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Southwest Climate Outlook Monsoon Tracker - September 2018

Friday, September 21, 2018

Monsoon precipitation varies considerably in space and time across the Southwest, as illustrated by monthly totals for various stations. (Fig. 1). Statewide patterns highlight widespread areas of both above- and below-average totals (see Fig. 4 on p. 5). The Fig. 2 plots of daily precipitation, temperature, and dewpoint temperature for the same stations as Fig. 1 capture the intermittent nature of monsoon precipitation as well as the persistent elevated dewpoint most locations experienced this summer.

In many years, breaks in monsoon activity (sometimes for extended periods) occur, characterized by decreased humidity and increased temperatures. While we had fewer such intervals this year, that was no indicator of consistent precipitation (El Paso and Phoenix are both good examples). As noted last month, sustained periods of high dewpoints without precipitation led to extreme heat warnings in the region and persistent warm overnight temperatures. Without storm-induced cooling, elevated dewpoint temperatures can be downright miserable, especially for households that rely on evaporative coolers for interior climate control.

At the time of this writing (Sept. 20, 2018), southern Arizona had just experienced a surge in tropical moisture that drove storm activity on Sept. 18-19, with preliminary reports of over three inches of rain in some locations, and widespread precipitation across southern Arizona (more on this and tropical storm activity next month).

A look at monsoon precipitation across the Tucson metropolitan area illustrates the spatial heterogeneity of monsoon events. Some areas receive frequent and/or abundant precipitation, while others–often nearby–do not (Figs. 6-7). Individual stations (e.g. the Tucson Airport) track long-term comparisons to normal at that single location, but cannot capture this variability. Higher elevation areas are expected to receive more precipitation, but the range found in lower elevation locations highlights how daily and cumulative totals can vary across a remarkably short distance.

CLIMAS has a new project in collaboration with the National Weather Service in Tucson exploring how to integrate citizen science rainfall observations into monsoon analysis and visualizations, and to compare these observations to official stations and radar derived estimates of precipitation. If you have any questions or want more information, contact Ben McMahan at - RainLog: - CoCoRaHs:

Online Resources

  • Figures 1-2 - CLIMAS: Climate Assessment for the Southwest -

    • Figure 1 Data:
    • Figure 2 Data:
  • Figures 3-5 - UA Climate Science Application Program -
  • Figures 6-7 - CLIMAS: Climate Assessment for the Southwest -

    • Data: NWS-Tucson,, & Pima County Flood Control District

Southwest Climate Outlook September 2018 - Climate Summary

Thursday, September 20, 2018

Precipitation and Temperature: Precipitation in August ranged from much-below average to average in New Mexico and from much-below average to above average in Arizona (Fig. 1a). August temperatures were mostly above average across Arizona and New Mexico, with swaths of much-above average and isolated locations of record-warmest conditions (Fig. 1b). Most of the daily temperature anomalies (deviations above or below the average temperature) have been warmer across the region since July 1 (Fig. 2). Summer (June-July-August) precipitation has varied considerably, from mostly average to below average in New Mexico, and from mostly average to above average in Arizona (Fig. 3).

Monsoon Tracker: Every monsoon is different, and this year has been characterized by abundant moisture available over much of the period, yet with a wide variety of precipitation totals (see Monsoon Tracker for details). This variability is characteristic of the monsoon, but we have seen fewer extended breaks in the monsoon compared to previous years.

Drought: Water-year precipitation continues to highlight persistent moisture deficits in Arizona and much of New Mexico–particularly in the Four Corners region (Fig. 4). These conditions are reflected in the Sept. 18 US Drought Monitor, which identifies exceptional drought conditions (D4) in northeastern Arizona and northern New Mexico as well as less-intense drought designations (D1-D3) across the remainder of the two-state region (Fig. 5). The overall drought picture in the USDM characterization is actually improved from a few months ago, with smaller areas characterized as experiencing exceptional drought, but this in part reflects the highly localized nature of monsoon precipitation. The storms brought drought relief to localized areas, but others received below-average precipitation during the same time period. Despite the short-term upticks in precipitation observed locally, the entire region is still impacted by the longer-term, cool-season precipitation deficits that have characterized drought conditions in the Southwest over much of the past few decades.

