Real-time Weather-Climate Discussion and Predictive Insights – 18 April 2005

Edward Berry, NWS and Klaus Weickmann, CDC

Since our last discussion dated 23 February 2005 tropical convective forcing shifted from the equatorial date line region to the Indian Ocean with a developing MJO (see estimate of MJO activity). During February 2005 tropical convection was strongly enhanced around the date line and suppressed over Indonesia, as part of an atmosphere-ocean coupling supported by relatively warm sea surface temperatures (SSTs; ~1-1.5C above normal) across the Nino 4 region. Once the coupling ended, tropical convection initiated across Africa and the Indian Ocean, and developed into a moderate to strong MJO by 20 March (click here for Wheeler phase plot). Part 1 describes briefly the current tropical forcing and the  recent circulation anomalies.  Predictive insights follow in Part 2.  For information on the status of El Nino and the MJO see the following links:

Latest CPC ENSO Advisory

Latest CPC MJO Discussion

Part 1.   Weather-Climate Overview

Figure 1 (below) shows Hovmoller plots of outgoing longwave radiation anomalies (OLRA) for a meridional band centered on the equator.  OLR anomalies are used as a proxy for deep tropical convection anomalies.  The contours isolate three coherent OLR modes, including the Madden Julian Oscillation (MJO), the Kelvin wave and the equatorial Rossby wave.  The red (blue) shading denotes (positive) negative OLR anomalies meaning enhanced (suppressed) convection. The numbers 1-3 highlight recent major changes in positive tropical convective forcing.  The horizontal lines give approximate dates for the start of the new forcing. Numbers 1 and 2 highlight the start of persistent tropical convective forcing from the west Pacific and the central Pacific, respectively.  Number 3 highlights the start of anomalous Indian Ocean convection associated with the recent MJO.  Since then, negative OLR anomalies have been systematically progressing eastward and are now concentrated over the western Pacific Ocean.  For the latest satellite imagery see: Latest Indian Ocean Satellite Picture;  Latest Western Pacific Satellite Picture ; Latest GOES West Satellite Picture

Figure 1.

Figure 2 (below) shows 150 mb vector wind anomalies for two 7-day averages (4-10 April and 11-17 April 2005), and a daily mean (17 April 2005). The first 7-day mean occurs when tropical convection with the MJO was moving east from the Indian Ocean and into the western Pacific. Prominent twin subtropical anticyclones are still present over the Indian Ocean while twin subtropical cyclones cover the subtropical east Pacific.  There is a hint of a residual wavetrain extending from convective outflow over the west Pacific to North America.  This pattern represents Stage 1 of the SDM (see Fig. 3), favoring troughs coming into the western USA, as has been the case for at least the past 3 weeks.  The second 7-day mean map shows a major change in the circulation as it interacts with the new region of tropical forcing over the west Pacific.  The Indian Ocean anticyclones have weakened and been replaced with near equatorial twin cyclones.  The east Pacific subtropical cyclones have shifted east and westerly anomalies now cover South America.  Southeasterly outflow from the west Pacific convection into the northern hemisphere has intensified.  

Figure 2.

The daily mean for 17 April (Fig. 2 above, panel 3) shows features similar to the 11-17 April mean.  Twin anticylones are now better established over the west-central Pacific and a wavetrain extends from them into both hemispheres.  The embedded westerly anomalies near 35 degrees latitude reflect an extending Pacific jet stream that is arelevant issue for the forecast, discussed in the next section.

2.   Predictive Insights

In the 23 February 2005 discussion, we suggested Stage 3 of the subseasonal Synoptic Dynamic Model (SDM; shown below in Fig. 3) would prevail for the first 2 weeks in March. For week 3, uncertainty in the evolution of the central Pacific atmosphere-ocean coupling led us to a persistence forecast with split flow across the USA and an active southern storm track.

Figure 3.

Our outlook was generally good for weeks 1 and 2. There was an active southern storm track and Arctic air did penetrate into the north central and northeast states. Once the central Pacific forcing ended toward the middle of March, existing circulation anomalies retrograded, and then weakened (including the North Atlantic Block and the ridge west of North America). The circulation then exhibited characteristics typical of Stage 4 of the SDM through much of the rest of March, which included an anomalously intense subtropical jet (STJ) and southern storm track into the southwest and southern states. This evolution toward Stage 4 was not predicted. Also not predicted was the development of a moderate to strong MJO by the mid to late part of March.

