Tuesday, January 30, 2007

Right on track, but not El-Nino!

Tropical convective forcing has consolidated around 140-160E along the equator while only sporadic convection persists across the South Pacific. In addition, tropical forcing has also been increasing across northern South America and South Africa. The latter is a response to the remnant dynamical signal of the December-early January MJO interacting with the Southern Hemisphere (SH) extratropics.

A strong Rossby wave energy dispersion (RWD) linked to the west Pacific tropical forcing is exciting the positive phase of a western Pacific wave train as I type (with SH symmetry). This pattern resembles the positive phase of the Pacific-North American teleconnection (PNA) but shifted west by about 20 degrees of longitude. As has been expected for about 10 days, bitterly cold Arctic air from central Siberia is currently being transported across the North Pole and will plunge into the CONUS by the end of this work week. The initial surge of cold air should be into Montana, with the Northern Plains, Upper Mississippi Valley and Great Lakes states being impacted the most severely.

So, what is the most probable evolution of our weather-climate situation during the next few weeks, given uncertainty and inadequate model guidance? Specific diagnostic factors to consider include transports and tendencies of relative angular momentum (AAM), the evolving SST and SST anomalies, the subseasonal tropical convective forcing and the blocking becoming established at the northern polar latitudes (with a possible warming of the stratosphere).

Poleward propagation of zonal mean zonal wind anomalies continues, with moderate westerly anomalies throughout the subtropical atmospheres, easterly anomalies around 50N and westerlies again farther north. The global signal of relative angular momentum is quite impressive with positive anomalies of roughly 2 standard deviations as of January 27th based on the reanalysis climatology. However, as discussed in past postings, this westerly flow did not evolve into a strong combined North Pacific jet typical of a warm ENSO. Strong easterly anomalies in the subtropics preceded the convection increase near the date line and may have interferred. Only during the last few days has there been a weak reversal of AAM transports with westerly flow being fluxed equatorward from the midlatitudes.

Looking farther out, a renewed active phase of the MJO may develop from South Africa into the southwest Indian Ocean during roughly weeks 2-3. Coupling with the warm west Paciific SSTs looks probable centered ~10S/160E while at least diurnally intense convection occurs across much of Brasil. This would suggest a return to a GSDM Stage 4-1 (La-Nina like) response meaning zonal mean easterly flow anomalies should re-appear across the deep tropics while the tendency of relative AAM becomes negative. For PNA sector, the large ridge currently developing west of Canada may shift northwest to Kamchatka during week 2 while the above mentioned anomalous subtropical westerly flow ”undercuts” the east Pacific ridge. Going into the middle of February there may be a situation of a cold trough extending from central Canada to just off the USA west coast interacting with a moist subtropical jet.

As mentioned above, the cold regime is on track for most of the USA for week 1. While there is likely to remain a cold air source for especially the northern states weeks 2-3, moderation of temperatures is expected. Intense convective lake effect snow is a good bet for week 1, with only light snow events across the Northern Plains on east. Portions of the Deep South to the east coast will need to be monitored for possible significant wintery precipitation. This whole pattern should shift north and west weeks 2-3, with possibly an active southwest flow storm track across the Plains by the middle of February. Much of the USA west coast should also finally get some decent precipitation weeks 2-3.

Please note: These are probabilistic statements, which we will try to quantify in future posts.

Ed Berry and Klaus Weickmann

Friday, January 26, 2007

El Nino weakening, west Pacific convection not the answer

The following is a posting that was placed on the HMT Blog at


Please go to that link if you would like to see some figures.

An MJO excited deep convection near the date line in early January 2007 but a "combined" Pacific jet and eastward shifted storm track did not develop. A persistent pattern of subtropical easterly and mid-high latitude westerly flow anomalies may have intervened. Date line convection is now weakening; a portion is shifting southeast into the southern hemisphere and another is shifting west toward the west Pacific. Combined with eastward shifting convective activity over the Indian Ocean, a consolidation of positive convection anomalies around Indonesia has already occurred. This activity may eventually spread over the warmest SSTs currently in the west Pacific, south of the equator.

