Edward Berry, NWS and Klaus Weickmann, CDC

At the time of our last update (3 November 2004) a MJO was moving east over the Indian Ocean with its active convection  located at ~80E.  At the same time new convection was developing over the equatorial date line region where SSTs are ~1-1.5C above normal.  Around 6-7 November, the MJO weakened substantially, and the convection farther east over the Pacific intensified as part of an equatorial Rossby mode.  The latter shifted westward toward the west Pacific and weakened producing some tropical cyclone activity there.  Strong subtropical westerly flow developed in association with the Pacific Ocean convection, and contributed to a split flow pattern across North America and most of the Northern Hemisphere.  During the last week, convection has once again intensified across the Indian Ocean and become suppressed west of the date line.

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 (below) shows Hovmoller plots of outgoing longwave radiation anomalies (OLRA) for the equatorial (bottom) and northern (top) tropics. The solid dots highlight convective flareups near the date line and the circled pluses over the Indian Ocean. The contours isolate three coherent OLR modes, including the MJO, the Kelvin wave and the equatorial Rossby wave. Both regions show westward propagation of negative OLR anomalies starting near the date line around 3 November. Meanwhile the convection with the MJO weakened abruptly in both regions as strong suppression of convection developed over Indonesia.  The MJO contours on Fig. 1 show a weak signal and the (Wheeler Phase Space Diagram) has a very weak MJO signal.  The suppressed convection over the Indonesian region (~100-120E)moved in from the west with a MJO and has added to a persistent signal of suppressed convection centered ~120E for the last 2-3 months. This is more probable during a El Nino.

At the present time (21 November 2004),  convection has re-intensified over the Indian Ocean. and continues sporadically over the Pacific Ocean, especially near 160W.  The latest Pacific satellite imagery in Fig. 2 (from 11/20/2004) shows convection coming eastward over western Indonesia suggesting Indian Ocean convection is moving east, possibly as a MJO.  Also, convection is seen near 160W, which may become a new equatorial Rossby wave.  The latter would follow on the heels of the wave whose residual moisture is still evident over the west Pacific and Phillipines in the Fig. 2 satellite image.  A more substantial consolidation of convection may occur as these two features interact.

For current satellite images see: Latest Indian Ocean Satellite Picture;  Latest Western Pacific Satellite Picture

Figure 1  latest images 

Figure 2

Figure 3 is a time-latitude plot of 200 mb zonal mean zonal wind anomalies.  The superimposed lines highlight a pattern of split flow that has been prevelant since early September 2004.  Some important events in the northern mid-latitude circulation are listed on the right.  The time of date line and Indian Ocean convective flareups are marked using the same symbols as on Fig. 1. In this short record, highly amplified wave patterns in mid-latitudes (30-60N) appear to accompany west Pacific convective flareups (the three solid dots). The first one was linked to a baroclinic wave packet that traversed the hemisphere while the last two were linked to anticyclonic (LC1 type) wave breaking events, one over the Pacific/North American region and other over the North American/Atlantic sector, respectively. A breakdown of the persistent split flow occurred during the second half of October 2004 when stronger mid-latitude westerly flow was linked to two intense troughs that affected California.  Recently, the split flow re-intensified.  From about 9-13 November the subtropical westerly flow  (~25N) was ~10m/s stronger than normal in the zonal mean.  At the same time  the mid-latitude (45N) westerly flow (~45N) was weaker than normal and westerlies were strong and moving south around 60N. 

There was a brief period during this time when the atmosphere was in Stage 3 of the Synoptic-Dyamic Model   (SDM), with an "El Nino" like base state over the Indo-Pacific region (link to discussion with SDM).  Since about 15 November, substantial weakening of the subtropical westerlies has occurred, with weak easterly wind  anomalies appearing north of the equator.

Figure 3

Figure 4 shows daily mean 150mb vector wind anomalies valid 11 and 17 November 2004.  The tropical convective forcing was just west of the date line on the 11th, and centered around 80E on the 17th.  On the 11th there is a signature of twin subtropical anticyclones around the date line with anomalous subtropical westerlies, particularly from Africa to the Americas, typical of Stage 3 of the SDM.  The large anticyclone over the North Atlantic is a remnant of the second wave breaking event noted above.  Wave energy from the event is moving east across Asia at this time.  By the 17th subtropical anticyclones have reformed across the Indian Ocean, tied to the divergence associated with the convection there.  The anticyclones are part of a wavetrain that has amplified across the subtropics of the eastern hemisphere.  Over higher latitudes, large scale zonal flow anomalies have developed, westerly at 60N and easterly at 80N.  The westerly anomalies over Asia are shifting south.  The winds two days later on 19 November show this as the high latitude wavetrain extending from Asia to the Pacific Ocean becomes dominant and the anomaly pattern over the North Pacific more closely resembles a response to the recent suppression of convection over the west Pacific (see Fig. 1). The patterns on 17 and 19 November are transitional ones, and have similarities to Stage 4 of the SDM. The interested reader can view an animation of daily mean 150mb vector wind anomalies  here .

