Thursday, December 4, 2014


An Active Pacific

Since this weekend, areas in the Sierra and around Tahoe received up to 10" of precipitation, with most areas receiving 1-5" (Figure 1). Some of the precipitation did fall as snow with snow levels varying between 5000' and 8500' during both storms. Resorts received anywhere from 14-33" of new snow in the past week (with the bulk of it accumulating on the upper mountain portion of the resorts).

Figure 1: 5-day precipitation totals from the CNRFC. 

Respectable precipitation amounts made it over the Sierra and into the Reno area. Spillover precipitation does not always occur and the rain shadow of the Sierra usually causes more precipitation to occur along and west of the crest than east of the Sierra. However, on Tuesday when the conditions satisfied much of the criteria necessary for spillover, areas east of the Sierra crest received almost as much precipitation than along and west of the crest. The aforementioned criteria for spillover (thanks Dr. Kaplan):
  • An upper level jet (positioned over or just to the south of Tahoe)
  • Low level jet (southerly)
  • Atmospheric River 
  • Mid level jet (SW-W)
  • Convective/frontal lifting
  • Orographic lift
All of these conditions were met to a certain extent, but the strong southerly flow and deep moisture helped the precipitation to spillover. When we have a quiet week (hopefully not soon), we will dedicate a full post to the spillover precipitation phenomenon. 

Figure 2: Snow level and 1-hour precipitation valid Friday evening at 10 pm via CEFA-CANSAC.

We do have another storm that will arrive Friday afternoon and continue overnight into Saturday. The storm track is to our north, which is where the coldest air and heaviest precipitation will be too. However, any precipitation, especially snow, is welcomed. Another storm will arrive Sunday and into Monday. It is slightly weaker than the Friday/Saturday storm.
  • 2-6" of snow will fall Friday and into Saturday and another 1-3" Sunday into Monday
  • Snow level will be between 7000 and 8000 ft. for both storms
The Pacific appears to stay active for the next week. The Madden-Julian Oscillation (MJO) is in Phase 5 and is forecast to go to phase 6 before weakening as move towards phases 7 and 8 (Figure 3). MJO phases 5-8 tend to favor the western United States for moisture transport and atmospheric wave activity. Another strong cross North Pacific upper level jet is forecast to form next week, which also equates to a more active weather pattern for the West Coast. 

Figure 3: MJO phase diagram. The green solid line and yellow thin lines represent the model outcomes for the MJO. If the line is outside the inner circle, MJO is active enough to impact weather. The phases for the MJO indicate locations and to a certain extent impacts of the MJO. Image via CPC. 

We will have another update this weekend about the upcoming storms and examining the MJO and El Nino-Southern Oscillation moving forward. 


  1. well I'm enjoying all these storms, any chance we get some cold ones soon?

    1. Hey sorry Alex, missed this comment. Looks like this week's storm will be colder.