Wednesday, March 29, 2006

Enhanced Seasonal Transition?

SDM Stage 1 persists (consistent with a La-Nina composite), with tropospheric relative AAM still ~2 standard deviations below the 1979-1998 climatology. The upper tropospheric pattern across the PNA sector continues to respond with an east Asian trough-central Pacific ridge (with a subtropical low to the south as part of split-flow configuration)-east Pacific/west coast USA trough. Tropical convective forcing remains strong centered ~5S/110E with weekly mean anomalies of at least minus 50-70 w/m**2. Please see the following links:

In the following, I will fill in some of the details of our current weather-climate situation. Afterwards, a few insights into the future (weeks 1-3) for primarily the weather across the CONUS is offerred.

Equatorial SSTs remain cooler than normal from ~160E to just west of the South American coast with magnitudes as low as ~minus 2C around 110W extending to depths of at least 200m. The warming along the immediate coast of South America may be related to the seasonal cycle. Comparable warmth is still observed just west of the date line and particularly over the subtropical South Pacific Ocean. Time-longitude plots of SSTAs do indicate some very slight eastward shifting of the negative anomalies across the equatorial cold tongue during the last 2-3 weeks. This may be linked to recent surface westerly wind anomalies from the IO/Indonesian tropical convection. Only time will tell if there is any importance to this behavior. Links below provide more detailed SST information.

As discussed in many of my previous postings and pointed out by others including CPC, this SST distribution does project onto a La-Nina composite (matches up spatially).

Tropical convection is currently concentrated ~5S/110E, and extends from the central IO across Indonesia, with weaker storm clusters east-southeast to near Polynesia. Tropical cyclone Glenda was located just northwest of Australia at the time of this writing (see, for details). As shown by the coherent modes Hovmoller plots to isolate tropical convective variability via time filtering (link below), after a consolidation about 7-10 days ago, some eastward movement has been occuring. There is a projection onto a weak MJO, and a back of the envelope phase speed computation gives me ~3-4 m/s. Whether or not it is a MJO, we will see. My own thought is we are seeing another 20-30 day mode that has dynamic linkages to the extratropics via subtropical wavetrains. One response to all this convection has been enhanced trade winds to its east and westerlies across the tropical IO and Indonesia, with anomalies ~5-10m/s contributing to above average low-level convergence (part of a positive feedback mechanism).

Recent full-disk satellite imagery has also revealed some flare-ups along both the North and South Pacific ITCZs (NITCZ and SITCZ, respectively). Interactions with the subtropics downstream from the twin upper tropospheric subtropical cyclones (discussed below) may be a contributor. The NITCZ has allowed a moist STJ to extend into Mexico and the Gulf of Mexico.

Finally, intense frontal and diurnal thunderstorm activity persists across northern South America and Brasil. This activity has been supporting STJs acros the Atlantic.

Animations of fields such as 150mb and 250mb daily mean vector wind anomalies (link given below) reveal the general SDM Stage 1 circulation pattern currently featuring twin subtropical anticyclones across mainly the region of Indonesia with downstream twin subtropical cyclones east of the date line (leading to split flow across the North Pacific -- ridge to the north of low). The northern member of these twin lows has been pounding Hawaii with excessive rainfall for at least the past 2-3 weeks, while also maintaining a moist STJ linked to South Pacific convection into the southwestern USA. Additionally, there have been synoptic events involving both baroclinic wave packets and fast RWDs, with "wave packet envelopes" circling the globe about once/every 8-10 days. Time-longitude plots reveal an essentially stationary zonal wave number 5-6 pattern across the subtropics (10-40N) with oscillations between waves 3-5 to the north (30-60N) since roughly mid February. Tied to all of this has been a general negative phase of the PNA teleconnection and "bursting" central Pacific anticyclones into the Arctic leading to projections onto the negative phases of both the AO and NAO (and possibly leading to the final warming of the northern stratosphere).

Time-latitude sections of upper tropospheric zonal mean zonal wind anomalies still present a general meridional symmetry (typical of strongly forced situations from the tropics). While zonal mean anomalous easterlies still persist north of 60, the magnitudes have weakened to ~minus 5 m/s. Anomalous westerly flow continues around 30N, while also increasing throughout equatorial and northern subtropical atmospheres. Animations suggest the latter is tied to the recent eastward movement of the tropical forcing (discussed above). Hence global relative AAM has been increasing for about the past week, from a low of ~minus 3.5 standard deviations to about minus 2 (1979-1998 climatology - see link below)

As seen from the plot, in terms of a vertical and zonal average much of the global increase has been from the removal of anomalous easterly flow throughout the northern subtropics.

Model performance has been improving during the last few days (see links below), after a recent drop linked to the IO tropical convective flare-up about 7-10 days ago. However, given the split-flow pattern across the North Pacific and the liklihood of more tropical convective bursts (in addition to seasonal transition), models may continue to struggle.

Where the atmosphere goes from here is always a good question. However, I think our SDM Stage 1 state is most probable for at least the next 10-14 days. Depending how far east the tropical convection moves, Stage 2 may be possible late week 2 into week 3. Most models (links to 2 of them below) are now capturing the signal for another strong baroclinic storm development initially impacting the USA west coast late this weekend, then moving into the Rockies leading to possibly intense baroclinic development on the Plains by ~middle of next week (days 7-8). That is consistent with the previous (botched) Blog posting on March 26th, and I would agree. An important but transient trough may also deepen across particulary the east and northeast states as well (large amplitude pattern) early-mid next week. For week 2 I think at least one more strong trough (relative to climatology) is probable for the western and central states. Should Stage 2 evolve, troughs may initially impact the Rockies late week 2 into week 3 instead of the USA west coast.

Active weather is likely to rule most of the country through week 2. As mentioned before, thanks to La-Nina, a climatologically active time of the year is being enhanced. The storm track may remain slightly south of normal, extending from the southwest states into the Great Lakes and Ohio Valley. Probable impacts include late season winter storm concerns from the Rockies into the perhaps the Upper Mississippi Valley, with severe local storms and excessive rainfall from the Central/Southern Plains into the Ohio Valley. Parts of the country that have been dry (eastern states, Southern Plains, etc.) should see at least some needed precipitation.

Same story remains for southwest Kansas. At least some opportunities of precipitation are currently presenting themselves. However, most will remain to the east, north and southeast due to the problem of dry intrusions, etc., with each passing low. Some precipitation looks reasonable for this upcoming weekend with perhaps more around the middle of next week. Opportunities of precipitation should continue to present themselves week 2. Temperatures will probably tilt toward above normal for the next 1-2 weeks.

I will try to do another update this weekend.
Ed Berry


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