Saturday, May 27, 2006

Catching Up With the Atmosphere

Repeating from the May 17th posting, our latest weather-climate discussion for the ESRL/PSD MJO international web site has been posted dated May 17, 2006. Please see view it.

Also, the following is a link to our recently accepted paper by MWR which discusses the GSDM.

I have been on a lot of travel since my last posting on May 17th, and am catching up on the current situation. The following is briefly what I have learned so far.

SSTs across most of the central and east equatorial Pacific are slightly above average, with anomalies ~ plus .5C to even 1.0C around the date line. SSTs range from ~25C near the South American coast to around 31C just south of the equator at 160E (recall that we use SSTs of 29C as a threshold for supporting persistent tropical convection). SSTs of 29C and higher also extend from the southwest Pacific into the IO. At depth, as was true about 2 weeks ago, anomalies around plus 1-2C extend from 50-250m east-west along the equatorial cold tongue, meaning a slightly deeper than normal oceanic thermocline. SSTAs from the IO into the west Pacific are at least plus .5-1C, with the South Pacific horseshoe experiencing ~ plus 1-2C. The Caribbean into much of the NTA also has SSTAs ~ plus 1-2C. Latest prediction from CPC expects ENSO-neutral conditions to prevail during the next 3-6 months (see latest TAO data here, ESRL/PSD data here ).

The following are links to ENSO discussions.

The signal from the MJO remains weak. However, tropical convective forcing has returned into the EH. This behavior was expected 2 weeks ago, along with the current response of the global circulation. Satellite imagery (eastern hemisphere, full-disk west Pacific, mtsat, IO, Africa; other imagery here ) indicates attempted organization with the most robust region of enhanced rainfall extending from the Arabian Sea into western Indonesia, about 5-10 deg north of the equator. The latter reflects the northward shift due to seasonal transition, including the onset of the Indian and South Asia monsoons. A secondary area of tropical thunderstorm enhancement was centered ~0/150E in the region of the very warm west Pacific Ocean SSTs. Finally, some enhanced activity remains from the east Pacific ITCZ into at least northern South America, linked to a convectively coupled Kw. The first named tropical cyclone (Aletta) for the 2006 East Pacific Hurricane season has formed just south of Mexico (see If interested, the latest 3-day averages of OLR total and anomalies, and other data can be found here.

Statistical and numerical models of the MJO (see ESRL/PSD MJO tools , BMRC MJO tools, CPC MJO tools) are inconclusive given the weak MJO signal. My own suspicion would be for some consolidation of the 2 areas of enhancement discussed above during the next week or so, with some eastward shift afterwards. The seasonal cycle including the Indian and Asian monsoon systems will play a role to whatever evolutionary route the tropical convective forcing takes during the next few weeks.

Since ~ May 14th the interplay between SSTs, tropical forcing and circulation response-feedbacks have lead to a substantial change in the global circulation. For instance, zonal mean upper tropospheric anomalous westerly flow has propagated off the equator (mainly into the Southern Hemisphere (SH)) and is being replaced by easterlies. Anomalous zonal mean easterly flow persists across the subtropics and mid latitudes of both hemispheres. Plots of recent 150mb and 250mb daily mean vector wind anomalies clearly present a signal of twin subtropical anticyclones centered ~ 40 deg. east longitude, where twin cyclones were a couple of weeks ago. These IO twin anticyclones are linked to a rapid wave energy dispersion that is approaching the USA west coast. Finally, also hooked up with the EH tropical convection and baroclinic wave energy dispersion, a large anomalous anticyclonic gyre located around Alaska is breaking anticyclonically helping to flux AAM out of the subtropics.

AAM anomalies are now ~ minus 1 standard deviation below the 1968-1997 climatology, with anomalous zonal mean easterly flow across the subtropics of both hemispheres. AAM tendency is just above a negative minimum (~ 3 standard deviations below the reanalysis climatology; see plot ), with much of that due to the SH frictional torque and north-south mountain ranges including recently the Andes and those across the tropics (see plot for mountain torque and plot for the frictional torque; see for all AAM plots). GSDM Stage 1 best describes the global circulation at this time.

As mentioned above, a baroclinic wave packet tied to the EH tropical convection is dispersing across the North Pacific Ocean basin as I type. As most models now show, the most probable response appears to be amplification and progression of the current PNA wave train week 1. During week 2 (well into June), uncertainty becomes higher than has been the case recently for forecasts for ~ weeks 2-3. Sources include the future of the tropical convective forcing not only from the EH, but flare-ups from other basins such as the North Atlantic and western Pacific. Also adding uncertainty are the roughly 20-day variations of the global mountain torque, especially from the Andes Mountains this time of year. My own thought is to key on the tropical forcing from the IO and Indonesia, suggesting GSDM Stage 1 to be most probable during week 2. During week 3 GSDM Stage 2 may evolve. The following outlooks have low confidence for mainly weeks 2-3.

Week 1 (28 May – 3 June 2006): GSDM Stage 1 is most probable, with transient synoptic variations. Initially, the western part of the country will be cooler than normal and unsettled while the central and eastern states have well above normal temperatures (possible excessive heat problems). Severe local storms are a good bet for especially the Central and Northern Plains and Upper Mississippi Valley. As the trough-ridge pattern across the USA moves east, while the western states warm the rest of the country should gradually cool down. The severe local storm hazard is probable to shift into parts of the Southern Plains into at least the Ohio Valley.

Week 2 (4-10 June 2006): GSDM Stage 1 continuing would be most probable. At the start, much of the Intermountain west and Central Plains may see well above average warmth while the rest of the country has near or somewhat cooler than normal temperatures. A situation similar to the present may return for days 10-14 (7-10 June). The wettest areas of the country may include the Pacific Northwest-Northern Plains/Upper Mississippi Valley on into the mid Atlantic and Northeast States.

Week 3 (11-17 June 2006): Given the above average west Pacific SSTs, robust tropical convection may emerge into that region, allowing a transition to GSDM Stage 2. Like was observed during especially the summer of 2004, that situation tends to favor cool/wet across the central part of the country with warmth/dryness along the west coast and southeastern USA (please see

for example).

Specifically for southwest Kansas, decent opportunities of rainfall exist through at least the first half of week 1. Overnights Monday night and Tuesday night may feature good MCS activity and locally heavy rainfall (along with the usual severe local storm hazard). By late next week into next weekend, dryer weather should return. Much cooler temperatures are probable by the middle of next week, with maximums perhaps remaining in the 70s for at least a day, only to warm quickly by next weekend. I suspect next weekend will be quite warm and dry, possibly turning windy and hot (with maxes above 100F) again for much of week 2. By that time the westerlies with their troughs are likely to pass well north of us and the Southern Plains may be under a subtropical ridge. Of course, what STJ there is may initiate relatively isolated high based storms along the dryline and higher terrain in general. Let's see if GSDM Stage 2 returns week 3, as discussed above. That circulation pattern is favorable for rainfall across southwest Kansas during the summer.

Latest CDC Ensemble Forecast

Latest NCEP Ensemble Forecast

Additional NCEP Ensemble Output

Latest Canadian Ensemble Output

Latest Deterministic ECMWF Forecasts

Please see the CPC Drought Monitor for areas of dryness and the latest official outlooks and statements from Storm Prediction Center not only for severe storms, but also fire weather concerns. Finally, the CPC USA Hazards Assessment for offers additional insights not only for possible week 1 high impact weather, but week 2 as well.

I will try to do an update around Wednesday, 5/31.
Ed Berry

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