The KeeneWeather.com preliminary 2009-10 winter outlook has been posted on the main page.
Click here for full details!
Saturday, September 19, 2009
Tuesday, September 15, 2009
PRELIMINARY 2009-10 WINTER OUTLOOK
Summer Analogs:
The summer of 2009 had a number of distinct features including height anomalies associated with a predominantly negative NAO and AO while an El Nino built in the equatorial Pacific. The top analogs for the observed H5 pattern were 1950, 1951, 1958, 1960, 1965, 1977, 1982, 1985, 1998, and 2004. Filtering for the prefered ENSO conditions (details below), the list is condensed to 1951, 1958, 1965, 1977, 1982, and 2004.
Pacific:
Between April and June, SSTs in the equatorial Pacific warmed rapidly triggering a lot of "strong" El Nino talk. However, in late June, trade winds strengthened over the eastern Pacific, subduing the high SST anomalies. There have been multiple west wind pushes west of the dateline, but propogation into the western hemisphere has been minimal.
So what we've seen is a stall in the warming of the EPAC while central Pacific SSTs have been on the increase. At the start of September, the highest totals were located between 160E and 180. SST anomalies have the greatest effect in the western Pacific where totals are the highest and the exponential relationship between saturation vapor pressure and temperature dictates that evaporation rates are increased significantly with small deviations in SST. The west-based versus east-based argument matters with respect to where forcing is positioned, so even though SST anomalies may be slightly higher at times in the east Pacific, their effects are not as great and the anomalies near the dateline will have much more significance. Overall, I expect this El Nino episode to at least remain within the moderate threshold.
The convergence of the trade wind anomalies with the westerly wind bursts has provided focus for tropical convection, and we have seen this summer a number of OLR minimas just west of the dateline. Also, it should be noted that the summer analog composites display this well too.
In the north Pacific, there has been notable cooling near and south of the Aleutians. The -PDO has weakened to it's highest state since September 2007. However, at the same time, SST anomalies have cooled in the Gulf of Alaska. This has influence over the location of a trough over the north Pacific. It is clear that the north Pacific pattern will be a crucial battle ground this winter.
Atlantic:
SST anomalies in the north Atlantic have been tilted squarely toward a winter +NAO configuration. Using a regression with the two regions of interest yields a DJF NAO of +.85. However, the equatorial Atlantic is looking more favorable. There has been a general cooling of waters between Africa and South America. This should have overall weakening effect of the Atlantic hadley.
In addition, with primary tropical forcing occurring near the dateline, and some weaker forcing through the eastern Pacific, this forces an equatorial cell with sinking air over South America which again, weakens the Atlantic Hadley, and assists in the development of a -NAO.
Stratosphere:
Stratospheric easterly wind anomalies continue to make their descent, and the QBO took a steep dive this summer. Easterlies will continue to descend this winter, and the QBO will likely bottom out sometime in the spring. The descending, negative phase along with a warm ENSO period is highly supportive of a weakened polar vortex.
Preliminary Outlook:
There are definitely inconsistencies between each analog. I do not intend on using them directly, rather just as guidance for the outlook. Overall, there is a decent consensus for a dominant +NAO in December, a transition phase in January, and a dominant -NAO from late January through February. January and February could certainly feature a couple major winter storms in the eastern US. One significant x-factor is the North Pacific pattern. I think we will see the PDO become more neutral toward the winter, but another question is what happens with SSTs in the Gulf of Alaska.
The summer of 2009 had a number of distinct features including height anomalies associated with a predominantly negative NAO and AO while an El Nino built in the equatorial Pacific. The top analogs for the observed H5 pattern were 1950, 1951, 1958, 1960, 1965, 1977, 1982, 1985, 1998, and 2004. Filtering for the prefered ENSO conditions (details below), the list is condensed to 1951, 1958, 1965, 1977, 1982, and 2004.
Pacific:
Between April and June, SSTs in the equatorial Pacific warmed rapidly triggering a lot of "strong" El Nino talk. However, in late June, trade winds strengthened over the eastern Pacific, subduing the high SST anomalies. There have been multiple west wind pushes west of the dateline, but propogation into the western hemisphere has been minimal.
So what we've seen is a stall in the warming of the EPAC while central Pacific SSTs have been on the increase. At the start of September, the highest totals were located between 160E and 180. SST anomalies have the greatest effect in the western Pacific where totals are the highest and the exponential relationship between saturation vapor pressure and temperature dictates that evaporation rates are increased significantly with small deviations in SST. The west-based versus east-based argument matters with respect to where forcing is positioned, so even though SST anomalies may be slightly higher at times in the east Pacific, their effects are not as great and the anomalies near the dateline will have much more significance. Overall, I expect this El Nino episode to at least remain within the moderate threshold.
The convergence of the trade wind anomalies with the westerly wind bursts has provided focus for tropical convection, and we have seen this summer a number of OLR minimas just west of the dateline. Also, it should be noted that the summer analog composites display this well too.
In the north Pacific, there has been notable cooling near and south of the Aleutians. The -PDO has weakened to it's highest state since September 2007. However, at the same time, SST anomalies have cooled in the Gulf of Alaska. This has influence over the location of a trough over the north Pacific. It is clear that the north Pacific pattern will be a crucial battle ground this winter.
Atlantic:
SST anomalies in the north Atlantic have been tilted squarely toward a winter +NAO configuration. Using a regression with the two regions of interest yields a DJF NAO of +.85. However, the equatorial Atlantic is looking more favorable. There has been a general cooling of waters between Africa and South America. This should have overall weakening effect of the Atlantic hadley.
In addition, with primary tropical forcing occurring near the dateline, and some weaker forcing through the eastern Pacific, this forces an equatorial cell with sinking air over South America which again, weakens the Atlantic Hadley, and assists in the development of a -NAO.
Stratosphere:
Stratospheric easterly wind anomalies continue to make their descent, and the QBO took a steep dive this summer. Easterlies will continue to descend this winter, and the QBO will likely bottom out sometime in the spring. The descending, negative phase along with a warm ENSO period is highly supportive of a weakened polar vortex.
Preliminary Outlook:
There are definitely inconsistencies between each analog. I do not intend on using them directly, rather just as guidance for the outlook. Overall, there is a decent consensus for a dominant +NAO in December, a transition phase in January, and a dominant -NAO from late January through February. January and February could certainly feature a couple major winter storms in the eastern US. One significant x-factor is the North Pacific pattern. I think we will see the PDO become more neutral toward the winter, but another question is what happens with SSTs in the Gulf of Alaska.
Sunday, September 6, 2009
Beautiful weather to continue
The streak of sunny dry days will continue this week. Strong high pressure over the region and northward will suppress disturbances to the south. Daytime highs will reach the 70's with lows in the 40's to low 50's.
I have been away from the site for a little while with college starting, but I hope to continue a steady stream of updates (as the weather warrants).
To start, I have been working on my winter 2009-10 seasonal outlook which I hope to post by September 15th.
I have been away from the site for a little while with college starting, but I hope to continue a steady stream of updates (as the weather warrants).
To start, I have been working on my winter 2009-10 seasonal outlook which I hope to post by September 15th.
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