Thursday, February 16, 2006

Baby Girl Wakes Up Old Man Winter

First, the URL for the recent weather-climate discussion Klaus and I posted (dated February 15,2006) is

All of our postings can be found at

In our recent report a review of the circulation anomalies and tropical convective forcing since late last fall was given. We also give an overview for about the past year documenting a transition from El-Nino to La-Nina. With all of that, perhaps I will be able to keep this Blog posting a bit shorter (no luck!).

Central and eastern Pacific SSTs continue about .5-1.0C below normal from ~160E-120W, and extend to depths of about 250-300m, with values as low as minus 3C. In contrast, SSTAs of +.5-1.0C continue over the west Pacific, with anomalies of ~ +2C at near 100m/160E along the equator. Actual SSTs along the equatorial cold tongue are as low as 25C while still in excess of 30C across portions of the west Pacific warm pool. The recent moderation of the cool equatorial SSTs, with even slightly warmer than average west of South America, may be related to the seasonal cycle and recent MJO activity. In any case, there continues to be an enhanced east-west gradient of SSTs maintaining the cold event conditions, including a resumption of enhanced surface easterlies from the tropical west Pacific into the South Indian Ocean (SIO).

The MJO is weak. Monitoring suggests a diffuse signal is currently re-emerging into the eastern hemisphere. Satellite imagery indicates that while a slow weakening of thunderstorm activity across the Amazon Rainforest and Brasil has been occurring, a slow increase has been in progress across the SIO, all during the past week (in an area that has seen very strong suppression for at least 2 weeks). Fairly warm SSTs across the SIO should continue that trend for more convection. Additional thunderstorm activity has also been on the increase across Indonesia (partly due to a westward shift from the SPCZ), and may be starting to interact with the convection farther west. Experience suggests the tropical forcing to become robust ~100-120E within the next 7-14 days, and there are statistical tools to support that notion. This pattern of east IO enhancement with central equatorial Pacific suppression would be consistent with La-Nina. Also consistent would periodic enhancement along the SPCZ, and above average SSTs in that region would support it.

Animations of northern extratropical circulation anomalies suggest the recent retrogression is linking up with the increasing eastern hemisphere tropical forcing. That is, northward displaced upper tropospheric twin anticyclones (with downstream twin cyclones) have phased nicely with a Rossby wave train across the north Pacific leading to our current negative phase of the PNA (western USA trough). In the context of the SDM, a transition from Stage 4-1 of the circulation may be in progress. Once established, SDM Stage 1 may persist at least into early March.

With the developing pattern of tropical convective forcing expected to persist for at least the next couple of weeks, so should the reverse PNA. Most models are in general agreement for this scenario; it is just a matter of monitoring amplitude variations. On going research work and synoptic experience suggests there may be zonal to meridional vacillations within this quasi-stationary pattern of a ~150W ridge and 120W trough. That could include anticyclonic wave breaking events of lows just off the Pacific Northwest coast, and possibly farther south.

In short, tropical forcing consistent with La-Nina has allowed an extratropical response across the PNA sector that would be expected from a cold event. So far the result has been a resurrection of winter for much of the north and west CONUS, including the coldest temperatures so far this winter. An active storm track across the central part of the country has also materialized. There should be at least one more storm along this track during the next 5-7 days, except a bit farther south than the current one (2/16), and the east Pacific STJ will likely contribute to it.

Should the convection become very intense in the region of Indonesia, an amplification of the central Pacific ridge may lead to an intense cyclonic storm over or just offshore from the Pacific Northwest by week 2. That system may then propagate downstream and maintain an active storm track from the Pacific Northwest into the Plains. While temperatures remain below normal across the north and west, above normal readings may occur across the southeast.

For southwest Kansas, I wish I could offer more optimism about precipitation. The second storm mentioned above gives some hope for light amounts on about Sunday. That may be in the form of freezing drizzle in addition to any light snow. After that, with the storm track staying to our north and systems remaining progressive, unfortunately little or no precipitation for at least the next 2 weeks. I hope this is wrong. Well below normal temperatures will continue through at least early next week, then moderate to at least about normal through week 2.

Ed Berry

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