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Intro:
Everyone has their own opinion about longer range and seasonal forecasting. There is no doubt it is still a fledgling science but one that has saw some advances and improvements in recent years. I try to use a blend of seasonal indices, synoptic factors and signals, and experience to put together a seasonal forecast. My last few have fared well, which means I am probably due for a bad one :), but regardless of the outcome it will be interesting to see how this Summer plays out. This discussion will focus first on ENSO, as I believe a correct ENSO prediction is a very important factor in a seasonal forecast, some other seasonal indices, the current drought situation, and then will conclude with monthly forecasts and a tropical forecast. I personally believe if you have a good idea what phase the ENSO and PDO will be in a given season your forecasts will be better, not perfect or always right, but better.
Seasonal
Indices:
ENSO:
Key Assumption:
We will see our current weak La Nina transition into a borderline
neutral/weak La Nina phase
during the early Summer.
This Winter we saw moderate to at times strong La Nina conditions in the equatorial Pacific. I have posted below the latest SST anomaly plot from Unisys and one from January 6th of this year.
Latest SST Anomaly Map courtesy of Unisys:
SST Anomaly Map from January 6th 2008
We will refer back to these later when we discuss the PDO and the state of the Atlantic, but for now notice in the equatorial Pacific between Ecuador and Indonesia the expanse of ocean with below normal sea surface temperatures has shrunk over the central equatorial Pacific (ENSO Region 3.4) and warmed considerably in the eastern eq. Pacific off the coast of South America. This is indicative of a weakening La Nina signature. Now in looking back at past data, a weakening of a La Nina from Winter into Spring is not uncommon. What we must try and forecast is whether the weakening trend is done or about to cease, or if this is a trend that will continue through the rest of Spring and into Summer. This is a key assumption for my Summer and Hurricane Forecast. Let's examine the subsurface. The following images are courtesy of the TAO/TRITON site. The first image is the subsurface water temperatures in the eq. Pacific from April 27th and the 2nd is an image from January 22nd 2008.
April 27th 2008 Subsurface
January 22nd 2008 Subsurface
Notice the big change in the subsurface over the last 3 months. The large pool of well below normal temperatures below the surface in the central Pacific is gone and in general there is much less colder than normal water in the region. What I take from this is that there is less below normal water, for any upwelling process to bring to the surface. I would say there is still enough to maintain a cooler than normal sea surface temp profile in the eq. pacific, perhaps even a weak La Nina, but nothing stronger than that.
I have also posted below the latest computer model forecasts for ENSO region 3.4 from both the IRI and the ECMWF.
IRI Plume
ECMWF Plume:
These are computer model forecasts of how the temperature in the key ENSO region 3.4 will change in the coming months. Many of the models show a borderline Neutral/weak La Nina state for the summer. These models are not always accurate but have shown some skill in the past, so they are certainly worth factoring into your forecast.
It is my assumption/forecast, based off of the above data and my own opinion that we will see an ENSO state this Summer that is in a borderline neutral to weak La Nina state. I do not at this time think we will see La Nina re-strengthen nor do I think we will see an El Nino form this Summer. Below is a table of years that featured moderate to strong La Nina Winters and what state they went to in the following Summer using region 3.4 and the Multivariate ENSO INDEX (MEI) as a guide to La Nina state.
As of the latest update in late April, region 3.4 had a departure of around -0.8C (weak La Nina) and an MEI of -1.546 (moderate La Nina). The MEI can sometimes lag a bit the SST data, as the atmosphere may sometimes lag in it's response to SST forcing. I expect the MEI to decline through the Spring as the SST anomalies have warmed. None of these analogs are perfect, they never are, but it is helpful to try and use them from a qualitative perspective. My assumption is that the ENSO is going to be in a borderline Neutral to weak La Nina state from a SST perspective and probably the same for the MEI. If one looks at the 11 years above, I would subjectively say 6 years stayed in a La Nina state in varying intensities through the Summer (1950, 1955, 1956, 1971, 1975, and 1999), 3 went neutral to at times weak La Nina (1974, 1985, and 1989), 1 was solidly neutral (2000), and 1 was actually transitioning into an El Nino (1976). Therefore, of these 11 years, 1974, 1985, and 1989 fit the subjectively predicted course of the ENSO I think it will take and I think 4 others are worth of some consideration as well (1956, 1971,1999, and 2000). Of those 7 years, 1974 and 1989 best fit, in my opinion, the way our current La Nina has evolved and the timing within the larger scale oscillations it is in. I will say this is a change of thinking for me that has just came about in the last couple of weeks. I was thinking weak/moderate La Nina for the Summer till just recently, which has altered my forecast. Below are the composite June, July, and August maps for those 5 summers. June offered up mixed results, but July and August were both cooler in the east and Midwest. For now I do not think we will see ENSO evolve into a full-fledged neutral profile (ala 2000) or evolve into a weak El Nino by late Summer (ala 1976). Obviously that is possible, but for now I dont see it happening.
