El Nino as arrived; summer heat to return

Well, it’s now official–the fairly rapid warming in the tropical Pacific ocean over the past three months is of sufficient magnitude and duration to be considered an El Nino (ENSO warm episode) event. Basin-averaged sea surface temperature anomalies are current pegged around 1 degree Celsius, and subsurface anomalies are approaching 4 C in some areas. Anomalies continue increase, and are expected to continue to do so for at least the next 3-4 months. This persistent and continued warming will probably allow the current warm event to rise into the “moderate” category by early fall. Since continued warming is expected after that, the 2009-2010 El Nino event is expected to be at least notable. The CFS–the primary coupled dynamical model utilized by the CFS in seasonal forecasts–is explicitly predicting a strong El Nino by November. The present CFS forecast brings Nino 3.4 anomalies to a peak of around +2.2-2.4 C, an no runs predict anomalies above 1.5 C. A few runs call for a much more powerful event, with anomalies peaking near or even above 3 C. For reference, 1.5-2.0 C anomalies are considered “moderate”, and anything above that is generally classified as “strong.” The CFS is subject to significant errors, and a bias corrected interpretation results in lower explicitly forecasted peaks. It is also important to keep in mind that the climate models hugely underestimated the magnutide of the massive 1997-1998 event, though they did accurently predict the development of El Nino conditions. Some improvements have been made in ocean-atmosphere and deep ocean-atmosphere models in the intervening decade, but I still look at model intensity forecasts with skepticism. The models have already underestimated the magnitude of the ongoing event, so it is possible that this continues to be the case. Other international models are not forecasting as strong of an event, but these models have failed to capture the present magnitide of warming accurately. Therefore…there is rather high confidence at this juncture that we are looking at at least a moderate El Nino event, which may become strong by winter. There is little else to mention at this point, and the most prononuced effects on California and the Western United States won’t occur until winter. However, if the El Nino event becomes strong earlier than anticipated, the forecast for this summer and early fall may change. Stay tuned.

As for our current weather, there are a few items of note. The remnants of T.S. Blanca have held together fairly well in the Pacific, and are beginning to resemble a small extratropical storm. This system is expected to weaken significantly well before reaching the CA coast, but mid and upper level moisture is currently streaming over the state. This will continue fot the next 48 hours or so, bringing variable amounts of cloud cover and perhaps some isolated thunderstorms over the mountains. There is a small chance of some some convective development anywhere if there is a source of elevated instability, but at present this doesn’t appear likely.

After a period of cool and troughy weather over the west coast (consider the showers and thunderstorms over OR and WA from a cutoff low presently off the OR coast), the more seasonable heat ridge will build in from the desert southwest over the next week or two. Hot temperatures more characteristic of summer will develop by Monday, and these look likely to persist for quite some time. The position of the Sonoran high will be favorable for heat waves, and eventually monsoonal outbreaks, by the third week in July. It will certainly feel more like summer…

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