Saturday, July 01, 2006

Westerly Flow is Increasing -- Useful Predictive Information???

The following is a link to our recently accepted paper by MWR which discusses the GSDM (Weickmann and Berry 2006).

From taking into consideration the interactions of 4 different subseasonal time scales, a sequence of maps depicting a coherent set of repeatable events has been derived for the Northern Hemisphere cold season from November-March. This set is broken up into 4 stages, referred to as GSDM (for Global Synoptic-Dynamic Model) Stages 1-4 in the text of my Blog. Figure 13 in our paper presents a schematic of the GSDM. Ideally it would be advantageous to post our weather-climate discussions (link at the bottom) with greater frequency to provide additional detail while having a more complete weather-climate record of attribution and prediction. In these discussions I adapt the GSDM for the warm season. Our list of work includes a seasonally adjusted rendition of the GSDM.

Overall, there is not a lot of additional information I can provide since my posting on June 28th. It is currently very difficult to latch onto any coherent evolving signal, tropical convective forcing or otherwise, that would allow anyone to make a week 1-3 prediction which would be statistically useful for all interests (not just for the USA). I make this statement even if there is good numerical model agreement. This is partly related to the seasonal cycle.

We are seeing the realities of making weather forecasts that need considerable scientific attention. Folks can look at models and offer lots of scenarios; but how useful and accurate is all that? Recognizing week 1-3 forecasts of opportunity, including extreme weather events, requires a solid detailed understanding of the dynamics of the given weather-climate situation (including daily monitoring) and the resources, support and commitment to do that. Okay, enough of my preaching! In what follows I will offer what I hope is helpful information.

SSTs have changed little during the past few days since my last posting. Please see global SST information from the latest TAO data here, ESRL/PSD data here, and CPC data

The following are links to ENSO discussions.

There are at least 2 regions of enhanced tropical convective forcing. Monsoon system enhanced rainfall continues generally from the Arabian Sea into Southeast Asia. This represents a north and eastward displacement from the equatorial Indian Ocean starting about 3 weeks ago, initiated by a convectively coupled Kelvin wave. Convection is also very active across the west central and northwest Pacific (including Tropical Storm Ewiniar ) from both SST boundary and dynamic forcing. Per coherent modes Hovmollers and other plots, enhanced convection appears to be stalling over the warm SSTs around 0-10N/150E (OLRAs ~ minus 50 W/m**2 and lower) while convectively coupled Kelvin waves propagate east along their merry way (~10-12 m/s). One Kelvin wave is already well into the Eastern Hemisphere enhancing convection along the east Pacific ITCZ while another is approaching the date line.

My own offering would be to “expect” the dynamically forced tropical convective signal to continue moving into the Western Hemisphere while SST boundary forced and monsoon system convection remains generally stationary across the Eastern Hemisphere. The dynamical signal may continue to be dominated by Kelvin waves. Perhaps by the end of week 2 we will see the spatial distribution of upper tropospheric velocity potential favoring rising (sinking) motion across the Western (Eastern) Hemisphere.

Empirical, statistical and numerical prediction tools are still inconclusive for useful information about the future evolution of the tropical convection. Please see ESRL/PSD MJO tools , BMRC MJO tools, CPC MJO tools, and for the details (and draw your own conclusions).

Zonal mean 200mb westerly wind anomalies of ~ 10-20 m/s have propagated off the equator to about 15N during the past couple of weeks. In fact, 200mb zonal mean westerly flow is above normal throughout the tropics and subtropical atmospheres of both hemispheres, linking with the extratropics of the Southern Hemisphere. Zonal mean anomalous easterly flow remains around 35-45N.

Starting around June 1st, there has been a nice evolution vertically averaged zonal mean anomalous easterly flow propagating poleward from the equator into the subtropics and midlatitudes, followed by the current westerlies. Global tropospheric relative AAM dipped to ~2.5 standard deviations below normal by mid June recovering to around normal as of 3 days ago based on the 1968-1997 reanalysis climatology (more said below). This behavior is the result of the tropical convective forcing as well as the mountain and frictional torques previously discussed (and would be expected from the GSDM – see below). The point is that about the only thing we can currently state is there has been a nice signal of tropical convective forcing and circulation response in the tropical and subtropical atmospheres. The future impacts particularly for the northern extratropics are unclear.

Animations of daily mean 150mb and 250mb vector wind anomalies show westerly flow to be quite extensive east of 150E throughout the equatorial and subtropical atmospheres, with anomalies generally 15-30m/s. The most impressive equatorial westerly anomalies extend from northern South America into extreme western Africa. With well above average trades across much of the North Tropical Atlantic, this has created a vertical shear profile that is hostile for tropical cyclogenesis (generally speaking). Rossby wave energy dispersions from both subtropical and northern Asia (with a contribution from the East Asian mountain torque) have led to the formation of a large cyclonic circulation anomaly across the North Pacific linked to the anomalous subtropical westerly flow. A ridge exists across western North America with a downstream eastern USA trough.

