Saturday, February 11, 2006

Moving Waves

First, we are still in the process of working on a weather-climate discussion for the PSD MJO web page. A draft has been written, and we hope to post the final version next week. The link is:

La-Nina continues, with subnormal SSTs along the central and eastern equatorial Pacific extending to depths of at least 250m. Recent 5-day averaged surface anomalies are ~minus 1.5C near 160W, with actual SSTs near 25C. The coolest anomalies have been moving westward along the equatorial cold tongue. Above average tropical Pacific SSTs are confined near 10S/160E, with actual temperatures ~30C. We are approaching the time of year when, for the cold tongue region, actual SSTs are warmest, climatologically March and April. This may impact the anomalies in the coming weeks.

For SST information, please see:

Please see the latest CPC ENSO advisory for additional information on the current cold event:

A weak to moderate MJO was in progress from about mid January into the first few days of this month. This event initiated north of Australia, and resulted in very intense convection along the SPCZ by late January. Although much weaker, the signal has moved into the western hemisphere (WH). This can be observed on satellite imagery by noting the enhanced thunderstorm activity across northern South America (Amazon Rainforest and southeast Brasil) and South Africa. The intensity of the SPCZ convection has weakened, with the most intense core back to ~0/140E. During the next couple of weeks a probable scenario would be for the convection with the MJO signal to consolidate with the area north of Australia. The result may be an intense region of forcing from ~100-140E similar to the first half of January, and this would be consistent with La-Nina.

The extratropics have also continued to do their own thing with the wave 0-2 (to keep things simple) retrogressive transient that has been going on (likely linked to a recent major sudden stratospheric warming). With the most significant lobe of the polar vortex displaced toward east Asia, a zonal wave number 3 pattern of troughs and ridges has evolved. Troughs are present across Eastern Asia and North America, and Europe. This has lead to a strong positive phase of the PNA (for now).

As the tropical forcing intensifies around Indonesia during the next couple of weeks, I would think the final stages of this retrogression should "lock in". Consistent with La-Nina in a composite sense, and just as the models show, a central Pacific ridge and western North American trough would be expected. This would project onto a negative phase of the PNA. While blocking across Alaska and cold regime for particularly the western and central USA seems likely per models including the CDC ensemble, amplitude details are uncertain especially for the western North American trough.

There are also other sources of uncertainty from, for instance, the future course of the above average zonal mean westerly flow across the equatorial and subtropical atmospheres (anomalies ~5-10 m/s) and the easterly flow on the poleward flanks. I would expect this westerly flow to at least locally propagate into the midlatitudes via subtropical jets (STJs, which are already present), and be replaced by deep zonal mean easterly thoughout the tropics and subtropics during the next few weeks. This would decrease the globally averaged AAM, also consistent with La-Nina.

In terms of the SDM, we are currently going into Stage 4, with a transition to Stage 1 most probable by the end of week 2 into week 3. For the PNA sector, this would translate to a northward shifted storm track with time.

When making week1-3 predictions of any kind, the above is only a small subset of the considerations attention must be paid. These are just a few of the sources of sensitivities for numerical models, and is why you will see run-run inconsistencies, model differences, etc., especially in a regime such as this.

I think much of the CONUS is in for a relatively cold regime week 1 (variations understood), perhaps shifting more into the Pacific Northwest, northern and central Rockies and Plains week 2. There already is evidence of cross-polar flow of Arctic air from Asia, and this will likely come into the western USA this week. During week 2-3 it is possible the southeastern states will warm to above normal temperatures.

With the trough developing across the western USA, an active pattern is probable for much of the country for the next 2-3 weeks. The focus may be the Plains. However, we will need to watch how far along or off the west coast one or 2 synoptic-scale troughs may initially dig. I do think there may be 1-2 cold/wet episodes for the west coast during roughly the next 10 days. That could include snow for locations such as Seattle. Afterwards, mainly the Pacific Northwest may be impacted. For the Plains, while significant winter weather occurs across the north, locations such as the Ohio Valley may see a lot of rain/thunderstorms (with icing in-between).

Now comes southwest Kansas. Even with all the above, I am not at all optimistic about significant precipitation for at least the next 2-3 weeks. The heaviest will easily be to our north and east. In this type of pattern, low pressure systems are going to tend to be progressive, with our location favored for the dry slots. The notion of shallow Arctic cold air with a few hundredths liquid (perhaps) of freezing drizzle/snow has some merit by the end of this upcoming week into the weekend. However, I cannot rule out a STJ system bringing some precipitation anytime. I hope the atmosphere surprises me, and I am wrong about our precipitation chances. After a warm start, temperatures should lower to below normal the end of the week.

Ed Berry

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