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Notes from an Applied Climatologist: Monsoon End Q&A

Monday, September 29, 2014

How Do We Know When the Monsoon is Over?

Across the southwest United States, the start of the summer monsoon season is pretty easy to recognize once you have experienced it firsthand a few times. Typically, one week it's hot and dry, and the next week, it's hot and sticky, but hopefully raining. This is a predictable part of the southwestern summer, and typically happens in late June or early July across Arizona and New Mexico (often first in New Mexico with Arizona trailing just behind). The start is relatively clear cut, but calling an end to the monsoon season is a bit trickier, because there isn’t a rapid and clean transition back to some other non-monsoon weather conditions in the fall, and the presence of monsoon breaks can complicate this transition.

The monsoon circulation pattern and resultant precipitation across the Southwest is largely governed by the position and strength of the subtropical high, also known as the monsoon ridge. This high pressure system builds north through Mexico into the Southwest from June into July, causing the winds to shift from a dry westerly flow in mid-levels of the atmosphere to a moister, subtropical easterly flow. This shift in wind direction is a key to defining the beginning, as well as the end of the monsoon season. The monsoon ridge typically starts to weaken and retreat through the month of September, but can build and subside throughout the month. September is also often a time when tropical storm moisture can interact with early season autumn storms moving in from the north Pacific, sparking widespread thunderstorm activity. These ‘transition events’ are further indication that the monsoon season is almost over and that fall weather is on the doorstep.

Monsoon Summary (June 15 – Sep 18)

Friday, September 19, 2014

We are nearing the end of the 2014 season, and while it is difficult to characterize the highly variable day-to-day storms of any monsoon as “normal,” we have had a fairly typical if not above-average monsoon season in terms of precipitation. Regional assessment is complicated by the effects of a few extreme events that amplified precipitation amounts in parts of Arizona and New Mexico and caused an entire month’s or year’s worth of precipitation to fall in a single storm.

Southeast, southern, west-central, and the high-elevation areas of central Arizona have all seen impressive monsoon totals, with precipitation ranging from 200 to 400 percent of average. Most of Arizona, in fact, has seen above-average seasonal monsoon precipitation (100-200 percent of average) with the exception of the Four Corners region, which is struggling with below-average precipitation and long-term drought (Figure 1). The intensity of these storms, measured as the ratio of total precipitation over the time period to the number of days observing rain, in inches per day (Figure 2), reveals that some areas—western Arizona in particular—received a significant portion of their monsoon precipitation in a few extreme events, and in some cases a single storm. These intense storms offer little in the way of long-term drought relief but pose major threats in terms of their destructive potential, especially in urban/metropolitan areas. Figure 3 (the percentage of days observing 0.01 inch or more) illustrates which areas received more consistent and steady rain.

New Mexico has seen a strong monsoon as well, with most of the state receiving well-above-average precipitation, and large portions of central and southern New Mexico receiving 200 percent or greater of average precipitation. As with Arizona, the Four Corners region is below average, as is the northeastern corner of the state (Figure 4). Maps of the intensity and frequency of monsoon precipitation in New Mexico (Figures 5 and 6, respectively) show larger areas of more frequent, less intense storms. This precipitation should help mitigate short-term drought conditions, but long-term deficits remain.

Tropical storms have been active in the Pacific, and while early-season storms veered into the Pacific Ocean, recent storms (Marie, Norbert, and Odile) have followed the later-season pattern of re-curve into the Pacific coast, boosting precipitation in the Southwest (albeit in a highly variable way). Norbert caused considerable flooding in Phoenix and to a lesser extent in Tucson, and on September 17 Odile caused most of southern Arizona and New Mexico to brace for the worst, with projections of 3-6 inches of rain for those in the direct path. The storm eventually swung south, and most of the impacts were felt in northern Mexico and far-southern Arizona (Figure 7).

