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Extreme Winter Weather?

March 22, 2012

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Mike Crimmins

An impressive late winter storm moved across the Southwest earlier this week dropping over two feet of snow across higher elevations and over half an inch of rain to much of the low desert areas across Arizona. A highly amplified (loopy) jet stream guided the storm directly from the Gulf of Alaska to the Southwest, along with an ample supply of cold air tailing the storm system (see image of jet stream below). This combination of jet stream energy and cold air were the ideal ingredients for a late winter storm across the desert Southwest.

Was this type of late winter storm an unusual outlier in terms of extreme weather? Not really. Very few records fell across Arizona over the past week. A handful of record low high temperatures fell across northern Arizona as the storm system passed, but no precipitation records fell anywhere in the state. This storm system stood out as unusual because it punctuated a string of unusually warm and dry days that characterize the dominant weather pattern that has gripped the region for much of this winter. Tucson has already experienced several days with above-average temperatures well into the 80s over the past thirty days and has observed only 0.56” inches of precipitation (with most of it falling with this latest storm) since the beginning of the calendar year. This total is about 1.5 inches below average for this time of the year. What’s more, Phoenix had seen no traceable precipitation since the beginning of the year until this latest storm.

The current, but waning, La Niña event has been implicated in driving this recent (and temporary) highly amplified jet stream pattern that brought winter weather to the Southwest and early summer and record temperatures to the eastern U.S. More importantly, La Niña is the most likely culprit behind the exceptionally dry weather pattern we have observed across the Southwest during the January-February-March season for two years. With fewer than average winter storms like the one this past week, much of the Southwest has been bogged down in moderate to severe drought for over a year now with the expectation that drought conditions will continue to expand and intensify across the region through the rest of the spring. We seem to be getting used to warm and dry winters in our region…as maybe we should.

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Many places in the southern

Submitted by Mike Schwarz on March 24, 2012 - 3:04pm.

Many places in the southern half of Arizona had only small rainfall amounts from the March storm. Douglas airport received .02" total. In Phoenix, although some places on the eastern edge of the metro area had close to an inch of rain, the airport got only .25", bringing their year-to-date percent-of-normal rainfall up to a whopping 10 percent of normal. Perhaps the main thing I'd like to note is the tendency of the NWS to over-forecast the amount of rain and cloudiness we receive in Phoenix. They had forecast Phoenix would receive 1-2 inches of rain on March 18. Over and over again, the amounts we actually receive are much less than forecast. In January and February, forecasts for significant rainfall yielded trace amounts at the Phoenix airport. And many, many times, they forecast partly to mostly cloudy skies here, and I see nothing but nonstop sunshine from sunrise to sunset. I think with the jet stream going further north, they are not taking our increased tendency for aridity into account, in the forecast models they are currently using.

Many thanks for your comment,

Submitted by Michael Crimmins on March 27, 2012 - 9:16am.

Many thanks for your comment, Mike. I talked with a colleague at the National Weather Service about the performance of the forecasts with this winter storm. Records show that the official forecast for most of the Phoenix metro area were about 0.75" for the entire event with the expectation that higher amounts would be observed in the higher elevation areas north and east of town. If you look at observations reported on http://www.rainlog.org, http://www.cocorahs.org/ and http://water.weather.gov/precip/ they all show that precipitation amounts were pretty close to the forecasted amounts. There were definitely dry spots with less across the metro area which were probably due to the way the precipitation formed in bands as the storm system moved through the region. The west side of Phoenix definitely saw less as well, which is also due to how and where the precipitation bands form. Another interesting aspect to this storm system is that the long-range forecast models (7-10 days) picked up on this storm about a week out and did a pretty good job of correctly forecasting the final track and speed of the storm system. This is an amazing advancement in the accuracy of forecast models in just over a decade. Still, forecasting is not for the faint of heart and still a very tricky business, especially in the Southwest. Storm systems have to connect with an ample supply of moisture, which is often difficult to predict many days out. This is where forecast models often stumble in longer-term forecasts. They do a really good job of gauging the position of the jet stream and track of storms, but will the moisture be available...that is another question.