Seasonal Outlook for the 2012-13 Cold Season
Severe to extreme drought persists over eastern Utah and western Colorado, in spite of a robust monsoon season from early July into early September. A second dry winter could have devastating impacts on the region. Click here for the latest Drought Monitor.
Beyond the seven day forecast, long range forecasters look to the state of the oceans for clues to storm track and intensity. In spite of many advances in climate science, El Niño Southern Oscillation (ENSO) remains the most powerful tool for climate outlooks. After two previous winters in the cold phase, or La Niña, El Niño is developing for this winter.
Colorado is sandwiched between stronger ENSO climate signals to the north and south. Though Colorado’s precipitation is highly variable it has some cold season response to ENSO. El Niño tends to produce a wetter fall and spring that often produces a rain/snow mix or maybe just rain. The cold core of winter during El Niño seasons tend to be drier than normal. So El Niño winters tend to produce below average snowpack. An exception to this is in the San Juan and Front Range Mountains that have a tendency towards above normal snowpack.
One theory we have been exploring this season and last, does the previous season’s ENSO state still affect this season? Last winter we found that the second La Niña in a row tends to be drier than the first. For this season, does last year’s La Niña have some affect on this upcoming El Niño? Looking back through the climate record since 1950, there have been only five seasons where El Niño followed La Niña. Here they are:
These five seasons may add some further information to El Niño patterns for this upcoming winter. Notice these seasons are relatively rare and have not happened in recent years. First observation, four of these five seasons were dry overall, with the latest season 1976-1977 being exceptionally dry. The one wet season 1957-1958 gained most of its precipitation in the fall and spring. The three middle seasons are possibly more representative of the upcoming season with matching Pacific Decadal Oscillation
conditions (not discussed here).
Here is a graph of these data for six Colorado mountain valley sites:
The green line, the five selected seasons, shows a tendency for below average snowfall when compared to all El Nino years in red, and the 30 year average in dashed black.
This and other climate data brings an increased probability of:
1) a wet October and November favoring the south,
2) a dry January and February (with December on the cusp of dry or wet)
3) a wet March and April mainly north,
4) overall snowfall totals will tend to be below normal across the northern and central mountains, near to above normal snowfall for the San Juan and Front Range Mountains,
5) an extremely dry season is possible while a wet season is unlikely,
6) the regional drought will likely see little or no relief through the winter.
Now these are just tendencies. Our region regularly experiences large variations from these climate signals. And a changing climate regime leads to question if years past are a good model for future seasons. So there is always hope for a snowy winter. Let it snow!