Edward Berry, NWS and Klaus Weickmann, CDC

At the time of our last update (13 October 2004) a convective flare-up was occurring in equatorial regions around 160E.  Since then, suppressed convection has moved into this region from the west and a new region of enhanced convection has developed over the Indian Ocean. A large scale amplification of the circulation accompanied the convective flareup at 160E. It was followed by a retrogression of the wave pattern and two intense full-latitude troughs that impacted the weather in the western USA, especially California.  

For information  on the status of El Nino and the MJO see the following links:

Latest CPC ENSO Advisory

Latest CPC MJO Discussion

1.   Weather-Climate Overview

Figure 1 shows Hovmoller plots of outgoing longwave radiation anomalies (OLRA) for the northern (top) and equatorial (bottom) tropics. The contours isolate certain coherent OLR modes, including the MJO. Both regions agree on the representation of MJO #6 and #8 (MJO #1 is the strong event of December 2003). MJO #7 is more complicated since there are two distinct flareups over the Indian and west Pacific Oceans in equatorial regions but only one in the northern tropics. However, both regions show the most recent convective flareup over the west Pacific that preceded MJO #8 and coincided with amplification of the circulation. At this point, the contours indicate that MJO # 8 is the strongest event on the equator since last spring.  As of 1 November 2004, the core of the MJO is at 80E and has been moving east at ~ 3 deg of longitude per day. For an alternate MJO index, see Wheeler Phase Space Diagram.  For current satellite images see: Latest Indian Ocean Satellite Picture;  Latest Western Pacific Satellite Picture

Figure 1 latest images 

Figure 2 is a time series plot of the observed and 7 day forecasts (NCEP GFS ensemble) of the Pacific-North American teleconnection index (PNA).  The period of interest is from about 9-19 October 2004.  The circulation amplification referred to above was transient and projected briefly onto the positive phase of the PNA as seen in the figure. Enhanced convection centered around 150E associated with MJO #7 (see Fig. 1) contributed to the amplification.  However, the tropical forcing ended quickly and the amplified wave pattern was "sheared apart" in strong zonal mean anticyclonic shear present in mid-latitudes (not shown).  The resulting subtropical troughs and high latitude ridges over the North Pacific Ocean then projected onto the negative phase of the PNA - effectively a retrogression of the pattern westward.  Zonal mean shear also became cyclonic around 30N (not shown), partially as a consequence of the "shearing" process. Subsequent perturbations coming off of Asia amplified and now tilted northwest-southeast helping to reinforce a ridge east of the dateline and a downstream trough along the USA west coast. This wave pattern continued to project onto a negative PNA. Two such events gave rise to the strong California storms that occurred around 19-20 and 26-27 October 2004.  The 7-day forecast did not capture the timing of the transition from positive to negative PNA (see vertical line on Fig. 2).  An animation of daily 250 mb vector winds can be viewed here by the interested reader. The animation is for the last 30 days and is updated daily so the amplification that initiates the events described above can be seen until about 13 November 2004. 

Figure 3 latest image

Fig. 3 is a plot of daily mean 150mb vector wind anomalies valid 1 November 2004.  The latest trough to effect western North America is evident but it deepened farther east than those cited above. This was the result of a strong zonally-oriented jet streak that propagated across the Pacific rather than amplifying over the western North Pacific.  A wavetrain extends from an east Pacific ridge, across the Atlantic and into Saudi Arabia. The divergence from MJO #8 is evident over the Indian Ocean centered around 0/60E.  Anomalous westerlies across southern Asia and near Australia are tied to twin subtropical anticyclones near 110E, likely a response to the Indian Ocean convection.  The downstream troughs over the west Pacific are distorted by cross-equatorial flow into the Northern Hemisphere, which may be related to a recent increase of tropical convection near the  anomalously warm SSTs near the dateline.  The pattern shown in Fig. 3 suggests a transition from stage 4 to stage 1 of the Synoptic-Dynamic Model (SDM).  The SDM is shown at the end of a prior discussion (Link to discussion with SDM)

At the current time, convection has been increasing across the western Pacific region around 0/160E, especially in the last two days. This is well to the east of the emerging MJO (see Fig. 1 and satellite pictures) and projects onto an equatorial Rossby wave.  This second area of tropical convective forcing must be closely monitored as the MJO moves into that region.  Experience indicates it will merge with the MJO and lead to a consolidation and larger area of convective forcing over the western Pacific.  The additional area of forcing and the possible consolidation will be important for the weather across the lower 48 states, and adds an additional source of uncertainty for the prediction.
 
The latest MJO forecasts from two statistical models predict the convectively active core of the MJO to be at around 140E by the end of week 2.  This equates to a propagation speed of 3-4 deg long/day (~4 m/s). Based on the Synoptic-Dynamic Model (SDM), the circulation across the PNA sector is expected to be in stage 1 by the end of week 2.  This means a trough across the western USA and a ridge across southeast.  However, the circulation across the PNA sector may quickly evolve into stage 2 even before then should the tropical forcing from the western Pacific become dominant.  The operational ensemble predictions from NCEP, CDC and CMC lend some support to these ideas, although they likely are underestimating impacts from both the MJO and the west Pacific tropical convection. Confidence is low to moderate in the outlooks that follow. 

Week 1 (4-10 November 2004):  This week is expected to be a period when the atmosphere continues its transition from stage 4 to 1 of the SDM.  Superimposed upon that should be the progression of synoptic systems, leading to a general trough across eastern North America and a ridge along the west coast by this weekend.  A second synoptic system is expected to help shift the trough into the central portion of the country by the end of this period.  Much of the Upper Mississippi Valley, Great Lakes and Northeast states are expected to have colder than normal temperatures this weekend into early next week, as some Arctic air (which has been building up across the northern high latitudes for the past several weeks) circulates south and southeast.  Also, portions of the Northeast states may get an early season snowstorm this weekend should baroclinic development along with a cold air source be present in that part of the country.
 
Week 2 (11-17 November 2004):  This is the period when retrogression of the west coast ridge to around 140-150W longitude should occur, allowing a trough once again across western North America, and a return to the negative phase of the PNA.  This trough may have a source of cold air from the Arctic.  Thus the western and central states would be expected to become colder and wetter than normal (with the possible exception of along the west coast) while the east and southeast states warm to above normal temperatures.  Given the uncertainties mentioned above, there should be a concern of at least one opportunity of significant baroclinic cyclonic development across the central USA, which could give rise to heavy snowfall across the northern Rockies/Plains and strong to severe thunderstorms over the south central and southeast states.

Week 3 (18-24 November 2004):  The situation described for week 2 may persist into week 3.  However, during this period the MJO may lead to a significant enhancement of convection across the western Pacific area in the region (if it already has not) of the anomalously warm SSTs.  That may allow the atmosphere to evolve into stage 2 of the SDM, meaning anomalous cold for particularly the central part of the country.

Latest CDC Ensemble Forecasts

Latest NCEP Ensemble Forecasts

Latest Canadian Ensemble Output