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Monthly Archive | CLIMAS

Monthly Archive

Extreme Heat in the Southwest - June 19, 2017

Monday, June 19, 2017

At the time of publication (Jun 15, 2017), an extreme heatwave is forecast to hit the Southwest beginning later this week and extending into next week the week of June 19, peaking on/around June 19-20, 2017. Tucson is currently forecast to reach 114, while Phoenix may see temperatures reach 120 – both of which are approaching the record high temperatures for Tucson and Phoenix, respectively. Southwestern summers have a well-earned reputation for extreme temperatures, and compared to most of the country, even a ‘normal’ summer day is often much warmer than record high temperatures in more temperate locales.

It is important to note that the current forecast represent temperature extremes that can be dangerous or even deadly, as a result of direct exposure, or associated with the accumulated effects of heat stress, particularly when nighttime temperatures remain elevated and it is harder to cool off at night.

The Phoenix NWS office is piloting an experimental heat extremes tracker/map that highlights the risk potential associated with direct exposure and more sustained heat events. (Figs. 1a-ab).

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CLIMAS SW Climate Outlook - ENSO Tracker June 2017

Friday, June 16, 2017

Oceanic and atmospheric indicators of the El Niño-Southern Oscillation (ENSO) are still within the range of neutral (Figs. 1-2), although sea-surface temperatures more consistently hint at borderline El Niño conditions compared to atmospheric indicators. Outlooks and forecasts generally agree that ENSO-neutral conditions will persist through the summer and is the most likely scenario for the rest of 2017. A lingering possibility remains of an El Nino event developing later this fall, but forecasts since last month have shifted further from that likelihood.

On June 6, the Australian Bureau of Meteorology maintained its El Niño Watch with a 50-percent chance of an El Niño event in 2017, but noted indicators have remained mostly unchanged for multiple weeks, “suggesting El Niño development has stalled for now.” On June 8, the NOAA Climate Prediction Center (CPC) observed that oceanic and atmospheric conditions were consistent with ENSO-neutral conditions, but recent model runs led CPC forecasters to shift to a 50-55-percent chance of ENSO-neutral conditions in 2017 and a 35-50 percent chance of El Niño. On June 9, the Japanese Meteorological Agency (JMA) identified a continuation of ENSO-neutral conditions with a 70-percent chance of El Niño conditions until fall 2017, noting that oceanic and atmospheric conditions “indicate no clear signs of El Niño development.” On June 15, the International Research Institute for Climate and Society (IRI) and CPC identified ENSO-neutral as the most likely outcome in 2017, with a 40-to-45-percent chance of an El Niño in 2017 (Fig. 3). The North American Multi-Model Ensemble (NMME) is borderline weak El Niño as of June 2017, and while the model spread indicates a wide range of possible outcomes for the rest of 2017 (Fig. 4), the ensemble mean indicates ENSO-neutral as the most likely outcome (but with a weak El Niño event still within the range of plausibility), which is reflected in the uncertainty in the CPC and IRI/CPC outlooks.

Summary: The lack of atmospheric indicators of El Niño and the borderline status of sea-surface temperature anomalies have strengthened the forecaster consensus that ENSO-neutral is the most likely scenario for the remainder of 2017. It is too early to entirely rule out an El Niño event later this year, but the timing and intensity of this plausible but increasingly unlikely El Niño event is still relatively uncertain. There are two key takeaways from the current outlooks and forecasts. One, there is a near-zero probability of a La Niña event in 2017. Given that the Southwest shifts toward warmer and drier winter conditions in La Niña years, this is a welcome alternative. Two, given the relatively weak correlation between cool-season precipitation and weak El Niño events, whether ENSO-neutral or weak El Niño conditions ultimately prevail, the overall seasonal outlook for the Southwest would look relatively similar.

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CLIMAS SW Monsoon Outlook - June 2017

Friday, June 16, 2017

In 2008, the National Weather Service changed the definition of the start of the Southwest monsoon from a variable date based on locally measured conditions to a fixed date of June 15. Prior to 2008, the start date reflected the seasonal progression of the monsoon (Fig. 1). This is based on larger seasonal atmospheric patterns and the establishment of the ‘monsoon ridge’ in the Southwest (Figs. 3a-b, also see sidebar for link to NWS pages).

