Saturday, March 11, 2006

Same Circulation Different Day

No significant weather-climate changes since my last two postings (March 4 and8). SSTs remain ~ minus .5-2.0C below average from aboout 160E-120W along the equator and extend to depths of at least 200m. Comparable positive anomalies exist from the IO into the subtropical Pacific horshoe. This is all consistent with La-Nina, and below are links for additional details.

The MJO signal is virtually gone (see links below). Quasi-stationary tropical convective forcing remains quite intense centered on Indonesia ~0/140E with OLRA ~minus 50-70 w/m**2. The areal extent of these thunderstorms is from just east of the Phillipines into northern Australia. Convective flare-ups have also been occuring across South Africa and along the western portion of the SPCZ. Finally, there are weak signals that suggest convection over the IO may increase during the next few days.

The circulation is loosely responding as would be expected. A linear combination of SDM Stages 1-2 (see

for the most recent version) may be representative. Trades remain above average east of the Indonesian convection while weakly anomalous surface westerly flow is still present from the east IO to north of Australia. In the upper troposphere, while zonal mean anomalous westerly flow has been propagating poleward through the subtropics of both hemispheres, anomalous zonal mean easterlies have returned to the tropical atmosphere. These easterlies have lead to a rapid decrease of global relative AAM during the past week, as seen from the link below (derived from operational data using a 1979-1998 climatology).

This decrease of AAM, in particular the large negative tendency, is a behavior that is not quite consistent with our current situation. However, the feeling is that the atmosphere may shift back into SDM Stage 1 during the next 1-2 weeks. If interested, the link below will direct the reader to plots of the AAM budget (including the tendency) which utilize the NCEP/NCAR reanalysis data and a 1968-1997 climatology.

In general little change to the overall negative PNA circulation state is expected for at least the next 2 weeks (considering the seasonal cycle). Synoptic events of about every 3-5 days would be expected, with larger variations in amplitude perhaps every 10-20 days. On the latter, events such as Rossby wave energy dispersion linked to the above discussed tropical forcing (specifically the twin central Pacific subtropical cyclones, discussed in previous writings) lead to the onset of the current western CONUS trough about a week-10 days ago. A similar behavior is possible starting next weekend. Many numerical model ensemble prediction schemes are now starting to support this notion (which was not the case a few days ago).

In short, several more episodes of the same general weather that much of the country is currently getting is probable for at least the next weeks. That includes heavy precipitation for the west coast, particularly the Pacific Northwest, active winter and springtime weather on the Plains (thunderstorms and winter precipitation north/heavy rain and severe storms south) and possibly very heavy rainfall with severe storms for the Ohio and Tennessee Valleys. The coldest temperature anomalies may reside across the northern Plains and Upper Mississippi Valley with sumertime temperatures across the Deep South.

Still no respectable precipitation is in sight for southwest Kansas. In fact, while typing this Blog all I could think about was that we were having "thunderstorms in spirit" with the altostratus virga mamma cumulogenetis due to strong dynamic lifting but with little moisture. As discussed previously, we may see events like this until May. Strong surface winds leading to dust storm and fire danger hazards will be a concern until decent rounds of precipitation occur.

Ed Berry

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