Sunday, April 02, 2006

Maybe Some Slight Sub-seaonal Changes

In my past postings I have provided links to SST information, plots of OLR and OLRA, AAM, NWP models, animations to various fields, etc. Please refer to them. The purpose of this writing is to update my March 29th posting, and I do need to keep this relatively brief.

There do appear to be some slight changes, at least on the subseasonal time scale, occurring as I type. These include:

1. The below normal SSTs ~minus 0.5-1.5C along the equatorial cold tongue region have been shifting east for at least the past 2-4 weeks. Additionally, actual SSTs of at least 28C have also propagated east to ~160W along the equator during this period. This behavior may be tied to the seasonal cycle, and also suggests our cold event may be starting to decay. The latter would be consistent with several dynamical and statistical tools which suggest ENSO may become at least neutral in ~ 6 months. However, that needs to be monitored, and where the tropical Pacific SSTs go from here is still unclear. For our purposes, warmer SSTs shifting east may also allow EH tropical convective forcing to also once again move into regions such as the west central Pacific;

2. There has been a consolidation (flare-up) of very intense (OLRA ~minus 70 w/m**2 and lower) tropical convective forcing ~10S/120-140E during the last 3-5 days. An eastward moving component from the IO has been a contributor, and a back of the envelope calculation gives a phase speed of about 4-5 m/s (~3-4 deg long/day). At this time the centroid is at about 140E, and may, in fact, contain at least a weak MJO component. Indices such as the Wheeler plot and time-filtered OLR/OLRA Hovmoller plots lend some support to that notion. In any case, to me what we are seeing is at least another 20-30 day mode of tropical convective variability, and this will impact the weather for the USA for at least both weeks 1 and 2;

3. SDM Stage 1 still rules, with global relative AAM ~2 standard deviations below the 1979-1996 climatology. Anomalous vertically averaged zonal mean easterly flow continues throughout the subtropical atmosphere, particularly south of the equator. Anomalous zonal mean easterly flow across the north polar latitudes has lead to a southward displaced storm track across the CONUS (and some split flow) with STJ interactions, contributing to the recent active weather pattern for the past 3 weeks; and,

4. Several monitoring aids including Hovmoller plots of upper tropospheric winds verify that our stationary SDM Stage 1 regime has persisted for at least the last 4-6 weeks. Again, tied to the tropical convective forcing currently moving east and also the seasonal cycle, this pattern looks to shift ~10 deg to the east by week 2. This matter was already discussed in my previous posting. Whether or not we SDM Stage 2 by late week 2 or 3 is unclear. However, at least for weeks 1-2, the recent pattern of troughs first digging into eastern Pacific and then getting kicked inland looks to change. Similar to what what observed about 3 weeks ago (~March 11-12, after an eastward shift of the IO/Indo convection at at that time), I think there exists a decent possibility of at least 1-2 strong troughs digging more into the western Rockies perhaps starting as early as next weekend. Indeed, models such as the NCEP ensemble and even the 1200 UTC 2 April ECMWF are starting to suggest this. Again, details are unclear and any predictive information beyond day 3 in a regime as complicated as this one must be expressed probabilistically. Consequences may include more strong baroclinic cyclogenesis across the Plains with all sorts of high-impact weather.

I think everyone knows the "weather consequences" of our active regime by now. For week 1, I like the trend of the recent model runs slowing down the eastward movement of our next western and central USA trough/baroclinic development. I can link this week 1 development directly to a current fast Rossby wave energy dispersion (RWD) due to the 140E tropical forcing. The models are playing catch up as the impacts from this RWD become better sampled into their initial conditions. Once this storm system emerges from the Rockies by about this coming Thursday, the ECMWF solution of a closed low moving to east across the central USA south of high pressure across Canada is reasonable.

As active as week 1 may be for much of the USA, week 2 may be even more so. Going along with climatology with some enhancement, storm season across much of the central part of the country may get down to some "real serious business". Initially the west coast would get the weather impacts, with progression through the Rockies and then emergence out into the Plains. The Ohio/Tennesse Valleys would also be affected. Places like the southeast states may have an early start to summer.

The beat goes on for southwest Kansas. Fortunately much needed precipitation has been occurring for about the past couple of weeks. Troughs landing initially in the east Pacific instead of the western states have helped, allowing an opportunity for some decent moisture transport this far west from the Gulf of Mexico (and tropics). I think there will be another chance roughly Wednesday night and Thursday. However, we will have to watch out for the dry intrusion/mixing down of the EML. Most substantial precipitation will continue to be to our east and north. Starting next weekend through the following weekend, our best opportunities for precipitation may be just ahead of the dry slots/dry lines since there should be rapid moisture returns. However, there may be a higher probability of one or two events featuring "southwest winds and blowing dust" similar to what we saw March 12th (understanding we now have better soil moisture). Warmer than normal temperatures are likely for at least the next couple of weeks, which should include at least a few days with highs well into the 80s and minimums in the 50s to lower 60s.

I will be on travel much of this upcoming week. I will try to do another posting about mid-week.

Ed Berry

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