In Southern Arizona, the start date was based on the average daily dewpoint temperature. Phoenix and Tucson NWS offices used the criteria of three consecutive days of daily average dewpoint temperature above a threshold (55 degrees in Phoenix, 54 degrees in Tucson) to define the start date of the monsoon. As shown in Figure 2, the dewpoint temperature criterion produced start dates ranging from mid-June to late July over the period of record (1949-2016). The average daily dewpoint temperature is still a useful tool to track the onset and progression of conditions that favor monsoon events, and the National Weather Service includes a dewpoint tracker in their suite of monsoon tools.

Thirty-year averages for daily dewpoint and precipitation demonstrate the gradual increase in dewpoint temperatures during the monsoon season, as well as the variability of precipitation observed over the same window (Fig. 4).

The updated definition of the monsoon identifies a season that lasts for 108 days with defined start and end dates of June 15 and Sept 30. In the Southwest, however, the majority of monsoon storm activity occurs in July and August (Fig. 5), with some lingering activity into September (occasionally augmented by eastern Pacific tropical storms). Dewpoint and precipitation may provide a more granular assessment of monsoon activity, but the seasonal designation allows for easier comparisons between years, and focuses planning activities on a discrete monsoon season (see NWS video about Monsoon Awareness Week).

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Southwest Climate Outlook June 2017 - Climate Summary

Thursday, June 15, 2017

Precipitation & Temperature: May precipitation was variable across the Southwest, ranging from average to much-above average in Arizona and below to above average in New Mexico (Fig. 1a). Similarly, May temperatures were average to above average across Arizona and ranged from below to above average in New Mexico (Fig. 1b). Taking a longer view, spring (March-May) precipitation was mostly below average in Arizona, while New Mexico ranged from below average in the southwestern region to above average in the northeast (Fig. 2a). Spring temperatures were much-above average across most of the Southwest (Fig. 2b). So far in June, temperatures have ranged from 0 to 8 degrees above normal across much of Arizona and New Mexico, with extreme heat forecast for the week of June 19. June precipitation has been sparse in most of Arizona, with infrequent storm activity mostly in southern and eastern New Mexico.

Snowpack, Streamflow & Water Supply: While snow is mostly—if not completely—now gone from the Southwest, some Colorado River Upper Basin snow water equivalent (SWE) values remain well-above average (Fig. 3). Above-average temperatures have amplified melting and runoff, leading to impressive streamflow forecasts across much of the West (see last month’s outlook for details) and, in many cases, higher reservoir volumes compared to one year ago.

Drought: The transitional period between cool-season precipitation and the monsoon is one of the driest times of year for the Southwest, but prior to this dry period, seasonal temperature and precipitation patterns had been above and below average, respectively, since mid-January. This led to both short- and long-term drought designations in southern Arizona and the southwestern corner of New Mexico (Fig. 4). Monsoon precipitation can be impressive in its intensity, but these events vary considerably in both their spatial extent and their temporal frequency, therefore they typically do not provide as much drought relief as sustained regional cool-season precipitation.

Wildfire, Environmental Health, & Safety: In the late spring and early summer, warming temperatures, low relative humidity, and sustained and gusting winds all contribute to increased risk of wildfire (Fig. 5). Accordingly, the Southwest has seen higher fire activity over the last month. Many of these fires are lightning-caused, as is common when the emergent monsoon brings convective activity, often in the absence of measurable precipitation. As the monsoon settles in, heavy precipitation events and increased relative humidity help suppress existing fires and reduce seasonal wildfire risk. In fact, the midpoint of the wildfire season follows the seasonal progression of the monsoon (see Monsoon Tracker), and a late start to the monsoon can extend the fire season just as an early start can help shorten it.

El Niño Southern Oscillation: Current forecasts suggest an increased likelihood of ENSO-neutral conditions in 2017 (50-55-percent chance), with a slightly lower chance of an El Niño event (35-50 percent chance) during the same period (see ENSO Tracker for details).

Precipitation & Temperature Forecast: The June 15 NOAA Climate Prediction Center’s outlook for July calls for equal chances of above or below average precipitation in Arizona and New Mexico, and increased chances of above average temperatures across the Southwest. The three-month outlook for July through September calls for equal chances of above or below average precipitation in Arizona and New Mexico (Fig. 6, top). Increased chances of above normal temperatures are forecast for the entire southwestern region (Fig. 6, bottom).

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