SUMMER 2009 OUTLOOK

--- WINTER REVIEW ---

The winter of 2008-9 started off slowly in New England through the first week of December, and then it struck in all its fury. A major storm storm system dumped 2 to 5 inches of water over much of the eastern US December 9-12. For many this was in the form of rain, however across central Massachusetts up through New Hampshire, much of this fell as freezing rain, with major ice accretion causing record power outages. The National Guard was brought in, and some schools were closed for weeks. That storm marked the start of yet another wintry December. On the following weekend, southern New England was given a quick introduction to snow. Two back-to-back storms dumped totals of 15 to 25 inches across the region. Ocean effect snow kept flakes falling non-stop for over 48 hours. Through the end of the month, two more significant snowfalls, one on the 21st and one on the 31st affected southern New England with a general 6 to 12 inches from both. Overall, the Northeast into the Ohio Valley ended with much above normal precipitation and normal temperatures. Negative PNA ridging over the Aleutians was coupled with positive anomalies in the Gulf of Alaska. Cold air was locked up in Canada for the most part, although the northern Plains did get a taste of it.

This cold all slid south into the Great Lakes and Northeast in January. Starting in mid December, a cycle began that kept this cold locked over us for 30+ days. A -PNA trough over the west forced downstream ridging in the east, however -EPO anomalies over Alaska countered it. When the EPO phase switched, -NAO anomalies over Greenland countered it. The -NAO spiked around New Years, with 2009 dawning on a very cold note. Finally, when the -NAO quit, the +PNA ridging was developing in the west, which delivered a few tremendously cold days and nights in New England during January. As for snow, a wintry mix hit the region on the 7th through the 8th, significant snowfalls occurred on the 11th, 18th, and 28th, all in the 6-12" range across SNE. Monthly temperature departures ranged from -4 to -8°C in the northeast. On the whole, precipitation was normal for SNE, with averaged storm tracks laying a streak of positive anomalies along the Appalacian Mts. Otherwise, the rest of the nation ended well below normal with the CONUS experiencing its 5th driest January in the last 115 years. Averaged height anomalies over North America were positive along the west coast and negative across the northeast US and southeast Canada.

February was a much different story with regards to wintry weather in the northeast. Between January 28 and February 19, no measurable snowfall was recorded in Keene. Only two moderate snow events were recorded; one on the 19th and one on the 22nd. This time, it wasn't warmth that did it but rather a lack of storminess. Cold air was available for the most part, with New England ending between 0 and 2°C above normal. However, persistent ridging over the center of the nation (southern Plains ended 11th warmest on record) kept a dry northwest flow over the eastern US. Nine states in the east had their top 10 driest February in the last 115 years, and overall the northeast saw its 7th driest on record, and the CONUS saw its 8th driest on record. As previously posted, The January-February period was the driest start ever recorded for the CONUS.

Toward the end of the month, while the short term weather was uneventful, all eyes were on the developing potential for a major east coast storm to start off the month of March. By the afternoon of March 2nd, A swath of 6-12 inches extended from the Carolinas up through New England. Beyond this point, SNE received one last moderate snowfall on the 9th and then winter ended, prematurely for most.

The one consistent feature throughout the December, January, February period was overall above normal heights across the north Pacific. The ridge axis fluctuated between 180W and 140W, with the three month time averaged maximum located along 155W, in between the locations of the PNA domain and the GOA domain. This general tendency is due in part to a strongly negative Pacific Decadal Oscillation. SST anomalies of 1.5°C to 2.5°C were present through the winter between 30 and 50N and 170 and 140W. Calculated PDO values (post-1948 ranks) were -0.87SD (17) in December, -1.40SD (11) in January and -1.55SD (9) in February, with a seasonal average of -1.27SD (12). The resultant positive sea level pressure anomalies were varied through the winter: December, displaced toward the western Pacific; January, weaker and split toward WPAC and west coast of North America; February, strongest and located more inline with the PNA domain. There is no reason for a moderate to strong negative PDO not to continue through the summer.

DJF 2008-9 Time averaged height anomalies:


--- PACIFIC ---

In the tropics, sea surface temperature anomalies followed a smooth transition to a moderate La Nina in the central Pacific (Nino4 and Nino3.4) as the effect of +SOI driven easterly wind anomalies that were present through the autumn. Changes in the eastern Pacific were more volatile through the winter. CPAC Cold anomalies shifted slowly westward through the winter while cooling peaked in the eastern Pacific in January. The trans-Nino index (TNI) which is a measure of the west-east gradient in SSTA (specifically Nino1.2-Nino4), took a notable turn from a persistently positive (cold-warm) phase present since late 2007, to a negative gradient in late February. The general trend of warmer anomalies shifting eastward through the central Pacific argues for a continuation of the negative gradient. And IF we transition to a weak El Nino, it will likely be west-based.

