Tuesday, December 22, 2015

December 26-29 Potentially Significant Winter Storm

Model guidance is beginning to pick up on the idea of a strong winter storm impacting the Plains.

The above image shows today's 06z GFS snowfall forecast from the time of posting to midnight on December 29th, 2015. We can see the large swath of snowfall extending from northern Texas through Kansas, Nebraska, and into Minnesota. The heaviest-hit region in the deep south appears to be extreme northern Texas, with amounts in the 12" to 24" range, particularly along the border with Oklahoma, as well as in a spot near New Mexico.
Portions of the Oklahoma Panhandle, as well as southwest Kansas and eastern Colorado, may be in line for up to 36" of snow, according to this forecast model. These are very extreme amounts, and although they have been showing up multiple times on previous forecast runs, extreme caution should be used with these maps, especially given the timeframe.
Amounts in excess of 24" are found in spotty areas to the north and east, particularly eastern Nebraska into southeast South Dakota, with lesser (but still significant) amounts in Minnesota.

The above image shows the 12z GFS interpretation of the coming storm, with total snowfall from posting time to 6 AM December 29th shown. Snowfall amounts are still incredibly high in northern Texas, the Oklahoma Panhandle, and much of Kansas into southeast Nebraska, but trail off from there. Amounts in the 12-16" range can still be found in Iowa and Minnesota, but these amounts are substantially lower than those in the 06z forecast. This variability, both in amounts and track (as you'll notice the heavy snow axis has shifted east) expresses the extreme uncertainty still associated with this system.

Tropical Tidbits
Lastly, we'll analyze today's 12z Canadian model, the GEM model. Amounts on the order of 20-24" are found in north and west Texas into the Oklahoma Panhandle, even in south-central Kansas, but overall snowfall totals are drastically less than the two GFS runs we analyzed earlier. This is a red flag that may indicate one model is either over-doing snowfall (GFS) or one model is under-doing snow (GEM). There's also a plausible chance that they're both incorrect- the point is, there's a lot of uncertainty here.

But just where is this system now?

Tropical Tidbits
Our system is currently traversing the Aleutian Islands, and will drop into the west coast of North America to produce a very strong trough, as per latest model guidance. It remains to be seen if this is model error, as medium and long-range guidance has a tendency to over-amplify storm systems, but for now, it's merely a system to monitor.

To Summarize:

- There is the threat of a winter storm in the Plains over the December 26-29 period.
- Model guidance is expressing high snow totals in the South Plains, but high uncertainty indicates that these models should not be taken at face value right now.


Upper Stratospheric Polar Vortex Warming in Short Term

(Note: This post discusses the stratospheric polar vortex in the short term (out to Day 10). The other stratospheric post discusses expectations next month.)

The upper stratospheric polar vortex is forecasted to experience some intensive warming, in the atmosphere's attempt to get this winter started.

The image above shows a graph of the forecasted Wave-1 temperature outlook from the ECMWF model, 10 days from today. While daunting at first glance, it can be explained.
The different levels of the atmosphere are listed on the left-hand legend. We can see the graph covers the atmosphere from the 1000-millibar level up to the 1-millibar level. Note the warmest colors centered right around the 3-millibar line. The bottom legend shows lines of longitude.

Let's depart this forecast for a moment and discuss different modes of stratospheric vortex disruption. There are two primary modes of disruption: Wave-1 and Wave-2. In a Wave-1 stratospheric polar vortex disruption event, a single ridge / body of warm temperatures forms aloft and singlehandedly tries to displace the polar vortex. This singular body of warmth / ridging is why it is called a Wave-1 event- Wave-1 events are also the strongest type of disruption event. Similarly, Wave-2 events involve two bodies of ridging / warmth trying to squeeze into the Arctic Circle, usually to split the polar vortex into two vortexes. This is weaker than a Wave-1 event, but still wields significant power in the atmosphere.

This temperature chart above shows a temperature spike at the 3-millibar level. Remembering this chart specifically identifies Wave-1 events, we can then deduce that the ECMWF model is expecting a Wave-1 disruption attempt at the 3-millibar level of the atmosphere. We can look at this on a 3-millibar forecast map below, valid for the same timeframe as the graph above:

Note how we see a single body of warm temperatures on the right-hand side of the hemisphere (if you look closely, the warmth is centered over Eurasia), visually showing that Wave-1 pattern I discussed earlier.

Why is this important? It means that the atmosphere is applying pressure to the stratosphere to try and make the pattern more conducive for wintry weather here in the troposphere, something that's been lacking this December. Now, vortex disruption at the 3-millibar level won't do much down here at the surface, but if it can expand to lower levels of the stratosphere (ideally 30, 50, and/or 100-millibar levels), the influence becomes greater on us here at the surface.

To summarize:

- The upper portion of the stratospheric polar vortex looks to experience warming in the next 10 days and beyond.
- While insignificant at that height of the stratosphere (3-millibar level), it could indicate a pattern more conducive for wintry weather may set up down the road (i.e. into next month).