The current short term amplification and retrogression of the circulation anomalies over the Pacific Ocean will dominate the weather patterns during the next 1-2 weeks. Prospects do not look good for rain along the west coast during this time. The behavior of the circulation beyond week 2 is partially linked with the strength and location of tropical forcing. The Indian Ocean SSTs continue quite warm and we expect convection to redevelop there, possibly aided by a dynamical component from the recent MJO. Convection should also stay active over the west Pacific. If the west Pacific convection dominates (~GSDM Stage 2), a very boring weather pattern may be in the offing for the USA west coast beyond week 2. Increased activity over the Indian Ocean would lead to a better chance for a trough along the west coast and more favorable prospects for the American River Basin (ARB) (~GSDM Stage 4-1).

Please see our January 23rd posting for our weeks 1-3 outlook for the rest of the USA (CONUS). The cold regime scenario along with the precipitation concerns expressed in that discussion remain unchanged. Most models and their ensembles have come into good general agreement through at least days 7-10. We will try to do another posting early next week.

Klaus Weickmann and Ed Berry

Tuesday, January 23, 2007

No fooling around; possibly severe cold regime for a large portion of USA appearing more probable by week 2

Tropical convective forcing has been persistent across the South Pacific along the SPCZ for roughly a couple of weeks. This response was expected given the December 2006-early January 2007 MJO and warm SSTs (~1-2C anomalies) associated with ENSO. The global circulation has been responding with the recent large tendency in global relative atmospheric angular momentum (~30 Hadleys) and the appearance of twin upper tropospheric subtropical anticyclones around 160W. Rossby wave energy dispersion linked to these twin anticyclones (with inter-hemispheric symmetry) may be contributing to the onset of North Atlantic blocking (negative phase of the NAO). There has been poleward propagation of zonal mean westerly wind anomalies resulting in 5-10m/s anomalies at 200mb in the subtropical atmosphere. GSDM Stage 3 best describes the current global circulation pattern. However, as discussed in our January 19th posting, this El-Nino-juiced forcing has been evolving in a more persistent regime that developed just prior to December 1st. The regime has been characterized by transport of westerly momentum out of the tropics into the higher latitudes. The forcing from the South Pacific has not changed this La-Nina like (GSDM Stage 1-2) pattern.

Rossby wave energy dispersions within the regime have led to a revival of tropical convection across the East Indian Ocean into Indonesia. Fast baroclinic wave packets moving through South Asia are interacting with this forcing leading to storm development across the west and central North Pacific. A discontinuous retrogression and amplification of the North American ridge into the Arctic is expected within the next week. While a convective signal in the South Pacific is expected to persist (allowing a subtropical jet across the Deep South at times), tropical forcing may become quite robust near 120E by sometime week 2, in which case, the ridge may retrograde to 150W. The ESRL/PSD and other model ensembles lend support to this scenario. In fact, blocking may develop all across the polar latitudes as we go through February. Another MJO may also develop across the Eastern Hemisphere next month which could lead to a further demise of our warm ENSO event.

The screaming message for a good part of the country is a turn to much colder temperatures by about a week from now. Bitterly cold Arctic air that has been “bottled up” across much of Siberia is likely to plunge into the USA with a few surges, likely centered on the Northern and Central Plains/Upper Mississippi Valley. Strong winds and brutal wind chills will be probable. Best opportunities for significant wintery precipitation should be from the central-southern Rockies into the Tennessee Valley, Deep South into the mid Atlantic States. Lighter snowfall events may occur with the Arctic surges (along with intense convective lake effect snow). This whole precipitation pattern may shift northwest later weeks 2 and 3 (troughs across the Rockies/western states) while temperatures slowly moderate. By around week 3 the ridge should be far enough northwest to allow strong and moist westerly flow (including “undercutting”) to impact the west coast with significant precipitation especially California.

Please note: These are probabilistic statements, which we will try to quantify in future posts. The decay time scale for momentum transport anomalies is on the order of 1-2 days. Thus this is the time scale on which a reversal of the transports could occur

Ed Berry and Klaus Weickmann

Friday, January 19, 2007

El- Nino is trying, but too little too late?

The following is a version of a posting to the HMT forecast at


Figures to go along with the discussion are on that Blog.

The strong MJO that came out of the Indian Ocean in early January has weakened considerably. As expected, it excited convection near the date line and attempted to generate a strong combined Pacific Ocean jet stream. This MJO also helped force the cold regime over the USA that we discussed on 29 December 2006 (on the HMT conference call). However, two factors indicate our scenario of a strong combined jet stream over the Pacific for this winter is now less likely.

