The University of Arizona

Monthly Archive | CLIMAS

Monthly Archive

June 2018 SW Climate Outlook - Monsoon Tracker & Tropical Storm Bud

Friday, June 22, 2018

Was our mid-June precipitation the monsoon? 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). This allowed for a clear delineation of the period of monsoon activity (108 days) and focused NWS’s messaging strategy as it pertains to the expected hazards during that period, which include extreme heat, strong winds, dust storms, flash flooding, lightning, and wildfires (see NWS Tucson monsoon information hub). Prior to 2008, the flexible start date reflected the seasonal progression of the monsoon, with a considerable temporal gradient across the region (Fig. 1).

This gradient is linked to seasonal atmospheric patterns and the establishment of the “monsoon ridge” in the Southwest (Figs. 3a-b, also see sidebar for link to NWS pages). The heating of the complex topography of the western U.S. with the increasing sun angle and contrast with the cooler water of the adjacent Pacific Ocean lead to the establishment of this upper-level ridge of high pressure over the Southwest U.S. (also known as Four Corners High). The flow around this upper-level ridge shifts from a dry southwesterly fetch in May to a moisture-rich southerly-southeasterly fetch in late June/early July (see figure below).

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. As shown in Figure 3 the dewpoint temperature criterion produced start dates ranging from mid-June to late July over the period of record (1949-2016).

Notably, by this metric, the earliest start date on record was June 17, but early July is far more common. In any case, June precipitation is relatively rare in southern Arizona (Fig. 4), thus the recent incursion of moisture from Tropical Storm Bud was a welcome change to our typical mid-June dry heat. Arriving in southern Arizona precisely on June 15 – the now official start of the monsoon – Bud raised the question: Did this moisture qualify as “monsoon”?

The moisture from Tropical Storm Bud was key to the widespread event, but the rain also was dependent on a low-pressure system that happened to be nearby. Bud was caught between a trough of low pressure off the coast of California and the subtropical ridge which was displaced well to the east over the Gulf of Mexico. The flow pattern over this event resembled a “transition” pattern typically seen at the end of the monsoon season (Fig. 5), when the mid-latitude jet stream becomes more active and the monsoon ridge starts to retreat south. Together these features helped guide the storm into southern Arizona.

The approaching trough of low pressure was also critical to cooling upper-level air temperatures, increasing the instability of the very moist airmass at the surface, and providing wind shear to help organize any storms that formed. This kind of assist is possible at the beginning of the monsoon in June but is much more common in late summer when we are transitioning out of the monsoon. It rarely occurs in the middle of the monsoon because of the dominance of the subtropical ridge pattern that limits how close mid-latitude storms can get to the Southwest. Ultimately, the storm was less dependent on the exact track that Bud took in mid-June (Fig. 6), and more dependent on the coalescence of larger atmospheric patterns that came together to bring welcome – if unexpected – precipitation to the Southwest.

The following images are examples of different data visualizations of this regional variability.

Online/Image Resources

  • Figure 1 - International Research Institute for Climate and Society -
  • Figures 2a-2b - Monsoon Definition & Progression - National Weather Service - Tucson -
  • Figure 3 - CLIMAS: Climate Assessment for the Southwest -
  • Figure 4 - CLIMAS: Climate Assessment for the Southwest -
  • Figure 5 - National Weather Service Tucson -
  • Figure 6 - National Hurricane Center -
  • Figure 7 - UA Climate Science Application Program -
  • Figure 8 - Rainlog Citizen Science Monitoring Program -
  • Figure 9 - CLIMAS: Climate Assessment for the Southwest -

Southwest Climate Outlook June 2018 - Climate Summary

Thursday, June 21, 2018

Precipitation and Temperature: The Southwest was characterized by below-average precipitation in May, ranging locally from record driest to near average (Fig. 1a). Temperatures were above average to much-above average across most of the Southwest, with small pockets of record-warm conditions in the northwest corner of New Mexico and along the eastern edge of the state (Fig. 1b). The March through May period exhibited similar patterns of mostly drier-than-average to record-dry precipitation (Fig. 2a) and much-above-average to record-warm temperatures (Fig. 2b). Water-year precipitation to date (Oct 2017 – May 2018) highlights how dry most of the region has been at a longer timescale, with below-normal to record-dry conditions across Arizona and above-normal to record-dry conditions in New Mexico (Fig. 3).

Monsoon & Tropical Activity: The Pacific tropical storm season got off to a strong start with Aletta and Bud, the former as an early start to the season in May, and the latter bringing well-above-normal June precipitation to parts of the Southwest (see Monsoon/TS tracker).

