The University of Arizona

Monthly Archive | CLIMAS

Monthly Archive

July 2018 SW Climate Outlook - Monsoon Tracker

Friday, July 20, 2018

In 2008, the National Weather Service (NWS) 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 (and a fixed end date of Sept 30). Prior to 2008, the flexible start date reflected the seasonal progression of the monsoon, with a considerable temporal gradient across the region (Fig. 1).

In Southern Arizona, the monsoon 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 (Fig. 2). Using that definition, the monsoon began in Tucson and Phoenix on July 8 this year. While an imperfect measure, this increase in dewpoint temperature contextualizes the slightly later-than-average start to monsoon activity compared to climatology, and roughly corresponds with upticks in precipitation activity across the Southwest (Fig. 3). Despite the relatively late start, the monsoon is now in full swing.

The seasonal totals to date (Fig. 4 on p. 5), the percent of normal precipitation (Fig. 5 on p. 5) and percent of days with rain (Fig. 6 on p. 5) all help characterize the spatial variability and intensity of the monsoon thus far. Tucson’s monsoon precipitation (p. 6) illustrates the amount of variability that can occur at a municipal scale.

Within the Tucson metropolitan region, an initial surge of activity associated with the remnants of Hurricane Bud brought considerable rainfall to the region June 15-17 (see last month’s Outlook for details). For much of the rest of June and the first week of July, however, there were few storms. Around July 8, rainfall picked up with widespread activity on a nearly daily basis, but with a wide range of daily totals from site to site (Fig. 7). Cumulative totals for the first month of the monsoon further reveal the extent to which some areas have received frequent and/or abundant precipitation, while others locations–often relatively nearby–have not (Fig. 8).

Online Resources

  • Figure 1 - International Research Institute for Climate and Society -
  • Figures 2-3 - CLIMAS: Climate Assessment for the Southwest -
  • Figures 4-6 - UA Climate Science Application Program -
  • Figures 7-8 - CLIMAS: Climate Assessment for the Southwest - Data: & Pima County Flood Control District

July 2018 SW Climate Update - ENSO Tracker

Thursday, July 19, 2018

Oceanic and atmospheric conditions remained ENSO-neutral over the last month (Figs. 1-2) and most ENSO forecasts and outlooks reflect that. On July 10, the Japanese Meteorological Agency (JMA) identified neutral conditions in oceanic and atmospheric indicators, but with indications of warming oceanic temperatures in the coming months. The agency forecast equal chances (50 percent) of either ENSO-neutral or El Niño by fall 2018. On July 12, the NOAA Climate Prediction Center (CPC) continued its El Niño watch, observing neutral conditions at present, however the warming oceanic temperatures are seen as an indicator of increasing likelihood of El Niño this year. Its outlook calls for a 65-percent chance of an El Niño event developing this fall, increasing to 70 percent this winter. On July 12, the International Research Institute (IRI) issued an ENSO Quick Look that similarly noted neutral conditions in the oceans and atmosphere now and warming oceanic conditions increasing forecast probabilities for an El Niño event to nearly 70 percent by the end of 2018 (Fig. 3). The forecast predicts weak conditions in the initial event development but potentially reaching moderate strength during the fall and winter. On July 17, the Australian Bureau of Meteorology rated its ENSO Outlook at “El Niño Watch.” Most indicators were within the range of neutral, but they too noted steady warming of surface and subsurface waters in the Pacific Ocean that warrant a 50-percent chance of El Niño formation by the end of this year. The North American Multi-Model Ensemble (NMME) also demonstrates the steady trend in observations and forecast toward warmer-than-average ocean temperatures, and is zeroing in on a weak to borderline moderate El Niño event by the end of 2018 (Fig. 4).


Summary: Steady warming in surface and subsurface temperatures in the Pacific led to increasingly bullish forecasts for an El Niño event by the end of 2018, perhaps even sooner. Given the timing and uncertainty in the models and forecasts, the formation of this event is still far from certain, however as mentioned last month, a La Niña event is all but impossible this year. Additionally, the intensity and timing of the event will play a large role in how much it affects tropical storm activity this fall and cool-season precipitation this winter and spring. Most outlooks—for now—are calling for a weak to borderline moderate event.

