50th percentile, and 90th percentile (high end /10% chance of exceeding) 48-hr snow amounts from the NBM, ending Thursday morning (23-Feb). While the 10th-90th range is quite high, at least a few inches can be expected in the metros, but Wyoming will probably take the brunt of the system. These are more plots made with a google colab notebook on my github.
Western US Cold and Snow
We saw earlier, the Western US will be impacted the most, and teased some cold southwest temperatures, and even low-elevation snow potential. First, we’ll look glance at some of the cold.
For the most part, the above map of probability of lows colder than 32 degrees for Thursday morning (25-Feb) is not very impressive. What is impressive, and more impactful at the low to moderate probabilities of such temperatures in California’s central valley. While some may dismiss such temperatures as wildly off-base, these do happen rarely. The track and depth of this trough are certainly capable. What about snow potential?
While this system will be capable of low-elevation snow, a product like the NBM is not picking up on meaningful chances of measurable low-elevation snow. However, since the cold air is established first, and then the low retrogrades, allowing moisture to wrap back up over the established cold air, this would bear watching as details are settled.
Northern Plains Snow
Finally, let’s take a look at the snow potential in the Northern Plains. As was alluded to earlier, the setup for a long-duration, heavy snow event is there. Once again we’ll return to the NBM. First, we’ll look at percentiles again.
In the above percentiles from the NBM for 72-hr snow ending Friday morning (24-Feb), notice the large footprint of 6 inches at the 10th percentile, and the high floor of 8-12″+ for the Twin Cities! Also notice the high ceiling with around two feet at the 90th percentile! If we actually care about certain thresholds, we can look at those too!
In the above gallery, we can see NBM probabilities for 4″ (some impacts proxy), 8″ (moderate impacts proxy), 12″ (major impacts proxy) and 24″. These probabilities have proven to be quite reliable and useful for informing decisions. Once again, these can be generated from a Google Colab notebook. While there is certainly still a some uncertainty we can see in these images, we don’t have to wait until we are 2-3 days out, or for a couple of deterministic models to completely agree before we can start talking about this system, especially if we can start putting numbers down to bound the uncertainty. Lastly, while hazards, such as snow amount are great if you can link them directly to impacts, in reality, this will vary based on local factors like climatology, time of year, etc. To that end, WPC has developed the Winter Storm Severity Index, and its probabilistic counterpart. These are worth a look when trying to gauge just how bad will this be, taking into consideration things like local climatology.