Winter Precipitation Forecast for 2013-14 Season
Mammoth Mountain Weather Guy, MammothSnowman.com Senior Meteorologist
Released Thursday October 17th, 2013
The winter forecast favors near average snowfall for Mammoth Mountain Ski Area with a total snowfall forecast of 375 inches (average is ~350”) from November through April.
The forecast shows above normal snowfall to start the season in November with the second or third week of November the possible time frame for heavier snowfall.
The outlook for December is showing an average snowfall followed by slightly below normal snow in January 2014. The forecast also suggests that February will be the driest month with below average snowfall followed by a snowy and above normal March with over 100 inches of snowfall. The snowfall season should conclude with an average April 2014.
Overall, the season should start out snowy followed by dry spells in early December and then in again in late January and February.
The main difference between Winter 2013-14 and 2012-13 is the early winter won’t be as snowy, but the late winter and Spring should see significantly more snowfall than the record breaking dry mid-Winter and Spring of last season.
Monthly Forecast Totals:
November = 70” December = 60” January = 45” February = 25” March = 115” April = 40” November – April = 375”
Many factors were considered in the creation of this winter 2013-14 precipitation forecast. Generally, SST anomalies in the tropical Pacific or ENSO (El Nino Southern Oscillation) is the most important factor as it modulates the general atmospheric circulation pattern during the Northern Hemisphere boreal winter.
Other long term sea temperature based factors include the PDO (Pacific Decadal Oscillation) and AMO (Atlantic Multi-decadal Oscillation) while other indices like the PNA (Pacific North American pattern), AO (Arctic Oscillation), NAO (North Atlantic Oscillation), and others are associated with current and short term pressure patterns and have less influence over seasonal time frames.
The current global Sea Surface Temperature (SST) pattern below shows neutral ENSO conditions in the tropical Pacific along the negative or cool phase of the PDO and positive or warm phase of the AMO.
While there are precipitation correlations with the stronger incidences of ENSO (El Nino or La Nina), there is no strong correlation for California precipitation with the PDO, AMO or neutral ENSO conditions.
The latest Climate Prediction Center advisory on ESNO strongly favors (greater than a 60-70% chance) a continuation of ENSO neutral conditions through the 2013-14 Winter and into Spring 2014 which is based off the majority of both dynamical and statistical model forecasts (see image below), although most models predict a gradual warming in the tropical Pacific over the next six months.To put it bluntly, current techniques for seasonal forecasting (see image below) are still very unresolved and generally unreliable, but practice makes perfect, and that is why a forecast should be issued.
The most common method uses historical analogues under the assumption that it happened then under those particular conditions, so it could happen again if the initial conditions are similar.
Other techniques rely on seasonal trends or use very long range computer model simulations that often are not very accurate.
This winter forecast uses analogue years (1960, 1967, 1981, 1990, 2001) that featured a neutral winter within the ENSO 3.4 region (an area along the equator east of the dateline) and were also preceded by a neutral summer and previous winter (the exception being 2001). 2001 was included because the Pacific and Atlantic SST patterns were very similar to the current temperature pattern in those basins.
1981 was given the most weight since it matches current ENSO pattern the best and also had slow warming through the Winter and following Spring which is the expectation this year.
Overall though, this is a low confidence Winter forecast since ENSO is in a neutral state.
There are other sub-seasonal factors that will influence the winter season on smaller time scales that are not predictive in the longer term, but could have big impacts over short time frames and help shape the season as a whole.
One particular phenomenon associated with atmosphere teleconnections is high latitude blocking, which is associated with a weakening of the polar vortex in the arctic and is defined through the AO index and sometimes the NAO index as well.
When the AO index is in the positive phase, the polar circulation is strong with low pressure over the arctic region and not much meridional flow (north/south movement of the jet stream) that can bring storms southward into the mid-latitudes.
When the index is in the negative phase, the normal zonal flow is replaced by strong and persistent meridional flow due to an area of high pressure or blocking ridge over the arctic regions.
The normal eastward progression of storm systems is obstructed leading to episodes of prolonged extreme weather conditions in mid-latitude areas like Mammoth.
The most important factor associated with blocking episodes is where the block is established across the arctic region as that will determine where the downstream affects are most pronounced.
Last winter, there was a persistent block north and west of Iceland/Greenland that produced record snowfall and cold for Europe (downstream of the block) and a cold late winter for the Midwest (upstream of the block).
The previous winter, the blocking pattern was much farther westward and produced big snowfall for the West Coast as the block set up north of eastern Siberia and the Aleutians.
The NAO index can tell us if the block is setting up in the North Atlantic (wrong spot) and the WPO (western Pacific Oscillation) is related to the north Pacific (better spot).
Persistent and strong high latitude blocking doesn’t occur every winter and isn’t predictable beyond a 2-3 week time frame, so we will just have to wait and see if it occurs again this winter and hope it forms in the right spot.
Reasoning about why these blocks form isn’t well known, although there is anecdotal evidence that sunspot activity plays a role (I am suspicious) along with polar sea ice extent and sudden stratospheric warming events.
Sea surface temperatures related to the AMO may also have an influence on the block setting up in the North Atlantic (wrong spot).
The other major inter-seasonal phenomenon worth watching is the Madden Jullian Oscillation (MJO).
The MJO is characterized by an eastward progression of large regions of both enhanced and suppressed tropical rainfall, observed mainly over the Indian and Pacific Oceans on the time scales of 30-60 days.
It is usually first evident in the Indian Ocean with the enhanced/suppressed rainfall moving eastward into the Pacific (top panel of image below) which at times can affect the Northern Hemisphere jet stream, usually in the form of an extension of the east Asian jet stream (middle panel of image below) eastward across the Pacific into the West Coast or as injections of tropical water vapor into mid-latitudes.
episodes are usually sub-tropical and are often accompanied by an Atmospheric River (AR) which can produce extraordinary precipitation amounts over very short periods of time (bottom panel of image below). It is also sometimes known as a pineapple express.
MJO’s don’t create their own weather patterns for the mid-latitude regions like the blocking episodes, but if the atmosphere is already primed, then it can turn an ordinary precipitation pattern into a major event.
The MJO is an ongoing phenomenon, the question is whether it will affect the NH jet stream pattern at some point this winter, and then when/how it affects will be manifested. The lead time on the MJO with any decent confidence is about 2 weeks out.
The end result will be many short term storms that as a whole will define our 2013-14 winter season.
Snowfall amounts with these storms will be forecast out to 10 days with heads up about potential
patterns up to 3-4 weeks and will be part of the twice weekly powder forecasts.
First forecast is scheduled for Tuesday, November 7th.
Let’s hope this turns out to be a stellar powder year!