It has been so warm lately despite being October and Halloween is around the corner. It is like summer does not want to leave despite the calendar saying otherwise. Why is it so warm right now?
Let’s take a look at the upper atmosphere at the 500 millibar level or 18,000 feet.
There is upper level ridging over the Lower 48. It is mainly over the Central US. The ridging prevents any cold air from coming down south. There is troughing off the Pacific Northwest, which has kept things wet for them, including California. There is also ridging over Alaska, which is a negative East Pacific Oscillation (EPO). A negative EPO normally supports cooler weather. However, the ridging over Central US is blocking any cold air. There is also troughing over Eastern Russia, which is a positive West Pacific Oscillation (WPO). WPO is similar to EPO as a negative EPO favors cold air being shunted down south. A positive WPO like positive EPO and North Atlantic Oscillation (NAO) keeps cold air from being shunted southward. There is also ridging centered around Yuzhny Island and Svalbard. It is a large area of ridging. I am surprised no one has created an index for that area. I suspect it is an oscillation like NAO and EPO. Would be interesting to check it out. Where are the cool and warm spots?
Not surprisingly, the warmest areas are where the ridging is most strongest. It is also very warm over the Arctic. The coldest area is over Central Asia, mainly over Kazazhstan. It is cold over most of Siberia. The Eastern tip of Russia is very warm. Rule of thumb, if there is upper level ridging, it will be warmer and drier, while upper level troughing will be cooler and wetter. The upper level trough has deep cold air, there can also be snow fall. Case in point, let’s look at two snow events in Southeast Texas; February 12, 1960 and December 10, 2008. February 12, 1960 is the biggest snowfall event since February 14-15, 1895. The December 10, 2008 gave Houston an early start in the snowfall department before places that see snow like Chicago and New York. Let’s start with 1960.
There is a deep trough over Mexico and is moving eastward towards Texas before the February 12, 1960 snowfall. It is a very large upper level trough.
The trough has deep layer of cold air. That is very favorable for snowfall. Snow happens when there is a deep layer of cold air. One reason why snow can fall at above freezing temperature. Shallow layer of cold air is not conducive for snowfall. More likely there will be sleet or freezing rain. One reason why Southeast Texas does not get snow often is cold air is shallow, not deep. Let’s also look at December 10, 2008 snowfall.
The trough in the 2008 snow event is smaller than the 1960 snowfall event.
Like the 1960 event, it also had deep layer of cold air. If one wants to compare February 1960 snowfall, the closest is December 2008 in terms of atmospheric setup. Perhaps we will see snow this winter.