JJA Summer Departures for Key Cities:
Washington DC -0.1
New York -0.33
Boston -0.5
Chicago -0.7
RDU +0.1
Atlanta +0.17
St. Louis -0.1
Dallas +0.8
Denver +0.5
Los Angeles +1
Seattle -1.3
This Winter we saw moderate to at times strong La Nina conditions in the equatorial Pacific. I have posted below the latest SST anomaly plot from Unisys and one from January 6th of this year.
Latest SST Anomaly Map courtesy of Unisys:
SST Anomaly Map from January 6th 2008
We will refer back to these later when we discuss the PDO and the state of the Atlantic, but for now notice in the equatorial Pacific between Ecuador and Indonesia the expanse of ocean with below normal sea surface temperatures has shrunk over the central equatorial Pacific (ENSO Region 3.4) and warmed considerably in the eastern eq. Pacific off the coast of South America. This is indicative of a weakening La Nina signature. Now in looking back at past data, a weakening of a La Nina from Winter into Spring is not uncommon. What we must try and forecast is whether the weakening trend is done or about to cease, or if this is a trend that will continue through the rest of Spring and into Summer. This is a key assumption for my Summer and Hurricane Forecast. Let's examine the subsurface. The following images are courtesy of the TAO/TRITON site. The first image is the subsurface water temperatures in the eq. Pacific from April 27th and the 2nd is an image from January 22nd 2008.
April 27th 2008 Subsurface
January 22nd 2008 Subsurface
Notice the big change in the subsurface over the last 3 months. The large pool of well below normal temperatures below the surface in the central Pacific is gone and in general there is much less colder than normal water in the region. What I take from this is that there is less below normal water, for any upwelling process to bring to the surface. I would say there is still enough to maintain a cooler than normal sea surface temp profile in the eq. pacific, perhaps even a weak La Nina, but nothing stronger than that.
I have also posted below the latest computer model forecasts for ENSO region 3.4 from both the IRI and the ECMWF.
IRI Plume
ECMWF Plume:
These are computer model forecasts of how the temperature in the key ENSO region 3.4 will change in the coming months. Many of the models show a borderline Neutral/weak La Nina state for the summer. These models are not always accurate but have shown some skill in the past, so they are certainly worth factoring into your forecast.
It is my assumption/forecast, based off of the above data and my own opinion that we will see an ENSO state this Summer that is in a borderline neutral to weak La Nina state. I do not at this time think we will see La Nina re-strengthen nor do I think we will see an El Nino form this Summer. Below is a table of years that featured moderate to strong La Nina Winters and what state they went to in the following Summer using region 3.4 and the Multivariate ENSO INDEX (MEI) as a guide to La Nina state.
