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SWCO Aug 2016 - La Niña Tracker

Friday, August 19, 2016

Oceanic and atmospheric indicators of the El Niño-Southern Oscillation (ENSO) remain in the range of neutral conditions (Figs. 1-2). Seasonal forecasts and models identify the most likely scenario being a weak La Niña event forming sometime in late summer or fall 2016 and lasting through winter 2017. Some uncertainty exists regarding the specific timing of this event, as the equatorial sea surface temperature (SST) anomalies have not yet dropped into La Niña range and there is a lack of coordination between ocean and atmosphere (and in particular the lack of enhanced trade winds).

A closer look at the various forecasts and seasonal outlooks provides insight into the range of expectations for this La Niña event. On August 10, the Japanese Meteorological Agency saw below-normal equatorial convective activity and enhanced trade winds as indicative of the start of La Niña-favorable conditions, even while the SST anomalies were slow to swing more negative (cool) after the sustained warm period associated with El Niño conditions. The agency forecast a 70 percent chance of a La Niña event developing, but not until fall 2016. On August 11, the NOAA Climate Prediction Center (CPC) highlighted ENSO-neutral conditions in both ocean and atmosphere and continued to focus on tension between statistical and dynamical models, the former predicting a later onset and weaker event than the latter. The CPC forecast remains at a 55–60 percent chance of a weak La Niña event starting sometime between August and October 2016. On August 16, the Australian Bureau of Meteorology maintained its La Niña watch, albeit with slightly reduced confidence compared to the previous month, hedging slightly by stating “if La Niña does develop it is likely be weak.” The bureau maintained its forecast probability at a 50 percent chance of a La Niña event developing, noting that is approximately twice the normal chance of a La Niña forming. On August 18, the International Research Institute for Climate and Society (IRI) and CPC forecasts indicated the probability of a “borderline/weak” La Niña forming was just less than 60 percent (Fig. 3). The IRI forecasters noted that until the atmosphere cooperates (i.e., the enhanced trade winds show up), the ENSO-neutral holding pattern will remain. The North American multi-model ensemble characterizes the current model spread and highlights the variability looking forward to 2017, but the ensemble mean hovers close to weak La Niña status for fall and winter of the coming year (Fig. 4).

The Southwest is in a holding pattern regarding La Niña but will likely see the effects of a weak La Niña this winter. Forecasters are likely already integrating the influence of the drier-than-average signal associated with La Niña into long-term precipitation and temperature forecasts and seasonal outlooks (Fig. 5a-b), and researchers are preparing to track both the timing and intensity of this event in relation to precipitation, temperature, snowpack, and water supply over the coming year. 

Given the dry climate of the Southwest, a weak La Niña event does not necessarily deviate far from the seasonal climatology of the region, but while ENSO-neutral winters have a wider range of precipitation values observed over winter, La Niña winters skew more dry. Even a weak La Niña event is likely to suppress winter precipitation totals in the Southwest, which is unwelcome news given the underperformance of last year’s El Niño winter and the longer-term effects of multi-year drought.

SWCO Aug 2016 - Monsoon Tracker

Friday, August 19, 2016

The Southwest saw the first strong burst of widespread monsoon activity near the end of June.  Most of the first half of July was characterized by a distinct break in monsoon activity, as atmospheric circulation patterns and lack of available moisture limited opportunities for widespread storms to develop, especially at lower elevations.  As July progressed, there were increasingly favorable conditions for storms to develop and spread, culminating in an extended period of widespread activity during late July and early August. Tropical Storm Javier helped jumpstart activity in mid-August, just as the previously mentioned extended run was winding down, and provided a brief extension to storm activity via a surge of moisture from the Gulf of California. The remainder of the monsoon window will be a waiting game to see if favorable moisture and atmospheric circulation patterns develop, as well as the potential influence of eastern Pacific tropical storm activity that could supplement storm activity and provide additional moisture to fuel storm activity.

Based on cumulative totals since the official start of the monsoon, most of Arizona recorded average to below-average precipitation, even while clusters of the state have recorded above-average precipitation as a percent of average (Fig. 2a) and in terms of outright totals (Fig. 3a). The percent of days with rain highlights the spatial variability of the monsoon and emphasizes the clustering of storms in the southeastern corner of the state (Fig. 4a). Precipitation plots from specific stations further highlight this variability, with Douglas, Flagstaff, and Tucson stations all on pace for average to above-average monsoon totals (Figs. 5a-c) but with the Tacna station in the southwest corner of Arizona yet to record a precipitation event (Fig 5d).

