An arctic air mass plunged down the east side of the Rocky Mountains in the past day. This air mass will cause record low temperatures for the Denver, CO area. According to the NWS, the record low maximum temperature for April 9th is 27F, which was set in 1973. The record low minimum temperature for April 9th is 12F, which was set in 1959. The temperature at DIA at midnight this morning was 24F. The maximum temperature during the day today will not be higher than 20F, which means the calendar day’s maximum temperature has likely already been set. It’s 15F right now, which is quite frigid for April in Denver.
The storm system that brought this cold air to the area was also supposed to bring considerable snow. Yesterday’s forecast predicted up to 12″. Because of the chaotic nature of the atmosphere, Denver will not receive 12″ of snow. The upper level low split into two smaller pieces as it tried to traverse the intermountain west. This development is not unusual, but numerical models have a hard time handling this behavior due to their limited resolution. When upper level lows split, the energy associated with the storm also splits. So instead of 12″ over the Denver area, lower amounts will be spread over a larger area. The timing of vertical lift and the passage of a series of cold fronts through Denver also affected the beginning of precipitation. Rain was supposed to fall starting around 6P last night, then switch to snow between 9P and midnight. Instead, light snow started to fall around 10P.
This storm system is part of a different pattern than what occurred last year. During early April 2012, record maximum temperatures were set. Most of the change is due to simple interannual weather and climate variability, including low-frequency climate oscillations like the El Nino-Southern Oscillation and the Interdecadal Pacific Oscillation. We can attribute part of the change to the underlying warming climate, which impacts those climate oscillations.
Climate change skeptics will likely point to this storm and the record lows that the NWS will record as “proof” that a warming climate is not occurring. To the contrary, there is a climate-related reason why this storm system is impacting the western US today and bringing record warm temperatures to the eastern US. The following plot shows today’s jet stream configuration:
The strength of the jet stream is characterized by the speed of the winds. As the figure shows, there are very fast winds on the west side of the trough over the western US (red-filled contour where winds are in excess of 125 knots). There are very slow winds south of Louisiana and east of northern Florida. I have included an arrow on this figure to highlight the climate-related impact. As the Arctic warmed more than the equatorial region, the temperature gradient weakened. Temperature gradients cause pressure and density gradients (Ideal Gas Law). As the average annual equator-pole temperature gradient weakens, the average pressure gradient similarly weakens. This reduced pressure gradient causes the west-to-east movement of storm systems to slow down. The arrow above highlights the amplitude of the current wave traversing North America. This wave’s amplitude is characterized as high due to its large latitudinal extent (it stretches from Mexico to northern Canada, which is a very large distance). This high amplitude simultaneously causes cold air to move from the Arctic to more southerly locations, such as Denver, CO, and warm air to move from the sub-tropics to more northerly locations, such as the eastern US.
Absent long-term anthropogenic climate change, this storm system would be much less likely to move slowly and bring record low temperatures to the middle of the US. Instead, the storm would move quickly across the country. Denver would receive cooler than average temperatures, but not record cold temperatures. The cold air would remain further north and impact Canada and the northern US.
To summarize, climate change will not banish record low temperatures. They will become more rare, however. Winter will still occur in the mid- and high-latitudes. But those winters will, on average, become warmer in the future. Precipitation that would have fallen as snow in the 20th century will be likelier to fall as rain as the 21st century progresses. More precipitation will likely fall during each event, but there will be longer time periods between precipitation events. Overall, aridity will increase and flash flooding could become a more common problem for communities.
Thankfully, the NWS predicts temperatures to return to normal by this weekend. I’m sure happy to receive the precipitation, but I wish it came as rain and left the Arctic air up in the Arctic.