I thought it would interesting to look at the past 5 months (September-January) as there have been some notable trends at the seasonal level which have led to major impacts within the US. Some of this is driven by the cool phase of the El Nino Southern Oscillation (La Nina) in the tropical Pacific, while in the longer-term, they are being driven by increasingly more powerful influence of anthropogenic climate change on global temperatures and natural variability.
The most notable climate anomalies the past several months since the beginning of Meteorological Autumn (Sept 1st) have been very abnormal heat…particularly in the Western US and increasing dryness across much of the nation.
The rapid increase in drought conditions since late Autumn was initially caused by the jet stream favoring the northern tier states and southern Canada, with strong upper-atmospheric high pressure over the West contributing to the abnormal warmth. However, by January, the pattern changed with the jet stream becoming much more higher in amplitude over the US. The strong ridging remained over the Eastern Pacific and Western US, but strong dips in the jet stream have thus far brought abundant cold air masses into the center and eastern third of the country. These continental Arctic air masses have also been quite dry, making it difficult for many places to recover from drought conditions. In many cases, the situation has worsened.
One thing to notice as far the heat is concerned for January 2018. Although there was a wide swath of the US with below normal temperatures, the 9 states out West had their Top 10 warmest January on record vs. no states with a Top 10 coldest. So even with a high amplitude jet stream opening up Arctic air to much of the US, high heat (by winter norms) still dominated the US average with the 35th warmest January on record, in the top 30% out of 124 years. In addition, Alaska witnessed very abnormal warmth. Barrow had its warmest November on record (more than 16 F above normal) while the whole state had its warmest January on record. Also in January, Ketchikan, AK measured its (and Alaska’s) highest daily January temperature on record of 67 degrees F in the Panhandle. While La Nina and other “teleconnections” (multi-month and sub-seasonal atmospheric circulation patterns) are creating conditions favorable for these abnormal conditions, anthropogenic climate change is clearly having an impact on the intensity of warm regions over cold regions and the tendency for more frequent drought conditions (and longer wildfire seasons), especially in the Western US.
It appears February will be a repeat of January, although it may end up warmer overall if long-term models work out. And with this, meteorological winter may end on very abnormally warm and exceptionally dry note.
–Meteorologist Nick Humphrey
2/11/18: Edited to add February 8th Drought Monitor.