Tropical Storms: The scale and impact of Hurricane Florence in the Carolinas has occupied much of the storm-related attention recently, but the eastern North Pacific hurricane season has been quite active, with 16 named storms at the time of this writing, including 6 major hurricanes (category 4 or above). 2018 is currently the fourth most intense Pacific hurricane season, with an Accumulated Cyclonic Energy of 218 (the record is 295 in 1992). These storms have generally moved westward into the Pacific Ocean (Fig. 6), but we are approaching the part of the season when these storms are more likely to curve back into the Southwest, where they can provide supplemental moisture for convective activity or even drive precipitation events directly, as we have seen in the past with Norbert, Odile, and others.

El Niño Tracker: ENSO-neutral conditions still characterize the oceanic and atmospheric indicators, but the most recent models and forecasts continue to point toward the emergence of a fall-into-winter El Niño event as the most likely outcome. However, expectations have been scaled back from earlier predictions, with the recent forecasts suggesting a weak event rather than a possible moderate-strength event. Adding further uncertainty, the possibility also remains of an El Niño event fizzling out before it even gets started, with neutral conditions simply continuing through the fall and winter. Whether weak El Niño or ENSO-neutral, both scenarios give the Southwest a better chance of improving on the drier-than-normal conditions associated with La Niña events.

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

Online Resources

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

Southwest Climate Outlook El Niño Tracker - September 2018

Thursday, September 20, 2018

With little change from last month, the Southwest remains in an ENSO holding pattern. Oceanic and atmospheric conditions are still within the range of ENSO-neutral (Figs. 1-2), but the forecasts point toward the likely emergence of an El Niño event this fall and lasting into the winter. On Sept. 10, the Japanese Meteorological Agency (JMA) identified continued ENSO-neutral conditions in oceanic and atmospheric indicators, with a 60-percent chance of El Niño developing this fall. On Sept. 11, the Australian Bureau of Meteorology maintained its “El Niño Watch;” most indicators are within the range of neutral, but models suggest a warming tropical Pacific in the coming months. The agency predicts a 50-percent chance of El Niño formation by the end of this year. Similarly, on Sept. 13, the NOAA Climate Prediction Center (CPC) continued its El Niño watch, identifying persistent neutral conditions now but with seasonal outlooks and models predicting the emergence of an El Niño later this year. Based on these models, the current forecasts call for a 50- to 55-percent chance of an El Niño event developing this fall, and a 65- to 70-percent chance of El Niño conditions this winter. On Sept. 13, the International Research Institute (IRI) issued its ENSO Quick Look, also indicating continued neutral conditions in the oceans and atmosphere currently, but calling for a nearly 70-percent chance of an El Niño event by the end of 2018 (Fig. 3). The North American Multi-Model Ensemble (NMME) has stabilized over the past few months but continues to indicate warmer-than-average ocean temperatures and the likelihood of a weak El Niño event by the end of 2018 (Fig. 4).


Summary: Despite persistent ENSO-neutral conditions, most outlooks have held steady in predicting the most likely outcome for late 2018 to be the formation of an El Niño event. With forecast probabilities between 50 and 70 percent and the continued discussion of this event, it starts to feel as though an El Niño is likely. Given the effectively zero-percent chance of a La Niña event, the remainder of these forecast probabilities call for an ENSO-neutral fall and winter (2018-2019), with a 30- to 50-percent chance of neutral conditions through early 2019. A few of the forecasts noted that while the current oceanic and atmospheric indicators are neutral, they are seeing something in the models that gives them (relative) confidence of an El Niño event by the end of 2018. Such events have fizzled before, however, so given the relative weak strength of the current trends towards El Niño conditions, we would be wise to just wait and see how conditions develop over the next few months.


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 -