Friday, April 27, 2007

Storms to return Rockies and Plains by Week-2

Global tropical SSTs remain above average with exceptions of coolness around Indonesia and more importantly the equatorial east Pacific. Magnitudes are ~0.5-1.5 C. Subsurface anomalies as low as minus 4.0C exist down to about 100m at 140W per TAO buoy array data with cool anomalies down to about 200m at the equatorial date line. Total SSTs are generally near or above 30C across the Indian Ocean and from the west central-South Pacific. It is still unclear whether or not a La-Nina is developing since May is a seasonally critical month.

Tropical convective forcing is getting better organized ~90-120E. Monitoring tools including the Wheeler phase space plot suggest a MJO may be developing in that region. The latter has been discussed as a possibility in recent posts. Other clusters of intense tropical thunderstorm activity remain across the South Pacific and equatorial Africa. A convectively coupled Kelvin wave, possibly forced by the extratropics a week ago, is supporting the activity across Africa. This forcing may contribute to additional enhanced rainfall across the equatorial Indian Ocean into Indonesia week 1. Whether or not a coherent eastward shift of tropical convection occurs across the Eastern Hemisphere afterwards remains to be seen. However, those with interests in that region should be concerned about excessive rainfall and possibly Bay of Bengal tropical cyclone development for at least weeks 1-2.

GSDM Stage 0.5 still best describes the current-weather climate situation. An evolution to ~ Stage 1.5 may occur during the next few weeks particularly if a MJO develops. Zonal mean easterly wind flow anomalies of ~5m/s loosely exist across the equatorial and subtropical atmospheres with magnitudes as high as 10m/s around 25N. Global relative AAM tendency is still weakly negative while actual AAM is a good 1 standard deviation below the R1 climatology as of April 25th (~1.5 sigma including the stratosphere presumably due to the easterly QBO phase). There are also indications of flux convergence of AAM ~40-50N in the presence of a developing sink across the northern subtropics. In fact, this flux convergence of AAM transport has been shifting northward during the last couple of weeks, probably related to the seasonal cycle.

Much of the USA (the CONUS has generally been my focus in these postings) has been through a very active period this past week. After a brief respite, this situation is probable to return. Particularly with the week-2 ensemble means from various operational centers, there is a lot of model spread in regard to the locations of troughs, ridges, possible so-called omega blocking north of the Great Lakes, etc., etc., etc.. These are the situations where an understanding of the weather-climate situation from rigorous daily monitoring utilizing tools such as the GSDM is needed to make an intelligent evaluation of all other numerical, statistical, etc., tools to formulate week 1-2 (week 1-4?) predictions. I view this procedure as a complete forecast process to provide the best possible information to sophisticated users.

Probablistically, more deep troughs and closed lows in the Rockies are likely by week 2 (pattern shifting slowly north). These lows should eject northeast into the mid and upper Mississippi Valley having the kind of impacts (warm sector severe thunderstorms and snowstorms across the Rockies) already observed for the past several weeks. Heavy rainfall and strong storms may start across the Southern Plains by early next week with the remnants of a subtropical jet. By week 2 the Rockies and Plains should experience exceptionally active May weather. I do think a blocking GSDM Stage 1 USA pattern is a possibility for week 2. Other ramifications may include excessive warmth across the Deep South with cool conditions across the Northeast. This situation should slowly shift east week 3.


An experimental phase space plot of the GSDM utilizing normalized relative AAM tendency (Y-axis) and normalized relative AAM (X-axis) can be found at

These are probabilistic statements, which we will try to quantify in future posts. The WB (2007) paper on the GSDM has been published in the February issue of MWR. I will try to write an update sometime next week while on travel.

Ed Berry

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