The past couple months have been warm and dry in San Diego. Given the overall state of the atmosphere and Pacific Ocean, I think this trend will continue through the winter of 2012-2013. Average rainfall during meteorological winter (December, January and February) is 5.78". I'm expecting 2 to 5". The average high temperature for December, January and February is 64.7, 65.1 and 65.0 respectively. I'm expecting average highs to hover 1 to 2 degrees above these values. It is important to remember that these are averages - there will still be cold rainy days this winter.
The areas to watch this year will be the Pacific Northwest for flooding rains, and the northern Ohio River Valley into New England for potentially heavy snow events. In addition, I'm expecting drought conditions to continue/worsen for the High Plains and parts of the northern Midwest. The Southeast could also see increasing drought conditions.
For my snow-loving friends in Richmond, there should be several chances for a big snow this winter. For this reason, I'm going for above-average total accumulations for December through February (9 to 14").
I will start off talking about El Nino, or lack thereof. It was exciting when there was the potential for an El Nino this winter (remember 2009-2010 for the East Coast when AO/NAO was also negative?). However, things have changed and we are in neutral conditions.
You can see the sea surface temperature (SST) anomalies on the map below. There is nothing to be found along the equatorial Pacific Ocean.
In additon, when you look at the animation below (SST anomalies), there is no apparent trend.
A weak El Nino could still develop later this winter, but there are equal chances for neutral conditions to persist or even for a weak La Nina to develop. Regardless, it will not be a major impact on my winter outlook.
For the record, the effects of El Nino are within the maps below.
And now La Nina.
The Pacific Decadal Oscillation (PDO) has been in the cool (or negative) phase for the past few years. This means that the waters along the U.S. West Coast and Gulf of Alaska are below normal, while the central Pacific waters are above normal.
A cool PDO can enhance the effects of La Nina, or decrease the effects of El Nino. The opposite is true for a warm PDO, which enhances El Nino and diminishes La Nina. This is accomplished through an atmospheric bridge (shown below).
I've included two great examples of the ENSO/PDO connection below: November 1988 (La Nina) and November 1997 (El Nino).
The effects of the PDO are described below. (http://www.atmos.washington.edu/~mantua/REPORTS/PDO/PDO_egec.htm)
|Table 1: summary of Pacific and North American climate anomalies associated with extreme phases of the PDO.|
|Ocean surface temperatures in the northeastern and tropical Pacific|
|October-March northwestern North American air temperatures|
|October-March Southeastern US air temperatures|
|October-March southern US/Northern Mexico precipitation|
|October-March Northwestern North America and Great Lakes precipitation|
|Northwestern North American spring time snow pack and water year (October-September) stream flow|
|Winter and spring time flood risk in the Pacific Northwest|
This is going to be important for the West Coast and possibly the southeast U.S. outlook, which I will address later.
The Arctic Oscillation (AO) is an index which is used to monitor the polar vortex over the Arctic Circle. This pattern has a major impact on temperatures along the eastern half of the United States. It also has a minor impact on precipitation. This is summarized on the maps below (+AO on top, neutral in the middle, -AO on the bottom).
We have seen large swings in AO from September until now (shown on the first graph below), which is a change from the past two winters. In addition, most computer forecasts keep this trend generally going for the near future (all graphs below).
As I talked about in my previous winter outlook...there has been interesting research about early-season Siberian snow cover affecting the polar vortex and bringing cold and snowy conditions to the East Coast (a negative AO scenario).
This year features above average snowfall in these regions (map below), which could enhance the negative AO.
The North Atlantic Oscillation (NAO) is somewhat related to the AO. This index is highly dependent on the strength and placement of the Icelandic low and Azores high. This website does a good job explaining NAO:
Also, here is a link to the NAO index since 1950:
If you want snow along the East Coast, then you want positive height anomalies in western Greenland and eastern Canada (west-based negative NAO). This will decrease the westerly flow of the jet stream and allow cold air to plunge into the eastern half of the United States.
|(WEST-BASED NEGATIVE NAO)|
|(TEMPERATURES ASSOCIATED WITH WEST-BASED NEGATIVE NAO)|
Unlike the past two years, the September-November NAO has generally hovered in negative territory.
ECMWF ensemble forecast shows this persisting into the near future.
Remember, if you are on the East Coast and want snow, you want a west-based NAO. In other words, you want the blocking ridge to settle in western Greenland or eastern Canada. The recent computer forecasts show this a possibility - a pattern that could linger into this winter.
Averages temperatures across the entire state have mostly remained above normal for the past 30 to 60 days.
This has also occurred across most of the Western States.
However, the opposite is true for the eastern half of the country.
I think a lot of this can be attributed to AO. Because there have been large swings in the AO, the eastern U.S. has seen cold periods followed by mild periods. So the signal isn't too strong, but it exists.
It's feast or famine when it comes to precipitation in California. NorCal has seen an impressive amount of rain, especially in the past few weeks. SoCal has seen almost nothing.
The rainy pattern continues into the Pacific Northwest and Northern Rockies. This could be attributed to the -PDO along with the fluctuating AO. Even though there are neutral conditions, this map looks a lot like a La Nina pattern.
The rest of the country has been hurting for some precipitation for the past 30 days. When AO/NAO were negative, the storm track allowed for some impressive systems to develop. You can spot the path of precip extending from Texas through Mississippi and Ohio River Valleys and into New England. I expecting that trend to keep cycling as the AO/NAO fluctuates this winter.
You can also spot the overall pattern looking at the 500mb height anomalies for the last three months.
It's sometimes scary making a winter outlook when ENSO is neutral. Things aren't as clear cut. Or are they? Maybe the forecast is more straight foreward when you remove a variable. I'm taking this approach. I'm not expecting any major shifts in the current pattern, though my gut feeling tells me that a weak La Nina could develop by the end of winter and into early spring. This isn't based on any scientific analysis...just a nagging fear.
I'm forecasting temperatures to reflect a -AO/NAO winter. However, I'm blending a -AO/NAO, -PDO and persistence to forecast precipitation.