First, since about 1 December the atmospheric momentum transports have been moving momentum out of the subtropics and into mid-higher latitudes. This pattern has been so strong that the forcing produced by the MJO over the warm El Nino waters appears only as a small perturbation in a persistent flow regime. Regionally this zonal mean regime is characterized by split flow patterns, especially over the oceans. Convection is currently increasing over the Indian Ocean and Indonesia while a portion of the convection at the date line is moving westward at ~4 m/s as an equatorial Rossby wave. This combination suggests convection will become centered somewhat to the west of its current position during the next 2-3 weeks, possibly ~0/140E. Implications would be for a retrogression of the east Pacific-North American ridge (from week 1) to perhaps 140-150W (weeks 2-3) and a better shot at the subtropical jet undercut scenario favoring precipitation along the USA west coast, especially California.

The second factor is more relevant for the atmospheric circulation beyond week 3. The onset of the Southern Hemisphere monsoon (also associated with the strong MJO) has produced strong anomalous northerly flow across the equator. This appears to be linked with an amplification and deepening of the cold water (negative anomalies) below the equator in the Pacific Ocean. This cold signal may continue to deepen and eventually reach the surface putting an end to the basin wide aspects of this El Nino.

For the USA, after the storm system and surge of cold air this weekend into early next week, much of the western two-thirds should have a moderating temperature trend. There may be storm development close enough to the east coast that may lead to significant winter weather for inland locations. During weeks 2-3 (~late January-mid February), a return to a cold regime, similar to what has already been experienced for the last 7 days or so, appears probable. However, this time there may also be a source of Arctic air from Siberia (where temperatures are currently lower than minus 50C, but anomalies are still slightly positive) as well as Alaska, meaning possibly more severe cold than observed last week. Additionally, significant winter weather hazards would again be possible from the Rockies into the Plains with heavy rain and thunderstorms (possibly severe) across the Deep South. As discussed above, precipitation may also increase along the California coast especially week 3.

Please note: These are probabilistic statements, which we will try to quantify in future posts. The decay time scale for momentum transport anomalies is on the order of 1-2 days. Thus this is the time scale which a reversal of the transports could occur.

Klaus Weickmann and Ed Berry

Tuesday, January 16, 2007

The “Truth” lies with Understanding Reality

Tropical convective forcing continues to reorganize across the Eastern Hemisphere. The MJO signal has weakened significantly. The intense thunderstorm activity currently along the South Pacific Convergence Zone (SPCZ) was initiated by the downstream troughs associated with the MJO and not a coherent eastward propagating convectively coupled mode. In fact, the western portion of this tropical forcing is shifting west as a convectively coupled Rossby mode.

While there was a strong signal of the MJO during much of December into early this month, several complex forcing-response-feedback circulation variations occurred. To save space, what I hope is most relevant to the readers is discussed here (comments welcomed). First, there was a strong trade wind surge downstream of the MJO that led to significant cooling of the warm central Pacific SSTs around the date line (anomaly and actual SST tendencies ~negative 1C). In consideration of other monitoring tools such as relative atmospheric angular momentum (AAM) transports, my feeling is that the warm ENSO peaked during December and is now decaying. The recent anomalous low level westerly winds along the SPCZ (weekly mean anomalies ~15m/s centered on the date line around 10S) I feel is transient and not the beginning of a “coupled warm ENSO response” (which is different than what I would have thought about a month ago). Secondly, as was discussed in my posting dated December 15th, a regime transition to cold/wet pattern for much of the USA occurred starting about a week ago (more said below). In fact, a negative phase of the PNA teleconnection index evolved which is consistent with the GSDM Stage 1 response that occurred (and not with the composite warm ENSO signal).