| Year | ENSO State that Summer (region 3.4) | ENSO State that Summer (using MEI) | My ENSO Assessment |
| 1950 | Neutral/Weak La Nina | Moderate La Nina | Weak/Moderate LN |
| 1955 | Weak La Nina | Strong La Nina | Weak/Moderate LN |
| 1956 | Neutral/Weak La Nina | Moderate La Nina | Weak LN |
| 1971 | Neutral/Weak La Nina | Moderate La Nina | Weak LN |
| 1974 | Neutral | Weak La Nina | Neutral/Weak LN |
| 1975 | Weak/Moderate La Nina | Moderate La Nina | Weak/Moderate LN |
| 1976 | Neutral | Neutral to Weak El Nino | Neutral/Weak EN |
| 1985 | Neutral/Weak La Nina | Neutral | Neutral/Weak LN |
| 1989 | Neutral/Weak La Nina | Neutral | Neutral/WLN |
| 1999 | Weak/Moderate La Nina | Neutral/Weak La Nina | Weak LN |
| 2000 | Neutral | Neutral | Neutral |
As of the latest update in late April, region 3.4 had a departure of around -0.8C (weak La Nina) and an MEI of -1.546 (moderate La Nina). The MEI can sometimes lag a bit the SST data, as the atmosphere may sometimes lag in it's response to SST forcing. I expect the MEI to decline through the Spring as the SST anomalies have warmed. None of these analogs are perfect, they never are, but it is helpful to try and use them from a qualitative perspective. My assumption is that the ENSO is going to be in a borderline Neutral to weak La Nina state from a SST perspective and probably the same for the MEI. If one looks at the 11 years above, I would subjectively say 6 years stayed in a La Nina state in varying intensities through the Summer (1950, 1955, 1956, 1971, 1975, and 1999), 3 went neutral to at times weak La Nina (1974, 1985, and 1989), 1 was solidly neutral (2000), and 1 was actually transitioning into an El Nino (1976). Therefore, of these 11 years, 1974, 1985, and 1989 fit the subjectively predicted course of the ENSO I think it will take and I think 4 others are worth of some consideration as well (1956, 1971,1999, and 2000). Of those 7 years, 1974 and 1989 best fit, in my opinion, the way our current La Nina has evolved and the timing within the larger scale oscillations it is in. I will say this is a change of thinking for me that has just came about in the last couple of weeks. I was thinking weak/moderate La Nina for the Summer till just recently, which has altered my forecast. Below are the composite June, July, and August maps for those 5 summers. June offered up mixed results, but July and August were both cooler in the east and Midwest. For now I do not think we will see ENSO evolve into a full-fledged neutral profile (ala 2000) or evolve into a weak El Nino by late Summer (ala 1976). Obviously that is possible, but for now I dont see it happening.
PDO
Key Assumption: We will see
continued solidly negative PDO cycle this Summer.
If
you refer back to the SST
anomaly chart above, it is clear that we are in
a negative
PDO (Pacific Decadal Oscillation) cycle (cool water off coast of
British Columbia and US NW coast and warmer water in the North
Pacific). According to both the sites
I use for the PDO index, http://jisao.washington.edu/pdo/PDO.latest
ftp://ftp.ncdc.noaa.gov/pub/data/ersst-v2/pdo.1854.latest.ts
we are in a solidly
negative phase. A negative PDO phase typically favors cool troughiness
in the
Pacific Northwest and usually goes hand and hand with a La Nina. When
picking analogs for this Summer one needs to factor the
PDO into the equation, I think we will see a PDO in the -1 to -2 range
using the NCDC ftp calculation. In looking at the analogs above in the
ENSO section, here as how the PDO fared that Summer.
I have listed the 11 above ENSO analog year and the JJA PDO values from both sites. I highlighted 1976, 1985, and 1989 because all those years featured predominantly neutral or + PDO values. I dont think that we will see neutral or positive PDO values this Summer because we have been in a predominant -PDO pattern and at the moment it doesnt look to be changing. The PDO is a Decadal oscillation meaning once it is in a predominant phase (negative or positive) changes of phase are usually short term. It doesnt mean it cant happen though for several months. The years that seem to fit my prediction best are 1956, 1971, 1975, 1999, and 2000. Unfortunately 1974 or 1989 do not fit well, although 74 is better than 89, so that means I will have to weight this PDO consideration if I use those 2 years as analogs.