Snowpack & Streamflow Forecast: Snow was all but gone from the Southwest by June, and snow water equivalent (SWE) for the Upper Colorado River Basin remain below average, with only the upper Great Basin and Pacific Northwest having any semblance of above-normal snowpack. Warm and dry conditions continue to affect streamflow and runoff timing – a pattern that extends to the Upper Colorado River Basin, where streamflow forecasts are all well-below average.

Drought: Drought-designated areas continued to expand from last month. In the June 21 U.S. Drought Monitor, Arizona and New Mexico saw further increases in the extent and intensity of drought (Fig. 4). These designations reflect short-term precipitation deficits, above-normal temperatures at monthly and seasonal timescales, and longer-term drought that tracks the cumulative effect of extended periods of warmer- and drier-than-normal conditions. The surge of tropical storm activity (Bud) in mid-June brought a welcome reprieve from ongoing dry conditions, but the next realistic hope for drought relief is the summer monsoon. The extent of its impact will depend on when it starts and how much (and how regularly) precipitation actually falls.

Wildfire: The National Significant Wildland Fire Potential Outlook for June identified above-normal wildland fire risk across the Southwest except for eastern New Mexico and far northwestern Arizona, while the outlook for July calls for the fire risk to return to normal (Fig. 5a-b) in anticipation of monsoon moisture abating the risk. Southeastern Arizona and portions of New Mexico received precipitation linked to the remnants of Tropical Storm Bud, but regional patterns are not indicative of an early start to widespread monsoon activity (see Monsoon Tracker). A late start to the monsoon could extend the fire-risk window, especially if long periods of dry lightning—a major ignition risk in June and July—precede precipitation. The region has been relatively fortunate in 2018, with less lightning-caused fire activity than might have been expected (Fig. 6), given the exceptionally warm and dry conditions over the winter and above-normal fine-fuel loading and continuity.

El Niño Tracker: Neutral conditions are present in oceanic and atmospheric indicators, and longer-term outlooks indicate increasing chances of an El Niño event in 2018. Both the timing and the probability of an El Niño event are still uncertain, but most forecasts highlighted an increased chance of El Niño forming compared to last month, with now nearly twice the chance compared to ENSO neutral conditions (see ENSO tracker). Notably, there is nearly zero chance of a La Niña event in 2018.

Precipitation and Temperature Forecast: The three-month outlook for June through August 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/Image Resources

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

June 2018 SW Climate Update - ENSO Tracker

Thursday, June 21, 2018

Oceanic and atmospheric conditions remained ENSO-neutral over the last month (Figs. 1-2), and most ENSO forecasts and outlooks reflect these conditions. On June 5, the Australian Bureau of Meteorology maintained its ENSO Outlook at “inactive,” with neutral conditions likely to persist through summer. However, the agency noted that models forecast warming conditions in surface temperatures in the tropical Pacific Ocean—a precursor to the emergence of an El Niño event. On June 11, the Japanese Meteorological Agency (JMA) saw an end to lingering La Niña conditions in spring 2018, a 70-percent chance of ENSO-neutral conditions over summer, and a 50-percent chance of either El Niño or neutral conditions this fall. On June 14, the NOAA Climate Prediction Center (CPC) issued an El Niño watch even while short-term conditions were expected to remain ENSO-neutral. CPC indicates a 50-percent chance of an El Niño event developing this fall and a 65-percent chance of El Niño conditions this winter. Similarly, but with earlier timing, the International Research Institute’s (IRI) June 19 ENSO Quick Look calls for a 50-percent chance of an El Niño event this summer and a 65-percent chance of El Niño over the fall. IRI predicts the event to be weak initially but to potentially reach moderate strength during the fall and winter. The North American Multi-Model Ensemble (NMME) returned to ENSO-neutral conditions, and despite uncertainty over the latter half of 2018, also is increasingly suggestive of a weak to moderate El Niño event by the end of 2018 (Fig. 3).

Summary: As recently as last month, ENSO-neutral conditions were seen as a near-certainty over summer. Forecasters are increasingly bullish on the likelihood of an El Niño event by the end of 2018, and more recent outlooks have increased the chances of an earlier start. A closer look at how these forecasts compare to climatology captures how these seasonal forecasts compare to long-term patterns. The most recent IRI plots indicate a roughly 65-percent chance of El Niño (Fig. 4, red bars), which is approximately 30 percent higher than climatology (red line), while the roughly five-percent chance of La Niña (blue bars) is about 30 percent below climatology (blue line). This illustrates that 1) an El Niño event is increasingly possible by the end of 2018, but it is far from certain; 2) a La Niña event is all but impossible; and 3) the chance of ENSO-neutral conditions is roughly equivalent to climatology. It is still relatively early, but current indications are now favoring the formation of an El Niño in 2018.

Online/Image 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 -