Per ESRL/PSD reanalysis data plots, as of 3 days ago the global tendency of relative AAM was ~ plus 25 Hadleys. As the MJO shifted east, zonal mean westerly flow anomalies initially developed along the equator and then propagated poleward, contributing to this positive tendency. These westerlies are currently in the subtropical atmospheres of both hemispheres with anomalies ~5-10m/s at 200mb (contributing to the current subtropical jet across the southern USA). Hence a nice evolution from GSDM Stage 1-2 has occurred. However, unlike my thinking from about a month ago, it now appears unlikely that a coupled ocean-atmosphere response involving the tropical forcing east of the date line will occur, typical of a warm ENSO (leading to GSDM Stage 3). Instead, this process may happen ~10S/160E during the next couple of weeks and then shift to perhaps 120-140E during February. As I type tropical forcing is once again increasing across the Indian Ocean where SST tendencies during the past week were ~plus 1C, and twin upper tropospheric anticyclones are already appearing.

My outlooks for weeks 1-3 remain unchanged from our January 12th posting. It remains most probable for the trough-ridge-trough pattern from the west/central Pacific to the central USA to persist, with the usual synoptic variations of amplitude. The subtropical jet should continue to undercut the east Pacific ridge leading to split flow along/off the west coast and a cold/wet regime for much of the country particularly east of the Continental Divide. If my notions of the centroid tropical convective forcing shifting back to ~10S/120-140E during February are correct, the above circulation pattern would be expected to do the same allowing locations along the USA west coast to get precipitation especially California (loosely GSDM Stage 4-1).

We are planning on issuing another discussion this Friday, along with a parallel posting on the HMT Blog. The MWR WB paper on the GSDM is expected to appear in the February 2007 issuance.

Ed Berry

Friday, January 12, 2007

Reality Update

Tropical convective forcing remains stationary ~0/140E with development of a new region ~160W within 15 degrees of the equator. The latter is associated with downstream twin subtropical cyclones that were part of the response with the recent Madden-Julian Oscillation (MJO). Most diagnostic tools have shown a rapid weakening of the MJO signal during the last week. The remnants of the MJO combined with the pattern of tropical SST anomalies, including the warm ENSO signal, suggest organized tropical forcing during the next couple of weeks is most probable around 10S/160E while extending from the South Pacific back into Indonesia. Some shift slightly to the west may occur afterwards.

Zonal mean westerly flow aloft has been increasing throughout the tropical and subtropical atmospheres (within 15 degrees of the equator) during the past week. In fact, some of this westerly flow has already worked its way around through southern Asia and is enhancing the East Asian jet. This supports a GSDM Stage 1 weather-climate pattern more typical of La Nina conditions than El Nino. The current negative phase of the Pacific-North American teleconnection (PNA) is a charcateristic of this stage. Westerly flow should continue to increase throughout the subtropics. As shown by most models, the current trough-ridge-trough wave train across the PNA region should progress eastward leading to a ridge position just off the North American west coast for week-two (GSDM Stage 2). A strong subtropical jet is expected to “undercut” the ridge. During weeks 3-4 it is probable this pattern will shift westward and allow the subtropical jet to impact southern California. This scenario differs from an El Nino composite where a combined, extended jet across the North Pacific would be expected (GSDM Stage 3). However, this possibility cannot yet be ruled out for this winter/spring.

After the major winter storm of the next few days, much of the USA is in for a cold and dry regime, particularly the northern and central states during week 1. Locations in the Deep South will have freezing temperatures while Santa Ana conditions may develop across California. As the subtropical jet begins to undercut the ridge, another cold/wet storm system may impact locations from the Desert Southwest/Deep South into the Mississippi and Ohio Valleys and on to the northeast during week 2 (starting roughly next weekend). Afterwards, this storm track may shift northwest and locations such as the California coast may finally get some much needed precipitation going into February.

Please see http://www.esrl.noaa.gov/psd/programs/2007/hmt/forecast/
for images.

Ed Berry and Klaus Weickmann

Wednesday, January 10, 2007

Phantom El-Nino impacts, or are we dealing with reality?

Full disk satellite imagery and other monitoring tools suggest the recent eastward shift of the tropical convective forcing from the Indian Ocean into Indonesia has stalled. The centroid of the forcing is currently at ~0/140E while a faster component has excited convection downstream at around 10S, 180E, enhancing an eastward shifted South Pacific Convergence Zone. The overall convective pattern is losing its MJO characteristics. Previous thinking was for the MJO to intensify the convection in the region of the South Pacific leading to the “classic warm ENSO” global circulation response by about the middle of this month. We now believe this is not likely. Instead, coupling may occur west of the date line near 10S/160-170E while the Indian Ocean remains convectively active, roughly every 30 days. In fact, the South Indian Ocean may see intense rainfall in about 10-14 days since SSTs remain above average there. Convective forcing over the Indian and west Pacific Oceans may consolidate around Indonesia during weeks 3-4.