The operational data plot shows AAM has ~ 1.5 standard deviations below the 1979-1998 climatology (see However, per reanalysis data, the tendency is ~ plus 30 Hadleys (positive maximum, again) with large contributions from both the global mountain and frictional torques. The frictional torque is starting to decrease, which usually leads the mountain torque by roughly 6 days.

Within the framework of the GSDM, as discussed above we do have a signal generally in the subtropical atmospheres of both hemispheres. Given the zonal mean anomalous westerlies and the torques, we may be transitioning to a summertime version of GSDM Stage 3. Perhaps an extratropical response for the Asia-North American sector will be a northwest displacement of the split flow pattern expected during winter (our weather climate discussions document a boreal summertime Stage 2 response). The recent appearance of the anomalous low across the North Pacific would support that notion (see plot for mountain torque and plot for the frictional torque; see for all AAM budget plots, including tendency).

However, where we go from here is still about as uncertain as it gets (not all that “unusual” for boreal summer). We already know there are at least 2 regions of tropical convective forcing (discussed above), and other warm ocean basins such as the North Tropical Atlantic may also have an impact (and not just from the current Kelvin wave). In my last blog I offered the possibility of a transition to GSDM Stage 3, and that appears to be occurring. However, the added westerly flow can “go anywhere” in terms of where the troughs and ridges are going to be during a boreal summer situation. My outlooks will not change much from the last posting.

Week 1 (2-8July 2006): I think there will be more westerly flow coming into the western USA, suggesting a deamplification and downstream progression of the current western states ridge into the Central/Southern Plains and Southeast. Most models are now showing this solution, and I would agree. Some of the complexity discussed above is getting into the initial conditions. This should allow increased tropical moisture transport from the Gulf of Mexico into the central states during this period (due lee-side Rocky Mountain troughs). The East Coast ought to get a respite from the very heavy rainfall and the Southwest USA monsoon is likely to remain generally suppressed (there may be a few northward surges). Broadly, synoptically, we may observe split flow along or just off the North American West Coast (with a ridge in Alaska) and confluent flow across the eastern half of the country. Intense heat may start to build across portions of the Southern and Central Plains and spread east.

Active and possibly severe MCSs/Derechoes (along with areas of heavy rainfall) from the Northern and Central Plains into the Great Lakes and Ohio Valley will be a concern as moisture transport improves. Increasing upper tropospheric anomalous westerly flow across the Tropical North Atlantic may provide an unfavorable environment for tropical cyclogenesis in that region. Please see for the latest tropical cyclone information.

Week 2 (9-15 July 2006): This is the period when the dynamic tropical convective signal may become robust across the Western Hemisphere. While I can type many scenarios and anyone can take their pick (including from the models), it is too unclear to offer a predictive solution that is statistically useful.

Week 3 (16-22 July 2006): Same as week 2. However, should a dynamic tropical forcing signal (the Kelvin wave) propagate through the Western Hemisphere and begin to re-emerge into the Eastern Hemisphere like we observed during late May and early June, tropical cyclogenesis may be a concern for both the east Pacific and Tropical North Atlantic Oceans. By this time, at least for the subtropics, GSDM Stage 4 may be most probable.

The solution given by the models of a cold frontal passage, with at least one round of storms, roughly next Tuesday for Southwest Kansas looks good. Temperatures should cool to highs in the 80s. Later next week into week 2, at least seasonal temperatures. With more general westerly flow (strongest to our north) coming across the Rockies later next week and into week 2, a few diurnally driven rounds of thunderstorms would be expected. The rest I leave unclear.

The time -filtered coherent modes Hovmoller plots of OLR and OLRA are at, velocity potential Hovmollers at , and an animation of velocity potential overlayed on OLRAs are at

Satellite imagery and other information can be found from the following links: eastern hemisphere, full-disk west Pacific, mtsat, IO, Africa, ; other imagery here. Latest tropical cyclone statements can be found from, while the latest 3-day averages of OLR totals and anomalies and other data can be found here (animations of various fields from the operational data) (Global Tropical Hazards Assessment available from this site, along with other useful information)

Latest CDC Ensemble Forecast

Latest NCEP Ensemble Forecast

Additional NCEP Ensemble Output

Latest Canadian Ensemble Output

Latest Deterministic ECMWF Forecasts (link to our Weather-Climate discussions) (model performance measures; navigation will take you to more)

Please see the CPC Drought Monitor for areas of dryness and the latest official outlooks and statements from the 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 attempt another posting about the middle of next week.
Ed Berry

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