Thursday, March 23, 2006

I am Back

SSTs remain cooler than normal across the equatorial cold tongue with anomalies as low as ~minus 1.0C and actual temperatures ~25C. Anomalies of minus 2-3C continue to extend to depths of ~200m for the equatorial Pacific east of the date line, with comparable warmth to the west. The warm pool region remains at or slighly above normal, while positive SSTAs (weekly means) ~1-2C prevail across the subtropical regions of the South Pacific and South Indian Ocean basins (actual SSTs at least 29-30C). This spatial distribution of SSTs still projects onto a La Nina (aka a cold event). The links below provide additional information for global SSTs.

The MJO signal continues to be very weak. However, since late January, there has been roughly a 20-30 day variation of the location of the eastern hemisphere (EH) tropical convective forcing. Specifically, we have seen at least 2 episodes when the forcing oscillated between eastern Indonesia/western Pacific Ocean and the Indian Ocean regions. Time filtered hovmoller plots suggest, in addition to overall weak MJO signals, both eastward propagating convectively coupled Kelvin waves and westward moving Rossby modes have contributed to this observed behavior. The link below is an example of one of these plots.

Additionally, monitoring tools such as animations of upper tropospheric daily mean vector wind anomalies offer some evidence that this 20-30 day tropical convective variability has been linked to extratropical baroclinic wave activity including possible jet streak induced vertical circulations. This shifting around of the tropical convective forcing has feedback into the extratropics including the PNA sector. For instance, Rossby wave energy dispersions (RWDs) linked to the convection have not only allowed anomalously deep and intense troughs to slam the USA west coast and subsequently impact the Plains with heavy precipitation and severe weather, but has also contributed to high impact weather for Hawaii.

During the past 1-2 weeks we have also observed anomalous anticyclonic wind gyres across the central north Pacific tied to the RWDs (linked to the EH tropical convection) expand into the Arctic. Zonal mean upper tropospheric easterly wind anomalies in excess of 10m/s have developed north of 60 deg. It is possible that these easterlies may contribute to the final warming of the stratosphere, which is part of the boreal spring seasonal cycle. Indicies such as the AO and NAO have become negative as a result. Also, anomalous zonal mean northern hemispheric westerly flow has shifted southward (as a response) during the past couple of weeks, from roughly 50-30N. For the CONUS this has meant a southward shift of the storm track.

SDM Stage 1 best describes the current weather-climate situation. From both the operational and reanalysis data (and their climatologies), on about March 17th tropospheric global relative atmospheric angular momentum (AAM) dipped to more than 3 standard deviations below the 1979-1998 climatology (see link below). This is the lowest value since mid January, and perhaps for even at least the past year. Large contributions to these negative anomalies are from the deep tropospheric zonal mean easterlies from both the tropical and subtropical atmospheres, as well as the Arctic (with some symmetry with the southern hemisphere).

Per full-disk satellite imagery (see for example) strong tropical convection is present near the equator from ~70-100E. OLRAs are ~minus 70-90 W/m**2. This flare-up started about 5-7 days ago, and recently intensified during the past 2-3 days, as part of the tropical convective variability discussed above. I think many models will have difficulty on their predictions of especially PNA extratropics for particularly week 2 due to this convective flare-up. Secondary clusters of tropical thunderstorm activity continue over the southwest and south central Pacific Ocean due to warm SSTs. Anomalous lower tropospheric westerly flow from this convection has bred at least two tropical cyclones currently in the region Australia, as well as the recent severe cyclone Larry which impacted the northeast coast of that continent earlier this week.

SDM Stage 1 is most probable for at least the next 2 weeks. While synoptic and submonthly variations will continue (along with the circulation and SST responses due to seasonal transition), I think the tropical convective forcing and subsequent atmospheric response will remain generally stationary. This does include the anomalous northern high latitude easterlies and subsequent positive geopotential height anomalies. For the PNA sector we should continue to see the same general pattern of east Asian trough/central Pacific ridge/east Pacific into western USA trough, latter interacting with STJs due to the South Pacific convection. Because the high latitude blocking, the flow pattern across the CONUS may be split meaning a southward displaced storm track. This storm track may extend from the southwest states into the central portion of the country and then continue into the mid-Atlantic region. I think much of the USA, particularly the western 2/3rds, may become quite active again by late week 1 through week 2 (starting ~ March 29-30th).

The weather impacts to the country may include continued increased chances for much needed precipitation across the southwest USA extending into the southern and central Plains particularly week 2. I like the solutions being offered by most models of taking the system for this upcoming weekend farther north from the Pacific Northwest into the northern Plains. However, I differ with the idea of a ridge "setting up and persisting"across the western states during late week 1 and week 2 per solutions of several models (and other interpretive forecasts). One reason I feel this way is because of the above mentioned tropical convective flare-up. In reality, I think by later next week a series of more troughs may slam the west coast only to move east-northeast into the central part of the country. This would suggest more heavy precipitation for much of the western states, with severe local storms and heavy precipitation possible from the southern Plains into the Ohio Valley. Locations such as the Front Range into the Mid/Upper Mississippi Valley may still see wintery precipitation along with intense elevated thunderstorm activity. While cooler than normal for the east and opposite to that for the southwest looks good for week 1, that pattern may start to reverse week 2 (weekly mean).

For southwest Kansas, thankfully some widespread decent precipitation did occur with the last storm system. And yes, I do deserve to eat some crow (or a dead run-over road killed bull) for being persistently pessimistic. However, I do not like our precipitation chances through at least the middle of next week. We may get some "nickle and dime stuff" late this weekend and early next week. I do think there may be one or two opportunities for more beneficial precipitation roughly next weekend into the following week due to the above discussed possibly southerly displaced storm track. This may include both severe storms and snow.

Ed Berry

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