Zonal mean easterly wind anomalies have propagated into the subtropics and been replaced by equatorial westerly flow anomalies. The latter have been most robust across the East Pacific. Some of this anomalous westerly flow is coming back into the Eastern Hemisphere, and should lead to respectable intensification of the subtropical westerly jet during the next couple of weeks. However, considering the tropical forcing and other dynamical processes, this intensification does not appear to be enough to allow an extended combined North Pacific jet to reach the USA west coast. Instead, as most ensembles are showing and as is already happening, ridge amplification will occur across the north central Pacific into Alaska with a downstream western USA trough for week 1 and then shift slightly east week 2 (GSDM Stages 1-2). For weeks 3-4, the most probable scenario would a retrogression of the ridge-trough pattern while a subtropical jet “undercuts” the ridge and heads into California. This scenario is not consistent with El-Nino composites, possibly due to strong convective forcing from other regions such as the Indian Ocean.

The USA is in for a huge weather change. As Arctic initially plunges into western states, significant baroclinic development on the Plains is likely especially Sunday-Tuesday next week. The models are playing catch up. Impacts will include another blizzard from the Rockies into the Upper Mississippi Valley while heavy rain and severe thunderstorms occur across the Deep South and Ohio Valley. While week 2 should be generally cold and dry for much of the country, weeks 3-4 may once again become stormy with continued cold. Locations such as the California coast may not see decent chances for precipitation until after the 20 January.

We will try to do another posting this Friday. Please see past discussions for URLs.

Ed Berry and Klaus Weickmann

Friday, January 05, 2007

Update on our ENSO/MJO and Then Some Atmosphere

Because of my preparations to spend the first of 2 months at ESRL/PSD in support of the HMT project, I am unable to do a complete posting today. However, my discussion and predictive thoughts from last week (12/29/06) look very much on track. The tropical convection across the Eastern Hemisphere centered ~0/130E is becoming less coherent spatially. A convectively coupled Kelvin wave is leading to a flare-up over the warm SSTs near the date line and there is some evidence that the MJO component may be stalling ~140E. The trade wind surge east of this forcing has led to some cooling of the SSTs and I think our warm ENSO has peaked (which is consistent with seasonal cycle considerations). I have a thought going into weeks 2-3 there may be tropical convection “all over the place” with one region from the South Pacific-eastern Indonesia while South Africa-Indian Ocean and northern South America become active (along with the SACZ).

The East Asian cold air surge directly linked to the tropical forcing is occurring as I type. This will lead to the reverse PNA pattern by the end of next week (through downstream propagation of baroclinic wave energy and excitations of Rossby waves) with then a slight eastward shift by the end of week 2 (GSDM Stages 1-2). Most models show this. There are a few solutions that suggest this regime to breakdown quickly during week 2 (ex., the Canadian), and I think that is unlikely. Blocking structures across the north polar latitudes are a real possibility as these evolutions occur. In fact, as we go into weeks 3-4, I have a thought that as our combined extended anomalous jet becomes established across the North Pacific ~35N, there will be “undercutting” of blocking across Alaska. Given the multiple regions of tropical convective forcing which may be present by that time, my own thoughts would be for a GSDM Stage 3-4 like response, in many ways similar to December 2006 (which led to the intense closed lows across Desert Southwest).

In terms of the weather, a cold (with Arctic air) and stormy regime for much of the western and central USA looks very probable by week 2 (southwest flow storm track on the Plains) possibly lasting into week 3. The west coast should be generally dry (weak systems understood) while the Deep South remains warm. After week 2 (after ~ January 19th) intense fairly low latitude westerly flow may slam into the California coast leading to high-impact precipitation events there. Much of the rest of the USA should have moderating temperatures (there may still be an Arctic cold air source) while locations from the central/southern Rockies into the Mississippi and Ohio Valleys remain active.

Of course, who really knows what the exact timing will be and let’s just see what happens. I will try to start my more frequent/short postings ~Monday-Tuesday of next week.

Ed Berry