Online Resources

  • Figure 1 - Australian Bureau of Meteorology -
  • Figure 2 - NOAA - Climate Prediction Center -
  • Figure 3 - International Research Institute for Climate and Society -
  • Figure 4 - NOAA - Climate Prediction Center -

Southwest Climate Outlook July 2018 - Climate Summary

Wednesday, July 18, 2018

Precipitation and Temperature: Precipitation in June ranged from record driest to much-above average (Fig. 1a); the wetter-than-average areas were those impacted by the remnants of Hurricane Bud in mid-June. June is typically dry, barring an early onset of monsoon activity, so any pre-monsoon precipitation will boost percentile rankings. June temperatures were warmer than average across all of Arizona and New Mexico (Fig. 1b), including record-warm conditions in eastern Arizona and central New Mexico. Notably, unlike recent years, no extreme heat waves affected the region in June, although sustained warmer-than-average temperatures did occur throughout the month and into July (Fig. 2). Year-to-date precipitation and temperature reveal average to record-driest precipitation and much-above-average to record-warmest temperatures (Fig. 3a-b).

Monsoon Tracker: The monsoon is off and running with widespread activity across Arizona and western New Mexico. This uptick in activity is linked to increases in atmospheric moisture (dewpoint, precipitable water), wind patterns, and atmospheric circulations that must come together for storm development and progression. As is often the case in the Southwest, the location and intensity of rainstorms have varied considerably across time and space, with a wide range in observed precipitation across the entire region, and sometimes even within the same metropolitan area (see Monsoon Tracker for details).

Drought: Water-year precipitation to date reveals persistent deficits across most of Arizona and much of New Mexico, including record-dry conditions across the upper third of both states (Fig. 4). With the onset of the monsoon, precipitation and humidity have increased considerably, bringing a stark contrast to the dry conditions of much of the last 12 months. This highlights an annual conundrum: how much does monsoon precipitation mitigate drought conditions in the Southwest? The question centers on the nature of monsoon precipitation, which is often of high intensity and subject to runoff and evaporation, and has a high degree of spatial variability, resulting in “winners” and “losers” in terms of monthly and seasonal totals. The July 19 USDM does not reveal much in the way of improved drought conditions in most of Arizona and New Mexico, as the short-term uptick in precipitation with the onset of the monsoon was not enough to reverse months of precipitation deficits compounded by the impact of warmer- and drier-than-average conditions (Fig. 5).

Wildfire: Widespread moisture and monsoon activity helped tamp down fire risk in July. The fire season is not over yet but the period of highest wildfire risk is. Given the widespread dry, windy, and warm conditions observed in the Southwest this winter and spring, fire season was not as catastrophic as some feared it would be, especially in Arizona (see Fig. 6 for a summary).

El Niño Tracker: Neutral conditions are present in oceanic and atmospheric indicators and are expected to remain neutral through summer. Seasonal outlooks indicate increasing chances of an El Niño event in 2018, with El Niño conditions likely to emerge by fall or winter (see El Niño Tracker for details). Above-average winter precipitation is one characteristic of El Niño in the Southwest, but if the event develops earlier rather than later this fall, it also could help enhance eastern Pacific tropical storm activity. This in turn could promote increased precipitation in the Southwest this fall, especially if these tropical storms bend back into the Southwest and drive moisture into the region. By way of comparison, last year—a La Niña year—brought little tropical storm activity to augment precipitation totals for either the monsoon or the fall season.

Precipitation and Temperature Forecast: The three-month outlook for July through September calls for increased chances of above-normal precipitation in Arizona and western New Mexico, with equal chances in central and eastern New Mexico (Fig. 7, top). The outlook calls for increased chances of above-average temperatures for the entire Southwest (Fig. 7, bottom).

Online Resources

  • Figures 1,3 - National Centers for Environmental Information -
  • Figures 2,6 - Climate Assessment for the SW -
  • Figure 4 - Western Regional Climate Center -
  • Figure 5 - U.S. Drought Monitor -
  • Figure 7 - NOAA - Climate Prediction Center -