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Monthly Archive

Sept 2016 - La Niña Tracker

Friday, September 16, 2016

Climate Summary from the September issue of the CLIMAS Southwest Climate Outlook

Oceanic and atmospheric indicators of the El Niño-Southern Oscillation (ENSO) remain in the range of neutral conditions (Figs. 1-2). In the last month, seasonal forecasts and models further reduced their certainty of a weak La Niña event forming in late 2016 or early 2017. The current projections find ENSO-neutral conditions to be the most likely outcome for this fall and winter, although the chance of a weak La Niña event cannot be ruled out. As past outlooks have noted, there was ongoing uncertainty regarding the prospects for La Niña, especially as it appeared to be having difficulty organizing, with limited coordination between ocean and atmosphere.

A closer look at the various forecasts and seasonal outlooks provides insight into the range of expectations for this La Niña event. On September 8, the NOAA Climate Prediction Center (CPC) highlighted ENSO-neutral conditions in both the ocean and atmosphere, and given the persistent neutral conditions, it shifted its forecast from slightly favoring La Niña to a 55–60 percent chance of ENSO-neutral conditions during fall and winter 2016-17. On September 9, the Japanese Meteorological Agency identified La Niña conditions as being in place, and projected a 70 percent chance that they would remain through winter 2017. On September 13, the Australian Bureau of Meteorology maintained its La Niña watch, but noted the lack of connection between ocean and atmosphere and the generally neutral conditions. The agency identified a limited chance of a “late and weak La Niña” emerging at some point, and highlighted that the borderline weak La Niña conditions may lead to weather patterns more in line with La Niña even if the event is not designated as such. On September 14, the International Research Institute for Climate and Society (IRI) and CPC forecasts described oceanic and atmospheric conditions that were borderline La Niña, but that models indicated a likely retreat into ENSO-netural status during winter 2016-2017 (Fig. 3). The North American multi-model ensemble characterizes the current model spread and highlights the variability looking forward to 2017. In the last month, the ensemble mean returns to neutral conditions over fall and winter 2016-17, whereas previously it had remained close to weak La Niña during this timeframe (Fig. 4).

The Southwest remains in a holding pattern regarding La Niña, even while the current forecast suggests it is now increasingly likely the region will experience an ENSO-neutral winter. Forecasters were likely already integrating the influence of La Niña into long-term precipitation forecasts and seasonal outlooks (Figs. 5a-5b), and it will be interesting to see if and how these forecasts change given the current ENSO-neutral forecast.

It is important to note that while a weak La Niña event would be more likely than not to bring drier-than-average conditions to the Southwest over the cool season, there is no basis to think that ENSO-neutral conditions will bring increased moisture over the winter, as the range of precipitation values for ENSO-neutral cool seasons ranges from very wet to very dry (Fig. 6). This is particularly the case if oceanic and atmospheric conditions are borderline weak La Niña yet they exert some influence on seasonal weather patterns even if not officially designated as a La Niña event.

Sept 2016 Monsoon Tracker

Friday, September 16, 2016

Climate Summary from the September issue of the CLIMAS Southwest Climate Outlook

The monsoon in the Southwest U.S. officially starts June 15 and ends September 30. The National Weather Service began using these dates in 2008 to identify a discrete monsoon period as opposed to relying on varying assessments using dew point temperature and onset of precipitation events. Prior to 2008, the historical start date of the monsoon was based on observed conditions (primarily elevated dew point), which varied across the region in a way that reflected the generally westward migration of the monsoon onset (Fig. 1). Firm start and end dates may not perfectly align with regional variability (especially if monsoon conditions are not present at the calendar start to the monsoon), but standardized dates do provide a more consistent time frame to compare year to year and emphasize the spatial and temporal variability of the monsoon within and across years.

The southwestern monsoon is characterized by spatial and temporal variability. 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 timescale. 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. Totals should even out over the course of the season and vary around the long-term average, but outliers and extremes are always possible. 

Monsoon Recap

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.  By mid-to-late July, 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 again in mid-August, providing a brief extension to storm activity via a surge of moisture from the Gulf of California. The remainder of August and early September saw a decline in widespread monsoon activity, even while numerous areas did see intermittent precipitation, particularly at higher elevations. On September 7, Hurricane Newton generated significant precipitation activity in a swath across southwestern Arizona and extending into central New Mexico, although observed values were less intense and widespread than initially forecast. Recently, the monsoon has been in active decline, with activity shifting to the northwest and regional activity looking more and more like fall across much of the region, although eastern New Mexico has seen an uptick in activity based primarily on surges of moisture from the Gulf of Mexico.