At the same time, in the last few weeks SSTA have been warming in the western Indian Ocean near the Africa coast and expanding eastward. This is showing a recovery back toward the positive phase of the Indian Ocean Oscillation. The positive phase is associated with generally a warmer central Pacific. Moderate to strongly positive subsurface anomalies generally support this as well, advertising an aging La Nina. The deep thermocline anomalies have transitioned east to some degree which could help the EPAC progress toward overall neutral conditions in the summer.

The La Nina developed notably later this winter than the climatological mean. Overall, MEI peaked in February at -.725, and the March figure should display the start of warming. Monthly SOI for March came in at -.20: negative for the first time since May 2008. An assumption that I'm making in this forecast is that the Pacific transitions to neutral by early this summer.

IRI model forecasts:


--- ATLANTIC ---

In the Atlantic Ocean, temperatures have cooled through the late winter and early spring, with notable negative anomalies showing up west of Africa as well as across the Caribbean and west Atlantic. For the first time since 2002, the AMO and TNA were negative in February. We are still squarely in the long term +AMO cycle which began in 1995, so I do not expect the cooling to last. In the meantime, the Gulf Stream is pulling large positive anomalies to the east of New England and south of Nova Scotia.

--- ATMOSPHERIC WINDS ---

Tropospheric winds continue to average toward easterly anomalies (negative relative atmospheric angular momentum), which was one major feature of this past winter. Whether the magnitude of SST anomalies satisfied levels to declare a weak or moderate La Nina, global wind anomalies and the resultant pattern tendencies were definitely of a La Nina atmosphere. The global wind oscillation has been consistently displaced toward the La Nina attractor (phases 1-4) and AAM anomalies reached an absolute minimum in February with the lowest monthly average since November 2007. At the moment, the GWO has been spiraling toward more neutral territory and recently it rotated through an AAM minimum within the -1SD neutral designation for the first time since December. Continued positive frictional torque centered around 30N latitude will dampen easterly anomalies and should bring the atmosphere back to at least neutral by the late spring.

Westerly wind anomalies in the stratosphere associated with the continued +QBO have slowed in descent, with building easterly anomalies above 20mb. Easterly shear zones have become faster than westerly shear zones, and descent will likely accelerate toward the autumn, with a neutral QBO this summer. At the moment, a long positive phase continues with the equatorial anomaly in February averaging +12.33m/s. This 30mb anomaly should notably abate through the spring and summer.



--- FORECAST ---

Taking all of the discussed factors into consideration, I've come up with a list of analogues to help outline the expected pattern to be featured this summer. First I applied the time averaged pattern from this past winter and found analogues to match it. I focused solely on the positive anomaly in the north Pacific since its magnitude was more than twice as large as the next strongest anomalies. In the past, rolling these analogues forward has given a relatively accurate indication of the expected future pattern. The following years were used: 1955, 1957, 1962, 1979, 1985, 1989, 1990, 2000, 2005, 2008. Taking this list, I filtered it for the specific teleconnections that were experienced this winter and expected through the spring. This includes the DJF La Nina and negative relative AAM, as well as the persistent negative PDO and the expectation for this to continue through the summer months. A +PDO immediately eliminates 2005 and 1985. Then an El Nino and positive relative AAM removed 1979 and 1990 from the list. The final analogues include 1955, 1957, 1962, 1989, 2000, and 2008. Looking at SSTs in the Pacific, general a La Nina / -PDO is matched, although La Nina is based further west.

Monthly analogue height anomaly composites:

A composite of these years provides a guide to the summer outlook. I also looked at the soil moisture nation wide because it has a notable effect on boundary layer temperatures during the summer. As I've remarked upon already, through February, 2009 was the driest start on record for the lower 48. While official records aren't compiled yet, March featured some relief toward the end of the month, and the start to April has reintroduced rainy weather to much of the eastern US. The main areas that continue to be in poor shape are Texas and California. In general, this creates a warm bias in these areas for the summer. Meanwhile, significant precipitation events through the winter and spring has put the northern plains in a big surplus. In general, this creates a cool bias in this area.



So taking all things into consideration, here's my outlook for Summer 2009:

Monthly temperature anomalies:


Summer (JJA) Precip percent of normal:


--- HURRICANE SEASON ---

The general pattern that I have outlined so far is somewhat complicated. It involves strange combinations such as a strongly negative PDO with a strong descending westerly QBO, and the descending QBO in sync with (possibly) a warm neutral Pacific. At this time, I continue with my previous forecast of a below normal season (with respect to the post 1995 average): 13 named storms, 7 hurricanes, and 3 major hurricanes. The greatest activity may be centered early in the season as the westerlies reach the upper troposphere. I hope to do a more detailed write up later this month.