Figure 4 latest image

2.   Predictive Insights Verification:

In the 3 November discussion, consolidation of convection around 140E was predicted due to interaction of a MJO with an equatorial Rossby wave.  Instead, the MJO weakened abruptly while the convection with the Rossby wave drifted west to about 140E.  The split flow pattern persisted and during 12-19 November much of the northern USA was warmer and dryer than normal and the southwest and southcentral states were cool and wet with several closed lows in a subtropical storm track.  This weather pattern for the lower 48 states was not predicted in the 3 November discussion.

Currently convection continues to intensify across the Indian Ocean basin centered ~ EQ, 80E, per latest 3 day averages of OLRA, with concentrated areas across eastern Africa and Indonesia, per satellite imagery from 11/21/04.  Latest animations of 150mb vector wind anomalies show that while a subtropical wavetrain of gyres has been moving east across both hemispheres, divergence has remained centered around EQ, 80E, consistent with the 3-day OLRA average.  As discussed above,  a separate weaker area has redeveloped near the date line (see small dot on Fig. 1).  All available MJO indices give a very weak signal at most, thus statistical and numerical predictions of a MJO cannot be considered useful at this time (see lastest MJO forecasts ). Synoptic indications are that a MJO is redeveloping over the Indian Ocean (perhaps centered at ~EQ, 80E); convection near the date line remains a wild card in its future evolution

Confidence in the upcoming week 1-3 outlook is low and uncertainty is high.  In addition to the tropical convective forcing, processes leading to wave amplification (e.g., mountains, tropical convection, etc.) across the mid and high latitudes of the northern hemisphere as shown in Fig. 3 will play a role in what happens for weeks 1-3. The seasonal cycle will also be settling into a wintertime basic state, probably somewhat abruptly during the next 3-6 weeks.  The circulation pattern on 19 November 2004 contains features of Stage 4 of the SDM and it is possible the atmosphere is currently transitioning to a Stage 1, favoring a ridge ~140W and a trough across the Rockies.  This would support a southwest to northeast storm track across the Plains/central USA, and suggest an increased risk of high impact weather for at least the next 1-2 weeks.  

Week 1 (22-28 November 2004):  Stage 1 of the SDM is the most probable forecast at this time.  Nearly all of the available operational numerical models including their ensembles are trending toward this solution (see links below).  An active southwest-northeast storm track across the central USA is also more probable.  Indeed, there is a robust signal that a strong baroclinic cyclone will develop across the Rockies and central USA starting early in the work week and affect that part of the country perhaps into Thanksgiving, as it moves northeast.  Therefore a major winter storm must be concern from the central/southern Rockies to the mid/upper Mississippi Valley and Great Lakes states (latter area may also get lake effect snow in the wake of the storm).  Strong to severe local storms/heavy rainfall will be possible across the southeast.  Toward the end of this period, another closed low may be digging into the southwest states.  Much of the Rockies and Plains should have colder (with "some" Arctic air) than normal temperatures, with above normal temperatures possible southeast states.
 
Week 2 (29 November-05 December 2004):  During this week convection may once again become rather intense around 150E, favoring the development of some version of a western Pacific wavetrain; that is, Stage 2 of the SDM, toward the end of this period.  At least one or two more potentially significant storm developments may occur across the Rockies and Plains after the first one during week 1.  This would mean continued cold and stormy for much of the Rockies and Plains, including an increased risk for severe winter weather.  Above normal temperatures would be expected across the east and southeast states.  Portions of the southcentral and southeastern USA may continue to experience above normal rainfall, with strong to locally severe thunderstorms.

Week 3 (06-12 December 2004):  The evolution to the western Pacific tropical convection is very uncertain.  In any event, this period may begin with a deep trough across the central part of the country (~90-100W), suggesting colder than normal temperatures for that area (including Arctic air), as indicated by Stage 2 of the SDM.

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