In what remains of the monsoon, limited opportunity exists for moisture and atmospheric circulation patterns to develop into widespread events, although intermittent convective activity could always develop across the region. Eastern Pacific tropical storm activity has the potential to contribute directly via rainfall events or at least to supply additional moisture to the region that could fuel storm activity, but this period of activity extends well beyond the bounds of the official monsoon (and also highlights the ongoing debate as to whether tropical storm events should “count” when discussing the performance of the monsoon).

Monsoon Tracker

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 rainfall (Figs. 2a-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 showing that their seasonal average for monsoon precipitation had been surpassed by early September (Figs. 5a-c), while other stations such as Tacna 3 NE in the southwest corner of Arizona have only recorded a few events and are well below their seasonal total (Fig. 5d).

Cumulative precipitation totals since the start of the monsoon show New Mexico has caught up in the last month. Much of the state has now recorded average to above-average precipitation, although some areas still are below average (Figs. 3a-3b). Relatively widespread, uniform precipitation is indicated by the observation that most of the stations across the state recorded rain on at least 30 percent of days in which rain was recorded anywhere, and a slightly smaller yet still significant region recorded rain on 50 percent of such days (Fig. 4b). Station plots also demonstrate the lagging precipitation in select locations such as Albuquerque (Fig. 5e), while others such as El Paso, Texas, and the Animas 3ESE station recorded above-average precipitation (Figs. 5f-5g).

Southwest Climate Outlook September 2016 - Climate Summary

Thursday, September 15, 2016

Climate Summary from the September issue of the CLIMAS Southwest Climate Outlook

Precipitation & Temperature: August precipitation totals were above average across most of Arizona and New Mexico (Fig. 1a), buoyed by a surge of monsoon storms that started in late July and extended through the first week of August, and a surge of moisture linked to Tropical Storm Javier in mid-August. August temperatures were mostly average to below average in Arizona and New Mexico (Fig. 1b), a decline that did not alter the overall seasonal pattern of very warm temperatures observed during the summer months (Fig. 2). September precipitation to date (Sept 1 – Sept 14) ranges from well above average in southeastern Arizona and portions of New Mexico, tied mostly to heavy rains during Hurricane Newton, to well below average in other areas of the region that did not record as much activity during this time (Fig. 3). Water year precipitation to date (Oct 1, 2015 – present) is below average in much of the Southwest, particularly in Southern California, most of southern Arizona, and western New Mexico (Fig. 4).

Monsoon: With two weeks remaining in the 2016 monsoon (June 15 – Sept 30), seasonal precipitation totals to date are generally below average in much of Arizona, except for clusters in the southeastern corner of the state. During the same time frame, precipitation totals are average to above average in most of New Mexico. These totals gloss over the spatial and temporal variability of the monsoon, but they do provide a rough measure of the monsoon’s performance in terms of intra-regional comparisons and in comparison to prior years. Tropical storm activity (primarily Javier and Newton in 2016) provided a boost to seasonal monsoon totals (for more detailed discussion, see Monsoon Tracker).

Drought & Water Supply: Long-term drought persists across the Southwest (Fig. 5). The Sept. 15 U.S. Drought Monitor designated all of Arizona and much of western and central New Mexico as experiencing some form of drought. The northwest and southeast portions of Arizona, along with most of the designated areas in New Mexico, were classified as abnormally dry (D0), while much of southwestern and northeastern Arizona were designated as experiencing moderate drought (D1). The far southwestern corner of Arizona was designated as experiencing severe drought (D2), reflecting the persistent multi-year drought conditions in central and Southern California. 

La Niña & Drought: Sea-surface temperature anomalies and atmospheric patterns all generally indicate ENSO-neutral conditions. Previous forecasts pointed towards the formation of a weak La Niña. In the last month, model consensus has shifted toward an increased probability of an ENSO-neutral winter, with a declining (but still present) chance of a weak La Niña event (see La Niña Tracker). An ENSO-neutral winter would be less likely to lead to the drier-than-average conditions associated with La Niña, but given the wide range of observed precipitation totals over ENSO-neutral winters (see Fig. 6, La Niña Tracker), this shift does not necessarily lead to increased certainty in the seasonal precipitation forecasts.

Precipitation and Temperature Forecasts: The Sept. 15 NOAA Climate Prediction Center’s seasonal outlook for October calls for equal chances of above- or below-average precipitation for most of the Southwest (Fig. 6 top) and increased chances of above-average temperatures across the western U.S. and the Southwest in particular (Fig. 6 bottom). The three-month outlook for September through November calls for increased chances of above-average temperatures across the U.S., including the Southwest, while calling for equal chances of average, above-average, or below-average precipitation.