For about the past 2 weeks the circulation of the atmosphere has behaved as would be expected from Stage 1 of the SDM. However, as of 17 April 2005, animations of 150mb vector wind anomalies (link to latest 30-day animation) showed twin subtropical anticyclones around the date line, with twin subtropical cyclones centered across the Indian Ocean. Above average westerly flow has been increasing across the equatorial and subtropical latitudes. These behaviors indicate the atmosphere is quickly transitioning into Stage 3.

Currently, per satellite imagery, the MJO is centered at about 10S/180. It has been moving east at roughly 5-6 m/s (4-5 deg long/day) for the past 2 weeks. Convection is increasing along the SPCZ as well as along the North Pacific ITCZ and northern South America, indicating the upper tropospheric divergence with the MJO is already moving into the western Hemisphere.

Statistical and numerical models of the MJO generally have the convective signal moving into the western hemisphere for the start of week 2. There is suggestion for MJO convection to again develop across the IO during week 3 (see latest MJO forecasts ). The coherent modes Hovmollers are also generally supportive for these predictions of the MJO (see also additional MJO forecasts and tools). There is still a concern that another atmosphere-ocean coupling may occur given the still ~0.5-1.0C SSTAs (SSTs at least 29C) centered around 0/160E (see TAO / TRITON data display, visualization and animation for latest SST data).

Thus, while it is reasonable to go with the MJO time scale for a week 1-3 forecast, there is a fair amount of uncertainty. Furthermore, the split flow pattern that is expected to continue across the USA adds further uncertainty for a week 1-3 USA outlook. Hence confidence for the following outlook is about moderate.

Week 1 (19–25 April 2005): The atmosphere is expected to be in SDM Stage 3 (similar to an El Nino composite), meaning increasing split flow across North America including the USA. The trough currently coming into the country is expected to split, like so many of its predecessors. The southern portion should be the strongest. It is believed a closed cyclone will develop across the southwestern states, such as Arizona or Utah, for example, and then get kicked to the east by the upcoming weekend. This scenario would suggest an increased risk of severe local storms across much of the Central and Southern Plains/Mississippi Valley, with spring snowfall across the Rockies. The latter may include the Front Range of Colorado. By late this period, while the central part of the country dries out, much of the east and southeast states should get precipitation. While temperatures may be cooler than normal for much of the west, the central and southeast states should be seasonable or even above normal. The coolest temperatures may be in store for the northeastern part of the country. This scenario is generally supported by the operational ensemble output (see links below).

Week 2 (26 April-2 May 2005): The circulation is expected to remain in Stage 3, possibly transitioning to Stage 4 by the end of the period. Split flow across the USA should be most enhanced, perhaps best defined around 100W. Given the seasonal cycle and considering uncertainties, the northern stream ridge should reside from the northern Rockies into western Canada initially, than perhaps shift back into Alaska. Troughs should be present across the southwest states and along the east coast. One would expect the jet to extend across the North Pacific into the southwestern USA trough, possibly combining with a strengthening STJ, leaving its footprint most strongly on the west coast states of California and Oregon. An anomalously southern USA storm track would then be expected to impact the southwest states into the Central/Southern Plains, and then throughout much of the East. This would mean a probability shift toward late season above normal rainfall for locations such as California and Arizona. Additionally, snowfall would again be possible for the central and southern Rockies, including the Front Range. Severe local storms may be an on-going hazard for the Southern Plains and Deep South, with a possibility of a late season snowstorm for portions of the High Plains. Finally, as a continuation from week 1, dryness is still likely for much of the northwest and northern Rocky Mountain States. With the exception of the Pacific Northwest, most of the country may experience below normal temperatures, especially the East. Daily monitoring will be needed to pick up on synoptic events that could lead to high-impact weather.  

Week 3 (3-9 May 2005): SDM Stage 4 would be most probable. Hazards similar to the previous week would be expected; however, the storm track from week 2 may start shifting north. In the longer term, as the southern storm track shifts northwest with the seasonal cycle, the Pacific Northwest and Northern Rocky Mountain states should receive increasing and beneficial precipitation.

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