Wednesday, February 22, 2006

Eastern Hemisphere Tropics Rule

In concert with La-Nina, SSTAs across the equatorial central and eastern Pacific remain below average, with anomalies ~minus .5-1C (SSTs ~25C) , extending to depths as low as 200m where anomalies around minus 2-3C are observed. The horshoe of anomalous warmth continues from the IO into the subtropics of the north and south Pacific, with magnitudes up to +1.5C. The warmest SSTs extend from the SIO into the South Pacific, with readings from 29-31C. It has been over these very warm waters where deep moist tropical convection has rapidly increased during the past week.

Possibly through interactions with the extratropics, there is evidence from diagnostic tools that a weak signal of the MJO has been propagating through the WH during the past 2-3 weeks (which was possibly moderate in the EH during the latter part of January), only now to re-emerge into the IO. Animations of satellite imagery clearly show the robust eruption of thunderstorm clusters over the SIO during the past week, with OLR anomalies as low as minus 90 w/m**2. Hovmollers employing a time-filtering technique to isolate coherent modes of tropical convective variability are now showing a weak projection onto the MJO. Monitoring for the next few days should give more insight into whether or not there is a decent signal of the MJO developing.

There are also intense thunderstorm clusters across the warm pool region extending southeastward along the SPCZ. Joining these 2 regions is the area of convection across Indonesia, making for a large area of forcing. How this region of enhanced precipitation will evolve during the next 1-3 weeks is unclear; however, there are some statistical and numerical tools that suggest this area to become centered ~120E by around the end of week 2. Some of these can be viewed at

Experience would also support that prediction.

The circulation has responded accordingly. Zonal mean upper tropospheric anomalous westerly flow is propagating into the subtropical atmosphere and being replaced by easterlies across the equatorial regions. As upper tropospheric divergence increases across the IO, twin subtropical anticyclones are developing in that region, joining with the ones already present across Africa and the Atlantic Ocean. Meanwhile, twin anomalous subtropical cyclones remain near the date line. The latter are assisting with ventilating the westerly flow out of the tropics into the subtropics, leading to the formation of STJs. One of these STJs is currently racing across the southern part of the CONUS. In short, the atmosphere appears to be transitioning from SDM Stage 4 to Stage 1. The current slow decrease of global relative tropospheric AAM from ~1 standard deviation above normal (based on a 1968-1997 climatology) to near normal adds some additional support to that notion.

As nearly all the models are now showing, for the PNA sector, initially there will be trough development just off the North American west coast (details understood) by early next week. With the moist STJ in place, substantial precipitation is a possibility again anywhere from California to Washington and eastward to the Divide late this upcoming weekend through most of next week. Downstream across the CONUS will be the Rockies ridge and eastern trough. Thus colder than normal air is likely for at least the eastern half of the country by this weekend, but not as cold as a week ago.

Another feature that is important for the weather across North America for the next 1-3 weeks is another high latitude ridge retrogression from Scandinavia into at least Canada. This kind of behavior is not unusual as we go into spring. My thought is that during the next 7-14 days the initial trough which evolves off the west coast may dig southeastward into the southern Rockies, only to eject northeast into the Plains. Models such as the GFS and Canadian ensembles give support to this option. Other troughs may follow.

The point is that the storm track should be depressed southward across the CONUS with blocking across Canada, and any ridge development over the southeast states may link up with it (as well as any central Pacific ridge). That would allow a larger amplitude flow across the country. The possibility for STJ moisture to spead northeast across the central portions would also exist, in addition to low-level moisture transport from the Gulf of Mexico and lower latitudes. So, with synoptic-scale storm development, many regions of the central and southern Plains that have been experiencing drought may get some welcome precipitation say ~March 4-10. Going along with the idea of baroclinic development, one or two significant winter storms would be a concern from the eastern Rockies into the upper Mississippi Valley with severe local storms across the south central states into perhaps the Ohio Valley.

For southwest Kansas, still dry for at least the next 7-10 days. I would like to be somewhat favorable for precipitation by the period of ~March 4-10. However, the bulk of it is likely to be to our east and north. Nevertheless, if the "more meriodional High Plains southerly flow with ejecting troughs saga" discussed above actually pans out, there is some hope for respectable preciptation. Temperatures during the next couple of weeks may average out to near normal due to rapid changes.

Ed Berry

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