Seasonal Outlook through September 2007

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Herbert Watson
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wolter/swcasts/ El Niño / La Niña and the Spring Predictability Barrier Rest of May &

1 Seasonal Outlook through September 2007 Klaus Wolter (15may) University of Colorado, CIRES & NOAA-ESRL Climate Analysis Branch El Niño / La Niña and the Spring Predictability Barrier Rest of May & CPC forecasts for May - September 2007 Experimental forecast guidance

Current state of ENSO (bottom) compared to last month (top): after a sharp drop in early 2007, a lull earlier this spring (top), are we now seeing a resumption of the drift

2 Current state of ENSO (bottom) compared to last month (top): after a sharp drop in early 2007, a lull earlier this spring (top), are we now seeing a resumption of the drift towards La Niña (bottom)?! Opposing wind anomalies over the West vs. East Pacific will keep some warmth alive near the dateline, while the eastern Pacific looks ready for La Niña.

3 The European model s April forecasts (left) keep the central equatorial Pacific (Niño 3.4) in a holding pattern through July, while the east-central Pacific (Niño 3; bottom) will be more likely to transition into La Niña soon.

4 Latest ENSO forecasts from 12 numerical & 8 statistical forecast models: the Spring Predictability Barrier remains formidable, but El Niño appears out of the cards. Note continued split between statistical (nearnormal) and dynamical (LA NIÑA) models!

5 After a cool start to April, it has warmed up again, especially in the last week, leaving only New Mexico on the cool side. The snow-melt is under way (again), and should still lead to an early melt-out compared to normal. The last month has been wet in much of eastern Colorado, as well as parts of the San Juans, giving them a break. A steady diet of about weekly storms has made for a benign spring (with occasional flooding along South Platte) east of the Divide.

6 What about next fortnight? The rest of May appears to be on track for near- or above-normal moisture in most of Colorado. ~ 22nd?! 4-6 days 6-10 days 8-14 days out The color green translates into odds around 50% of getting moisture typical for the wettest third for this time of year. Even a dark brown refers to better than even odds.

7 What about June? Based on historical analog soil moisture situations, the outlook for June is not nearly as grim as last month s peak into May (which was extremely warm and dry for CO): warm for much of the southwestern U.S., tapering off to the east across Colorado, and a similar transition from dryish near the Utah border to moist over the eastern plains of CO. IOW, what you see is what you get!

8 CPC Spring Forecasts According to CPC s official forecasts from last month, May-July 2007 temperature (left) and precipitation (right) forecasts put western Colorado under slightly increased odds for a warm and dry spring, leaving the Front Range blank. Source (for CPC forecasts):

9 CPC Summer Forecasts According to CPC s official forecasts from last month, Colorado s July-September 2007 seasonal temperature (left) and precipitation (right) is anticipated to be on the warm side (update will probably look the same due to trend), but left EC for the latter. Source (for CPC forecasts):

10 Experimental CDC Forecast Guidance My final spring forecast (left) anticipated a dry season in western Colorado, juxtaposed with a wet forecast east of the divide (this forecast was based on data through March). My summer forecast (right) is slightly tilted towards a dry summer monsoon in NM and the northern Front Range, while both AZ (into southwest CO) and northwest TX may look at a wetter-than-average monsoon. If La Niña were to get under way this spring, the summer forecast would remain fairly similar for AZ/NM, but probably drier for Colorado. Source:

11 Executive Summary (15 May 2007) Final version at: While moderate El Niño conditions did briefly develop over the course of last winter, they never fully coupled the atmosphere to the tropical Pacific, and appear to have collapsed earlier this year. Though likely, a transition to La Niña is not guaranteed in If it develops soon, it will probably focus on the eastern Pacific. The last two months have seen a more active storm track than in early March. This has benefited the Front Range more than the Yampa valley. The rest of May promises more of the same, along with seasonal-to-warm temperatures. My last experimental forecast guidance for spring (April-June) gave eastern Colorado better odds for a wet season than western Colorado. My outlook into the monsoon season (July-September) is mild for Colorado: a slight tilt of the odds towards wet for the San Juans compared to dry for the northern Front Range. If La Niña were to take hold soon, a dry and hot summer would be slightly more likely than not in our state. Bottomline: ENSO-neutral conditions in the Pacific are not incompatible with the current storminess in the western U.S., but this appears to favor the eastern plains more than the west slope of Colorado. Drought concerns should focus on western Colorado where a lackluster snow pack and continued dry&warm conditions are setting the stage for a difficult summer.

Seasonal Outlook through September 2007

Seasonal Outlook through September 2007 Klaus Wolter (18apr07) University of Colorado, CIRES & NOAA-ESRL Climate Analysis Branch klaus.wolter@noaa.gov http://www.cdc.noaa.gov/people/klaus.wolter/swcasts/