| Year | ENSO State That Summer | NCDC PDO | UW PDO |
| 1950 | Weak/Moderate La Nina | -1.93,-3.54,-3.58 | -1.77,-2.93,-0.7 |
| 1955 | Weak/Moderate La Nina | -2.12,-3.14,-3.43 | -2.44,-2.35,-2.25 |
| 1956 | Weak La Nina | -1.56,-1.63,-1.58 | -1.7,-1.03,-1.16 |
| 1971 | Weak La Nina | -1.41,-2.6,-1.18 | -1.55,-2.2,-0.15 |
| 1974 | Neutral/Weak LN | -0.29,-0.37,-1.02 | -0.31,-0.08,0.27 |
| 1975 | Weak/Moderate La Nina | -1.39,-1.18,-2.33 | -1.16,-0.4,-1.07 |
| 1976 | Neutral/Weak EN | -1.04,0,0.18 | -0.67,0.61,1.28 |
| 1985 | Neutral/Weak La Nina | 0.18,0.16,-0.67 | 0.18,1.07,0.81 |
| 1989 | Neutral/Weak La Nina | 0.33,0.24,-1.25 | 0.36,0.83,0.09 |
| 1999 | Weak La Nina | -2.21,-1.92,-2.69 |
-1.3,-0.66,-0.96 |
| 2000 | Neutral | -0.62,-1.36,-2.49 | -0.44,-0.66,-1.19 |
I have listed the 11 above ENSO analog year and the JJA PDO values from both sites. I highlighted 1976, 1985, and 1989 because all those years featured predominantly neutral or + PDO values. I dont think that we will see neutral or positive PDO values this Summer because we have been in a predominant -PDO pattern and at the moment it doesnt look to be changing. The PDO is a Decadal oscillation meaning once it is in a predominant phase (negative or positive) changes of phase are usually short term. It doesnt mean it cant happen though for several months. The years that seem to fit my prediction best are 1956, 1971, 1975, 1999, and 2000. Unfortunately 1974 or 1989 do not fit well, although 74 is better than 89, so that means I will have to weight this PDO consideration if I use those 2 years as analogs.
QBO
Key Assumption: The QBO will be
in a moderate to strong westerly (+) phase this Summer.
I am generally not a big fan of the Quasi-Biennial Oscillation with regards to seasonal forecasting, unless it goes along with other ideas set forth from other indices. That is not to say it isnt important or does not have a legitimate physical application, I just dont use it at the moment extensively. However I do look at the phase to see if it matches any of my working analogs. We are currently in a weak westerly phase (+) that appears to be heading towards a solidly strong westerly phase for the Summer. Of the 11 analog years above, 1955, 1971, 1975 saw this occur in a weaker fashion, 1985 is a pretty good match, and 1999 is a decent match. So that gives SOME added weight to those years.
I am generally not a big fan of the Quasi-Biennial Oscillation with regards to seasonal forecasting, unless it goes along with other ideas set forth from other indices. That is not to say it isnt important or does not have a legitimate physical application, I just dont use it at the moment extensively. However I do look at the phase to see if it matches any of my working analogs. We are currently in a weak westerly phase (+) that appears to be heading towards a solidly strong westerly phase for the Summer. Of the 11 analog years above, 1955, 1971, 1975 saw this occur in a weaker fashion, 1985 is a pretty good match, and 1999 is a decent match. So that gives SOME added weight to those years.
Atlantic SST's
Key
Assumption: The Atlantic will remain above normal this Summer:
The Atlantic Ocean switched to a warmer than normal phase back in 1995 and remains above normal. This has corresponded with increased hurricane activity but also with warmer summers across the US. Since 1995, only 2 of those 12 Summer have been cool for the country as a whole, 1997 and 2004. 1997 was actually closer to normal while 2004 was well below normal for the nation as a whole. As I said above, my thoughts are that the turning of the Atlantic to a warm phase, has not only led to more active hurricane seasons, but also warmer summers for the US as a whole. This is a factor that must be considered when formulating a forecast for this Summer. The AMO (Atlantic Multidecadal Oscillation) value is usually used to determine the warmth of the Atlantic, and as of now it is very much in line with where it has been this time of the year for the last 11 years which is above normal. As of now I expect this above normal phase to continue.
The Atlantic Ocean switched to a warmer than normal phase back in 1995 and remains above normal. This has corresponded with increased hurricane activity but also with warmer summers across the US. Since 1995, only 2 of those 12 Summer have been cool for the country as a whole, 1997 and 2004. 1997 was actually closer to normal while 2004 was well below normal for the nation as a whole. As I said above, my thoughts are that the turning of the Atlantic to a warm phase, has not only led to more active hurricane seasons, but also warmer summers for the US as a whole. This is a factor that must be considered when formulating a forecast for this Summer. The AMO (Atlantic Multidecadal Oscillation) value is usually used to determine the warmth of the Atlantic, and as of now it is very much in line with where it has been this time of the year for the last 11 years which is above normal. As of now I expect this above normal phase to continue.
Drought Conditions:
Current Drought Conditions:
Although we are currently in a moderate to severe drought in the southeast, conditions have been improving this Spring. It has been wet in the long term for the plains, Midwest and northeast. Areas where a drought is in place in the Spring, can often times produce a heating enhancement due to the dry soil and the opposite can be true for wet soils. The years that best matched the drought conditions in the Spring among the 11 main analogs were 1950, 1999. Other years that were somewhat of a match but not perfect were 1974, 1985, and 1976.