Cumulative totals since the start of the monsoon show that much of New Mexico recorded average to below-average precipitation, but with wider areas of above-average rainfall as well (Figs. 2b – 3b). The percent of days with rain also highlights the more regular coverage seen in New Mexico, with a wider swath of increased activity that covers most of the state (Fig. 4b). Station plots also demonstrate the lagging precipitation in select locations, with Albuquerque and El Paso, Texas, lagging behind seasonal totals (Figs. 5e-f). Stations closer to southern Arizona, such as the Animas 3ESE station, are recording above-average precipitation (Fig. 5g).

The southwestern monsoon is characterized by a high degree of spatial and temporal variability. Over the course of the season, storm events are interspersed with breaks of limited activity as migrating high pressure systems and available moisture dictates where in the Southwest rain might fall. This results in highly variable precipitation totals on a daily or weekly scale. Regional climatology gives some indication as to the expected cumulative total precipitation any location might expect but says less about how those precipitation totals will be achieved. Any given year of monsoon activity is difficult to categorize on a week-to-week basis, and simple score-carding using seasonal precipitation to date will be skewed by recent runs of heavy rain or extended breaks in the monsoon. Looking at daily precipitation event maps, there are very few days in which no rain fell anywhere in Arizona or New Mexico; there are numerous locations that saw little to no rain while nearby regions saw extensive precipitation.  These totals should generally start to even out over the course of the season and vary around the long-term average, but outliers and extremes are always possible.  As stated above, the remainder of the season is a waiting game to determine if and when a favorable atmospheric circulation brings additional storm activity into the region, or if tropical storm activity can help jumpstart these storms with additional surges of Gulf moisture.

Southwest Climate Outlook August 2016 - Climate Summary

Thursday, August 18, 2016

Precipitation and Temperature: July precipitation totals were below average across most of Arizona and New Mexico in the past 30 days (Fig. 1a), due in part to an extended break in monsoon activity. July temperatures were above average across nearly the entire region (Fig. 1b), linked to global trends that saw record-warm temperatures in 2016 and to regional patterns of warmer and drier conditions that correspond to the aforementioned break in monsoon activity. August precipitation to date is average to above average for most of Arizona and portions of New Mexico (Fig. 2), partly linked to moisture associated with Tropical Storm Javier that resulted in heavy precipitation in parts of the Southwest. In August, temperatures have been mostly average to below average in Arizona and mostly above average in New Mexico (Fig. 3).

Monsoon: A strong start for the monsoon in late June was followed by an extended break in activity for much of July. The end of July saw another multi-day system that brought frequent storm activity to the region, particularly in southeastern Arizona, which has seen some of the heaviest monsoon activity this year (see Monsoon Tracker). Seasonal totals to date for New Mexico generally have been below average, although August rainfall has helped make up some of the precipitation deficit.

Drought & Water Supply: Long-term drought persists across the Southwest (Fig. 4), reflecting multiple years of drought conditions and accumulating precipitation deficits. In the Aug. 16 U.S. Drought Monitor, most of Arizona was designated as experiencing moderate drought (D1), while most of New Mexico was designated as abnormally dry (D0). This pattern is unlikely to reverse in the short term, especially with a weak La Niña event likely occurring this fall into winter, which could bring warmer and drier conditions to the Southwest. Water year precipitation to date is below average in much of the Southwest, particularly in Southern California, most of southern Arizona, and western New Mexico (Fig 5).

La Niña: Sea surface temperature anomalies and atmospheric patterns all indicate ENSO-neutral conditions. Uncertainty remains as to whether an actual La Niña event will occur, but current model consensus points toward the formation of a weak La Niña event between now and October that is expected to last through winter 2017 (see La Niña Tracker).

Wildfire: Cooler and wetter-than-average conditions linked to the lingering effect of El Niño tamped down early-season fire activity in April and May. Fire activity picked up in June and July, but precipitation from cold front, monsoon, and tropical storm activity, as well as increased relative humidity across the region, helped limit the risk of severe wildfire. As of August, wildland fires had burned approximately 220,000 acres in Arizona and 274,000 acres in New Mexico. Favorable weather conditions permitted fire managers to let a number of fires burn for beneficial use to reduce future wildfire risk.

Precipitation & Temperature Forecasts: The July 21 NOAA-Climate Prediction Center’s seasonal outlook for September calls for increased chances of above-average precipitation for most of Arizona and western New Mexico (Fig. 6) and increased chances of above-average temperatures across the western U.S. (Fig. 6). The three-month outlook for September through November calls for increased chances of above-average temperatures across the U.S.— and the Southwest in particular—while calling for equal chances of average, above-average, or below-average precipitation.