Wednesday, March 01, 2006

The Lion Sleeps No More

La Nina continues with SSTAs along the equatorial cold tongue varying from ~minus.5-2C, with the coolest near 120W having a temperature of around 24C. Below normal SSTs extend down to at least 200m east of the date line, with anomalies as low as -4C. From the west Pacific to the South Indian Ocean SSTs remain at least .5C above normal, with readings of at least 29c-30c, also extending down to depths of about 200m. Linked to this east to west gradient of SSTs, the trade winds have again increased during the past week with anomalies of at least 2-4 m/s.

As mentioned in the past couple of these postings, the most significant tropical convective forcing has returned into the eastern hemisphere. About a week ago a consolidation of this forcing involving a convectively coupled Rossby mode and weak MJO resulted in a flare-up near 90E along and just south of the equator. Since then a rather large area of tropical convection has been moving east fairly quickly, even projecting onto a Kelvin wave per coherent modes Hovmollers (see ). Severe Hurricane Carina developed in the South Indian Ocean after the consolidation.

Per full-disk satellite imagery the above mentioned tropical thunderstorm activity covers most of Indonesia, centered around 130E. Back over the warm SSTs of the South IO, another large area of intense thunderstorm clusters have developed during the past couple of days and extend from South Africa toward Madagascar and on east. Three-day averages of OLRA of at least minus 70 w/m**2 have been observed with this activity. Finally, relatively intense convection still persists along the SPCZ due to the warm date line SSTs (see From looking a plots of OLRA, a horshoe pattern of negative anomalies can be seen fanning from the deep convection into the extratropics of both hemispheres, including the connection into Hawaii and the USA west coast.

SDM Stage 1 does capture many behaviors of the current weather-climate situation, which is typical for a cold event. Twin anomalous upper tropospheric subtropical anticyclones are becoming more robust from Africa into western Indonesia, while northward displaced twin cyclones cover the west Pacific. In fact, linked to the above mentioned tropical convective flare-up, a Rossby wave energy dispersion currently arcs from the twin cyclones across the Pacific rim linking to the trough along the North American west coast. This response does project onto the negative phase of the PNA teleconnection. Finally, split upper tropospheric flow is present across both northern hemispheric ocean basins, which includes a retrograding blocking anticyclone across the North Atlantic.

My thought is we should see the tropical convective forcing become stationary and perhaps intensify in the region of Indonesia (120-140E) for at least the next couple of weeks, given the current distribution of SSTs due to La Nina. MJO variability is likely to be weak. Flare-ups that do occur back to the west may shift east as Kelvin waves and re-enforce the stationary pattern. I would also expect the SPCZ particularly from the date line to the region of Polynesian Islands to be active from time to time. Thus it is probable the SDM Stage 1 pattern will continue during this period (into the middle of this month). With the tropical forcing residing a little farther east during the next couple of weeks than has been observed recently, I would expect troughs to dig more into the western USA than the east Pacific by week 2 (~March 8).

Now come those always unclear particularly for week 2 synoptic details for the extratropical PNA sector. With all the above and in what follows, we need to remember that the "effects" from seasonal transition into spring are also a consideration. In a probable sense, retrogression of the blocking ridge into Canada is likely by the weekend, and may link-up with the central Pacific ridge during week 2. With most models now having many of the characteristics our base state in their initial conditions, the notion portrayed of at least 1-2 anomalously deep mobile troughs digging into the western CONUS then lifting northeast into the Plains I think is a confident prediction. The storm track is likely to be depressed southward, and a moist STJ, at times, may interact with it (linked to the SPCZ).

The trough currently digging into the east Pacific should push onshore and into the Plains by the weekend, and then affect the east coast early next week. Models have a good handle on the details of that. Starting about a week from now and continuing through at least week 2, the Rockies and Plains states may be in for a very active period (the "March lion"). Initially, the Pacific Northwest states would have a concern for heavy precipitation as storms dig toward the central and southern Rockies. With possible CO/NM cyclogenesis, synoptic systems would then lift northeast into the upper Mississippi Valley/Great Lakes. Implications include heavy rain/severe local storms from the south central into the Ohio Valley states, and winter storm conditions (with possible thunderstorms) from the central/eastern Rockies into the Upper Mississippi Valley. Below normal temperatures would be expected for much of the west and northwest CONUS while the deep south may have summer-like warm and humid conditions.

For southwest Kansas, I still have to be VERY careful about raising hopes for substantial precipitation. The heaviest precipitation will clearly be to our east and north. However, I think we have the best opportunity for at least some measurable precipitation during the next couple of weeks than has been the case for at least the last 4-6 weeks. Opportunities exist for some light rainfall through about Saturday and perhaps early next week (I really hope I am being too conservative). The probability for a "wet closed low" with STJ interaction to take a favorable track just to our south is not zero for week 2. Perhaps some snowfall may result. On average, temperatures should be about normal into the middle of this month, with likely a lot of day to day variation.

Ed Berry

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