Although we are currently in a moderate to severe drought in the southeast, conditions have been improving this Spring. It has been wet in the long term for the plains, Midwest and northeast. Areas where a drought is in place in the Spring, can often times produce a heating enhancement due to the dry soil and the opposite can be true for wet soils. The years that best matched the drought conditions in the Spring among the 11 main analogs were 1950, 1999. Other years that were somewhat of a match but not perfect were 1974, 1985, and 1976.
Putting It all together
Summer 2008 Rainfall
The Summer forecast this year
in my opinion, hinges on exactly how our current La Nina evolves. Up
until recently, I was solidly in the weak to moderate La Nina camp for
the Summer, and my Summer forecast was warmer. However, recent trends
in the subsurface data as well as ENSO computer models have caused me
to back off that. So my assumption is now that we will see a neutral to
borderline weak La Nina event and past years that have saw this occur
suggest an increased probability of a cooler forecast. I did factor in
the expectant continuation of a -PDO and a warmer than normal Atlantic
ocean, along with the current SE drought into the forecast as well
which led to a bit cooler NW forecast and warmer SE forecast than what
some of the past analog years indicated. Overall I think the warmest
Summer month in the east with respect to normal will be June. July and
August look to be below normal in the NE and slightly below in most of
the southeast. I also think drought conditions in the southeast will
ease somewhat over the Summer.
June 2008
Forecast
July 2008 Forecast
August 2008 Forecast
July 2008 Forecast
August 2008 Forecast
Summer 2008 Rainfall
JJA Summer Departures for Key Cities:
Washington DC -0.1
New York -0.33
Boston -0.5
Chicago -0.7
RDU +0.1
Atlanta +0.17
St. Louis -0.1
Dallas +0.8
Denver +0.5
Los Angeles +1
Seattle -1.3
2008 Hurricane Forecast
The
combination of a
continued warmer than normal Atlantic Ocean along with a neutral to
weak
La Nina means this is going to be a active hurricane season. A
look at the SST anomaly map posted above shows warmer than
normal waters off the coast of Africa (which if it continues would
favor tropical waves moving off of Africa and maintaining themselves),
but also shows cooler than normal waters just east of the Leeward
Islands and in the waters north of Carribean Islands and around the
Bahamas (which would suggest a reduced chance of storms forming in
these regions if the trend continues). However since we are still in a
predominantly positive AMO phase, I do not think Atlantic Ocean SST
anomalies will be a negative factor this season. Now we should not see
a season similar to 1995 or 2005 when the Atlantic was MUCH warmer than
normal, but we should still see an above normal season. Since
1995, when the Atlantic turned back to an above normal phase, the
average # of storms has been 15.1. The 58 year average is at 10.8. In
fact since 1995 the only years that have been below the 59 year average
were 1997 and 2006, which happened to have the highest July-October
MEI, meaning they were the strongest El Nino's in that period. This
year
there is no El Nino, and since 1995 7 seasons have averaged a
negative June-October MEI, and the average # of storms those years was
15.
My forecast:
13 named storms.
7 hurricanes
4 major hurricanes.
Hurricane Threat Map:
The way I expect the synoptic weather pattern to play along after looking at the 11 ENSO analog landfall years, would lead me to favor the central and eastern Gulf, the Carolinas coast, and the New England coast. I think there is a reduced chance this year of a hit in the western Gulf. Now this doesnt mean there wont be any storms to strike the western Gulf or that a storm will hit the increased risk areas, it just means that the way I see the predominant pattern playing out in the late Summer and early Fall would favor a higher chance in those areas.
My forecast:
13 named storms.
7 hurricanes
4 major hurricanes.
Hurricane Threat Map:
The way I expect the synoptic weather pattern to play along after looking at the 11 ENSO analog landfall years, would lead me to favor the central and eastern Gulf, the Carolinas coast, and the New England coast. I think there is a reduced chance this year of a hit in the western Gulf. Now this doesnt mean there wont be any storms to strike the western Gulf or that a storm will hit the increased risk areas, it just means that the way I see the predominant pattern playing out in the late Summer and early Fall would favor a higher chance in those areas.