This post was originally published as part of the September 2014 Southwest Climate Outlook

El Niño Tracker - Sept 2014

Thursday, September 18, 2014

The song remains the same this month with El Niño not quite here yet, but probably soon. This is now the seventh consecutive month since the NOAA Climate Prediction Center issued an “El Niño Watch” last March. The signs are a bit stronger once again, but it is getting late in the game since El Niño events take several months to build up and typically peak during the mid-winter months. Another slug of warm water (known as a “Kelvin Wave”) has been making its way across the Pacific Ocean from west to east just below the surface and is poised to emerge and help warm sea-surface temperatures in the eastern Pacific over the next month or so. There has also been some activity in the western and central Pacific called “westerly wind bursts” which can help move this warmer-than-average water to the east, but the bursts have been temporary and haven’t helped sustain a steady progression towards El Niño conditions. 

Forecasts models are predicting the current Kelvin Wave and associated warm water in the east Pacific will finally get this fickle event to organize and roll forward as at least a weak El Niño. The mid-September consensus forecast (Figure 1) issued by the International Research Institute for Climate and Society (IRI) and the NOAA Climate Prediction Center (NOAA-CPC) still indicate a greater than 70 percent chance of El Niño conditions developing during the November-December-January period and most likely persisting through early next spring. Most models indicate that the event will ultimately peak at a weak strength, with only a handful of models suggesting a moderate-strength event. The impacts associated with weak El Niño events are much less certain; past events have brought both dry and wet conditions to the southwest U.S. during the winter season. Seasonal precipitation forecasts still indicate an enhanced chance of above-average precipitation over the upcoming winter season, but confidence in this forecast has wavered slightly because of the expected weak nature of the emerging El Niño event.

This post was originally published as part of the September 2014 Southwest Climate Outlook

Notes from an Applied Climatologist: Precipitable Water Q&A

Friday, September 12, 2014

 

What are the different ways that moisture is tracked in the SW?

Measuring accumulated precipitation is a (relatively) straight forward process that mostly involves a ground-based network of rain gauges that measure the depth of rainfall that accumulates, although some gauges, like tipping bucket gauges, also measure intensity. 

Tracking the atmospheric moisture that sets the stage for precipitation events is a much more complicated endeavor. Atmospheric moisture varies at all levels in the troposphere - the lowest level of the Earth’s atmosphere and where our weather happens.  This requires a three dimensional approach (really four dimensional when you factor in these measurements are made over time) when measuring and tracking atmospheric moisture.

The primary way that the vertical dimension of the atmosphere is sampled is with weather balloons and radiosondes. These are instrument packages attached to helium balloons that measure atmospheric pressure, temperature and humidity, as they quickly ascend (about 1000 ft/minute) through the tropopause. This is done twice a day at approximately 1000 locations across the globe. The data collected through each radiosonde launch creates an atmospheric profile of the wind, temperature and humidity patterns above that location. This atmospheric profile is fed into global weather models and is a critical data component of accurate forecasting.

With respect to atmospheric moisture, the vertical profile gives an indication of where and how much moisture is available for supporting the development of precipitation. High amounts of low level moisture near the surface can be an important source of fuel in the development of unstable air and convective thunderstorms. High amounts of mid-level moisture can sometimes support high-based thunderstorms where the rain that falls evaporates while falling into drier lower levels, producing strong outflows and gusty winds at the surface. When moisture is present at all levels in the atmosphere, these conditions can support widespread, heavy rainfall events.

 

Precipitable Water

One way of quantifying the total amount of moisture in a vertical atmospheric profile is through the calculation of a metric called precipitable water. This is the amount of water (often in a depth of inches or millimeters) that could be condensed out of a vertical column of air extending up through the troposphere. This value is calculated with the data from each radiosonde sounding twice a day and is also estimated through satellite data as well (see image above).

