Seasonal Outlook through September 2007

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wolter/swcasts/ El Niño / La Niña and the Spring Predictability Barrier Rest of April

1 Seasonal Outlook through September 2007 Klaus Wolter (18apr07) University of Colorado, CIRES & NOAA-ESRL Climate Analysis Branch El Niño / La Niña and the Spring Predictability Barrier Rest of April & CPC forecasts for May - September 2007 Experimental forecast guidance

2 Current state of ENSO (bottom) compared to last month (top): after a sharp drop in early 2007, we are now seeing a slow drift towards La Niña?!

3 Eastern Pacific cloudiness (red ellipse) is enhanced close to Central America and reduced near Hawaii - typical for (late) El Niño stage; Western Pacific (blue ellipse) is enhanced near the Phillippines and reduced near the dateline - typical for (early) La Niña! The dice have not been cast just yet

4 The European model s March forecast (bottom) has slowed down its transition towards La Niña compared to February (left). However, by September, most forecast members indicate weak-tomoderate La Niña conditions.

5 New ENSO forecasts from 9* numerical & 7 statistical forecast models: the Spring Predictability Barrier remains alive and well, but El Niño is out of the cards (for now)! *: Ignore the NASA forecast: poorly initialized & predicting an ocean colder than ever observed!

While the last month continued warm west of the divide, eastern Colorado turned sharply colder in April (running colder than March in places), helping with P-E The last month has

6 While the last month continued warm west of the divide, eastern Colorado turned sharply colder in April (running colder than March in places), helping with P-E The last month has been wet in New Mexico and parts of eastern Colorado (reinforced in last 2 days), as well as parts of western Colorado (several ski resorts closed with their deepest base of the winter).

After a mixed first half of April, what s in store for us? The remainder of April appears to be on track for above-normal moisture in most of Colorado, especially the north.

7 After a mixed first half of April, what s in store for us? The remainder of April appears to be on track for above-normal moisture in most of Colorado, especially the north. 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.

8 What about May? Based on historical analog soil moisture situations, the outlook for May is grim : warm and dry for much of the southwestern U.S., especially in Colorado. Fortunately, the skill level of this monthly forecast remains close to zero <a similar forecast for April was off as well>.

9 CPC Spring Forecasts According to CPC s official forecasts from last month, April-June 2007 temperature (left) and precipitation (right) forecasts put most of Colorado under slightly increased odds for a warm and dry spring. Source (for CPC forecasts):

CPC Summer Forecasts According to CPC s official forecasts from last month, Colorado s July-September 2007 seasonal temperature (left) and precipitation (right) is

10 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 do the same due to trend), but left EC for the latter. Source (for CPC forecasts):

11 Experimental CDC Forecast Guidance My most recent spring forecast (right) has sharpened the trend towards a dry season in western Colorado, juxtaposed with a wet forecast east of the divide (this forecast is based on data through March, but matches recent storm outcomes). If this forecast verifies, watch for a (resumed) early snow-melt west of the divide, and below-normal demand east of the mountains. Source:

12 Experimental CDC Forecast Guidance Source: My early take on the summer monsoon is favorable for Arizona/Utah, but dry over much of New Mexico (and the northern Front Range). The former is consistent with the low snow pack over Utah in particular (which would favor an early monsoon) as well as the onset of La Niña conditions (for Arizona). The latter is consistent with above-normal soil moisture in New Mexico. Given the poor proven skill level this far out, take this with a very large grain of salt!

13 Executive Summary (18 April 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 this year. The last month has seen a considerably more active storm track (and colder weather) than earlier in March. This has stabilized the remaining snow pack after appearing to be on track for near-record early meltout. The remainder of April promises more of the same, with the best moisture potential around April 24th. My experimental forecast guidance for the spring season (April-June) has sharpened the outlook with an expected dry spring in western Colorado juxtaposed with a better-than-average odds for a wet spring east of the divide. The early outlook into the monsoon season (July-September) is either undecided or dry (around here) for Colorado. If La Niña were to take hold soon, a dry and hot summer would be slightly more likely than not. 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 (south-)western Colorado where a lackluster snow pack and an expected dry&warm spring are setting the stage for a difficult summer.

Seasonal Outlook through September 2007

Seasonal Outlook through September 2007 Klaus Wolter (15may) University of Colorado, CIRES & NOAA-ESRL Climate Analysis Branch klaus.wolter@noaa.gov http://www.cdc.noaa.gov/people/klaus.wolter/swcasts/