Precipitable water values are carefully monitored across the southwest U.S. throughout the monsoon season, because they help diagnose the amount moisture available to fuel convective thunderstorms and heavy rain events. Values through the monsoon season can vary widely across the Southwest, but daily values are typically between 1 and 1.5 inches in Tucson as an example. Higher elevation areas have less atmosphere to work with, so precipitable water values will naturally be lower.

To put these values in context, the precipitable water value at Tucson collected from the morning radiosonde on September 8th, the day that widespread flooding occurred across Phoenix and Tucson, was an even 2 inches. This was a record high value for September at the Tucson site. This value of 2 inches of precipitable water actually turned into much higher local precipitation amounts. This is because converging air can cause precipitable water values to pile up and feed individual storms, leading to much higher local precipitation amounts than would be achieved if simply squeezing the water out of a single column of air.


Michael Crimmins is an Associate Professor and Climate Science Extension Specialist in the Department of Soil, Water and Environmental Science

Figure 1: NOAA Weather Balloon - Source: NOAA
Figure 2: Precipitable Water in the Last 72 hours (as of 09.15.2014) - Source: Cooperative Institute for Meteorological Satellite Studies
Figure 3: Precipitable Water Jan-Dec, Tucson AZ - Source: NOAA

Norbert Brings Flooding and Record Rains to the Southwest

Tuesday, September 9, 2014

Yesterday (Monday Sept 08) brought considerable moisture to Southern Arizona and Nevada, as both the Tucson and Phoenix metropolitan areas saw considerable flooding, including record rainfall totals for both areas.  This moisture was brought in by Hurricane/TS Norbert, and had been anticipated since it began curving back into the Pacific coast.  There is precedent for this type of moisture, as Mike Crimmins laid out in his blog post addressing the moisture that TS/Hurricane systems bring to the Southwest, but it remains difficult to predict the exact effect of these storms.  The experience in both Tucson and Phoenix show how disruptive and fast moving these storms (and their runoff) can be, as well as how quickly these waters can subside once they move through.

Notes from an Applied Climatologist - Aug 2014 Rainlog Climate Summary

Monday, September 8, 2014

The monsoon season continued to roll along throughout August, bringing abundant precipitation to much of Arizona, yet some places were still between the rain drops, as is typical with summer thunderstorms in the southwest. The month started off strong, with the monsoon ridge pushing to the north over the first couple of days of the month.  This helped to usher in low-level moisture, and fueling scattered thunderstorms across the state.

This didn’t last long, and by end end of the first week of August, a low pressure trough off the coast of California weakened the ridge and shifted it to the east. This pushed the low-level moisture south, and caused a brief dry out across much of Arizona. During this mini-break, we saw limited thunderstorm activity in the higher elevation areas of the southeast corner of Arizona between Aug 5th and Aug 10th.

Beginning on the 11th, the monsoon ridge pushed north again, helping to restart the deep southerly flow of moisture into the Southwest.  August 12th was a (very) big day for rainfall, as organized thunderstorms brought several rounds of heavy precipitation from southeast Arizona all the way up to Flagstaff.  Parts of Yavapai County observed over 5 inches of rain on the 12th with many Rainloggers from northern Phoenix to Prescott and Flagstaff reporting 2 to 3 inches of total rainfall.

This deep moisture remained in place over the next 10 days, which helped fire off afternoon thunderstorms each day in higher elevation locations, and with the occasional storm wandering into lower elevation valley areas.  Rainloggers in Yavapai County noted another busy couple of days from August 17th to the 19th, with daily rainfall totals between 0.5 to 1.5”, and with multiple day totals of close to 3 inches.

The second major dry-out occurred the last week of the month as a very Autumn-like low pressure system moved across the western U.S., which knocked the monsoon ridge back south. August wrapped up with several days that were completely devoid of observed precipitation anywhere in Arizona.

For the month overall, most of the state observed average to above-average precipitation, with the driest spots being in Pinal County and far northern Apache County. Short-term drought conditions improved across much of the state with the rainfall, but long-term deficits still remain and will take several more above-average seasons to catch up.